| Author |
Message |
    
Coalburner
New member
Username: Coalburner
Post Number: 1
Registered: 4-2006
| | Posted on Monday, April 10, 2006 - 5:49 pm: | 
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Can a electronic circuit be built to regulate 25-28vdc down to 13.8vdc? |
Bruce
Senior Member
Username: Bruce
Post Number: 3733
Registered: 9-2003
| | Posted on Monday, April 10, 2006 - 7:37 pm: |
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yes it can how ever the device will get hot |
Coalburner
New member
Username: Coalburner
Post Number: 2
Registered: 4-2006
| | Posted on Monday, April 10, 2006 - 11:32 pm: |
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If I was to build such circuit what would I want the maximum overvolt to be? |
Bruce
Senior Member
Username: Bruce
Post Number: 3736
Registered: 9-2003
| | Posted on Tuesday, April 11, 2006 - 6:37 am: |
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The problem is HEAT you have to wast 50% of the power comming from the supply. Ok lets say you need 3 amps the regulator transistors would have to get rid of 36 watts of heat to give you the 12 volts @ 3 amps THEN you could place a high current 15 volt zener diode with a fuse back to the sorce and take your 12 volts across the zener .... as long as the voltage stays under 15 volts the diode just sits there.
Just keep in mind that 36 watts is about the same heat as a soldering iron ........ |
Tech237
Moderator
Username: Tech237
Post Number: 267
Registered: 4-2004
| | Posted on Tuesday, April 11, 2006 - 12:51 pm: |
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Bruce,
I seem to recall that most high amperage supplies run the transformer at 28-40v out before regulation hence the big heatsinks. In fatc I had a supply that used a 40v transformer and had around 40amps capacity.
Having said that I agree in that if you only want a small (say up to 5a) current than a smaller voltage into the regulator circuit is better.
Coalburner pm me the specifics of what you want and I'll check my files for a suitable circuit. |
Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 1408
Registered: 9-2004
| | Posted on Wednesday, April 12, 2006 - 12:12 am: |
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i am currently searching for a 20vac 35-40amp transfomer to build a high current supply with.
i still cant find one!
i am thinking that this would be good to use in a power supply that will put out 15vdc.
here's the circuit that i want to use:
http://www.aaroncake.net/circuits/supply2.htm
maybe this will help with your project.
bruce, do you think i am trying to dissipate too much heat also?
i will use a fan, but i dont want it to run too hot!
will you post the equation(s) you used to derive this number? (36 watts of heat)
i am new to power supply design, but really interested.
matt |
Bruce
Senior Member
Username: Bruce
Post Number: 3739
Registered: 9-2003
| | Posted on Wednesday, April 12, 2006 - 4:44 am: |
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"T1 should be about 5 volts higher than the desired output of the supply, and rated for about one amp more of current. The voltage overhead is required by the regulator section. The extra current is to keep the transformer from over heating."
Lets asume ( BAD WORD ) that you get 24 volts to the input of that regulator and 14 out of it the transistor Q1 would at 40 amps have to dissapate 400 watts ....... Also you would have to have a current driver amp between d1 and Q1 because the gain of Q1 would be 10-20 and you would need a 40 - 60 watt zener to directly drive Q1..... |
Jp1116
Intermediate Member
Username: Jp1116
Post Number: 106
Registered: 5-2002
| | Posted on Wednesday, April 12, 2006 - 8:10 am: |
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Have you thought of using multiple transformers. Some of the larger unregulated power supplies for linears use multiple transformers and bridges. Not sure if this will work for what you want to do.
If can't find the parts and choose not to build, Astrons and maybe Pyramids are internally adjustable up to 15 volts. Some models have an external adjustment. The 50 amp models may do what you want, I've picked up a few at swap meets for a resonable price. That would give you a nice regulated, protected power supply. |
Chad
Advanced Member
Username: Chad
Post Number: 781
Registered: 11-2004
| | Posted on Wednesday, April 12, 2006 - 9:37 am: |
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Multiple 3055's set up like a power amp wiht emitter resistors would work better IMHO.. That's the way my Acopian is set up. Don't forget some snubber coils around the circuit too to reduce RF problems.
Chad |
Tech237
Moderator
Username: Tech237
Post Number: 269
Registered: 4-2004
| | Posted on Wednesday, April 12, 2006 - 2:16 pm: |
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Bruce,
Chad is right. Every large capacity supply I have ever come across uses multiple pass transistors to handle the current. A handful of 3055 is pretty cheap.
THe 40Amp supply I mention above had around 10 of them and none ran too hot to touch, especially as they used the whole case as a heatsink. |
Bruce
Senior Member
Username: Bruce
Post Number: 3741
Registered: 9-2003
| | Posted on Wednesday, April 12, 2006 - 6:53 pm: |
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I know that HOWEVER you must still be able to drive them with your referance diode. It dosn't matter if you have 10 of them the referance zener would have to be rated at 10% of max current .... |
Tech237
Moderator
Username: Tech237
Post Number: 272
Registered: 4-2004
| | Posted on Wednesday, April 12, 2006 - 7:58 pm: |
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I dont know I seem to recall I used a zener rated less than 4w for 40 amp supply. lest see 40A x 13.6V thats 544 watts. I know I did not use a 55w zener.
I did cheat, I used an op-amp to multiply the current rating of the zener.
If the circuit is here and not still in the stuff I have left in my parents garage I'll look to posting it as it might be of interest. |
Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 1421
Registered: 9-2004
| | Posted on Wednesday, April 12, 2006 - 10:54 pm: |
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maybe one of you guys who has actually built a power supply before could lead me in the right direction for a design.
my parameters are 15vdc @30-40 amps continuous.
(it seems that the only transformer i can find will put out 14-16vac @35 amps, if this helps)
i am fine with breadboarding the regulator circuit, but the more detailed the desciption the better.
its the equations that bog me down, just not familiar with them at all. so if the math were somewhat done already that would be a plus.
OK, im done asking too much for the moment,
matt |
Tech237
Moderator
Username: Tech237
Post Number: 274
Registered: 4-2004
| | Posted on Thursday, April 13, 2006 - 1:36 pm: |
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Kid,
I am trying to locate my supply design and AS SOON as soon as I do I'll work out some way of posting it on here (That OK Tech808?).
I'll include the actuall calculation used to design it.
Off of teh supply I have mentioned preivously I ran a IC-706, IC-703 (both 100w out), a FM-900 (25w ex-commercial mobil on 2m), an 50w ex-commercial on 6m, 25w ex-commercial on 70cm, a Ft-212 (5w on packet), TNC, FT-290rh (5w 2m ssb) and a second 5w 2m SSB/CW unit.
14-16v out of teh transformer for a 13.8vdc supply is running a little close for me. Normally I'd use at least an 18-20v output transformer, just to ensure I have enough votlage to ensure a stabil regulation.
Over the wweekend I'll dig into my files and look for the design. |
Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 1430
Registered: 9-2004
| | Posted on Thursday, April 13, 2006 - 10:51 pm: |
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thanks 237,
thats what im learning, that the transformer needs to be able to drop a few volts for the regulator circuit.
the more i think about it, the $150 MFJ doesnt seem like too bad a deal.
i wonder if they'll sell replacement parts? (transformers?)
matt |
