Power Supply
Another power supply design based around the popular and versatile LM317 / LM337 ICs (you can easily find the datasheet on the internet). Nothing fancy here, it is just really useful to have a reliable supply. This design features 3 independent power supplies:
- 0 - 30 V variable, 0 - 1 Amp variable
- ± 3, 5 or 6 Volt switchable, current limited to 100 mA, 500 mA or 1 Amp
- ± 9, 12 or 15 Volt switchable, current limited to 100 mA, 500 mA or 1 Amp
The full circuit is shown here (click for full size gif):
The fixed switchable supplies are based around an LM317/LM337 with a p-FET / n-FET on the front to allow better control over current limiting. The switchable resistors set the current limit. This is based on a circuit from EDN, which also details inclusion of a resistor from the output of the LM317 to the gate of the FET to provide foldback current limiting. The article provides details on calculation of the resistor value, which works great for a fixed output voltage. The foldback action would also require switched resistors for this switched output voltage design. The output voltage is set by switchable resistors. A comparator is used to detect the rise in voltage on the gate of the FET when current limiting sets in, and switches a red/green LED to provide a visual indicator of current limiting to the user. Both fixed switchable regulator designs are similar except for the different output voltages and associated resistors.
The variable supply is single polarity, and uses two LM317 regulators, one for current limiting and the second for voltage adjustment. A small negative supply is required to allow voltage and current to be controlled down to zero. A separate transformer is used to generate this supply, and is also used to create a separate power line for the 200 mV panel meters (LA0310 from SurplusTronics), hence there are 4 transformers inside the case! A high side current-sense is used to read off the current using a special op-amp. Resolution is 1 mA, and hence the circuit measures the quiescent current of the Voltage setting LM317 (9 mA as shown in the photo below)! Again, a red/green LED is used to display current overload, picking up a change in the 1.24 V reference voltage of the LM317. The hysteresis of the current overload comparator means that when the current setting is below about 20 mA the red LED sometimes stays on. This isn't ideal but I haven't bothered to fix it.
The photos below show the front of the PSU in operation, and the inside of the case with all the LM317/LM337 and FETs mounted on a sturdy heatsink, and the four transformers. I haven't tested heat output with all three supplies independently supplying 1 Amp, but in practice I won't be driving the unit that hard.
© In the Light, 18 March, 2011 , Disclaimer, Son of Suckerfish drop-downs from HTML dog