Correct Way to wire two transformers to a Voltage Selector Switch

[Phrazemaster]

Hi all,

I've got a PSU I'm building that has 2 transformers, a toroid and a conventional type. They are both rated for 120/240 use on the primary, depending on how they are wired.

I have a voltage selector switch, 120/240, that I want to use to switch both with. I actually did this before, and it seemed to work fine, but then I thought since I'm redoing it I better ask. I can't seem to find anything on google about it.

My concern is, is this safe? The same switch wiring both transformers? Or would secondary currents flow between them due to different windings presenting different impedance? Obviously they are both at the same potential, so I *think* it's ok.

The PSU I did this with before worked, no smoke, and was stable, but it did hum.

Any thoughts? Attached pic of wiring.

Thanks,

Mike

 

[ruffrecords]

I think you 120V wiring is wrong. You need to connect 1 and 3 and also 2 and 4 to get the primaries in parallel.

The principle is sound. The mains is effectively a very low impedance source so it will drive both transformers OK.

Cheers

Ian

 

[Phrazemaster]

Hi Ian,

You are correct. The numbers on the voltage selector switch do not correspond; I just had them on the drawing because the voltage selector switch has them. But they are the voltage selector switch numbers and not the transformer wiring numbers.

If you look at the colors I do have 1/3 and 2/4, which is Blue/Violet, and Gray/Brown, from both transformers, going to the right voltage selector switch poles. The voltage switch internally connects poles  1-2, and then 3-4, which corresponds to Blue-violet and gray-brown...see the upper left diagram of the voltage selector switch.

I appreciate your insights about the mains impedance being low. I can see this would work fine for 120v, as mains drives both transformers in parallel - fine.

What concerns me is the 240 case, where the windings are in series.

So mains drives both transformers' first primary coil, which is then connected to both transformers' second primary coil. But here the output from each primary's first coil is mixed with the output of the other transformer primary's first coil.

Won't this result in a phase shift as the windings of the first primary coil of each transformer will be different lengths? Or a problem due to different inductances...not being an EE I'm in over my head.

I did use this scheme in this very same transformer, and no smoke, but I was using it in the 120 configuration only. And it hummed something fierce, although this is likely due to placement of the transformers...

I'm sorry if this is a stupid question; any insights are welcome and thanks so much.

Mike

 

[ruffrecords]

In the series case, both transformers each have their primaries wired in series but these are then wired in parallel with each other. You should not wire the primaries of two different transformers in series with each other. That is very unlikely to work.

Cheers

Ian

 

[Phrazemaster]

Thank-you Ian, much appreciated.

 

[Walrus]

I think I see what Mike is saying. When the system is set to 240 volts, the two transformers will have their Primary winding centre points joined to each other. i.e. the Gray Violet joint of one transformer will be joined to the Grey Violet junction of the other.
Will that not have some detrimental effect??

 

[[silent:arts]]

I always wondered the same looking at attached schematic taken from a well known manufacture - 3 transformers wired in parallel to the voltage selector switch ...

 

[Phrazemaster]

I would think there would be some kind of phase shift as each primary's first coil would be different inductances...?

Then when mixed to go to each primary's second coil, the second coil would have some kind of phase shift as compared to the first...but I don't know.

My solution is kludgy. I've just ordered another voltage selector switch from mouser, so I'll use one per transformer.

If anyone else knows the answer to this, it'd be great to understand!

Best,

Mike

 

[PRR]

> attached schematic taken from a well known manufacture - 3 transformers wired in parallel

I'm 93% sure this is ONE transformer with two primaries and three secondaries.

> the two transformers will have their Primary winding centre points joined to each other

Is OK, this time.

If you connect "two" coils on one core in Parallel, and they are not the SAME number of turns, they will fight. 120V vs 121V causes a 1V difference, which in the low copper resistance can cause a large useless current and heat.

I face the same thing on my house. I want 240V CT to 240V CT. If the "CT" is not *exactly* centered, say 119V:121V, there would be a large circulating current, up to 900 Amps(!), though probably about 9 Amps (bad for my electric bill).

But in this case, we KNOW the two 120V windings on each core are *exactly* the same number of turns, because they tell you to connect them Parallel for 120V operation.

With two transformers you have to think about matching. But again both transformers are rated for Parallel operation, so the two windings on each are exactly same-turns. When run Series for 240V, the two CTs will be *exactly* centered. The CTs may be connected together with negligible voltage difference (>0.1V) and insignificant circulating current.

Use a Lamp Limiter for experimenting and start-up!! Too many wires there, way too much current available from your wall, you do not want a mistake to burn-up the house (or even pop fuses in the cellar). With the lamp limiter, unloaded happy transformers is no-light, goofed shorted connections is just a 50W lamp and no sudden harm. (You probably can't run the final build on a lamp, just do the no-load tests with lamp.)

Hay-wire the Series connection but CTs not connected to each other. Carefully measure the voltage difference. Should be far under 1V, and probably under 0.1V. if so, you can connect the series CTs if that simplifies the switching. (I do not like two switches, that's just odd, and invites mis-settings.)

If you have one PT much bigger than the other (say 50VA and 10VA) and the large one is not fully loaded (say 35VA on a 50VA part), then just 120/240 switch the big one. Wire small loads for 120V and put them across one 120V winding of the big core. The big one will autotransformer 240 or 120 to 120 on one 120V primary. You can see this in some large guitar amps which feature a 120V cooling fan and 120/240 mains switching.

 

[Phrazemaster]

Thank PRR...I knew something was amiss with the scheme I hatched.

 

[[silent:arts]]

I'm 93% sure this is ONE transformer with two primaries and three secondaries.

Yes, but look at the right side of the schematic where you find two more transformers (one secondary each), primaries wired to J, K, L, M from the first transformer ...

 

[PRR]

> look at the right side of the schematic where you find two more

Ah!! Missed that completely!

Well, then (re-drawn so even PRR can see it) this is your cook-book solution. The 117V/234V switch is simple DPDT. The four primary wires on each transformer are labeled J L K M, *just* like the drawing, and joined to the switch.

Still much room for confusion, so do the lamp-limiter.

Or even bring it up with a 12V 1A heater transformer at the "120/240V" input. With the lower voltage, secondaries should run 1/10 or 1/20 of nominal voltage ("15V" gives 1.5V or 0.8V AC). The 12V 1A heater transformer won't burn-up fast or melt your wall wires. If it gets very-warm quickly, something is wrong.