crowbar or TVS ?

[JacquesBBB]

I want to protect the output of a bench power supply that is  providing  fixed 5V output.
I had the surprise once to see this output jumped to 15 V.

Now I understand that this can happen if the  transistor that is driving the main power transistor fails.

I   made on a prototype  board the crowbar that is given in the Art of Electronics (3rd) in Fig. 9.102C 
with an added fuse.



But will a simple TVS (Transcient Voltage Suppressor)  as the ICTE5  with an additional fuse  do the same thing ?
http://www.vishay.com/docs/88356/icte.pdf

[uncle_bob]

I want to protect the output of a bench power supply that is  providing  fixed 5V output.
I had the surprise once to see this output jumped to 15 V.

Now I understand that this can happen if the  transistor that is driving the main power transistor fails.

I   made on a prototype  board the crowbar that is given in the Art of Electronics (3rd) in Fig. 9.102C 
with an added fuse.



But will a simple TVS (Transcient Voltage Suppressor)  as the ICTE5  with an additional fuse  do the same thing ?
http://www.vishay.com/docs/88356/icte.pdf

Hi

If it is a real fuse and a real TVS, the TVS will go up in smoke before the fuse opens. 

Yes, you *can* have a TVS the size of your fist and a $10 50 ma fuse and it will work fine.

Bob

[uncle_bob]

Hi

If it is a real fuse and a real TVS, the TVS will go up in smoke before the fuse opens. 

Yes, you *can* have a TVS the size of your fist and a $10 50 ma fuse and it will work fine.

Bob

Good TVS devices are designed to fail short, not open. There is a thread here, someone got a cheap thermal camera, only to find it won't power up. It turns out the TVS was shorted as a result of overvoltage, then blew the fuse. After replaced those, it came back to life.

Hi

Some TVS's go short and then go open after the over current.. Like a fuse, they have an energy level they will deal with. Past that energy level, all bets are off.

Replacing both an expensive fuse and a TVS each time there is a spike seems silly. If you have a supply that spikes once, it's likely to do it on a regular basis.

Bob

[Gyro]

The 'problem' with TVSs is that they are not perfect clamping devices.

Firstly they have a tolerance that you can do nothing to change (other than selecting another sample). They also have three important parameters:

1. The holdoff voltage. That's the voltage where none of the devices, over the full manufacturing tolerance spread will conduct any significant current. This is the maximum voltage of the rail you would use them on.

2. The conduction voltage. That's the voltage where the TVS will conduct a specified current ( just like Vz @ Iz for a zener diode).

3. The clamping voltage. This is the point will conduct hard, again at a specified (large) current. This is the point where your fuse would start to blow.

In practice, for a 5V (holdoff) device, the spread of these three voltages will be quite high, you probably won't hit the clampiing voltage until about 8.5V... probably higher.

In contrast, your crowbar circuit with its nice accurate TL431 and trimmable resistive divide can be made to trip at, say, 5.5V. Even if you replaced the Thyristor with a big beefy transistor to act as a voltage clamp rather than a crowbar, it would still perform better than a TVS in your situation.