voltage divider for AC

[darrins]

I hope this isn't a stupid question.

I want to use voltage from AC to power a circuit, but at a reduced voltage level. I'm not sure exactly how much voltage I need, so I want to build a variable voltage divider with a resistor and potentiometer in series.

Is this a reasonable approach and, if so, what rating of resistors should I get? I estimate that I need anywhere from 25% to 50% of the AC to power my project.

If I get a pot with a metal knob, is there any danger of the knob becoming HOT?

Thanks for any help.

Darrin

 

[ante1]

Darrin,

I don

 

[steven2]

darins type voltage devider into your serch bar and click to find. somewhere in the forums is allso a link to some site possibly electronic calculators, electronics 2000. anyhow ive downloaded a few and theres not just resistor code calculators but allso led sieries resistor calculator and voltage devider calculators to, and others

 

[IanP1]

It is dangeres !!! but maybe you can use variable transformer? It is design to produce output from 0-100% of input

 

[ante1]

Ian,

With a variac you have the same problem it

 

[IanP1]

That I fully understand.
Yet we still don't know what voltage is Darrins going to devide/adjust.
Maybe he is after low voltage and in his case you can use transformer and SCR type dimmer and adjust voltage (this time isolated ) on the secondary from 0 to ...????
We just don't know..

 

[ante1]

Hi Ian,

What kind of SCR dimmer do you recommend for such device? The output waveform from the once I have seen does not work any good as input for a transformer. Usually the transformer gets overheated and destroyed since they are an inductive load and thus do not like the pulseshape (waveform) very much. At least this is my experience of dimmer / transformer circuits.

 

[darrins]

Thanks for the advice. I don't like to do anything that's dangerous, which is why I posted the original question. I had not heard of a variable transformer or a variac. They are a little pricey, but I could probably get a deal at my local discount electronics shop.

Thanks again.

Darrin

 

[IanP1]

Hi Ante,

I have 96 low voltage lights in my house and they are fed from 240/12V transformers and for most of them I fitted light dimmers.
These devices are using triacs (not scrs). Obviously, as they are phase control regulators, the shape of the "sine" wave is not sine any more but that doesn't matter.
Transformers? Who cares, they are as cold as ambient temperature...+ a little bit...

 

[Olihou]

I have some experience in powering low power circuits directly from 220V mains, without the use of a power transformer. Voltage regulation could be achieved by the use of zener diodes

Darrins needs to tell others what kind of device/circuit he/she intends to power before others can offer advice.

It could be rather dangerous unless every thing is well insulated, by one with good experience in high voltage circuits.

Oli

 

[ante1]

Hi Ian,

I experimented some time ago with a variable power supply based on a triac-controlled transformer. I tried this curse I wanted to avoid the heat losses created by the voltage drop over the power transistors. A feedback loop secured a constant 8 Volt drop and minimized the heat. It worked very nicely on the secondary side but not on the primary. The problem was excessive heat in the transformer and eventually damaging it. I did not manage to get it to work so I scrapped the project, this is the reason I am interested in a triac or SCR circuit that do not kill transformers. ;-)