# Now I am going to ask 3 stupid questions, please dont laugh. but i cant seem to find info

Discussion in 'General Electronics Discussion' started by Brad Hruska, Jul 6, 2017.

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Jan 26, 2017
My first stupid question is concerning a variac, as the voltage drops say from 120v down to 12v does the amps go down equally? lets say at 120v the variac is pushing 10 amps, so does this mean at 12v the variac would be pushing only 1 amp? or does it work like inverse as the voltage decreases the amps increase? or do the amps stay the same regardless of voltage??

My Second stupid question is concerning full bridge rectifiers, I know it switches ac to dc, and the there are maximum loads.
Like on the kbpc 8010 rectifier, I know the maximum voltage it can handle is 1000v @80 amps.
Can anyone point me to a rectifier that can operate from 0 -500v at about 60A

Now for my third stupid question if I hooked up the rectifiers to the output side of a variac, would the dc voltage go up and down as you turn the ac up and down???

Last edited: Jul 6, 2017
2. ### BobK

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Jan 5, 2010
1. A variac is a kind of transformer. The rule for transformers is that power in = power out. So, if the primary of a variac is drawing 1A at 120V, that is 120W. The secondary, to have equal power, must be 120 / 12 or 10A. There are, of course, some losses due to heating, so the input power will be slightly higher.

2. Diodes begin conducting at about 0.6V. Below that, no current flows. A bridge has two diodes in series, one on each side of the AC input, so it heeds about 1.2V before conducting. So yes, it will work at 3V, but you will get more more like 1.8V out. If the 3V is RMS, not peak, it will be higher by a factor of the square root of 2. At higher currents, the voltage drop across the diodes can increase to a little over a volt, further reducing the output.

3. Yes.

Bob

3. ### ramussons

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Jun 10, 2014
The current depends on the load connected. V / R = I.
If the load is unchanged, the current flowing will reduce when the voltage is reduced.
Since the variac is wound with the same gauge of wire, the Permitted current will be same, irrespective of the voltage.

Yes. But it will be highly inefficient since the Forward Voltage Drop will be about 1.2 to 1.4 Volts.

Yes.

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Jan 26, 2017
Sorry guys if I sound dense but I still don't get the amps in relation to volts. If I drop the volts down from 120vac to 12vac do the amps go down by 10th as well?

Do they stay the same
Or do they go up

I guess what I am trying to ask is If at 120v there is 10 amps available, how many amps can I draw at 12v

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Jan 26, 2017
I found a sheet on the kbpc 8010 it say a 50 - 1000v so does this mean 50v is the lowest it will operate at?

6. ### BobK

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Jan 5, 2010

I told you that if there was 120V 1A in the primary, there must be 12V 10A in the secondary. And you ask if the amps goes down. No, going from 1A to 10A is not going down.

Two of us answered that it would work at 3V, though inefficiently. Now you ask if 50V is the minimum voltage. No, 3V is less than 50V, so 50V is not the minimum.

Bob

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Jan 26, 2017
Sorry Bob, Yes o did read the answers, I was not clearly understanding how the variac worked since I understood that it had 1 coil.
So just to be perfectly clear it works like a standard transformer then, where by if the voltage drops by a factor of 10 the amps will increase by a factor of 10. So if the unit has wall amperage available of [email protected], when the voltage drops to 12v I (in theory) should have 100A available. Am I understanding this perfectly?

As far as the rectifier goes what is the 50v they are talking about?

I thank you for your help with this.

8. ### BobK

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Jan 5, 2010
Yes, it works like a standard transformer. If you had a variac that was actually drawing 10A in the primary with it set to 12V output, there would be 100A in the secondary. However, that would be a massive piece of equipment. If you think about the power, it cannot be otherwise. You cannot create power, so the power out cannot be larger than the power in. And if the power out is less than the power in, it has to go someplace, so it cannot be less than the power in.

50V. I can't tell you for sure, unless you link the datasheet, but I suspect that it is referring to a line of rectifiers with maximum voltages starting at 50 and going up to 1000.

Bob

9. ### duke37

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Jan 9, 2011
A 10A Variac (other autotransformers are available) will be a heavy lump, do not drop it on your toe.

Assume that you set have the input at 120V and a current into a resistance of 10A, then the resistance is 12Ω.
Dropping the output voltage to 12V will provide 1A into the resistance. 12W.
12W from 120V will need 0.1A input. P = I*V
There will be losses and inductive currents but the input current will be not much more than 0.1A

10. ### kellys_eye

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Jun 25, 2010
10A is the maximum current the variac will allow without burning out - regardless of the voltage you set it to.

But the current will depend on what's connected - NOT what the variac is set to. It's the connected load that determines the current that is drawn.

But using a variac as a variable power source is not a good idea anyway - there is no isolation from the mains supply and electrocution is a greater risk.

What is your intended use for the variac anyway? Most users apply them to repairing SMPS circuits or valve radios. It's use outside of these applications is very limited - primarily due to the aforementioned safety issue.

11. ### duke37

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Jan 9, 2011
I have seen it stated that using a Variac on a SMPS is not a good idea since it may not like a low voltage.
Running a valve radio gently is also not optimum, at low voltage the rectifier does not work then a little more voltage does not limit the current. I use a lamp limiter for running up old valve (tube) radios. A 40W bulb in series is about right.
My Variac is used to supply an old battery charger which rectifies the voltage and provides isolation. This gives me variable speed on my coil winder

Most of my experimenting is done with an earth leakage trip and an isolating transformer and I am still here.

12. ### kellys_eye

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Jun 25, 2010
Of course using a variac for any form of repair relies on the appropriate knowledge of how and why....

I'm aware of the lamp-limiter method for powering radios too - a variac/charger/rectifier is a bit 'brutal' for a speed control though given the availability of cheap controllers nowadays.

Still, we all tend to use what we have available so whatever fits.....

Kudos on setting the standard for safety by the acknowledged use of isolating transformer and ELCB - I suspect not many workshops go to that extent - more should.

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Jan 26, 2017
If the primary coil says 120V @ 10 amp max (I understand and agree that you should allow 100% headroom i.e. run it at 5A max) Why would the variac burn out if I ran it at 12v @ 100A?
I mean if you drop the voltage by a factor of i.e. 120 on primary down to 12v on secondary, the amps available on the secondary would go up by a factor of 10 also, ie. 10A imput on primary, 100A on secondary no??

So I don't understand why you say 10a maximum regardless of the voltage you set it to??
Please explain... I believe I might have missed something...

ps. I fully intend on isolating the system in 6 locations, ground fault socket, an isolation transformer, a foot peddle with relay combo connected to the mains out to the variac, between the variac and the rectifier, between the dc and the hotwire with a speed controller that is also hooked to the foot control.

I think I got the safety pretty well covered

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Jan 26, 2017
Thank you for clarifying that Bob.
So if the variac is a 120v @10 amp on primary and is capable of outputting 12v @100a on the secondary, Why is Kelly_eye in post 10 saying that " 10A is the maximum current the variac will allow without burning out - regardless of the voltage you set it to."

I don't get it and this is what is confusing the hell out of me... he is basically saying the Amp in on the primary needs to be the same as the Amp out on the secondary. Why?

Please let me know what is correct

I need to get a maximum of 50A @12v and be able to get 5A at around 90v Would a .5kva variac work or should I get a 1kva unit??

15. ### kellys_eye

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Jun 25, 2010
That variac will NOT deliver 12V at 50A - end of. The wire it is wound with has a maximum current rating and it is this that sets the maximum allowable current to be drawn.

The variac rating is its absolute maximum.

True, a 1.2kVA variac can deliver 10A @ 120V (1200W) but as the voltage drops you have to de-rate the power draw to maintain the 10A limit so the actual power dissipation drops as the voltage drops. Set it to 12V and you can still only draw 10A from it.

If you want low voltage at high current then you need a switched-mode PSU (standard practise these days as they are cheaper and more efficient than linear regulated supplies) and you can find them on eBay - new and used.

Getting a fully variable DC supply at those currents won't come cheap.

16. ### duke37

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Jan 9, 2011
A Variac is an autotransformer with one single layer winding around a magnetic torroid. This winding is machined flat and plated with noble metals and the output current is taken by a carbon brush placed on this winding at the appropriate voltage.
Excess current will distort the winding and the carbon brush will burn a patch on the winding. The output current all goes through the brush/winding interface and the limit is the same regarless of the voltage.

If you want [email protected], then use a 600VA transformer with output of 12V and feed it with a 10A Variac (0 to 120V). Big Variacs are very heavy and very expensive.

An ordinary transformer can be overloaded for a short time until it gets too hot but a Variac should never be overloaded.

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Jan 26, 2017
Ok thank you Kellys eye for that clarification.
as far as the switched power supply, it will not work for hot wire cutters, because of the short circuit protection, overload, over temp and some other thing they install so that it shuts down till the short is removed.

This is why I need to use a linear system.

18. ### duke37

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Jan 9, 2011
I made a box for my neigbour to control a hot wire cutter. This used a triac light dimmer feeding a transformer. I put a bulb in series with the primary to indicate the current and to protect against possible transformer saturation.
He has made some very good model aircraft using polystyrene wings.

Edit: These were made in parts so they could be got out of the house.

19. ### AnalogKid

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Jun 10, 2015
Because a Variac basically is a tapped inductor, its current rating is limited by the wire gauge. Winging some numbers here - let's say it is wound with #22 wire and rated for 120 V, 1 A. When you turn the tap down to the 10% point (12 V), you cannot pull 10 A out of there because that 10 A has to pass through the other 90% of the winding, and that still is only #22 wire.

One way to think of it is that the voltage rating is a max value determined by the wire insulation, terminal spacing, and other things that become less reliable at higher voltages and more extreme environments (high humidity, etc.). The current rating is a max value determines by wire size, terminal size, wiper contact area, and other things that become less reliable at higher currents and temperatures.

ak

20. ### BobK

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Jan 5, 2010
No I did not say that. You did not state the the Variac was rated at 10A, you stated that the variac was pushing [sic] 10A at 120V:
I said that the 12V secondary would have to be 100A if the 120V primary was pulling (not pushing) 10A. I guess I took your question too literally. I thought it was a theoretical question, not a question about the capabilities of an actual piece of hardware.

Bob