Author Topic: Unusual old german PSU transformers stage  (Read 1148 times)

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Offline Francenthusiast

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Unusual old german PSU transformers stage
« on: October 01, 2016, 03:58:58 pm »
Hi !

A friend gave me that old german power supply and I try to understand the hot stage before the bridge rectifier.
Why the two transformers are that big ? (I mean T1 is only powering the relay, why is it the same size as T2 ?)
How the relay is  acting on them ? I guess that the relay is here as a short circuit protection, but I can't decipher the schematic with the centred taps... Any good soul to help me ?

Here is the schematic I found on internet and the list of components :





Thank you very much !
 

Offline oldway

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Re: Unusual old german PSU transformers stage
« Reply #1 on: October 01, 2016, 04:51:49 pm »
Quote
Why the two transformers are that big ? (I mean T1 is only powering the relay, why is it the same size as T2 ?)
The two transformers are in parallel, they both are used the same way.
 

Offline Francenthusiast

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Re: Unusual old german PSU transformers stage
« Reply #2 on: October 01, 2016, 05:26:07 pm »
Ok, so I may understand: the voltage divider composed of R34 R35 & R36 bias the V12 zener at a certain voltage, then bias the base of the NPN V11, it shorts to ground the PNP V9 that activates the relay. And I guess that, as the relay controls the center taps of  both transformers, it doubles the output transformer voltage when the voltage divider reach the V12 bias voltage. Maybe to lower the voltage drop in the power transistors ?

Am I correct ?
 

Offline SeanB

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Re: Unusual old german PSU transformers stage
« Reply #3 on: October 01, 2016, 08:22:24 pm »
Yes, it allows full power at low voltage settings by dropping input voltage to the regulator stage. More modern ones will use a single transformer with more tap points, but use the same principle of trying to keep the voltage across the pass transistors as low as possible.
 

Offline Francenthusiast

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Re: Unusual old german PSU transformers stage
« Reply #4 on: October 01, 2016, 10:48:10 pm »
Thank you for the confirmation SeanB  :-+, I'm kind of proud of myself deciphering this even if it's very simple !
 


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