Introduction

The General Purpose Regulator board is a power supply board designed specifically for projects that need a 5V supply up to 2 Amps for digital logic and microprocessors, along with +9V and -9V rails up to 500mA for powering analog circuits that require dual rail supplies, such as operational amplifiers (opamps).

This is the perfect power supply for mixed signal products, for example digitally controlled audio electronics, radios, telecommunications and industrial electronics.

Circuit Description

Digital Supply

The 5V digital power supply output is based on the well-known, low cost LM2596S "Simple Switcher" integrated DC-DC buck regulator from Texas Instruments. The LM2596S-ADJ/NOPB is used in this particular case, with the output voltage set by external resistors R5 and R7. This makes it possible if you build the supply to use alternative resistor values in the feedback network so set the voltage to 3.3V or even lower as needed by your target circuit.

The LM2596S is mature stable technology and easy to get at low cost. The only downside really is that it's not quite as efficient as newer designed DC-DC controllers, with it's peak efficiency around 84% at 1Amp load. However this is not much of a problem given the intended use of this supply - small footprint, low noise and low BoM cost are all more important than efficiency.

The LM2596S has a built in power transistor for switching the input voltage (via D3) to the main energy storage inductor (L1) and current rises linearly in L1, supplying the load and sharging the filter capacitor (C6)

Analog Supply

The analog supply gets positive and negative voltage rails by using a "voltage doubler" network. From the single AC input (from the secondary of a 9 or 12V transformer), D1 provides positive rail rectification and D2 provides the negative. Adjustable linear regulators LM317 (and it's complement, LM337) have been the mainstay for analog linear power supply design for opamps for decades! And many engineers would say, they're still the best!

It's worth noting that most "opamp" power supplies would have +/- 12V or +/-15V rails, and I designed this one to output +/-9.3V. The reason is, 9.3V is more than enough headroom for most designs, and also it widens the range of choice of power supply transformer used. 9Vac transformers are often easier to come by in the form of wall-packs, and are also often cheaper than equivalent transformers with 15V secondaries.

Having said that, the output regulation voltage is set on the LM317 by feedback resistors R1 and R2, and on the LM337 by feedback resistors R3 and R4. Notice that these are kept matching for a symmetrical dual-rail supply. If you have a need for a higher output voltage, just change the values of the feedback resistors based on the formula:

Just make sure the unregulated input voltage to the regulator is at least 2V higher than your intended output (to overcome the drop-out voltage) and not exceeding the voltage ratings of the input diodes and capacitors, or the maximum rating of the regulators (40V).

It's important to note also that when using diode voltage doublers as in this case, the maximum reverse voltage across the diode is double the AC peak input voltage. Therefore, if you have a 9Vac(rms) transformer, the peak voltage is 9V x √2 = 12.73V. I'm using low forward voltage Schottky rectifiers, which under full load will have about a 0.7V drop across them. Therefore the bulk storage cap will have about 12.03V across it, when the transformer input goes to the negative half-cycle will result in the reverse diode voltage of 12.73V + 12.03V = 24.76V. Originally I had specified SS12 diodes, but at 20V rating realized they would not last, so I changed the design to specify SS13 schottkey diodes which have a Peak Inverse Voltage rating of 30V. If you want to increase the transformer voltage, you will need to change the input rectifiers also to withstand any increase in inverse voltage - the SS range is good for that, having parts that go much higher (like SS15).