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Power supply part design: AC DC convert problem

I have a PSU parrt design problem. It should receive 24V AC input to get 12V DC. It start with a snub circuit, which is a varistor. The second is the bridge rectifier, which should gives out full wave rectified DC. Then the bridge rectifier's V+ output connect to a 220uF capacitor, whose negative pin connect to the V-. Following the 220uF capacitor, is the 12V regulator.

The problem is when I supply the circuit with the 24V AC, or actually it is 27V AC, the regulator will enable the thermal shut down protection. No DC output.

But if I only give the 24V DC supply input, the PSU part is fine.

I figure out the problem is the AC/DC part problem. But don't know the exact answer.

The current consumption is 120mA. The 220uF is oversized, a 22uF might do the job, which I had a test and it seemed fine.

Reply to
DAXU
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27Vac (assuming RMS) gives you a DC voltage of about 35V. thats getting close to too much for a 7812 style regulator.

Then (35V - 12V)*0.12A = 2.8W. You will need something like a 10degree C per Watt heatsink, to stop the regulator getting too hot.

Its not a large heatsink, but its not tiny either.

Cheers Terry

Reply to
Terry Given

Hello! Terry,

Thanks for your answer.

Since the heat sink might be a problem in its size, may I have a resistor put after the bridge rectifier to drop some voltage down to like 23V. That resistor might be (35-23)/0.12=100Ohm, which power rate should be over 1.5W.

Is that solution OK?

Cheers, Jerry

Reply to
DAXU

yep, although dont forget that, say, a 2W resistor gets HOT.

Cheers Terry

Reply to
Terry Given

Hi Thanks

Will the 2w resistor break in the end if it is too hot, say for power up over months, years. Should that resistor put before the 22uF capacitor after the rectifier?

Cheers, Jerry

Reply to
DAXU

it can (literally) burn the PCB material. If its too close to the electrolytic capacitor, that caps lifetime will decrease (it halves for every 10C increase in cap temperature). Excessive temperatures lead to solder joint failure.

the trick is to ensure that the leads are nice and long (sometimes they are bent into funny shapes) and that the resistor is not running at its rated power dissipation (typically the temperature rise at rated power is 100C - 200C). If you were to dump 1.5W, then a 2W resistor will get extremely hot, but a 5W resistor wont, especially with long legs

in that position it also serves to reduce the peak current seen by the cap, again improving cap lifetime.

Reply to
Terry Given

This is gross overkill. The standard MO for a 12V DC output is to start with

14 - 16 V AC. If this is a one of a kind hobby project, just use a 12VAC transformer and an LM2940 regulator with a 1000 uF capacitor.

If your 27V transformer has a center tapped secondary, just used a full wave rectifier instead of the bridge.

Tam

Reply to
Tam/WB2TT

Hello,

Depending on your experience and time, you might consider a switching regulator (buck converter, for example: National Semiconductor "Simple Switcher" or the modern LM500X series).

Is your 24V coming from a small mains transformer? If so, you must also account for the no load voltage. It can be up to 20% higher than the rated voltage at nominal load, so the amplitude of the no-load voltage can be close to 40V. In addition, you should add a percentage for mains fluctuations (10%?).

Are you sharing the transformer with other devices? If so grounding the DC circuit may result in a short circuit of the 24VAC.

Best regards,

Wim

Reply to
Wimpie

Depending on your load demands, I'd probably put a zener after the resistor - call it a "preregulator", if you want. :-)

Just a raw resistor doesn't do any voltage dropping at 0 mA, so you're back to your 36V input. The zener would limit that - well, you know how a zener regulator works:

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Good Luck! Rich

Reply to
Rich Grise

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