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24 VAC to 12 VDC

sundy

Aug 5, 2016
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I am trying to rectify 24 VAC to 12 VDC without a step down transformer.
  1. I used one of my PCB's as an experiment. The circuit works but the 12 V regulator heatsink gets way too hot.
  2. I am also worried about loading the source PS as the unit I am working on are connected in parallel, otherwise I would do something like this http://www.brighthubengineering.com...9-make-a-transformerless-power-supply/#imgn_2
  3. I have searched all over for a SMPS that operates at 24 VAC input, I have not found a off the shelf option yet.
  4. I don't have the time or the skill to design a SMPS.
  5. The unit I am driving draws only 0.080 A at 12 VDC.
  6. I am thinking of using an isolation transformer designed for the audio industry since they are cheap.
I am looking forward to comments.
 

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AnalogKid

Jun 10, 2015
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Am I correct in assuming that the 24 Vac is distributed power in a master clock system, and already is isolated from the AC powerline?

If your numbers are correct, the max power dissipation in the regulator should be 0.38 W. That's 0.34 W for the output current and 0.04 W for the 7812 operating current. -ish. That is warm enough to notice but not melt fingerprints. "way too hot" is a circuit assembly problem, not a design problem.

12 Vac > 1 diode (1N4004) > filter cap > 7812 > output cap > output

Efficiency is around 70%, so I don't think the complexWithout isolation or a bridge rectifier, you will have to pay more attention to the AC connections.

ak
 

sundy

Aug 5, 2016
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Yes it is a clock system. I got my thermometer out and the heat sink is up to about 150 F at the hottest spot. I think it's going to be OK. I'm using full wave bridge rectifiers. I wonder what load the they present to the PS?
 

sundy

Aug 5, 2016
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Excellent point. The author did make that point in the article but you are correct to point out the safety issue.
 

sundy

Aug 5, 2016
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I am considering this question answered, solved, and put to bed.
 

BobK

Jan 5, 2010
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If your numbers are correct, the max power dissipation in the regulator should be 0.38 W.
I get a lot more than that.

24VAC will rectify to 32VDC. You now have 20V to drop in a linear regulator at 80mA which is 1.6W. A smallish heat sink should be enough for that, though. So if it is getting very hot, I agree with your conclusion that something else is wrong.

Bob
 

AnalogKid

Jun 10, 2015
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I magically converted 24 Vac to 12 Vac before starting. oops.

1.6 W in a bare TO-220 is going to get very hot, but even a small heatsink will bring that down to comfortable.

ak
 

sundy

Aug 5, 2016
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I am interested in this 'fixed impedance' of a capacitor. How could I overcome the other shortcomings of the circuit to utilize the fixed impedance?

Transformers are heavy and expensive, I am trying to avoid them.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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I am interested in this 'fixed impedance' of a capacitor. How could I overcome the other shortcomings of the circuit to utilize the fixed impedance?

Transformers are heavy and expensive, I am trying to avoid them.


Rectify, filter, and use a small DC-DC switch mode regulator.
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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I have just this week finally moved to my new home in Venice, Florida, but I haven't forgotten about you. It will take awhile to get setup here for electronics work, but since you said you had already hired someone to handle your electronics work, further examination of your secondary clock circuit is a zero priority for me right now.
 
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sundy

Aug 5, 2016
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Right now a test clock is running of the schematic I posted in the OP. The heat sink is not as hot as initially thought. If I can run 20 clocks off a 24 VAC 4 A transformer I will call it good.
 
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