Using SCR's as diode's - reverse blocking voltage - Page 1
Forum section: electronics / projects

author: adamcord
author_rank: Contributor
timestamp: February 12, 2020, 04:13:07 pm
content: In a project I'm working on, I'm thinking of using some beefy SCR's as diodes by tying the anode to the gate (a practice which is usually kosher). I want to use these SCR's due to their high blocking voltage and high current ratings. But since the Gate would be shorted to the Anode, I'd eliminate one of the P-N junctions that might otherwise be there to block the reverse voltage, leaving only one P-N junction, right?So my question is:Does shorting gate to anode on an SCR lower it's reverse blocking capability?(I guess similarly, what if my gate driver conducts between gate-anode during the reverse voltage condition? Again, would this pose a risk of reverse breakdown?)

author: schmitt trigger
author_rank: Super Contributor
timestamp: February 12, 2020, 04:38:56 pm
content: I randomly checked the very first power SCR that I found a Vishay VS-111RKI120PBF. This is a 110 amp device.Its data sheet indicates a VDRM/VRRM of 1200 volts, yet its maximum peak gate voltage is only rated between 20 volt and -10 volts.

author: Zero999
author_rank: Super Contributor
timestamp: February 12, 2020, 04:39:59 pm
content: Dumb question: why not use a diode?To answer the original question. SCRs can normally block the same reverse voltage as their off voltage rating. The only issue is the gate typically has a maximum reverse voltage rating of around 5V. Connect a small diode with a high enough reverse voltage rating in series with the gate and a high value resistor (100k to 1M) between the gate and cathode.You should use a diode, if you don't need a switch. It's much cheaper and easier.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 12, 2020, 04:40:53 pm
content: G-K has a low reverse breakdown voltage, like BJT B-E (and for the same reasons).IIRC, there are some SCRs made that way, with high breakdown on both terminals; I forget what special application they serve though. Or maybe it's what a TRIAC is internally, of course pasted together with its complement so the device switches AC without much concern over drive magnitude or sign.Tim

author: wraper
author_rank: Unknown
timestamp: February 12, 2020, 04:55:20 pm
content: You should first have tried to measure G-K resistance with multimeter and find that it conducts in both directions with tens to hundreds of Ohm resistance.

author: wraper
author_rank: Unknown
timestamp: February 12, 2020, 04:56:27 pm
content: Quote from: T3sl4co1l on February 12, 2020, 04:40:53 pmAround 0V to be exact, it's simply conductive both ways.

author: Zero999
author_rank: Super Contributor
timestamp: February 12, 2020, 06:16:35 pm
content: Quote from: wraper on February 12, 2020, 04:55:20 pmIt depends on the SCR. K-G is a diode connection. In small, sensitive gate SCRs it should measure the same as a diode, using a multimeter. Larger non-sensitive gate SCRs have a gate-cathode resistor built-in.

author: wraper
author_rank: Unknown
timestamp: February 12, 2020, 08:12:28 pm
content: Quote from: Zero999 on February 12, 2020, 06:16:35 pmIt's not a built-in resistor, it's an intrinsic feature of SCR. Small SCR may have this resistance on higher side but it's still there. And this resistance may easily vary 5+ times times within single part number (and so does gate sensitivity).

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 13, 2020, 12:16:31 am
content: Quote from: Zero999 on February 12, 2020, 04:39:59 pmI'm assuming he has the SCR lying around but not any large diodes and that it's for a temporary application. In that case, add a series diode and resistor combination from anode to gate to trigger it.

author: Circlotron
author_rank: Super Contributor
timestamp: February 13, 2020, 12:33:59 am
content: Quote from: NiHaoMike on February 13, 2020, 12:16:31 amThat resistor in series with the diode should be something like 10-50 ohms . The value is not critical, just that this lower value makes sure the SCR turns on before the forward voltage gets too high, and the on-stage voltage drop does not give too high a continuous gate current like it would if there was no resistor at all.