'Free' smoothing chokes for preamp PSUs

[Firebottle]

Suitable chokes can be a bit thin on the ground from component suppliers so I thought I would post up this little tip.
I say free because they all come from junk or ‘boot sale price’ stuff.

All these ‘chokes’ are in fact small mains transformers, some clamp mounted, some PCB mounted. They can be found in cheap computer speakers, old linear ‘wall wart’ adapters and the like. Just think how many of theses are lurking at your nearest tip.

They are typically rated between 1.2 and 6VA, so are only suitable for small current power supply smoothing, hence the title ‘for preamp PSUs’.

The maximum current rating can be guesstimated from the VA rating. Divide VA by 240 (as in the mains supply voltage):

1.2VA 1.2/240 = 5mA
2VA 2/240 = 8mA
3VA 3/240 = 12mA
4VA 4/240 = 17mA
6VA 6/240 = 25mA

The primary winding (as in the 240V winding) is used, making sure any secondary connections are insulated.
This picture gives an idea of the size of these ‘chokes’, note the valve in the middle.
I have shown the VA rating, the dc resistance of the primary and the measured inductance in Henries of each one.



Cheers, Alan

[Barry]

Great tip for DIY'ers.

If you can't measure the inductance, it can be calculated by measuring the off-load current drawn from the mains. Divide the mains voltage by the measured current to derive the reactance. The reactance will be largely inductive (almost totally), so divide the reactance by 314 (= 2*pi*50) to obtain the inductance.

Good one Alan.