two 24VAC Sources to one 5VDC

[bill12780]

Hello all,

This is my first post here. So just a bit of background. I am NOT a fully educated Engineer. (read--Associates in Electronics with 25 years of experience) However, I do play one at work. So somtimes I ask pretty stupid questions.

Anyway, I have a project that is really perplexing me. I have a control module (Another company) that outputs TWO(2) 24VAC Sources and share a common. One source actuates a valve open, the other, Closed.

I need to interface OUR sensor to controll these two sources via a ucontroller. These 24VAC sources are the only avaliable source of power to the circuit. So I want to rectify them and use them to make a 5V source. Problem is that they are never on at the same time and they switch back and forth depending on what the controller module is telling the valve to do.

I have worked out the details but It requires two relays and two isolation transformers, and two bridge rectifiers.

Anyone got any ideas how to do this other then what I have come up with?

Any advise here is greatly appreciated.

Best Regards,
Bill12780

 

[geekEE]

What about just using two auctioneering diodes to half-wave rectify your two supplies?

You should probably check that the control module just switches between the two outputs and that there isn't a dead period in between where neither one is on. If there is, then you may need some significant capacitance to keep your supply up during that time, depending on how much current you're drawing from it.

 

[itsmoked]

You just use a fullwave rectifier. Which ever coil is energized provides half-wave power to your filter cap.


Your coils continue to be hooked up directly across the two outputs.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[itsmoked]

You will still need a regulator and filter caps and maybe some form of snubbering to protect your 5V load.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[bill12780]

WOW! I forgot to close this window and I thought I would just refresh and BANG! Replies....thanks!

GeekEE,
OK so "auctioneering diodes" I have never heard this term. I did a quick google and found mention of them for what looks to be a DC_DC converter. But nothing else..Can you give me a small brief education on this? Or possibly point me to the right website?

itsmoked,
"You just use a fullwave rectifier. Which ever coil is energized provides half-wave power to your filter cap."

I am confussed...I have no coils just three wires comming from a controller. I have NO IDEA what is happening in the controller. I belive that there is some delay in the switching but I would bet its pretty quick. I understand the need for the regulator and associated filter/transite caps etc.. But from your schematic I would need to hook both "Hots" to the one leg of the transformer and the "Neutral" to the other. I am not sure what that would do to the controller. If I knew what was going on in there I might could do this.

Right now I have one hot hooked to the coil of a relay and jumpered to the common contact I then drag the 24VAC through an isolation transfomer to seperate the grounds and then to the Bridge.

there are two of these circuits so that no matter what Hot goes live. It will rectify the voltage.

Could you please explain this a bit more?

Just so you know why I am asking this question is that I am currently using a 24VAC isolation transformer that is used in phones. Its the low end of the freq response is about 300hz and its getting a bit warm (running it at 60hz)and I am having a heck of a time finding a small isolation transformer that will suit my needs.

thanks for your input! (both of you!)

best,
Bill12780

 

[itsmoked]

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[geekEE]

Actually, Keith's picture is pretty much what I had in mind. Your controller box would replace the transformer in the drawing. The three leads coming out of the secondary would be the same as the three leads coming out of your box. The top and bottom leads are the "hots" and the middle is the "neutral".

When you have two supplies hooked up with diodes in that manner, the supply with the higher voltage will "win" and supply the current. Auctioneer diodes are usually used with two DC supplies (or a DC supply and a battery), but in this case it would work well to both rectify and choose between one supply or the other.

 

[geekEE]

Ah! Keith beat me to a reply by a minute and with a pretty picture too!

 

[itsmoked]

Muwhahahahah!!

Ah, but you seem to understand his "box" thingy better than me!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[ScottyUK]

Half-wave rectifiers?! DC in transformer windings? Horrible! But almost ok if the load current is not large and we are in the realms of Chinese cheap & nasty. The cost of the larger filter cap to reduce the ripple to an acceptable level might make it more economical to use two fullwave bridges and fit the auctioneering diode on the bridge outputs.

I disagree that an auctioneering diode is the same as rectifier diode, unless you mean that they are likely to be the same component type. The role in the circuit is different: auctioneering diodes are common in pure DC systems where a rectifier would have no purpose.


----------------------------------

Sometimes I only open my mouth to swap feet...

 

[itsmoked]

Scotty...

Engineer..


Politician...


Lawyer...


Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[ScottyUK]

I was told by an old boss that my idea of diplomacy was to apologise just before whacking someone...



----------------------------------

Sometimes I only open my mouth to swap feet...

 

[itsmoked]

LOL

So you're maybe one of those "I'm sorry I have to do this", types.

Hahaha.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[MacGyverS2000]

Scotty: "This is going to hurt you a lot more than it's going to hurt me..." ;-)

Dan - Owner

http://www.Hi-TecDesigns.com

 

[bill12780]

itsmoked, (or keith)

that is brilliant. I would have never thought to do it that way. (lack of formal education I would imagine)

I get the "auctioneering diode" thing now. I have used this before for batt/AC operated units. But nice term "to the highest bidder" / "highest current" I get it.

I am gonna build this up and see if it works. I am however gonna use a full bridge and put the diode on the DC supply. Has ScottyUK mentioned.

Will let you all know in a hour or so...

thanks for all your help!
Bill12780

 

[logbook]

The key to this problem relates to just how much power you need at 5V. If it is quite a lot then you will want to use some transformers to step the 24V AC down to 5V DC.

The efficiency of the proposed schemes is so low that for every watt of 5V power you use, you are dissipating 5 watts.

You could use a 110V primary transformer at 24V you know.

 

[geekEE]

I think if you use two full-wave bridges, then you wouldn't need separate auctioneering diodes because the diodes in the bridges would serve the same purpose.

logbook makes a good point about efficiency. Bill, if you use a linear regulator and you draw a significant amount of current, your regulator could be dissipating a lot of heat. If this is the case, you may need to investigate using a transformer or perhaps a switching regulator to make it more efficient. Do you know how much current you'll be drawing from your 5V supply?

 

[bill12780]

Ok here is an Update...

I built the circuit and it smoked the rectifiers immediately and with out perjudice!

BUT...this idea lead me to exactly what logbook said. Then when I found the transformer I realized how I could do this with one bridge, one transformer and get the isolation I need.

I used a dual primary 115V/230 transformer. Instead of hooking the primaries as usual, I hooked one Hot to the top of the transformer and one to the bottom and used the two in the middle as the neutral. The transformer I have on hand is not the right ratio. But it proves my design.

I am not sure. But I think it has to have isolation from the two different grounds. (ac neutral and DC ground) Someone with more "Theory" then me might can answer that.

But at this point I think I have a resonable design. Thanks for all you guys input! Hope I can return the favor someday!

Best to all!
Bill12780

 

[logbook]

>I used a dual primary 115V/230 transformer. Instead of hooking the primaries as usual, I hooked one Hot to the top of the transformer and one to the bottom and used the two in the middle as the neutral.

Sounded good at first ... but one primary will strongly couple to the other. The ON circuit will drive the OFF circuit. If there is actually only one valve then you may get away with it.

 

[bill12780]

logbook,

Your right. But each valve is controlled seperatly so there would be one of "my" circuits for each valve. (All hooked to the same sensor)

One thing I did forget to mention the HOTS are wired throught a relay that is self-firing. So each hot is physically disconnected when not in use.

Thanks for the input thought. I do appreciate a second, third or forth pair of eyes! I learned long ago no matter how smart you think you are---there is always someone smarter!

Bill12780