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Small Voltage Regulator Answered

I am looking to buy or build a small voltage regulator that can fit in an O gauge electric train engine.
It needs to have a 18-20 volt AC input and 10-12 Volt AC 1 amp output.
I would like to buy one already built, buy a schematic or a schematic with the parts to build.
Thank You

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Downunder35m

4 years ago

A 78L12L should fit in the SOT-89 package but you need a transistor to do the current handling as it is only rated for 100mA.

For everything else you would need something in a TO-220 package.
But if your really need AC why not simply use a small transformer?

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gmoonDownunder35m

Reply 4 years ago

A stepdown transformer is indeed the best solution, as Downunder35m suggests. A 240 / 120 isolation transformer could be wired to use the 240 input, yielding about 50% of the primary voltage. That's a bit less than the 55-60% you're looking for (can the engine run on 9-10V?).

A brute-force approach would be to drop 8 volts with a series resistor. If the engine indeed draws 1 amp, that calls for a 8 ohm resistor...dissipating approx 8 watts. Since the engine would probably draw more current under load, a 20-25 watt resistor would be the minimum power rating, with some sort of heat sink. And devices in series draw the same amount of current, so you're doubling the current draw on the supply transformer...

Would I recommend this approach? No. But this might have been the standard approach for model railroading, once-upon-a-time, so it has some historical mojo. You'd certainly have enough waste heat to produce those fake smoke effects some fancy...

(Don't do it. ;-)

Diodes in series can be used to drop a little voltage, but for 8V that's like 11 or 12 diodes (times two for AC, one for each half cycle, or 24) which isn't practical. And they still dissipate heat.

There are ways to work with Zener diodes in AC, but again--same amount of heat...

Use the transformer, if you can find/make the right one.

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frankv6Downunder35m

Reply 4 years ago

Thank You,

I will try that and let you know how it works.

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frankv6Downunder35m

Reply 4 years ago

Thank you for the information, I never thought about a transformer but I doubt I would be able to find one that would fit, I only have an area about 2” X 2” and about 1/2” deep, I was thinking more like a small circuit board. What is a 78L12L? I am not familiar with that.

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Downunder35mfrankv6

Reply 4 years ago

What use is a small circuit board?
You want AC to AC, any voltage regulator can only work DC ;)
You
can simply experiment with some magnet wire and and old ferrite
transformer like found on basically all switch mode power supplies,
amplifiers and even ADSL filters.
As an example wind 100 turns on the primary, connected to your 20V and 80 turns on the secondary for the 12V.
Depending on what you are actually trying to do I am sure there are simpler ways....

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verence

4 years ago

A 78S12 can handle up to 2A in a TO220 package. But that would create (20V-12V)*1A = 8W of heat. Which is a lot in a small housing. And it would give you 12V DC.

For a quick and dirty solution (and if the output wave form is irrelevant), you could try a simple diode. This way, you get only one half wave of the input AC at the output. Technically the output voltage would be have of the input voltage if you only look at the RMS. And of course, it is not technically AC any more, just pulsing DC, but a motor may be happy with that.

If the output HAS to be AC, a small transformer may be the best option. To do it with semiconductors, you would have to rectify the AC, regulate the voltage down (maybe invert it) and then have a dual output stage to somehow create a +/- rectangle signal.

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frankv6verence

Reply 4 years ago

I should have explained myself better, first by a small circuit board I was looking for a 2” X 2” card I could solder the components on. I have a small smoke unit with a heater which looks like a wire wound resister with an AC fan motor smaller then the size of a gum ball. I like the idea of winding a small transformer, I have a box with some old componets I might be able to use. Good idea

Thanks

Fan Driven Smoke Unit.tiff
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verencefrankv6

Reply 4 years ago

I think you should try the simple diode trick. A 1N4001 (or anything up to 1N4007) should do the trick, maybe two in parallel (as they are rated only for 1A). You don't even need a board, just put them in one supply line.

If you try it, just check that the fan motor turns (duh!) and neither motor nor diodes get hot.