Variac ?

[65535]

I am trying to win an ebay auction for new old stock variac. Anyways I have a few thoughts. It's rated at 9 Amps I figure a 10Amp circuit breaker on the output should keep it safe from overload.

I was curious though, I would assume that as long as you stay below the voltage rating and the current rating you could use any input voltage in AC. Could I infact run the input at 12VAC and get a very precision output of 0-14VAC (rated 120VAC 0-140VAC variac)?

Which I could rectify to 0-14VDC and put a smoothing cap in parallel and make a variable voltage lab supply. Along with the breaker I should have a pretty safe setup. Am I missing something?

 

[Mr Happy]

I am trying to win an ebay auction for new old stock variac. Anyways I have a few thoughts. It's rated at 9 Amps I figure a 10Amp circuit breaker on the output should keep it safe from overload.

Put the circuit breaker on the supply side, I'm guessing?

I was curious though, I would assume that as long as you stay below the voltage rating and the current rating you could use any input voltage in AC. Could I infact run the input at 12VAC and get a very precision output of 0-14VAC (rated 120VAC 0-140VAC variac)?

Which I could rectify to 0-14VDC and put a smoothing cap in parallel and make a variable voltage lab supply. Along with the breaker I should have a pretty safe setup. Am I missing something?

I think if you decrease the input voltage like that, you will probably decrease the available current in proportion. So it will run 0.9 amps instead of 9 amps. But voltage-wise it should work fine.

 

[VidPro]

yes, its like a variable transformer, cool things they are, if you can afford the shipping
i wouldnt expect a lot of "precision" the old ones we used were a Wire wound with a slider, and an old one could be a bit jumpey like a old stereos volume control.
wouldnt they also waste a LOT of avilable power? if i remember right when turned lower they put out some heat, and had lots of passive ventilation.

http://sound.westhost.com/articles/variac.htm

what if you could get one that the "secondary" coil was more in line with the maximum voltage your intending, as opposed to a full 120V one
http://cgi.ebay.com/Lab-Volt-Variac...temQQimsxZ20090302?IMSfp=TL0903021410002r1835
then you get more amps on the secondary, but never the excessive votlages, that way your Rectification, and capacitors can be of larger amps without having to be mondo to cover the extreeme voltages.

i suspect this one has a 1-3 ratio type transformer on the 0-120 output, which is another option, get a 120-35 transformer, and put it on the ouput of the 0-120

 

[Benson]

I think if you decrease the input voltage like that, you will probably decrease the available current in proportion. So it will run 0.9 amps instead of 9 amps. But voltage-wise it should work fine.

I'm pretty sure 9A at 14V will be fine -- there's no theoretical limit to the current you can pull through a transformer, and the rated limit is set to prevent fusing a wire in the winding.

Which I could rectify to 0-14VDC and put a smoothing cap in parallel and make a variable voltage lab supply. Along with the breaker I should have a pretty safe setup. Am I missing something?

Well, rectifying 10VAC into a cap won't get you 10VDC -- with no load, the cap will charge to the peak voltage, which is 1.414 times the RMS voltage. Under load, the cap will drain, and get intermittently kicked back to the peak voltage, so the average voltage will be less, with a sawtooth ripple on top.

Basically, you'll have a 0-20VDC power supply.

 

[lctorana]

Full wave capacitor-input voltage multiplication - use the formula 1.3 * AC input less the voltage drop of your rectifiers, which for a bridge is anything from 0.8 - 2.2V depending on diode type and current draw.

This assumes that the capacitor is large enough not to affect the output voltage - it needs to be at least 160/V microfarads per milliamp output, where V is the DC output voltage.

The voltage regulation then depends on two factors only - transformer losses and rectifier internal resistance.

The Variac WILL work at the full 10A current with only 12VAC input, but magnetizing current, core losses and I-squared-R losses will rob proporionately more of the output voltage than they would at 120VAC.

And the fuse? If you intend to draw 9A of current, use a 25A fuse on the supply side of the variac. But if you persist with the 10:1 reduction, you should be able to use a 2.5A fuse in the mains lead. The transformer magnetising current is inductive, and to cater for the surge for any inductive load we usually reckon on a 2.5:1 or 3:1 headroom for fusing.
Use a mains-rated 3AG fuse in a proper holder, btw, please don't be tempted to use an automotive fuse, even though it looks identical.

***

In my experience, the old wirewound variacs are very smooth indeed, if in good condition and not burned out. Noisiy, jumpy volume controls in my experience are the carbon-tracked ones, particularly if the designer was lazy enough to allow any DC in the circuit...

Apologies for the length of this post.

 

[65535]

My thought on the circuit breaker was that I never wanted to pull more than 10 Amps from the output of the variac as that is it's upper limit for current. Since the voltage is variable IV(in)=IV(out) the current is inversly proportional to the voltage which would mean even if I am only drawing 9 amps which is the rated limit at the lower voltage output (say 4volts) that would be roughly 300 amps at the 4 volts. Which would destory the variac.

So since I want to control the output current I should but the breaker on the output which would limit it to 10 amps. Unless I'm wrong a breaker works at pretty much any voltage AC.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&ssPageName=STRK:MEBIDX:IT&item=120397036647

Here's the variac. No one steal my baby.

 

[VidPro]

well he is listing it by watts, but only the voltage ever changes, as you reduce the total number of "wire turns" on the "secondary" coil, you are not increasing or decreasing the wire size or its handling capacity.

so if it took a MAX of 10amps, you will only get 10amps, until somone makes one with the wire size changing as it gets neer the ends, and interesting idea, after 40 years of variacs mabey somone should have done that.

if it took a max amps of 10, is decades old, i would probably only run it at 5A anyways, but all it could do is heat up a bit, smoke some inuslation off then short out, then your homes AC breaker will go .

cant you find a 20A to run at 10A? would you really run a radioshack 3Amp transformer at a full 3amps?

look at the last picture, see the specs on the plate? single digit for the amps ? is that a 8 or a 9?

 

[65535]

From what I have been reading variacs are designed to run to a maximum current regardless of voltage. In this case that is 9 amps. Now obviously 9 amps on the low end draws fractions of an amp from the primary input. So if all I do is fuse/breaker the primary input for 10 amps or more I could potentially draw many times more than 10 amps on the output tap until something gave way. So I believe it needs to be protected on the output. I was hoping a breaker would work as fuses are annoying and expensive to replace.

 

[VidPro]

you can only draw on the output what the output wires and wiper, and connections, can handle when low.
but really , Aged old covering over the wires , dont ya think that stuff breaks down over time?
then you gonna run it at it MAX ? which is the MAX, not how most people would have applied it.

ever pull apart an old transformer and watch the insulation flake off the wires? even though when built it was clinging on enough to wrap the thing.
you would be luckey to get the listed amps out of it, and it is the amps, look at the STARTER in your car, the voltage is low but the amps is high, when it comes to wire size and heat its the amps themselves not the voltage that is the need. short out your spark plugs and its not a big deal, cause its the amps, not the volts.

 

[broadgage]

I would advise against the use of a variac as described by the O/P, for several reasons.

Firstly a variac is not an isolating transformer, if it is connected to line voltage then the output must be treated with the same respect as the line supply direct from a wall outlet.

Secondly, if the input to the variac is at a much reduced and therefore safe voltage such as 12 volts, then it wont work very well in practice.
All transformers have a voltage drop on load, this is caused by the internal resistance of the windings.
If a transformer is used at voltage much lower than intended, then the voltage regulation will be very poor, that is the voltage will drop substantialy on load.
For example consider the variac in the O/P. It is intended for a 120 volt input, with the output variable up to 140 volts.
If this unit is connected to a 12 volt input as suggested, then the OFF LOAD output will be variable up to 14 volts, BUT this voltage will not be maintained under any useful load, it will drop very significantly.

Also the output of a variac is an adjustable % of the input voltage (subject to the voltage drop noted above) Therefore if the line voltage increases by say 5% then the output will also increase by 5%, which is clearly undesirable for constant voltage battery charging or similar purposes.

To protect a variac against overload, it is best to use a 2 pole circuit breaker, with one pole in the input connection and one in the output.

A circuit breaker in the output only wont protect against an internal fault, and a breaker in the input only wont protect against overloading the output.

 

[65535]

Here's kinda a basic idea for my circuit breaker setup, which I haven't totally made clear yet.

http://www.tonystrains.com/technews/img/xfr(8)b.jpg

Just look at the input and output not the devices on the output. Each will be a 10 Amp breaker.

I have OLD US made equipment or had but got rid of, but most of it will last fine transformers if well made are very hardy. I'll poke around with my DMM but I seriously doubt that a new old stock variac would have diminished capacity.

As for my OP I was just curious if I could use it as a makeshift low voltage power supply by connecting the output of a 120VAC-12VAC transformer to the input of the variac to get a low voltage power supply.


Edit: I don't know why I didn't think of this before. But it would be a safer better idea to put a beefy 10:1 transformer on the output I could get better current handling due to a higher voltage and lower current on the variac.

Normally the variac will be for testing line voltage things like lite bulbs and light duty motors. Not for heavy use. If I wanted heavy use I'd refurb a 200 amp variac that's on ebay for $10 with $400 shipping.