Transformer Question

[hifileslie]

I have just completed a separate PSU for a power amp project. The box contains only a 500va transformer and switches/fuses etc. Several times when switching on the PSU the House Circuit RCD has triggered -this happens whether or not the PSU is connected to the amp. There are no obvious shorts . Could the inrush current trigger the RCD?

I also have a dual secondary 20-0-20 Toroidal transformer with no wiring diagram. How do you know which wires to connect together to get a 20-0-20 ouput? Grateful for some advice please.

[AndrewT]

Hi,
put a ohmmeter across the wires to find the 2 windings.
connect 2 wires, 1 from each winding and them measure the voltage at the 2 unconnected ends.
DO NOT connect two wires from the same winding!!!!!!
If the voltage is nearly zero then you have connected two 20vac ends together.
If the voltage is 40vac then you have connected a zero to a 20vac and this becomes the common for your +- psu.
In a recent similar thread I asked:-
How do you identify the ends of split primaries?
Anyone know????
regards Andrew T.

[jaycee]

If it doesn't always trigger the RCD then yes- you need an inrush limiter. RCD's are very sensitive and are often triggered by the 100A+ that is generated at inrush.

If you're not sure, connect up a lamp in series with the Live wire. If it flashes bright then goes dim, the supply is fine. If it stays bright, you have a short.

[Bikeman]

Since we're on the subject of transformers, I have what might be a stoopid question.

If you measure the dc resistance of the primary leads, it shouldn't be more than a few ohms, right?

I've got what I think is a dud trannie, I just want to make sure I'm not doing something wrong before I take it back and jump up and down...


Mark

[AndrewT]

Hi,
for a step down transformer the high resistance will be in the primary (more turns and thinner wire). The low resistance will be in the secondaries (sometimes very low, less than 1 ohm).
Check again. The wires will be enamelled & you have to measure onto clean copper, secondly make sure you check all the wires in case you are trying to measure two different windings which by design must be open circuit.
regards Andrew T.

[jacco vermeulen]

Originally posted by AndrewT
How do you identify the ends of split primaries?
Anyone know????

Another question for you : what the heck are split primaries ?

The second one: Are you a Scot or enjoying the lake district ?

[jacco vermeulen]

btw: with a phase checker you can identify the orientation of the secondary winding.
Like with connecting speaker cable some prefer to connect both secondaries the same way.

[jacco vermeulen]

Originally posted by hifileslie
There are no obvious shorts . Could the inrush current trigger the RCD?

You seem to have answered the question yourself.
You did not mention the type of transformer. Toroidal transformers have an inrush current for a few mS that is much higher than the continuous rating, more than twice the nominal value.
Any toroid of 300VA and over may trigger the RCD.

I use 4 separation toroids of 1600VA each between the main automatic fuses board and my amplifier powersupplies.
If all are switched on simultaneously, not only three 16Amp/230Volt switches trip, but also the primary 50 Amp one.

Also, you did not mention how the transformer is connected to the powersupply housing.

[AndrewT]

Hi,
split primaries;- two 120 or 110Vac windings that are used in parallel from 110Vac and in series from 220/240Vac. This allows use almost worldwide. A complicated transformer may have dual 100/105/110/115/120Vac primaries to allow any combination and to adjust for how close your mains transformer is to your house.
regards Andrew T.

[jacco vermeulen]

Ah, dual primaries.
I should think that if you reversed the measuring by powering a secondary winding you can apply the trick you advised the other gentleman.