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I am building a +/-15V Power supply to power a sensor, the sensor draws 25mA. Unfortunately, I'm having some difficulty with it. The design is as follows:

  1. 6V AAA battery pack plugged into a DC/DC Boost Converter
  2. Boost Converter set to output 30V from the regulated output
  3. Output is split into +/-15V using Linear regulators (LM7815/LM7915 pairs)
  4. +/-15V and Ground is plugged into sensor

The boost converter I am using is rated for output voltages up to 37V: https://www.jaycar.com.au/arduino-compatible-dc-dc-boost-module-with-display/p/XC4609

The 'data sheet' (if you can even call it that) can be viewed at: https://www.jaycar.com.au/medias/sys_master/images/9004196134942/XC4609-dataSheetMain.pdf

The circuit to split the 30V into +/- 15V has been adapted slightly from https://www.head-fi.org/threads/virtual-ground-regulated-and-rail-splitter-circuits.654485/

I have not included an LD1085 because the output of the boost converter is regulated.

The schematic is:

schematic

simulate this circuit – Schematic created using CircuitLab

I have used an LM317 regulator instead of the LD1085 Low drop out regulator. This was mainly due to price and what I had available. I have also used smaller capacitor values than what was quoted.

Simulating the circuit shows the correct values. However after building the circuit its not working properly.

But when I plug in the rest of the circuit the DC boost converter the screen turns off, I think its due to a short. The LM7815 gets proceeds to heat up, I think that it's trying to regulate from 15V to 30V.

The circuit itself is built on perfboard, and I ensured all connections were correct using a program called Fritzing. http://fritzing.org/download/

The circuit can be seen below

+/- 15V power supply

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  • \$\begingroup\$ Is C7 installed correctly? The (-) stripe should go towards -15V rail, not ground. \$\endgroup\$ – markrages Jan 23 '18 at 6:56
  • \$\begingroup\$ Yes, it is. The rails on the power supply I built goes: 1. GND, 2. +15V, 3. -15V, 4. GND, 5. 3.7V (but I haven't put the voltage divider in for the 3.7V yet). C7 has its (-) stripe on the -15V rail (strip #3) and the + side is on GND (strip #4) as per the schematic. \$\endgroup\$ – Brendan Jan 24 '18 at 7:28
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If you set your boost converter to produce precisely 30 volts (not 33 volts) then you have achieved the outer rails but not the mid rail. The mid-rail can be made with a single 15 volt regulator. I would choose a buck regulator for doing this because it will generate very little heat.

So, your isolated 30 volt output becomes +15 volts. Your isolated 0 volts becomes -15 volts and the buck regulator would produce a mid-rail of 0 volts.

It makes no difference if you regard these three voltage as 0, +15 and +30 OR -15, 0 and +15 - your circuit will not perform any differently.

Buck regulator idea. This one shows a 12 volt output but can be adjusted to provide 15 volts by altering R1 or R2: -

enter image description here

There are plenty to choose from.

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  • \$\begingroup\$ Thanks for your suggestion. That does make sense. I have purchased a bunch of TLE2426 rail-splitters instead. \$\endgroup\$ – Brendan Jan 24 '18 at 7:23
  • \$\begingroup\$ Also, it was supposed to say 30V. I've edited it to say 30 now. The 33V was a previous version, when I was using a LM317 to regulate down to 30V. \$\endgroup\$ – Brendan Jan 24 '18 at 7:24
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If you didn't match the regulators very closely, their offsets will cause large current flow through the 1Ω ballast resistors. Your DC-DC converter can't cope.

From a 7815 datasheet: 7815 datasheet excerpt

The output voltage won't be exactly 15V, it will be somewhere between 14.25V and 15.75V. The 7915 will have a similar output tolerance. Worst case you have 1.5V difference to drop across 2Ω. Things are going to get hot. The topology is extremely bad because it cannot function at all without device matching. You've witnessed that directly. If you make it with unmatched devices, it will virtually always get very hot and not work. A good topology can withstand component variations.

You can replace the whole thing with a single TLE2426 in a TO-92 package, or make your own discrete op-amp rail splitter. Either will work better, and both will always work.

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  • \$\begingroup\$ Thanks for your reply. I'm having a look at the TLE2426 now. I wish I knew about it earlier! \$\endgroup\$ – Brendan Jan 23 '18 at 6:30

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