Crowbar component ratings. - Page 1
Forum section: electronics / beginners

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 08, 2021, 05:33:35 pm
content: I have been trying to find the value of a zener diode for a high current low voltage power supply. What I can not find is the zener diode power rating. Looking to protect a power supply from over voltage, the supply will be 12.50 Volts with a maximum power rating of 25 Amps. I understand the concept of the crowbar circuit, but am unsure what power rating the zener diode needs to be. The SCR I'm going to use is Vishay VS-40TPS12A-M3, Thyristor 1200V, 35A, the gate current trigger is 35mA. But the only through hole zener diodes only go up to around 5 watts power rating, above that the zener diodes are stud mount devices. I was hopeful not to have to use a stud mount zener diode. The fuse in the output circuit would be a bit above 25 Amps, if the SCR fires, wouldn't the power passing through the zener diode reach the fuse blow power rating ? I really don't know what power rating the zener diode needs to be. The zener voltage I had in mind is 16 Volts.Any help appreciated, thanks for reading.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 08, 2021, 06:22:34 pm
content: Ohm's Law is inexorable: the voltage drop on the zener diode * fuse current = power dissipated on it.But crowbar, as it seems to me, it is better to do a varistor.

author: Mr. Wizard
author_rank: Unknown
timestamp: January 08, 2021, 07:09:52 pm
content: I’m not sure that’s right. I believe in a traditional crowbar circuit the current in the zener will be limited by the resistor placed in series with it. The SCR, which is in parallel with the zener and resistor, takes all the current (once the DC voltage exceeds the zener voltage plus a diode drop and turns the SCR on), not the zener. Please correct me if I’m wrong.

author: ledtester
author_rank: Super Contributor
timestamp: January 08, 2021, 07:44:51 pm
content: It would be helpful to see a schematic of the proposed crowbar circuit.

author: Benta
author_rank: Super Contributor
timestamp: January 08, 2021, 08:08:18 pm
content: In a classic crowbar circuit, the Zener is just there to trigger the SCR, which will then short-circuit the supply and (hopefully) blow the fuse. Unless someone has replaced the fuse with a nail, of course (happens more often than you think).

author: S. Petrukhin
author_rank: Super Contributor
timestamp: January 08, 2021, 09:16:24 pm
content: Quote from: Benta on January 08, 2021, 08:08:18 pmDo they really use nails in Germany too?

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 09, 2021, 10:37:39 am
content: Ok that makes sense, the zener diode isn't going to see huge current, as the resistors are of a low power rating. Any high current would destroy low power resistors. I'm just a bit confused with the SCR firing and blowing a 25 Amp fuse.The momentary dead short current would be huge, at least until the fuse blows. But it's over voltage is the mode of protection, and not an over current situation that blows the fuse. But more an over voltage scenario. I might not have all that right in my understanding.

author: Ian.M
author_rank: Super Contributor
timestamp: January 09, 2021, 11:18:20 am
content: OnSemi had a nice discussion of the considerations for selecting a crowbar SCR starting page 71 of'Thyristor Theory and Design Considerations Handbook' HBD855/DBasically its a matter of comparing I2t curves for SCRs and fuses, also considering the total energy available in the reservoir capacitors, and peak current from rail voltage and loop impedance, to select a SCR + fuse combo where the fuse can be guaranteed to clear before the SCR can blow.The Zener and series gate resistor ratings will be much lower as they only have to withstand the trigger current for the time it takes the crowbar to clear the fuse. Odds are a steady state power rating of 1/10 of the peak power will be sufficient, but you'll need to check their specific datasheets for limits on pulsed current and pulsed dissipation.

author: davelectronic
author_rank: Frequent Contributor
timestamp: January 11, 2021, 02:51:45 am
content: I've been looking different thyristor's and the specs on them. One thing i think i discovered was the RMS average current, and the average current can be considered as one for DC applications like crowbar circuits. I won't pretend i know everything in the data sheets, but i'm slowly getting it. when i started electronics some time ago now, i was interested in how these semiconductors could control AC wave forms. In particular motor control, i will have to try a few things to get a decent crowbar working, again my maths isn't great, but hopefully i can try and work it out. Thanks for the replys. and thanks for the links.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 11, 2021, 10:45:29 am
content: You don't need a massive thyristor. Its job is to blow the input fuse, so it only needs to be capable of dealing with your short circuit surge current.See Ian.Ms reply below. I'd plump for an IC controller for response time and some handy features like built in current monitoring and flags.You can get crowbar control ICs eghttps://www.analog.com/en/products/ltc1696.htmlorhttps://www.analog.com/en/products/ltc1642.htmlandhttps://www.microchip.com/wwwproducts/en/MIC2085.Sadly, the Onsemi MC3423 is now obsolete but you can still find it on Ebay. (Buyer beware).