Transformer stress, interesting article.

[pwdrive]

Hope I posted this in the right dept. worth a read.

http://www.w8ji.com/power_transformer_stress.htm

I came across this during a debate on choke input filters.

Lawrence.

[Radio Fixer]

I can add something to that Lawrence

One place that gives derating for transformers is the catalogue from RS Components Ltd and another is Hammond Transformers http://www.hammondmfg.com/pdf/5c007.pdf. Both quote 0.28 for half wave and 0.62 for a full wave bridge circuit, both into a reservoir capacitor. Basically this means that if the transformer is rated for say a secondary AC of 1A, into a resistive load, the DC current load for half wave, with filter capacitor input, can only be 0.28 X 1 = 280mA. The most efficient configuration was full wave, centred tapped, again into a reservoir capacitor, where the DC load current could be equal to the secondary current rating of the transformer.

Gary

[pwdrive]

Hell Gary, thanks for that, the link I posted was as a result of a heated debate on the yahoo groups HRO Receiver forum (I wasn't in the heated part...) it was about some one in the States who had a HRO 50 with a duff mains transformer and the only replacement he had was a transformer with a higher than normal sec. HT winding, the debate was about various ways to drop the excess voltage, it developed into a bit of a slanging match (I stayed clear of that).
Thanks for the info.

Lawrence.

[Geaney]

This is very interesting - thanks for posting the link. In my recent restore of a Grundig 3028 I replaced the selenium rectifier with a silicon one and inserted the dropper resistor, etc. However the transformer runs hot but I cannot find any excess current drain on the secondary side - which is what I assumed is causing the t/f to overheat. I decided that the T/F itself must be faulty. Now I assume I should apply the maths in the article to see how much of the heating is due to the silicon full wave rectifier, etc.
I've got a donor 3028 chassis (specifically to swap the t/f) but have not picked it up yet. Hopefully I'll get it done by end of April.

Cheers, John

[pwdrive]

That's ok John, just to say that the link I posted is not a definitive article on the whole subject of PSU issues but it came up in a forum discussion which i mentioned earlier...it would appear to have some substance and may be food for a bit of experimentation as the resevior capacitor does take a lot of current on the face of it. I will have a go when I get time with a thermometer and various values of resevior capacitors (taking into account the rectifier ratings) I suppose it would be tempting if say you had a duff resevior capacitor of say 4 uf to stick an 8 uf in if that was all you had to hand without realizing what its consequential effects might be.

Lawrence.

[pwdrive]

Here is a more in depth link on the subject, disspeling some popular myths.

Lawrence

http://www.w8ji.com/choke_input_power_supply.htm

[pwdrive]

Just a slight detour but maybe worth a thought....Energy saving light bulbs..Power factor...I bet the electricity companies have already whacked this on the cost of electricity.

Lawrence.