diyAudio (
-   Power Supplies (
-   -   Reducing transformer voltage... (

69CamaroSS396 25th May 2009 09:48 PM

Reducing transformer voltage...
I have a matched pair of NOS Triad power transformers that are 550-0-550@250mA and 5V@4A. Is there some simple way to safely reduce that output to the neighborhood of 400-0-400?


sawreyrw 26th May 2009 12:44 AM

You can't easily reduce the secondary voltage of the transformer, but depending on the load current, you may add a resistor to the secondary of the circuit that will drop the voltage to the level you want.

Bregg 26th May 2009 02:57 PM

Hi !

This resistors to drop voltage shoud be huge .You need 150 V drop , and 250 mA , it is a lot of power and temperature - 37,5 W per each branch . Make sure so you have proper cooling . Resistor should have 600 ohm , if of course you need this 250 ma . How much of current You need to provide for application powered by this tran ?

Values of this transformer are telling me it's a power supply for tube amp , and I not reccomend any such big resistor reductions in main supply voltage because of voltage swing which depends from anode and other changing currents . Power supp should not to swing voltages . Revinding this tran is a better solution , I think .


sawreyrw 27th May 2009 01:19 AM

I know the transformer is rated at 250 mA, but if he only needs a few mA, then a dropping resistor will be fine. As you stated, it would not be reasonable to drop 150 volts at 250 mA.

woody 27th May 2009 03:20 AM

Will you be using the 5v winding? If not you could use a 30 to 36v
transformer to "buck" your line voltage. But your 5v will also be
reduced to about 3.8v.... but if you only needed 5v at 2A you
could send the 3.8v through a schottky rectifier bridge and cap
to get ~4.8v

sivan_and 27th May 2009 04:28 AM

what abt a capacitor in series with primary winding ?

star882 27th May 2009 05:56 AM


Originally posted by sivan_and
what abt a capacitor in series with primary winding ?
The transformer is an inductive load (with an unknown inductance) so it would not be very easy to calculate the correct value. The easiest solution is to use another transformer to reduce the input voltage.

69CamaroSS396 27th May 2009 09:25 PM

Thanks for the replies. I am building Millett UniAmp monoblocks, which the 800VCT is recommended for. I'm buying some Edcors of the appropriate specs, but plan to build more UniAmps in the future, so that was why I was curious about this.

Thanks again, Greg

All times are GMT. The time now is 05:08 PM.

Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 17.65%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2019 DragonByte Technologies Ltd.
Copyright ©1999-2019 diyAudio