Consider - you want to draw 200mA from this circuit. All current must go through the 50 ohm resistor. 200mA through 50 ohms means a voltage drop of 10V across that resistor. It's not going to work, is it?
Instead of trying to do it with resistors, here's a better way. You have access to 12V. I'm going to assume that's a well-regulated +12V. Drop that to +10V using 2 1N4001 diodes in series on the +12V line. Then put in a 78L05 voltage regulator. Add a load resistor for the regulator of about 330 ohms.
Now you have 0V, +5V and +10V. If your input is isolated from your output then that can serve as -5, 0 and +5.
The 78L05 can handle 100mA, which may be enough depending on how much ground current the circuit you are powering generates. If not, use a 7805.
This will work if the circuit you are powering draws current fairly symmetrically from the + and - rails. If it draws more from the + rail the "0V" line will go high. Decrease the value of the regulator load resistor, or use a 79L05 regulator instead to generate the "0V".
Alternatively use a power op-amp to generate a very stable mid-rail. There is a circuit for this in the data sheet of the LM675 op-amp.
There is an even easier fix. Use a 12V to +/-5V DC to DC converter.
http://uk.rs-online.com/web/p/isolated-d...
That is really a pretty poor design, sorry. You will never get 200 mA out of it, as the drop across the 100 ohm resistor would be 20 volts, and you only have a 0.4 volt margin.
Get some 3 pin IC regulators, such as the LM7805 and use that instead.
but if you just want to clean up your existing design, best would be to drop that voltage divider and just have the two zeners in series and those in series with a 10 ohm resistor. That will allow a much higher set of currents. However, the 10 ohm would need to be a 1 watt unit, and the zeners rated at 2 watts each.
This assumes your 12 volt source is capable of 200 mA
Never use a Zener diode as a voltage regulator.. their dynamic impedance is only a few ohms -- that is where all your current is going.
Just use a linear regulator and cut the parts count down to about 2 parts.
The resistor values are way too high. You should use something like 7805 and 7905 regulators.
Hi. I am working on a +/- 5V power supply which can output max 200mA both on 5V and -5V rail.
It works fine when it is not loaded, but if the load increases then the voltage drops. My opinion is that I need to lower the resistor, but still I need your advice.
The schematic: http://i47.tinypic.com/23ku55e.png
Thanks!
