# 48v to 40v

Discussion in 'The Projects Forum' started by alecjcook, Mar 18, 2014.

1. ### alecjcook Thread Starter New Member

Jan 2, 2014
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0
Hi. I am designing a bench power supply. My transformer will kick out 48v (1ish amp) but the supply I'm spec-ing we only do 36. My regulator has a max input of 40v, so my question in, what is the best way to get my 48v down to 40 cleanly? Would a voltage divider be enough? Or is that just wasteful?

2. ### k7elp60 Distinguished Member

Nov 4, 2008
505
90
If the regulator is a 3 terminal regulator(input,output,and ground)you could put a zener diode from the ground terminal to ground. In other words connect the anode of the zener to the common terminal(ground) and the anode to ground, and a resistor from the cathode of the zener to the input.

3. ### ScottWang Moderator

Aug 23, 2012
5,444
857
What is the regulator?
What is the V,I,R of load?
How is the power supply, fixed 36V or a variable voltage power supply?

4. ### Plasmahunt3r New Member

Feb 6, 2014
28
2
There is a simple solution. Use a Transistor and Zener to create an Emitter Follower Voltage Regulator. Your Transistor can handle several Amps (I use MJE13007). Just pick a Zener voltage (+ .6V) as a reference voltage to apply to Transistor base and your output voltage would be that (minus .6v).

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5. ### AnalogKid AAC Fanatic!

Aug 1, 2013
5,583
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1. Max. output current?
2. Fixed or adjustable output voltage?
3. Regulation tolerance (+/-1%, 5%, etc.)

ak

6. ### alfacliff Well-Known Member

Dec 13, 2013
2,449
429
that 48 volt transformer will be more like 67 volts dc after rectification and filtering. dies it have a center tap? if so, a two diode bridge will get it down to what you are looking for.

7. ### crutschow Expert

Mar 14, 2008
16,209
4,333
That won't affect the voltage drop from input to output which is the critical value.

8. ### crutschow Expert

Mar 14, 2008
16,209
4,333
I would also recommend a pre-regulator such as Plasmahunt3r suggested.

A technique to reduce the losses somewhat is to use the minimum filter capacitance on the rectifier output so that the lower voltage of the pp ripple is only about 40V at full load. This reduces the dissipation in the pre-regulator and also reduces the peak current from the transformer which reduces I squared R winding losses.