Hi all

You have to help me stop going fetal on the floor! I am wanting to design a +/-17v power supply for an audio project. I can get the 17v from the 7815/7915 but I've been struggling to get both + and - rails at the same voltage. I'm very new to this so must be missing something simple.

I have a SMPS which can give me 40v DC which I was using to give me +/- 20V to go into the regulators. I was struggling so I simplified it and tried to get just the +/-15V using this from the datasheet:



The individual voltage regulators work fine on their own, the problem is when I put them together as above and I get -15V but + 9.5V

When I measure the input I get -30V and +10V referenced to ground. When I measure current into the regulators both sides are equal at 2.2mA.

If I ust put the SMPS output across two resistors then the voltage balances equally. It's only when I attach the regulators that theres a mismatch between + and -.

I've really tried to search everywhere but couldn't find this problem, the problem seems that the ground reference isn't sitting in the middle, is this because one side is drawing more current than the other? I've made sure teh ground point on the SMPS is connected to ground of the regulators.

The SMPS is this one - https://uk.farnell.com/xp-power/fcs40us36/power-supply-ac-dc-36v-1-11a/dp/2909130

Thanks in anticipation

Pat

 

Search LM325.

 

I'm not sure why you believe that your 40V is referenced to frame ground. I think you will need to generate a virtual centre reference.

 

That smps is a single output, you will need a negative voltage generator.

 

Thanks all. I can’t understand why the 10/30 split from the SMPS’s 40v. There’s some principle I guess I’m missing as to why the two voltage regulators don’t act like a resistor divider with the midpoint giving a +/- 20v reference

I’ll look into a rail splitter IC perhaps to get a virtual ground?

 

Thanks all. I can’t understand why the 10/30 split from the SMPS’s 40v. There’s some principle I guess I’m missing as to why the two voltage regulators don’t act like a resistor divider with the midpoint giving a +/- 20v reference

It would only work that way if the loads on both regulators were identical. If the current on V+ is 3 times that on V- then you are likely to end up with it split 10V/-30V.

How much current do you need to supply?

By the way, you would get a much less noisy supply from a transformer and rectifier.

 

The best solution would be to obtain
2 identical 18-Volt Power-Supplies and connect their Outputs in series.
This will give You a solid "Center-Tap".
It will also provide substantially more Current,
almost twice the Current of a single 40-Watt Supply.

The Regulators may, or may not be advantageous, depending upon your application.
They will reduce the overall efficiency, so if you're not building a sensitive Audio-Amp, don't use them.

An "L-C-Filter" will do a better job of cleaning-up any Switching-Noise than a standard Analog-Voltage-Regulator will,
and will do it with much less Power-Loss.
.
.
.

 

I have a SMPS which can give me 40v DC which I was using to give me +/- 20V

You can't get +/-20 from 40V. As you probably know by now, adding two resistors you can make a point that measures 20V but you cannot pull power from it.