A question about transformers

[Kala_12]

I wonder if I could ask what might be a silly question about using mains transformers.
I have two toroidal transformers, both with two windings, one at 12-0-12 volts and one at 7 volts.
My question is can I connect one end of one 12-0-12 to the “opposite” end of the other to give an output of 24-0-24 volts?

I ask this as I tried it and the unloaded output voltage varied from 48v to 60v on a DMM after displaying 1 for a number of seconds. (Just thought - would I need to take into account phasing?)
Stuart.

[Chrispy57]

Hi - yes, this is often done. I wonder if you have 120V rather than 240V primaries though to get those measured voltages?

Cheers
Chris

[Kala_12]

There two 120v primaries in series. I would have expected a steady 48 volts, but the measured output fluctuated.
Stuart.

[Skywave]

How much was that fluctuation? E.g. 47v - 48v - 47v? Or perhaps 48v ± 0.25 v. ? What instrument did you use to measure that voltage?
I am assuming that the measured voltage was on no-load.

Al.

[Lucien Nunes]

If you get the phasing wrong, the reading between the two ends will be near zero, as it will be the difference of the two 12-0-12V windings rather than the sum.

Could this be what you saw, not 48-60V but 48-60mV of difference? The fact that it is a small voltage and the transformers physically separate would account for the fluctuation, and also the DMM going over-range at switch-on as their transient behaviours would not be so nearly equal as the steady state voltages.

[Kala_12]

The variation was something like from 48v to 60v, but I will repeat the measurements in the morning. I will also measure the individual transformers.
The measurements were done with a DMM, on load and off load and report back.
Stuart.

[Kala_12]

A bit more checking done.
Measured across the ends of 12-0-12 off load = 29v on analogue meter, varying from 21 to 25 on two different (cheap) DMMs.
Measured with a 300ohm load = same as above.
Measurements the same on both transformers.

Measured across the ends of both 12-0-12 in series off load = 58v on analogue meter, varying from 41 to 58 on two different (cheap) DMMs.
Measured across both 300ohm loads = same as above.

The 7volt winding measured as just over 7volts on analogue meter, 7.4v on both DMMs.

Stuart.

[Skywave]

To me, your results throw doubt on the reading accuracy of the DMMs you've used. However, you have also used an analogue meter. What is it? Manufacturer, model type, etc., please.

Al.

[Cruisin Marine]

First things first, make sure the primary windings are wired in series, 2 x 110 volt to give 220v or whatever in those regions.
Second thing, measure each 12 volt secondary and are they both 12 volts each when measured individually?

I guess this is a state of confusion here; 12-0-12 will be 24 volts measured across the WHOLE of that individual secondary winding), so if wired in series with another 12-0-12 winding (in phase of course) will give a result 48 volts AC?

[Kala_12]

Al,
The analogue meter is a Radio Shack multimeter, the DDMs are cheap little things from Maplin. I am not particularly bothered about accuracy, as long as it’s close and consistent.

Cruisin Marine,
Quite right. See post #7. Note it is 12-0-12 off load.

These two transformers are about 120 VA each and are to supply two power amps. When I power one of the amplifiers - just to 2 or 3 watts - all is fine. When I power the other one, all is fine. (I must add this is with the lamp limiter is circuit). When I connect both amplifiers to the PSU, the lamp comes on almost full brightness and there is a clean sounding hum from the loudspeakers at about the level of a smallish transistor radio. Is this to be expected? Can I risk just put 3amp fuses in the AC lines to the rectifier or alternatively a pair after the rectifier / capacitor, to each amplifier and switching the lamp out of the circuit?

I’ve just read that - I hope it makes sense.

I have tried to create a diagram using LTSpice, but I can’t get it to do what I want.

Stuart.

[Kala_12]

Sorry. line 5 should read
Note it is 12-0-12 ON load.
Stuart.

[Silicon]

Could this be due to a 'hum loop'?

Do you have anything connected to the amplifier inputs?

[bigfathairyvika]

I would say, the results throw doubt on the way it is being measured not the measuring equipment.
Or, the transformer is faulty.
I have found that using a lamp limiter when measuring transformer output gives false results.
Eg: A transformer with a 700v secondary without a lamp limiter and open circuit secondary measures around 700v.
With the lamp limiter the measurement is more like 500v or less.

[Ed_Dinning]

Yes, measuring a transformer with a lamp limiter in circuit will give reduced voltage readings, BUT, the ratio between windings will remain constant. Hence if the Pri is know then the exact sec voltage can be calculated

Ed

[bigfathairyvika]

Hmmm..
Maybe the next thing for the OP to do is measure the voltage across the primary, which is one of the first things I would have done after checking offline resistances of the windings and between the windings.

Edit: As the winding ratio is fixed and if the transformer is not faulty and the measurement is done correctly, then the only possibility is that the mains input is changing.

[Kala_12]

Silicon – I have tried with and without a CD player connected, volume controls at minimum.

Bigfathairyvika – aaah, never thought about the limiter when doing the measurements but as said above, I’m not too bothered about exact measurements; ballpark is good enough. The max DC voltage for the amps is 45-0-45. I have 4 or 5 of these transformers so I’ll try different ones. Regarding the mains voltage changing, that would be a rather large swing in value to give such a large swing on the output. I’m happy to accept the fact that the DMMs are the original problem and that I shall forget about them.

Ed – Would you confirm (or otherwise) there is no problem my using two transformers like this.

Could someone comment on the middle bit of post #10 please.

Thanking you for your help so far.
Stuart.

[Silicon]

If I refer to the 12-0-12 windings as one 24V AC winding (with a centre tap), I am assuming that you have connected two of these in series (and in phase) to give approximately 48V AC.

What are you connecting this AC voltage to?

How is the signal 'Ground' connected to the power supply and amplifiers?.

Also, how is the mains Protective Earth connected to your amplifiers?

[Ed_Dinning]

Hi Post10 seems to imply that there is a cross connection between the 2 amps somewhere.Try reversing the phase of the mains supply to one transformer

In theory no problem in connecting 2 transformer like this, but the devil is always in the detail

Ed

[Kala_12]

Not sure what you mean Ed.
I hope this diagram answers questions.
All voltages approximate.
Stuart.

[Kala_12]

Sorry it’s taken so long to get this sorted, but I can now let you know what happened.
First things first. The attachment shows how things are connected up, remembering that the transformer is actually two 29volt (off load) transformers, one for each side.

It eventually clicked what Ed said about the lamp limiter in circuit (…yes, I know!) so I bit the bullet, connected a couple of sacrificial loudspeakers with a small series resistor and tried it without the limiter. It worked. Shorted out the series resistor and all was well. Next, I tried the system as a whole using larger loudspeakers. It worked as required.
I still don’t understand why when I try one or other amplifier all is ok but both together didn’t work.
Lesson learned – THINK. So thanks for the comments and suggestions giving me the confidence to go for it.
Stuart.