Electrical – Calculating the voltage drop of two Zener diodes in series? (reverse conducting)

diodesvoltagezener

I'm building a circuit powered from a 12V 55mAh (A23 battery). Needless to say that power consumption is a primary concern. The circuit is an ATtiny85 which is driving 13 LED's (in 5 groups). I initially used a voltage divider circuit to reduce the 12V down to 5.5V to supply the ATtiny85. This worked but I found the voltage divider was eating up too much current, 5mA, even when the microcontroller is put to sleep and all LED's are turned off. When active it consumes 5mA to 7mA. I replaced the voltage divider with two 1N4733A 5.1V Zener Diodes in series:

schematic

simulate this circuit – Schematic created using CircuitLab

This circuit works beautifully for saving power. Power consumption with this circuit is in the micro-amp range when the ATtiny is sleeping (too low for me to measure) and consumes only 0.1 mA to 0.3 mA when the ATtiny is awake and pulsing the LED's. A FIFTY fold decrease in power consumption.

The trouble with the new circuit is that it is very sensitive to the supply voltage. At 12V input (using a bench power supply) the circuit fires up and works well. I get around 5.1V at Vcc. At 11.5V supply voltage the circuit will not start.

When I was using a voltage divider instead of the diodes the circuit worked well down to about 8V supply voltage. With the diode setup it needs 12V to start and will keep running down to about 11.5V before it shuts off.

Is there a way to modify this circuit so that it works over a wider range of supply voltages? Ideally down to around 6V (as the battery discharges). The datasheet's I've looked at for the Zener diodes don't mention anything about the expected voltage drop across the diode when it's conducting at the breakdown zener voltage. If I replace D1 and D2 with 3.3V Zener's should I get the same voltage drop? I'm not sure how to calculate the expected voltage drop at Vcc.

Best Answer

Since no-one has tackled your basic question, I'll attempt to explain:

You cannot calculate the voltage across a Zener at low current. The reverse voltage is a characteristic of the device and depends on the construction/architecture.
There is no better reference I've found than the OnSemi Theory of Zener booklet and you should start by reading this.

Zeners come typically in one of two distinct architectures, Zener effect and Avalanche effect, You are using Zeners down in the Zener effect region, which can be characterized down to very low currents and the voltage across them will vary somewhat predictably.

From the OnSemi document, here is one very pertinent graph of the Vz characteristic:

enter image description here

Notice that Zeners below 6V Vr have quite different Vr curves. Those below 6V have a very large slope and as you lower the current toward the leakage limit the Vr voltage varies. The leakage current is normally stable (at a given temp) for voltages over 1V.

Now to your problem space, putting 3 Zeners in series.

The leakage current for each of the Zeners will be different. The Zener device acts as a constant current source once it has sufficient Vr and until tunneling begins. Leakage current will be typically in the 10-200uA range and you will find the maximum specified in the datasheet for some devices.

For your devices (1N728A and 1N729A) the leakage current will be below 100uA @1V and will begin Zener effect between about 1V and the rated voltage.

enter image description here

For your configuration ONE of the Zeners will have the lowest leakage current which will restrict the other two below their leakage current. This means that at very low currents (whatever that leakage value is) the voltage across the two of the Zeners will be much less than their rating, and may fall below 1V.

First bump in the road is an ATTiny with much lower current when asleep

In your configuration the ATTiny85 will draw a very low current when in sleep modes and from the datasheet you can see that when asleep with the Watchdog timer used for wakeup you can expect less than 10uA.

enter image description here

This very low current for the ATTiny85 will restrict the current through the Zeners to below the leakage current of any of them.

This may result in the VCC for the ATTiny rising to above the Absolute maximum rating of 6V and result in device failure. And indeed you already showed that the VCC rises to 5.1V.

schematic

simulate this circuit – Schematic created using CircuitLab

This low voltage drop across the Zeners explains why your circuit works down to 7VDC on the battery, but it is not a good mode of operation and you could easily damage the MCU. My assumption here is that you actually have a capacitor across the MCU you have not shown, and you are effectively running the ATTiny from that when it wakes up.

Viable Solution based on a regulator

Your best bet would be to invest in a linear series regulator. There are many variants available that have very low quiescent current such as the MCP1703 which would provide a single chip solution at low cost (

It's doubtful that you will find a switching regulator for the same price and low power.