# interesting transformer

#### matty204359

Joined Apr 6, 2011
105
I have inherited a really cool transformer. The markings indicates it is a variable transformer. I've tested it a bit and seems to work as expected. I'm curious about a few things. one is could I use a bridge rectifier to make a DC supply? two is how does it work? My understanding of transformers is it has to have two different windings with a different number of turns to create a voltage difference. sorry about the huge images, i wanted the markings to be clear.

#### #12

Joined Nov 30, 2010
18,224
It's an autotransformer, commonly called a Variac. You could use it to make a DC voltage, but not on this site. It has no isolation from the mains.

#### matty204359

Joined Apr 6, 2011
105
It's an autotransformer, commonly called a Variac. You could use it to make a DC voltage, but not on this site. It has no isolation from the mains.
I understand talking about non isolated transformers is a not allowed here but can I ask how to isolate my transformer, one idea I had to isolate it was to use an isolated transformer prior to the variac and feed the output to the variac would that be isolated?

as a consequence would the variac have more resolution? suppose it was 2 volts per a degree rotation at mains voltage. if I put a lower voltage first (say 12volts) would it be like 0.2 volts per a degree rotation.

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#### #12

Joined Nov 30, 2010
18,224
Put the isolation transformer on the output of the Variac. It works better there. If it's a step down transformer, you will get better adjustability.

#### matty204359

Joined Apr 6, 2011
105
Put the isolation transformer on the output of the Variac. It works better there. If it's a step down transformer, you will get better adjustability.
It is a 120 to 16 volt transformer, I will remove the variac and show the transformer in all its glory. so variac >transformer > bridge >output, is this considered isolated and acceptable to discuss?

#### #12

Joined Nov 30, 2010
18,224
I believe it is.

#### crutschow

Joined Mar 14, 2008
33,318
Put the isolation transformer on the output of the Variac. It works better there. If it's a step down transformer, you will get better adjustability.
It also allows a higher output current from the step-down transformer since the full output power of the Variac can be utilized.

#### crutschow

Joined Mar 14, 2008
33,318
It is a 120 to 16 volt transformer, I will remove the variac and show the transformer in all its glory. so variac >transformer > bridge >output, is this considered isolated and acceptable to discuss?
And, of course, you will need a large filter capacitor at the bridge output to smooth the DC.

With a center-tapped output transformer you could make a tracking dual output supply.

#### matty204359

Joined Apr 6, 2011
105

These are the transformers I currently have in my possession, where could I buy a transformer with a higher voltage rating 24 volts is nice but I think I could use some higher voltages maybe 40-50volts. would they sell a transformer at a place like RadioShack?

#### #12

Joined Nov 30, 2010
18,224
I think the limit at Radio Shack is 25.2 volts, center tapped but, you can get on the internet. Why don't you look up Radio Shack?

#### matty204359

Joined Apr 6, 2011
105
I did look at radio shacks website already, I'm going to call an electronics distributor that is a few blocks away from my house. does anybody have any suggestions of places that might carry transformers?

sorry if i have a lot of questions but i'm starting school on september 4th and want to have as much practical knowledge as I can get.

#### #12

Joined Nov 30, 2010
18,224
There's a thread on here somewhere about internet vendors. Farnell, mouser, jameco, digikey. Just google "electronic parts". You can buy all sorts of transformers on the internet. For a quickie, try the store down the street or buy (2) two 25.2 volt transformers at Radio Shack to get 50.4 volts, RMS. That will get you about +/- 70 Volts DC and a few other combinations. More than enough for hobby work.

Now that you know a 25 volt transformer can produce 35 volts of DC, maybe you don't even want 2 of them.

#### matty204359

Joined Apr 6, 2011
105
There's a thread on here somewhere about internet vendors. Farnell, mouser, jameco, digikey. Just google "electronic parts". You can buy all sorts of transformers on the internet. For a quickie, try the store down the street or buy (2) two 25.2 volt transformers at Radio Shack to get 50.4 volts, RMS. That will get you about +/- 70 Volts DC and a few other combinations. More than enough for hobby work.

Now that you know a 25 volt transformer can produce 35 volts of DC, maybe you don't even want 2 of them.
wait! you can combine transformers? will that reduce the wattage rating if you do? and if you combine two of them can you use the point that you combine them as a center tap and get +25.2 and -25.2 volts

#### #12

Joined Nov 30, 2010
18,224
yes, no, yes, except you can't get + and - volts out of a transformer, only AC volts.

#### matty204359

Joined Apr 6, 2011
105
yes, no, yes, except you can't get + and - volts out of a transformer, only AC volts.
you would use a bridge rectifier to get dc. I have to research how these split supplies work. I'm imagining you would have the common be the positive and the negative of the dc output of two different rectifiers so the "ground" would be in the middle? I believe I have much research to do before attempting a split supply because I don't want to smoke my variac or burn my house down.

#### #12

Joined Nov 30, 2010
18,224
That's what fuses are for.

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#### #12

Joined Nov 30, 2010
18,224
I use fuses because I've been collecting them out of old equipment for 40 years (they are basically free to junk collectors like me). Still, you must use something! and a circuit breaker will do the job.

I install circuit breakers in some equipment, like battery chargers, so I don't hate all circuit breakers. I just use what's convenient.