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How to lower the DC voltage of a transformer. Answered

Hi, at last i have got a 45-0-45 transformer @5amps, but now i have a problem, the VDC output i am getting is 74vdc but my 400watt rms mono amp's maximum input VDC is 56vdc, so i don't think its a good idea to feed the 74vdc, so is there a way to lower the 74vdc, BTW please don't advice me to unwind the secondary coil because i am new to electronics and the transformer didn't come cheap, so i don't want to break it.

i am using IN4007 diode and 4700uF 63V cap for the bridge rectifier.

​I just forgot to give some details on this topic. The transformer is to use with the 400watt rms mono amp. The amp has 8 MOSFETS side by side, 4 IRFP9240 and 4 IRFP240 and 6 transistors.



BTW the transformer shows 45-0-45 VAC but when i test with a multimeter, the VAC is 53volts.

Thank you.




Best Answer 1 year ago

Strange you do not mention the explosion of the 63V cap on 74VDC ?

Here is a lower value output of your XFMR transformer..

Click pic to see whole image..


I thought i wrote 74"VDC" not 74"VAC"(: anyways the output of the transformer is 53VAC but after rectifying it, i am getting 74VDC BTW the 400watt amp is a 56-0-56 VDC so the rectifier needs to be a center tapped. I did some experiment with diodes to lower the voltage but it took me 80 IN4007 diodes:D(i am bit despaired because this project is costing me a lot of money and lots of time wasted) to lower the DC voltage to 63VDC i can add some more diodes to get close to 56VDC but as you know, this is not a practical way, i am not sure if i can use resistors because on load the resistors might get too hot and smoke nu(: Thank you so very much my good brother, for your kind help and time, appreciated.

Here is the ckt and a 1N3309 10 volt Zener Diode 50 watt


Keep in mind lowering the voltage will also cut further back on the available power to your amps !


Just thought of a way to lower the voltage a lot cheaper too !!

Get a small 24 Volt 2 Amp Radio Shack or eBay transformer.

See the last schematic drawing....


ok, i will try that, thanks once again, appreciated.

That 400 watt amp will use I=P/V = 400 / 56= 7.15 amps

But your transformer is rated for a maximum of 5 amps

This means you can only power P=VxI = 56 x 5 = 280 Watts maximum

And if there are two channels that is 140 watts per channel..

You can put a 50watt 11Vz Zener diode in the center ground return of my transformer schematic which lowers the voltage to a 53 VDC.

This is a stud 1/4-20 power device that needs a heat sink to throw away 55_watts to lower your output voltage and costs $35..

ok, this project is costing me lot nu(: anyways thank you so very much, appreciated.

Right now, its giving 74V. Can you load it and see what it gives you at 5A ?

You have run into something called the transformer's "Regulation". An expensive transformer has a lower regulation - like 5% or so. Yours has a regulation of around 30%.

It may be you'll have to get a high power regulator to get down to a safe voltage for your amp

ok, i think i know how to load the TR for test(: You meant something like this Yes(:

Yep!! thanks to you and all the cool brothers out here, on the amazing and the coolest diy/education/knowledge based website i have ever known :)INSTRUCTABLES(: Thank You.

Load into the 400watt amp or... BTW i was thinking about that but i don't have any electronics device to test the 74V TR, i have only two amps the other one is a 200watt rms mono amp which is also rated at 56VDC max. BTW if i feed the 74VDC to one of the amps, will it cook the amp(: Can i ask what is a Power Regulator, i mean to use with the TR, anyways thank you so very much my good brother, for your kind help and time, appreciated.