# Reduce voltage from transformer [closed]

I am trying to build a variable power supply using a transformer with a 12V 0 12V - 5 ampere output. I am using a BR1010 rectifier. I have connected the 2 - 12V wires from the transformer to the AC input of rectifier. At the output of the rectifier I have connected 2 - 25V 2200uF capacitors in series. The output I am getting through the capacitor is 40V. How is this voltage increasing?

I want to pass this into a XL4015 step down module so that I can control the voltage in a more proper manner but the input range for this module seems to be maximum 36V.

What can be done in this case? Am I doing something wrong? Any help is appreciated. I am posting few pics of my circuit.

Yellow wire is 12V-12V output from the transformer that is connected to input of the BR1010. The red and black wires are connected to the capacitor in series (2*25V). PS: Ignore the red wire to the XL4015 module.

I am trying to build a variable power supply using a transformer 12 0 12

The other option is to use this type of configuration: -

Picture from here.

Notice that the centre-tap is 0 volts and the peak voltage on the DC output is 12 volts x $$\\sqrt2\$$ = 17 volts. With a 0.7 volt drop in the diodes, you should get 16 volts no problem. If you needed a negative supply you can double up on the rectifiers and reservoir capacitor and liberate the same DC supply (in addition to the positive supply) but one that is negative negative. On your current setup you can measure from the centre-tap to either side of the bridge rectifier output and see this for yourself: -

Picture from here.

But the question remains; is this now too low for your XL4015 module and what you are trying to achieve? You haven't stated the minimum expected level you require so only you can answer this. The XL4015 will work down to about 8 volts I believe.

The output I am getting through capacitor is 40v. How is this voltage increasing

Your AC secondary output is 24 volts and, when rectified and smoothed, you will get a DC voltage of 24 volts x $$\\sqrt2\$$ = 34 volts but, this can be a lot higher on an unloaded transformer hence you see 40 volts DC.

• Hi Andy, thanks for such a quick and detailed response. I will be using this variable power supply for various purpose such as using my wired trimmer that needs 3 volts or powering my 775 motor that will accept anywhere between 12-24 volt. I am trying to achieve this voltage regulation using xl4015 module. But as I mentioned using a full bridge rectifier gives me 40v, which is higher than the max input for the module. – yoyo Aug 23 '20 at 8:58
• I tried using half wave rectifier configuration using the 0 tap on transformer and 12v tap but output is of 20 v max. And that is too low for me. A ideal setup for me would be to be able to feed around 30-35 volt into xl4015 module. Now I am planning to use a resistor of 39/47kilo ohm to reduce the original output voltage of 40v to 35 volt. Will that be any good I doubt. – yoyo Aug 23 '20 at 8:58
• @yoyo What current will you be taking on full load and, how much does the 40 volts drop when you are taking two thirds to three quarters of full load current? 39 to 47 kohm will likely be far too big. You need to use a different method but that can't be determined until you report back with the values I asked for. – Andy aka Aug 23 '20 at 9:05
• Hi Andy. The max current of the module is rated 5amp, also the transformer is of 5amp. Also the nature of work I'll be doing using 775 motor can require 5 amp. So max current of 5 amp would be perfect in my scenario. Regarding resistor, I plan to have my connection with combination of 10k ohm and 39/47k ohm. I used the formula to get this value. See example for formula learningaboutelectronics.com/Articles/… – yoyo Aug 23 '20 at 16:47
• There will only be power loss to consider. Your output power to input power will be about 85%. – Andy aka Aug 24 '20 at 7:22

The transformer is rated at 12-0-12 VAC at 5A, so the open circuit voltage without a load is higher than that.

• Thank you so much for the reply. Since I'm using both 12 v taps from transformer as input to rectifier I think it's generating 40v after rectification. But how can I reduce that 40 v to match the input for my xl4015 module (it's rated max 36v). Will a simple resistor do the job? – yoyo Aug 23 '20 at 6:42