Zener Diode Clipper Circuits For Both DC and AC Transients - Page 1
Forum section: electronics / beginners

author: KJ-90
author_rank: Contributor
timestamp: December 14, 2016, 05:24:13 pm
content: This may sound like a simple question, but I am wondering about zener diode clipper circuits of the back-to-back anodes type. It involves keeping transients out of test equipment powered by AC wall adapters, which output DC voltage. As we know, TVS diodes are a larger, more powerful version of ordinary zener diode clippers. They can be either unidirectional ( DC ) or bidirectional ( AC ). In an AC circuit, the TVS diode will clip both positive and negative AC peak voltages that it is rated to. For example, a 120 VAC TVS diode might have a clamping voltage of 209 volts peak. The unidirectional DC diode will have a clamping voltage that varies with the reverse stand off voltage ( for example, a 5V TVS diode might clamp at 9V ). Note that of course the breakdown voltages are usually somewhat lower.However, if I want to protect a Fluke 8040A vintage LED DMM that runs at 7.5 Vdc, how would I connect TVS devices to block both positive and negative DC transients, and AC transients? For example, what voltage ratings of zener diodes connected anode to anodewould I use to block out such transients above say 10 Volts DC negative and positive, and AC as well? What voltage rating of a bidirectional TVS diode would I use also? How are the voltage ratings of devices determined for this application?

author: Gyro
author_rank: Super Contributor
timestamp: December 14, 2016, 07:43:45 pm
content: A Unidirectional TVS will automatically block negative voltage transients - it's a zener diode, it automatically conducts when forward biased (negative transients) and avalanch clamps on positive overvoltage. In this respect the bidirectional TVS is the 'difficult' one because it rquires back-to-back dies in the same package so that it can tolerate both positive and negative supply voltages.A simple unidirectional TVS (or zener) is all you need to block both positive and negative transients on a DC supply.

author: TimFox
author_rank: Super Contributor
timestamp: December 14, 2016, 08:36:03 pm
content: You can also use a four-diode bridge and a single Zener diode. The clipping voltage will be the Zener voltage plus two diode drops, but you don't have a difference between the positive and negative Zener voltages found with two Zeners.

author: Gyro
author_rank: Super Contributor
timestamp: December 14, 2016, 08:54:16 pm
content: Quote from: KJ-90 on December 14, 2016, 05:24:13 pmReading again, I'm not sure why you want to block the 7.5V DC supply against "10V DC negative" or AC. Surely you want to blockeverythingother than +7.5V DC. A single unidirectional TVS or Zener (and a fuse) will do this.

author: macboy
author_rank: Super Contributor
timestamp: December 15, 2016, 07:25:33 pm
content: Quote from: Gyro on December 14, 2016, 08:54:16 pmI agree, a single zener of suitable voltage (say 10V or so) across the supply will, in combination with a fuse or polyfuse, protect against positive overvoltages as well as negative voltages, which may be even more damaging. Why allow -10V to pass before clamping?Or are we misunderstanding the application?