# Computing stuff tied to the physical world

## Forward voltage drop on a diode

In Hardware on May 14, 2012 at 00:01

With all this tinkering with solar panels, little batteries, supercaps, etc, you often need to prevent current from leaking away. The usual approach is to insert a diode into the circuit.

Diodes conduct current in one direction and block the current in the opposite direction.

Well, that’s the theory anaway. In real life, diodes only conduct current once the voltage is above a certain level, and they tend to leak minute amounts of current when blocking the reverse voltage.

For ultra-low power use and the low voltage levels involved, you need to be careful about the type of diode used. A regular 1N4148 silicon diode has a forward drop of about 0.65V, quite a bit when supplies are 2..3V!

The Schottky diode has a much lower voltage drop. It’s usually specified as 0.3..0.4V, but it really depends on the amount of current passed through it.

To see the properties of the BAT43 Schottky diode I’ve been using, I created this simple test setup:

A 10 Hz “sawtooth” voltage is used to create a signal rising from -3V to +3V in a linear fashion, 10 times a second. This means that the current through the 100 kΩ resistor will go from -30 µA to +30 µA. We can then watch the voltage over the diode, and how it goes from a blocking to a conducting state:

The yellow trace is the sawtooth signal applied to the circuit. The blue trace is the voltage over the diode. Note the difference in vertical scale.

You can see that with negative voltages, the diode just blocks. As it should. With positive voltages up to 1.2V, i.e. a current up to 12 µA, the voltage drop over the diode is under 0.15V, rising slowly to about 0.175V at 30 µA.

Changing the resistor to 10 kΩ to increase the current by a factor of 10, we get this:

Same picture, different scale. At 300 µA, the voltage drop is now about 0.23V, and it’s fairly flat at that point.

For comparison, here’s a run-off-the-mill 1N4148 “standard” silicon diode:

Again: different vertical scale. About 0.53V at 300 µA. More importantly, it’s already 0.4V at 60 µA.

So when losses matter at low voltages and low currents, it’s better to use Schottky diodes.

1. always wanted to build one but never got around to it. might be more components than it’s worth, but when you simply must have a very low voltage drop…

• Ooh – that’s an interesting one. Similar to the idea of an “ideal diode”. i.e. a MOSFET with smart logic to sense the direction of the applied voltage and then explicitly switching on or off. The result is a diode with almost no forward voltage drop at all.

In the case of solar cell driving a JeeNode, I suspect that the forward voltage drop will not be a problem. We’ll see – it’s definitely a circuit to keep in mind and try out at some point, thanks.

2. There is one drawback with schottky diodes, especially regarding ‘low current’ solutions. Their leakage current is in the order of two to three magnitudes higher than with ‘normal’ diodes. With the BAT43 (or BAR43) some uAs can actually flow in reverse direction (highly dependant on junction temperature).