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.
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
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
http://www.national.com/an/AN/AN-181.pdf#page=3
Generally with an opamp and a 3055 in the same sentence you mean 'current booster'. Are you sure that's not what you mean?
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: current limiting
RE: current limiting
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
RE: current limiting
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
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
RE: current limiting
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: current limiting
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
RE: current limiting
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
www.retallickeng.com.au
Was told it couldnt be done, so
i went and did it!
RE: current limiting
RE: current limiting
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
Dan - Owner

http://www.Hi-TecDesigns.com
RE: current limiting
RE: current limiting
Dan - Owner

http://www.Hi-TecDesigns.com
RE: current limiting
RE: current limiting
Dan - Owner

http://www.Hi-TecDesigns.com
RE: current limiting
http:/
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
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
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
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:/
http:
http:/
http://cg
http://c
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: current limiting
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
I agree with Keith, buy something designed for the purpose.
RE: current limiting
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
RE: current limiting
RE: current limiting
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
RE: current limiting
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
RE: current limiting
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: current limiting
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
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
http:/
www.retallickeng.com.au
Was told it couldnt be done, so
i went and did it!
RE: current limiting
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
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
----------------------------------
If we learn from our mistakes I'm getting a great education!
RE: current limiting
RE: current limiting
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
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
RE: current limiting
RE: current limiting
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
As you can see its very simple.
www.retallickeng.com.au
Was told it couldnt be done, so
i went and did it!
RE: current limiting
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
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
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
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
RE: current limiting
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
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
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
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
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
RE: current limiting
Do you have a sketch?
www.retallickeng.com.au
Was told it couldnt be done, so
i went and did it!
RE: current limiting
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
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
RE: current limiting
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
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
I'm not really sure what you'd want to know.
RE: current limiting
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
RE: current limiting
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
RE: current limiting
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!