Do you sometimes need an inexpensive precision power supply? This Design Idea presents a precision power supply that has thermal overload protection and short circuit protection, and can supply 100 mA of current. I needed such a power supply, and I started by using Google to look for a suitable circuit that might be available on the Web. I didn’t find an acceptable solution, but I did get some ideas that got me started on this design.
This circuit is designed around an LM317L voltage regulator. Yes, this is an old regulator type, but it is still in active use, is cheap, and is readily available. This circuit, Figure 1, uses a rail-to-rail (input and output) op amp in a feedback loop from the LM317L output to its adjustment pin. An inexpensive LM4040BIZ precision voltage reference is used to establish a 0.2% voltage reference for the op amp.
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| Figure 1. | This power supply circuit is designed around an LM317L voltage regulator. | |
I found a SPICE model of the LM317 on the LTspice groups.io website [1], and ran some simulations to see if a stable configuration could be realized. The resulting configuration is, according to LTspice simulations and the SPICE model, conservatively stable with about 70 degrees of phase margin. The 3.3 ohm resistor in series with the LM317L output may be counterintuitive, but it works well to increase the stability of the circuit, and the power loss in the resistor is only 33 mW with 100 mA of output current.
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| Figure 2. | The feedback loop keeps the output impedance of the circuit very low. | |
The feedback loop keeps the output impedance of the circuit very low (Figure 2), so the 3.3 ohm resistor is not a concern. The output filter capacitor, C4, contributes to the loop stability. The loop is stable with a value of C4 equal to 10 µF, but the larger value of 22 µF provides more margin for a conservative design. The low values of R7 and R8 ensure that the minimum required output current is realized, and they also make the feedback loop more stable.
The schematic of the LM317L varies somewhat, depending upon the manufacturer, and, also, the LTspice model of the LM317 may not exactly model the device, so I opted to use an abundance of caution when stabilizing the loop.
Note that the R9 – C5 filter on the output of the reference may not be required. It is there to eliminate most of the noise generated by the LM4040BIZ. The heavy filtering provided by the op amp circuit and the output capacitor, C4, is probably sufficient.
I built and tested this circuit, and it performed as expected. I used two matched resistors with nominal values of 243 ohms for R7 and R8, and the circuit was accurate to 0.2%, with an output voltage of 4.99 V. And, as predicted by the LTspice simulation, the feedback loop was stable with a load of a less than 10 mA and also with the maximum load of 100 mA.
The worst case accuracy of the output voltage is 1.2% when 1% resistors are used for R7 and R8. When a matched pair is used for R7 and R8, the voltage accuracy is close to 0.2%. For more precision and a little more cost, you could use an LM4040 specified with a 0.1% voltage accuracy. My results show that the circuit works well as a low cost precision power supply with up to 100 mA output current.
