# Unexpected rectifier waveform

I've built a simple half-wave rectifier using the following components.

• $10$k$\Omega$ resistor

• 1N914 diode

• 1 kHz sinusoidal source

I'm measuring the voltage across the resistor with an oscilloscope.

This is the wave I get with the source set to a magnitude of 1.5 V:

\

Bust as soon as I increase the magnitude of the source above 3.0V, this starts to happen. \

Can anyone explain what is happening here? Is this expected behaviour, or have I made a mistake?

• Definitely seems odd at first glance. Tell us more about how your circuit is constructed. Is this on a solderless breadboard, a PCB, airboarded? What's your source? – Stephen Collings Jan 30 '14 at 19:41
• This is on a breadboard, with a function generator as the source. – Chris A Jan 30 '14 at 19:49
• Solderless breadboard? The parasitic capacitance of that may be causing you difficulty at those frequencies. I'd be interested to know if this still happens if you solder the circuit together as an airboard. – Stephen Collings Jan 30 '14 at 19:52
• Try a second diode : I would either suspect some damage, or someone slipped a 4V7 zener into the 1N914 drawer... – Brian Drummond Jan 30 '14 at 21:09
• Bottom line, the component kit I was using was supposed to have two 1N914 diodes and one zener. Naturally I assumed the two identical diodes were the 1N914's, when in fact I had two zeners and only one regular diode. Everything is working as expected now. – Chris A Jan 30 '14 at 23:36

## 1 Answer

This has happened to me once and I struggled with it only to find that what I thought was a 1N4148 glass diode (like a 1N914) was in fact a 3.3V zener diode - try reverse biasing it like a zener to see if it draws current and acts like a voltage regulator.

Even if you assumed 10pF cross capacitance this would be an impedance of nearly 16 Mohm at 1kHz and not enough to do what the screen shots suggest. It has to be a broken diode or the wrong diode.

• Also, they should see the graph flip, which would pretty much nail it. – scld Jan 30 '14 at 21:28
• I suspected something like that. Will verify this evening. – Chris A Jan 30 '14 at 22:18