# Bipolar supply +/- help (Single transformer)

#### oidium45

Joined Apr 24, 2010
138
I am trying to get both a positive and negative supply from one transformer. The bigest problem that I am having is that the positive needs to be +24-48v and the negative needs to be -10v. Will the below drawing work (raw power supply for regulators)? If not can someone please recommend an alternative?

I have included the basic circuit that I am trying to build as well.
Thanks

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#### bertus

Joined Apr 5, 2008
22,217
Hello,

I would take a separate small transformer for the - 10 Volts and a large transformer for the power section.
The standard LM317 can only handle upto 40 Volts at its input.
There is also a high voltage type of the LM317, the LM317AHV.
That one can handle upto 60 Volts.

Bertus

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#### oidium45

Joined Apr 24, 2010
138
Thank you Bertus! I meant 32-40v input. My mistake... Actually I will probably be using a maximum of 36v for the input. I am still having difficulty figuring out how to get the negative supply voltage, but a small transformer is definately something that I do not mind adding to the circuit.

#### bertus

Joined Apr 5, 2008
22,217
Hello,

The "help" transformer for the - 10 Volts can be really small.
A transformer that handles 100 mA would probably be enough.
Something in the range from 7 to 9 Volts will do.
The Fets act as current sources, the voltage is not that critical.

Bertus

#### SgtWookie

Joined Jul 17, 2007
22,230
Ask yourself - what circuit might you want to power from less than 1.25 Volts?

You really don't want that large of a voltage differential across an LM317 anyway (I'm speaking of 36v in, and a lot less out) because for a given current, power dissipation in the LM317 will get much higher as it's output voltage goes lower.

For a 36v input, tell me what the power dissipation in the load and the LM317 will be, when the load requires 1 Ampere of current; first at 33V (output of the LM317, input to the load), second at 18v, and thirdly at 3V.

#### oidium45

Joined Apr 24, 2010
138
Bertus, I thought about using a smaller transformer for the negative but I still run into the problem of not knowing how to connect it into the circuit. Let me explain a bit better.
1) Could I power the second transformer off of a winding from the main transformer? Or do I need to use a 120-10v transformer?
All information that I have found suggests that if I use a single transformer with two bridge rectifiers they will short each other out. But I have found no information about using one of the windings for a second transformer instead.

2) How would I connect the second transformer output into the circuit? Would I connect the negative to the transistor, ground to the main ground of the circuit, and leave the positive "hanging"?

3) Can I create a 36v power supply from one winding on the transformer, and use a seperate center tapped winding on the same transformer to generate the -10v?

Sgt Wookie,
I understand the issue with heat dissipation. I believe I have that problem solved for now. "hopefully"... I am planning to try using a variable input adjustable through a rotary selector switch. This will allow me to adjust the input voltage to the LM317 to more closely resemble the output voltage requirement.

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#### Wendy

Joined Mar 24, 2008
23,276