unidirectional vs bidirectional diode

[ulix]

Hello,
I want to add this circuit to my transistor tester, for the protection of the avr pin protection.
Can I use a bidirectional diode as well for the P6KE6V8A? => like the P6KE6V8CA?

[Simon]

no, unless your circuit can take a negative voltage equal to the breakdown voltage of the TVS, most devices only take -0.6V on inputs for a reason.

[rs20]

I'd use the unidirectional device if I could, but I can't contrive a situation where the bidirectional diode wouldn't serve perfectly well, at least as far as being robust to bad inputs on TP1-TP3 is concerned.

Fwiw, if you only have the bidirectional device to hand, you can somewhat fake a unidirectional device by adding a standard diode in parallel with the bidirectional device.

no, unless your circuit can take a negative voltage equal to the breakdown voltage of the TVS, most devices only take -0.6V on inputs for a reason.

How is a negative voltage going to end up on VCC with respect to ground as a consequence of even the most malicious inputs on TP1-TP3?

[ulix]

Is there a alternative to the P6KE6V8A? I cant get one 😐

[rs20]

Is there a alternative to the P6KE6V8A? I cant get one 😐

Note that as I mentioned above, the bidirectional one is fine to use if you have trouble finding the unidirectional one. But since you asked, here's about six hundred alternatives: link

[Simon]

How is a negative voltage going to end up on VCC with respect to ground as a consequence of even the most malicious inputs on TP1-TP3?

I have no idea what this thing is connected to and am just thinking along theoretical lines. the classic situation is back emf that has a negative and positive spike, on the positive side it needs limiting, on the negative side it needs clamping to ground.

[ulix]

Well, I'm not sure. Can i use every diode listed?

[rs20]

I have no idea what this thing is connected to and am just thinking along theoretical lines. the classic situation is back emf that has a negative and positive spike, on the positive side it needs limiting, on the negative side it needs clamping to ground.

Right -- even in the theoretical motor case, the negative spikes get dumped to ground, and the positive spikes get dumped to VCC. Either way, VCC is becoming more positive with respect to ground, not decreasing let alone going negative.

Well, I'm not sure. Can i use every diode listed?

That link has had parameters preselected so it's only showing unidirectional diodes with a breakdown voltage in the 6-7V range, just like the one you specified in the first place. If you find a specific one that you can source somewhere (up to and include actually ordering from Digikey, probably not the optimal solution in Germany), feel free to post the datasheet here and we'll double-check it for you if you want. It's actually difficult with microcontrollers, because you might be operating the microcontroller at 5V, but the absolute maximum rating can be 6V (taking the ATMega8U2 as an example). So you need the TVS to be definitely completely open-circuit at 5V, but definitely completely closed-circuit at 6V. The part you originally specified won't actually do that, it doesn't start breaking down until 6.45V at least.

This is a good question, worthy of a new forum thread on its own: how do you protect a chip that nominally runs at 5V, but will be damaged by 6V? Because this sort of zener clamp isn't precise enough to do that. If you can run the micro at 3.3V instead, that'd be much easier.

If it doesn't interfere with the design, you should consider adding resistors on the test lines; that will limit the amount of current that can travel up through the protection diodes and into the VCC of the circuit. Otherwise, connecting a high-current 10V source to the pins will just blow up the TVS.

[timb]

Bidirectional Diodes, also known as copper wire.


Sent from my Tablet

[Simon]

Bidirectional Diodes, also known as copper wire.


Sent from my Tablet

So a copper wire has a fixed voltage drop of 0.6 volts or more does it? A bidirectional TVS diode will clamp spikes in both directions. Hence it is called bidirectional. The single directional one will act as an ordinary diode going the other way which is often preferred if you are trying to clamp back EMF. If you have a TVS diode across power supply to a circuit it is very unlikely in the case of ADC system that you want to be able to reject negative spikes over a certain voltage. You would in fact want to reject all negative spikes which could be coming from something else in the system so you use a unidirectional TVS diode. If it is or you have other requirements you can use a bidirectional TVS diode where the breakdown occurs on both polarities.

[timb]

It was a joke. The common description of a diode is a "one way valve" of sorts. So a bidirectional diode would just be a piece of wire. (And yes, if the wire was thin enough it could have that much of a voltage drop.)


Sent from my Tablet