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current limiting
3

current limiting

current limiting

(OP)
I once had a circuit diagram for adjustable current limiting using a ua 741 and 2n3055 pass transistors.
No longer to be found.  Anyone  have a similar one?  its to limit up to 30 amp.

www.retallickeng.com.au

Was told it couldnt be done, so
i went and did it!

RE: current limiting

Is this to be part of a power supply where the 2N3055 transistors are already part of the voltage regulating circuit, or an external stand-alone series current limiter?

If the former, then the ARRL Handbooks normally have more than one low voltage, high current power supply circuit that includes current limiting using the pass transistors with one more op-amp and a couple of transistors to control the current limit by means of the pass transitors.

Personally, I don't recall ever seeing a stand-alone current limiter.

RE: current limiting

Do you want current limit or current foldback?

RE: current limiting

(OP)
The exact terminology in not sure on.  I have a 40V powersupply being used for anodising. amperage must be held to a constant value.  I have a voltage regulator atm, using a LM317 and 4 of mj2955 transistors. unfortunatly i have to constantly monitor the amps being drawn as resistance in the bath changes.

I dont need to regulate the voltage as that will be set by the  resistance, so a seperate circuit to replace the voltage reg will be needed.



www.retallickeng.com.au

Was told it couldnt be done, so
i went and did it!

RE: current limiting

This should be listed in the constant current notes for the LM315.  Basically the four PNP transistors are in parallel.  The input pin of the regulator is fed power through a resistor that also supplies the base current for the transistors.  The out pin and collectors are connected to a current sense resistor sized to give you regulator voltage at the desired current.  The regulator common pin is connected to the other side of the resistor and id also the power supplies output. Transistors should have a small load sharing resistor in each emitter.  Actual design values based on desired current.

RE: current limiting

Maybe you could swap power supplies with 'JohnBreen'.

http://www.eng-tips.com/viewthread.cfm?qid=210823

He has some huge 7.5V 300A power supplies unsuited for his 13.8v purposes, and it appears that your power supply is missing a key feature (adjustable current limit) for your purposes.

RE: current limiting

(OP)
300 Amps!!   sounds like a large welder!! My electroplating and anodzising setup doesnt warant such capacity.
Im in Australia so not really suitable, even tho i do have 3phase (415v)
I built my supply from scratch, rewound microwave transformer.
I had a good look at the 317 data sheets, and might go this way!  Thanks OperaHouse.  Myself being very low on the ladder of electronic design could you point me to a circuit diagram?

www.retallickeng.com.au

Was told it couldnt be done, so
i went and did it!

RE: current limiting

Well here it is, only because you got me feeling sorry because  you have to eat kangaroo jerky and probably run this on solar panels.  I have a friend who tried to import that Billybong stuff and ran into all sorts of problems with alegator regulations.  You can keep switching in resistors in a binary fashion to get any current.  Use at least 5W wirewound resistors.

RE: current limiting

(OP)
Thankyou OperaHouse!

Kangaroo is acutally rather nice, much better than possum soup.

I take it i cant just change the resistors to a Pot for variable adjustment?

itsmoked,  thankyou for your contribution, but i wouldnt know where to wire in any extra pass transistors.

www.retallickeng.com.au

Was told it couldnt be done, so
i went and did it!

RE: current limiting

Where are you going to find a pot that will take 15A and is under an ohm!  Current is defined as 1.2/R.  The values I gave you result in about half the current listed.  More than 20 years since I used a LM317, thought it was about 1.7V and not 1.2V.  I could add a pot but there are many downsides to it.  You can add additional switches, but don't go below .5 ohm on each switch.

RE: current limiting

Hiya-

If this is a "one off" type and you don't feel comfortable with doing a large amount of engineering, you can have several automotive type lamps/headlamps wired in parallel with switches to do the current regulation and act as loads for you.

If you search the web for anodizing, you will run across one of these circuits.  Just a bunch of lamps and the current is adjusted (with the aid of a meter) by turning off and on various lamps.  Since you are running around 40V, you might want to string a couple of lamps in series for each load as you don't want to accidentially burn out a 12V lamp with 40V across it.......

If you want some more "robust" types of loads, one might want to salvage the reistance wires from old toasters, electric wall heaters, etc.  Note that you will want to consider compression type connections for the wire used.  Again, these can be switched in or out via parallel circuits.

Also, out on the web, there is a rough calculation for the current/surface area of the part being anodized. I don't have immediate access to the info. It had something to do with anodizing telescope parts.  Was a pretty good article.

I have a back burner project doing anodizing but will be using a car battery and a large wire wound variable resistor for my efforts.  Since mine is just about a one-shot affair, it will be crude, but hopefully effective doing small parts.

Doggone it. I couldn't find the link on google directly.

Maybe a wander over to:

http://groups.yahoo.com/group/anodizing101/

Might find some additional input.

Best of luck on your efforts.

 Cheers,

   Rich S.

RE: current limiting

(OP)
This supply wil be used for Anodising, type 2 and 3,  and also hard chrome plating, so it will need to be robust and reliable!  If only i could locate the circuit i once had.

www.retallickeng.com.au

Was told it couldnt be done, so
i went and did it!

RE: current limiting

You said you wanted up to 30A.  To be more reliable, I would use more than 4 transistors.  What is your open circuit voltage and normal electrode voltage.   At 10V across the transistors that would be pushing 80 watts on each transistor.  300W is a lot to dissipate. The LM317 has a lot of protective features.  This circuit is simple and robust, you are just pushing it if you go over 15A.

RE: current limiting

You don't say what the resistance of your load is, but keep in mind, with a 40 volt supply and a 30 amp limit, there's a chance that your limiter will dissipate 1,200 watts, mostly in the power transistors.  In that case, with an absolute maximum dissipation of 115 watts for the 2N2955 (in a TO-3 case), you'll need 10 or more devices with lots of heat sinking (hopefully with a fan) to scale up OperaHouse's circuit.

If there's no chance of the full voltage appearing across the current limiter, you can reduce the number of transistors, but you need to consider the base drive current.

The DC current gain of 2N2955 transistors can be a minimum of 20 @ 4 amps and 5 @ 10 amps.  That means that with 8 parallel transistors each handling about 4 amps, the LM317 will need to supply a total of 1.5 amps to the bases.  The TO-3 package LM317 has that as a minimum spec, but at a maximum voltage across the device of 15 volts.

With only 4 PNPs, each will have up to 7.5 amps of collector current, the current gain could be as low as 10, so you need 3 amps to supply the bases.  So with 8 pass transistors, the design is marginal for 30 amps, with 4 transistors you're depending on luck that you have 2N2955s in the top end of the hFE range, and the power dissipation could be a killer.  Consider an extra transistor to drive the parallel bases in a Darlington configuration.

You might also think about using FETs as the pass devices.  They parallel fairly easily, and require minuscule gate drive.

RE: current limiting

I'm also going to suggest FETs instead of BJTs (or at least IGBTs)... paralleling BJTs is a good recipe for thermal runaway.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: current limiting

All that stuff still wastes heat.  You could do better with just an old SCR. If you are going to have a FET at least include a switching regulator.  I gave him just what he asked for, a LM317 and four PNP MJ2955 transistors. Obviously this worked in the past for him because he knew the components.

RE: current limiting

The discussion turned to "using FETs as the pass devices."  I have an old adjustable SCR power supply that puts out 30A over a wide voltage range that produces little heat.  Old technology, but it worked.  Using a FET in a passive role only complicates everything for this guy who is having problems with a couple resistors.

RE: current limiting

I'm okay with keeping it simple, but you stated that an SCR would produce less heat than a FET, and I was asking how you came to that conclusion.

Dan - Owner
http://www.Hi-TecDesigns.com

RE: current limiting

Would this be the type of thing you are looking for?

http://www.zen22142.zen.co.uk/Circuits/Power/add-on.html

You have to adjust the components to get a higher current. Stay within the limits of the transistor. In the end, you may end up with a 4A or 5A current limit per transistor or some such thing. Then, just parallel the circuits up to get your total current limit.

If you want to go further then build an adjustable regulated supply for the base that is capable of supplying the base current for multiple transistors and use it to drive all the transistors in parallel.

If you haven't got it figured out yet, the current limit is the base voltage minus 0.7V divided by the emitter resistor.

I think Operahouse is referring to using a phase controlled rectifier via SCR's. If so, the phase control part is not simple and it's hard to filter the output for a constant DC. However, if a lot of ripple voltage is OK in this application then it could be done.

RE: current limiting

(OP)
Operahouse's  circuit is great, but i need to have linear adjustment, not just a few steps.  my old, now lost circuit used a pot to do this.  

With the voltage regulator, using nichrome wire to load it, at 18 amps, and 30 volts yes the 4 of TO3's  and heatsink were too hot to touch, even with fan cooling.  
Im all open to ideas, one mate mentioned using a 555 and mosfet as a switching supply. Im not sure how smooth the DC output would be, i need very little to no ripple.  

Im far from an EE, but enjoy building things myself as its the best way to learn more about it.


www.retallickeng.com.au

Was told it couldnt be done, so
i went and did it!

RE: current limiting

You can expand it up, but it will still produce the same heat.  A 2:1 pot could be added wasting  more heat in the resistors.

Using about the same parts it could be converted to PWM.  See Nationals AN181 fig 9.  Increase C3 to operate with a suitable inductor, R4=0, put an opamp at the bottom of R5 with a cap to ground, and current sense in the common return leg.  Problem is getting an inductor to handle 30A.  I found some big inductors in some sodium lamps that could be paralleled.  You're between standard look up designs.

RE: current limiting

If you are serious about doing plating... You need to pitch this "make it yourself" thought.  Get a robust reliable powersupply that has already dealt with the packaging, heat management, safety, indication, distribution, and adjustability, and focus on the plating,(the ultimate point!).   The path you are going down is like a guy learning to fly a plane so he can pick up some parts without paying UPS.

To wit here is what I see on the first two eBay pages I looked at.

There is no way you can come close to competing with the logic of buying one of these verse making one.  You can't  come close.


http://cgi.ebay.com/MASTECH-LINEAR-DC-POWER-SUPPLY-VARIABLE-0-30-V-0-20-A_W0QQitemZ260217764939QQihZ016QQcategoryZ58286QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

http://cgi.ebay.com/Kepco-KO-25-50-DC-Power-Supply-0-25V-0-50A_W0QQitemZ230229836286QQihZ013QQcategoryZ58286QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

http://cgi.ebay.com/Sorensen-Nobatron-DC-Power-Supply_W0QQitemZ320225050292QQihZ011QQcategoryZ58288QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

http://cgi.ebay.com/Power-Ten-4600D-40125-DC-Power-Supply-40V-125A_W0QQitemZ220209803145QQihZ012QQcategoryZ58286QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

http://cgi.ebay.com/Power-Ten-163C-60110-DC-Power-Supply-60V-110A_W0QQitemZ220209803665QQihZ012QQcategoryZ58286QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: current limiting

JimKirk - "...limiter will dissipate 1,200 watts..."

I understand why I've never seen an external current limiter at this sort of power level. The advantage of an internal one is that it can turn down the voltage. Thus it would be far easier to add the current limiting circuit inside the existing power supply than to build an external limiter.

Of course, it would be even easier to buy a cheap PS with the feature already built in.

RE: current limiting

Yep, that was sort of my point.  To throw around that kind of power, a switch mode supply, or something with an SCR pre-regulator is a huge advantage.  In any event, none of the possibilities are for the faint hearted or inexperienced.  I built a current limiter that handles up to 20 amps or 50 volts, using 10 parallel FETs heat sinked for about 100 watts continuous dissipation.  It's powered by a 9 volt battery.  Scaling that up for a dissipation 12 time as much is certainly doable, but it'll heat up a room pretty fast.

I agree with Keith, buy something designed for the purpose.

RE: current limiting

(OP)
To buy would be a nobrainer, but being in Australia its very slim pickings!   USA voltages and Frequency, by the time i added a stepdown transformer, and assuming the supply will handle 50Hz instead of 60Hz.  

Is there anything wrong with homebrewing ?   
I do realize there is going to be a heat issue, we are used to heat down under hehe.

hmmmm,   what do to.



www.retallickeng.com.au

Was told it couldnt be done, so
i went and did it!

RE: current limiting

If you really want it adjustable a 1.5V battery can be added to subtract out the 1.2V reference.  Connect a 2.2K resistor to the battery - and the other to a 10K pot.  The wiper now connects to the LM317 common pin. Te other end of the pot connects to the battery + and to where the common pin of the LM317 used to connect to, the output of the regulator.  This will be adjustable to the upper limit of the current sense resistor selected.  A relay can be used to turn on the NEGATIVE lead of the battery.  A dead battery results in maximum current.  Battery life should be quite ling.

RE: current limiting

Adding a current limiter (foldback of the voltage) into an existing power supply would only require a handful of components. And the only added component that would experience the full current would be the low value series resistor (which can be made with a suitable chunk of wire). Everything else would be low power op-amps and transistors and similar. If you can match your existing power supply architecture against one with a current limit, then the design would be almost cut-and-paste.

RE: current limiting

(OP)
VE1BLL,
I thought all current limiters work on the principle of reducing voltage. But i dont know much. I would like to see this circuit.

Operahouse, would this modification still have the 3055's making lots of heat?

www.retallickeng.com.au

Was told it couldnt be done, so
i went and did it!

RE: current limiting

Which modification?  Converting the LM317 to PWM operation will greatly reduce the heat. Any analog solution will produce the same heat. The number of transistors will have to increase to 7 (one in darlington config)regardless and there will be design issues you will likely be unable to work out with PWM.  So many questions from someone who won't give a star.

RE: current limiting

"I thought all current limiters work on the principle of reducing voltage."

An EXTERNAL current limiter would essentially DROP the voltage to limit the current and thus generate huge quatities of heat.

INTERNAL current foldback circuits just "reach over" and tweak the voltage down using the existing voltage regulator circuit in a manner that is virtually no different than adjusting the voltage.

That's why external current limiters are a non-starter for high power systems. But internal current limiters are very common on power supplies.

RE: current limiting

I imagine your current power supply for this is just a transformer, rectifier and capacitor. Still, you are asking to  build a 1KW supply with minimal parts.  A 30A tank doesn't seem like a hobby application. Can you break it up in multiple smaller tanks?

RE: current limiting

Why not a rectifier bank.  A choke.  Some capacitors.  And a Variac?   No wasted power at all.  Variable adjustment.

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

RE: current limiting

Another idea...

If your existing power supply has remote sensing terminals, then you could fiddle with that signal to foldback the output voltage without modifying the internals of the power supply.

In other words, you sense the current in the usual manner with a low R resistor, run that signal through an op-amp or two, and then use it to control the power supply via the remote sensing terminals.

Of course, you'd have to consider all the circumstances, and maybe mix in some consideration of the output voltage as well.

RE: current limiting

(OP)
I think VE1BLL has come up with something that sounds like it will work a treat, Internal .  Maybe my original circuit worked this way, with the UA 731.  What are remote sensing terminals?  I can modify the internals as much as i need to as i built it from scratch.  

25 amps minimum needed to hard chrome plate @ 3Amps/Sq in.

Ok a star for you Operahouse.  
Yes it is transormer, rectifier and capacitor, with the voltage ajustment atm.  

I dont need the voltage adjustment.

www.retallickeng.com.au

Was told it couldnt be done, so
i went and did it!

RE: current limiting

You are not understanding the basics.  In any analog regulator circuit heat will be produced.  It is the difference between the input voltage and the electrode voltage. That difference times the output current will be the amount of heat produced.  Having a large surface area to get rid of that heat and distributing that heat in multiple transistors will make it reliable.  

You have a perfectly good circuit now that with additional transistors could handle the current.  Your application appears to require a large differential voltage that will waste a lot of heat.  That will turn any circuit you likely will find to toast in a short period.  There is nothing out there that will magically solve your heat problem except using a Pulse Width Modulator.

I am all for experimentation, but there is a big difference in building a PWM circuit that supplies 3A and one that will provide 390A. A lot of people have built little PWM regulators and think they work great.  On closer look they have problems because of layout and component selection.  I don't believe you have the equipment and knowledge to develop PWM ans support quickly wains here as things get really technical.

RE: current limiting

I misunderstood the starting point. I didn't realize that the existing power supply was completely home brew (including the regulator circuit design). I was making an assumption that the existing circuit already included some key features, and that those features were of mature design (capable of dealing with the worst-case heat for example).

At these power levels an analog solution is impractical, and a switching solution is non-trivial and fraught with hidden difficulties.

So the simplest solution would be e-bay; even in Australian there should be something available. Or look to the local Amateur 'Ham' Radio clubs for cheap surplus power supplies. At commercial PS meeting your specs might be US$100-$250 on the surplus Ham radio market.

RE: current limiting

Look for supplies by Farnell / Advance, Power Ten and HP / Agilent as starters for high current, low voltage switching power supplies. The really heavy current low voltage stuff is a minority interest so it tends to turn up way below true value. I picked up an 8V 580A HP unit for under £1000 when list is maybe ten times that. There's plenty of big PSUs out there, and most aren't too fussy about 50Hz or 60Hz.
 

----------------------------------
  
If we learn from our mistakes I'm getting a great education!

RE: current limiting

I wouldn't exactly say impractical.  I have an ACDC EL-300 electronic load that is a similar item in not a very large package.  Rated at 60V, 60A, 300W it has six transistors taking all the load.  Without the fan it can handle 60W.  Any voltage and amperage combination can not exceed 300W.  I had searched the anodizing community message boards and people there are using a car battery, lamps, and battery chargers with light dimmers.  I have asked for the supply voltage and normal electrode voltage and never received an answer. There was some mention of 30V which may mean he is starting with an unusually high starting voltage.  I we don't get better details of the process he will soon be left alone with the dingoes.

RE: current limiting

My Lambda 15v (regulated, adjustable +/-5%) 80A power supply was only $40 Cdn at a local Ham radio flea market. That's about $0.50 per lb of weight.

And since my PS more-or-less covers the highly desirable 13.8Vdc car battery replacement voltage range, it is more of a sellers' market. Units that don't cover this 12 to 13.8Vdc range are a buyers' market in the Ham radio world. I've seen units that were a '2-man lift' being sold for $10-$20 range.

Getting one with a current limit knob narrows the market a bit. But (for example) my company once pushed an old surplus test rack out on the loading dock heading towards the dumpster (skiff). I removed the 0-40A 0-30A HP PS and brought it back into the lab. If I hadn't, it would have been landfilled.

So, there's lots out there if you look in the right places. And they shouldn't cost much if you're lucky.

RE: current limiting

(OP)
Dingoes wont cause any harm!
the howling does get annoying tho
ebay australia is very very dissapointing
Ok, i WAS hoping to use the one supply for anodizing (decorative color), hard anodizing and hard chrome plating.

I now see that the large range of voltage is causing you blokes all kinds of headaches trying to sort this out.

Decorative anodising, up to 25 volts.
Hard anodizing uses from 25 to 50V,
Hard chrome 5 volts.

i think i need 3 supplies!!   Thats ok, i can do that.



 

www.retallickeng.com.au

Was told it couldnt be done, so
i went and did it!

RE: current limiting

Use the same limiter, just adjust the input voltage going to it.  A variac would be ideal to adjust the input voltage to the transformer.  Other transformers you have around could be as a buck transformer to lower the line voltage 12,24,36 and 48 volts.  These would not have to be large, the secondary current only needs to be that of the power supply primary.  Keeping the differential voltage of the limiter to around 10-15V will keep the heat reasonable, minimum 5V.  300W would be about right for maximum watts.  Just add two more transistors in parallel and one more to drive them.  E of driver goes to base of the parallel transistors.  A voltmeter or two small 12V lamps in series from input to output of the limiter would be helpful in adjusting the right voltage.

RE: current limiting

One thing that has bothered me since your first posts is your requirement of little ripple. I have seen most chemical reactions benefit from pulsing, batteries love it. One site uses a PWM speed control modulator they got off ebay for anodizing to adjust average current.  There seems to be little  agreement about anything on anodizing message boards.

RE: current limiting

(OP)
Hard chrome needs less than 5% ripple. your description of low ripple may be alot different.
I had a win!!  found the original circuit today, will post the circuit tommorow for you viewing pleasure.
UA741 wont take 40 volts so theres a prob already.

I have learnt so much with this thread, thankyou to all!!!





www.retallickeng.com.au

Was told it couldnt be done, so
i went and did it!

RE: current limiting

Hi kenre;
I just looked at this thread. As I read it, I kept thinking, Ohm's law, I=V/R. Use a shunt resistor and control the voltage across the shunt. This will give accurate current control with minimum losses.
VE1BLL said basically what I was thinking.
If your power supply has a voltage regulator, you may need to change only one or two connections in the power supply.
If you use a 1 ohm resistor in series with the load, then the current will equal the voltage. Use a 0.1 ohm shunt (more practical) and the current will equal 10 X the voltage across the shunt.
If you can post the circuit diagram I am sure someone will be able to identify the wire that needs to be reconnected.
With a 0.1 ohm shunt resistor, the loss in the shunt at 30 amps output will be 90 watts. (I^2*R)
This will maintain the set current even when the load resistance changes.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: current limiting

Hi Bill,

That's what the circuit posted above is supposed to do. The non-inverting input of the uA741 is set to a reference voltage by the pot, the circuit regulates the current until the voltage fed back to the inverting input of the uA741 from the shunt resistor is equal to the reference. Looks like there might be a few components missing from the sketch but the underlying principle isn't unreasonable.
 

----------------------------------
  
If we learn from our mistakes I'm getting a great education!

RE: current limiting

(OP)
Scottyuk,  no components missing other than the obvious transformer and bridge rectifier.  
What has me stumped now is the ua741 only handles about 18 volts, can i supply this with 18 v via a regulator and still have it work?  Speaking to an anodiser recently, they start at 30v and ranp up to 80v to supply the required current.

Ive been looking for a PWM circuit, but nothing with higher current limiting.

 

www.retallickeng.com.au

Was told it couldnt be done, so
i went and did it!

RE: current limiting

Kenre,


Not as it stands - the uA741 supply shares the same reference as the main power circuit, and it has to be able to deliver an output on pin 6 which is two Vbe drops greater than the output voltage, so the supply range of the uA741 limits the output voltage available. That circuit is going to need quite a few mods (i.e. re-design) to work over a larger voltage range. If this is anything other than an interesting hobby project I would seriously look for a commercially built lab power supply. There have been a few on UK eBay which would have done this job since this thread started - wrong continent I know, but there's bound to be one sooner or later.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: current limiting

This circuit really isn't any different than the one with the LM371.  The LM371 just builds the op amp voltage ref and driver transistor into one package.  The transistors in your circuit won't share the load evenly and you could use LM347 with a single supply.

RE: current limiting

I doubt you will find an off the shelf design for a 30A, 0-70V adjustable current PWM supply. Honestly, if you are having some difficulty with the linear design I don't think designing a switching supply is for you. Not trying to offend you but a PWM will require more engineering than a linear.

I don't agree with Scotty's assessment of the power supply though. You can use a separate supply for the control circuitry. For 30A in the load the op-amp needs to generate about 3V or so on it's output. As you turn the current down the voltage required also goes down. You also should be able to find a better opamp that will run single ended at about 5V to do what you want. The transistors all need to be rated for the supply voltage plus some margin so they don't fail (I'd use at least 1.5 times personally).

I do agree with Scotty when he says you need more circuity to be able to parallel the transistors and get them to load share. You generally can't just parallel bipolar transistors because they do not share well. I think a shunt for each emitter and then take the feedback off each shunt with a resistor (say 10k) all joined at the opamp would likely work. Like combining a summing circuit and the control in one opamp.

 

RE: current limiting

(OP)
Oops, there is something missing!    there should be a .1 ohm resistor (5watt) on each emitter of the 3055s.

No offence taken, i didnt design this circuit myself.  I wish i was able to hehe.

 

www.retallickeng.com.au

Was told it couldnt be done, so
i went and did it!

RE: current limiting

That sounds more likely to work. Combining them to one resistor to measure the current will likely be more accurate.

I'd say find a single ended opamp that will run on something between about 5V and 12V. Then, build a power supply to match the op-amp and the rest of the circuit as you've shown and see how it works.

Is this for the RC business?

RE: current limiting

Hi Lionel,

Regarding the opamp supply... yes, you're absolutely right that a separate supply could be used which gets around my 'non-problem': I'd mentally inserted something which isn't there in the drawing. I've even been back to check!
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: current limiting

(OP)
Lionel ,
 Eventually i will add this to the business, as part of another much larger project. Im no good at designing electronic circuits, but really enjoy putting them together from a circuit diagram or even a kit.  

Scotty, are u reffering to my post about supplying the opamp a lower voltage?







 

www.retallickeng.com.au

Was told it couldnt be done, so
i went and did it!

RE: current limiting

Did you ever try putting a battery bias in the common of the LM317 to subtract off the reference voltage and make it adjustable?  I don't think you can beat using the LM317 that adds some current and thermal protection with that low a part count.  

RE: current limiting

(OP)
I havent had a chance to get back to this yet,  been busy doing what im better at hehe,  making metal swarf.

Do you have a sketch?

 

www.retallickeng.com.au

Was told it couldnt be done, so
i went and did it!

RE: current limiting

ken,

No, refering to my earlier post timestamped 6 Apr 08 0:54. Lionel corrected something for me.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: current limiting

(OP)
operahouse,

im going to try your circuit with the 1.5 v.

you posted,

Te other end of the pot connects to the battery + and to where the common pin of the LM317 used to connect to, the output of the regulator.  

Which pin are you referring to as the common pin?

www.retallickeng.com.au

Was told it couldnt be done, so
i went and did it!

RE: current limiting

That should be the center pin or case depending on the package.  What you are doing is subtracting out the reference voltage of the regulator.  The battery should last a year at that current.  A weak/dead battery will be indicated by inability to go to zero.  A 1K with a 220 ohm would make the pot more linear with a little shorter battery life.  The resistor makes the pot useful the full range.

RE: current limiting

I guess I was confused as to your requirements.

You posted you need up to 50V for certain types of anodizing. This means you need more than 50V input to the current regulator to get 50V output. If you build that circuit and short the output while running at this voltage level you'll blow up the typical 40V max input LM317.

It's kind of like the power dissipation levels. Sure, you can say I'll only allow a 10V drop across the current regulator making at most 300W of power losses. But, what happens if you run outside of this design, ie a short occurs or you forget?

I would try to design a circuit that can handle a short circuit at the highest voltage myself, but that's just me I guess.


 

RE: current limiting

(OP)
I think i have had a short circuit in my brain!

I have rewound the transformer so i have 38V DC output, to protect the 317.

This lower voltage will fine for my mostly small jobs, anything larger will just take much longer.

Any opinions on this circuit ive found?







 

www.retallickeng.com.au

Was told it couldnt be done, so
i went and did it!

RE: current limiting

The voltage and current don't meet your requirements???

I'm not really sure what you'd want to know.

RE: current limiting

(OP)
Is this a good design?  The current can be increased easily, maybe the voltage also with a Lm317 in place of the 7812.

What does have me interetsed is the single FET,  i cant see how it could be a PWM circuit, so can the single FET handle it?
 

www.retallickeng.com.au

Was told it couldnt be done, so
i went and did it!

RE: current limiting

I know it is tempting.  I look at those adds that tell me I can increase my gas mileage by 70%.  But, you haven't been listening.  Single devices have power limits and if you get anywhere near to half the rating, your design is often in real trouble.  The variable current part of that design has no current sense and that fet would easily turn to toast under certain operating conditions.

RE: current limiting

(OP)
Im listening!  but i cant learn without asking.
So the design has major flaws, i wasnt too far off, asking how one Fet could handle all that current. It Cant.

 

www.retallickeng.com.au

Was told it couldnt be done, so
i went and did it!

RE: current limiting

It doesn't appear that you have read any of the application notes of the devices you are using.  None of the questions have been technical in nature or show a basic understanding of of the application.  Your interest has been more in having someone supply a complete design.  Given that you intend to sell this as a kit, you should be reading all you can. There are a lot of questions that you should be asking that you are not.  

 

RE: current limiting

(OP)
"Given that you intend to sell this as a kit..."

Where have i mentioned this?  It is for my personal use only.
I manufacture and sell metal and fibreglass products, i understand these.    

www.retallickeng.com.au

Was told it couldnt be done, so
i went and did it!

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