Correct me if I'm wrong. A generic 20mA LED has a max safe reverse voltage of only ~5V. In a system with higher voltages, even a resistor to limit reverse current to, say, 1mA will still not protect the LED. If it breaks down at all, it's toast. Is this right?

Such an LED can be protected by adding a normal diode in series with it. [UPDATE: reverse parallel is better]. Using a 1N4007 would protect the LED at voltages approaching 1000V. The arrangement of the two diodes with respect to each other doesn't matter as long as they're in series. Right again? (I have no intention of doing that, I'm at 12V, just making a point.)

 

Correct me if I'm wrong. A generic 20mA LED has a max safe reverse voltage of only ~5V. In a system with higher voltages, even a resistor to limit reverse current to, say, 1mA will still not protect the LED. If it breaks down at all, it's toast. Is this right?

Such an LED can be protected by adding a normal diode in series with it. Using a 1N4007 would protect the LED at voltages approaching 1000V. The arrangement of the two diodes with respect to each other doesn't matter as long as they're in series. Right again? (I have no intention of doing that, I'm at 12V, just making a point.)

I would put that 1N4007 in parallel with the LED but in the opposite direction.

 

A lot of devices are tested for reverse breakdown voltage with a microamp, so I think it's a continuum. A microamp probably won't kill an LED, but a milliamp or a dozen milliamps might. The 1N4001 has a reverse leakage of 5 ua to 30 ua and a reverse recovery time, so I would tend to put the diode in parallel with the LED.

 

Correct me if I'm wrong. A generic 20mA LED has a max safe reverse voltage of only ~5V. In a system with higher voltages, even a resistor to limit reverse current to, say, 1mA will still not protect the LED. If it breaks down at all, it's toast. Is this right?

Such an LED can be protected by adding a normal diode in series with it. Using a 1N4007 would protect the LED at voltages approaching 1000V. The arrangement of the two diodes with respect to each other doesn't matter as long as they're in series. Right again? (I have no intention of doing that, I'm at 12V, just making a point.)

Once or twice I've accidentally hit LEDs with considerably more than reverse 5V and got away with it, but its better not to tempt fate.

Most people use an inverse parallel clamp diode - a series diode has reverse bias junction capacitance that could couple any spikes into the LED junction.

 

I've played with this on a 'scope and there is a tiny glitch with a series diode, easily taken out with some pfs across the LED.
The parallel method doesn't have a glitch, isn't susceptible to power line spikes being capacitively coupled, and doesn't need to be "fixed" with a capacitor...but it wastes current or needs another diode in series to stop the waste current.

 

Yeah, reverse parallel makes a lot more sense, especially for my particular situation where I have current limited to ~0.5mA. A little waste current is trivial.

 

I've played with this on a 'scope and there is a tiny glitch with a series diode, easily taken out with some pfs across the LED.
The parallel method doesn't have a glitch, isn't susceptible to power line spikes being capacitively coupled, and doesn't need to be "fixed" with a capacitor...but it wastes current or needs another diode in series to stop the waste current.

That tends to be the case with the type of dropper we aren't allowed to discuss here - I usually use an inverse parallel pair of LEDs and use the "waste" current. If that gets too messy - I use a bridge rectifier.

 

A generic 20mA LED has a max safe reverse voltage of only ~5V.

It depends on the LED. I have some white SMT LEDs that have a low reverse breakdown voltage and the manufacturer included a red LED anti-parallel to protect it. Said LEDs are also static sensitive.

Most of my LEDs specify 12V for max reverse voltage.

 

... the manufacturer included a red LED anti-parallel to protect it. ...

[slaps forehead] That's the perfect solution for my application, two LEDs antiparallel. Whichever is conducting will limit the reverse voltage on the other one to Vf.

 

[slaps forehead] That's the perfect solution for my application, two LEDs antiparallel. Whichever is conducting will limit the reverse voltage on the other one to Vf.

I said that in post 7........................

 

Spent yesterday in agonizing pain from throwing my back out the day before. I guess I was a little slow to pick up on the obvious. Thank God for oxycontin and chiropractors.