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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 16th Oct 2021, 7:51 pm   #1
samjmann
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Default Transformer output current.

I'm replacing a mains transformer that's no longer available with a standard type. There are two secondaries of 19v, feeding two 200mA fuses.

The only transformer that will physically fit is rated at 6VA. Each secondary winding is rated at 3VA at 19vAC. From what I know the maximum secondary current per winding would be 3/19 = 157mA. Are my sums correct in this?

Assuming the manufacturer may have allowed 50% extra on the fuse rating:
Normal current draw 133mA plus 50% extra gives about 200mA the rating of the fuse.

I know what you're all thinking 'why doesn't he just measure it'? The reason being my DMM has now died on the current ranges!!

The transformer I'm looking at is RS stock code:504757

Thanks for any help or advice. SJM.
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Old 16th Oct 2021, 9:30 pm   #2
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Default Re: Transformer output current.

I would consider it unwise to count on the fuse having been oversized relative to the actual load current.
A 200ma fuse might be for a load current of 180ma or perhaps a bit more.

I would measure the load current, purchasing a new basic multimeter if need be.
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Old 17th Oct 2021, 4:59 am   #3
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Default Re: Transformer output current.

When relying on a VA rating, the current rating is 157mArms with PF=1 (ie. a resistive load). If your load is resistive and the resistance is not overtly temperature sensitive (like a valve heater resistance which has a high in-rush), then you could well be ok. If you are rectifying the 19V and have 3W of DC loading then that may be a step too far for the secondary winding.
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Old 17th Oct 2021, 10:36 am   #4
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Default Re: Transformer output current.

Thanks for the helpful replies. I could put a 1ohm resistor in series with one of the fuses and measure the AC volts across it, this would give me some idea. All that is being fed is a small graphic equaliser. The existing transformer is much the same size as the proposed replacement. All that's wrong with the original is the internal thermal fuse in the primary is open circuit. The unit does work when on the 220V setting when i used it with a variac.
For some reason the fuse is just in the 240v primary winding.

Regards, SJM.
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Old 17th Oct 2021, 10:38 pm   #5
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Default Re: Transformer output current.

You could still run the equipment from the 220V tap by inserting an impedance in series with the primary winding that effectively drops 20V. Even a resistor dropper in that situation is not going to dissipate much heat and so may be quite practical.
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Old 19th Oct 2021, 9:10 pm   #6
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Default Re: Transformer output current.

Thanks trobbins, that's a good idea. The total current drawn can't be that high, so as you say the dissipation would be quite low. Thanks. SJM.
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Old 25th Oct 2021, 9:34 pm   #7
bigfathairyvika
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Default Re: Transformer output current.

Can you not just replace the thermal fuse in the original?

Edit: if you can't see the fuse, they are sometimes hidden under the first layer of insulation. Just cut it back and join the wires, then you can wire up a suitable replacement externally if you need to.
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Old 26th Oct 2021, 3:35 am   #8
broadgage
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Default Re: Transformer output current.

A thermal fuse is intended to detect overtemperature and not overcurrent.
To do this it must be in close thermal contact with the transformer winding. Bypassing a thermal fuse and fitting another one externally is therefore unwise.

Arguably acceptable for a brief test or investigation, but not for long term unattended operation.

Thermal fuses in small transformers are often intended NOT to save the transformer, but to operate when the transformer is already defective and prevent a fire.
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Old 26th Oct 2021, 1:25 pm   #9
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Default Re: Transformer output current.

Depending on the age of the transformer I understand that thermal fuses can blow not just as a function of over-heating but simply through too many thermal cycles. I've no figures for 'how many' is 'too many' but it's just another age related symptom of vintage equipment i.e. thermal fuses will fail eventually

Thermal fuses are often inaccessible without major disassembly of the transformer and its windings, so by-passing the thermal fuse and using one of the other Primary inputs and a suitable in-line fuse may be an option. You won't, of course, have any over-heating protection and pay attention to Post #8 from broadgage above.

All the usual caveats apply and you do so entirely at your own risk!
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