In my attempts to resolve some logic level problems with the Yellowstone Tester, I decided to take a crude approach, and reduce the 5V supply to about 4.7V using a simple zener diode voltage regulator. From my previous tests with a variable voltage supply, the reduced voltage at 4.7V was enough to get the tester working nicely. But now when I try combining a fixed 5V supply with a resistor and a 4.7V zener, I find that it doesn’t work as expected. Zener regulators apparently don’t behave the way I thought they did, and for this specific circuit, they may not work at all.

The problem is that the rated voltage of a zener diode is only valid for a specific level of current. I knew this, but I thought the voltage would only change a small amount over a wide range of current: perhaps 100 mV of change for the 10 mA to 100 mA range where my circuit operates. In other words, I thought the IV curve for the zener would be very steep, nearly a vertical line.

This proved to be a bad assumption. Using a 1N4732A zener, at a current of 90 mA I do see 4.7V on the zener, but the voltage is only 4.506V at 40 mA, 4.459V at 30 mA, 4.335V at 20 mA, and 4.212V at 10 mA. That’s not good. It’s non-linear, but if it were a resistor then its value would be about 6.1 ohms. I’m not sure how I was meant to know this from the datasheet, which doesn’t include any IV curves. The datasheet does include a dynamic resistance (impedance?) number, but I thought that was for AC applications since it’s specified for a frequency of 1 kHz.

 
Voltage Regulator Says What?

So how are you supposed to build a voltage regulator using a zener with this amount voltage of variability? The standard way to build a zener regulator is with a resistor in series with a zener, and a load in parallel with the zener:

If V_out is ever higher than V_Z, then the zener will pass more current, which also increases the current through the resistor, which increases the voltage drop across the resistor and lowers V_out. So V_out gets “regulated” to the value of V_Z, so long as the current through the zener remains constant.

If the circuit has a fixed load, then the value of the resistor can be chosen to get the desired current through the zener to achieve the nominal zener voltage. But if the circuit has a variable load, there’s a problem. In order to maintain a regulated output voltage, the voltage drop across the resistor must remain unchanged while the load current changes. That means the current through the resistor must also remain unchanged. The only thing that will change is how much current flows through the load and how much flows through the zener. If the load draws less current, then the zener must draw more, in order to keep the total current constant.

Let’s say the load current can vary between 10 and 100 mA, and the zener can theoretically handle 150 mA before it burns up. For safety’s sake you probably don’t want to push the zener all the way to 150 mA though, so you might limit it to 120 mA. You could choose a resistor value to achieve a constant current of 130 mA through the resistor, which can be divided between the zener and the load 120/10 mA for light loads anywhere up to 30/100 mA for heavy loads. That’s a 90 mA range of currents through the zener, so for a 6.1 ohm equivalent resistance you would see a 0.549V change in the output voltage. That’s not very well regulated at all.

If I’ve analyzed this all correctly, then a zener voltage regulator basically doesn’t work unless the load current never varies by more than about 10 to 20 mA. That’s a pretty lousy regulator. I’m not sure how I never realized this about zener regulators before.

 
Plan C

So now what? I still need to get the Yellowstone Tester working. The original 5V circuit has problems with the logic levels, and Plan B for reducing the supply voltage to 4.7V with a zener seemingly doesn’t work either. Would I be better off with a simple Schottky diode in series with the 5V supply input? This wouldn’t be perfect either, since the nominal 5V supply might actually be closer to 5.1V or 4.9V, and the voltage drop across the diode will change with the load current. But I still think it would be enough to keep the voltage within a range of about 4.8V to 4.5V, which is better than I’ll get with the zener.