SCRs - Page 1
Forum section: electronics / general

author: Australianstig
author_rank: Contributor
timestamp: August 02, 2016, 04:49:07 am
content: How easy is it to parallel SCRs if one can't find the right current rated one?

author: wraper
author_rank: Unknown
timestamp: August 02, 2016, 05:26:43 am
content: You would need to add resistors in series with each of them (additional power loss and heat), otherwise current would not share equally.EDIT: and they need to be close to each other on the same heatsink. Because the more it heats, the less voltage drop will be on particular SCR -> more current flows through it -> it heats up even more, increasing imbalance.If your load can be divided in halves, this is the way to go.

author: tautech
author_rank: Super Contributor
timestamp: August 02, 2016, 05:32:26 am
content: Quote from: Australianstig on August 02, 2016, 04:49:07 amWow, you must have a BIG project, SCR's are available to 1000's of amps ratings.It'd be wise to state your requirements so an appropriate device can be suggested.

author: Australianstig
author_rank: Contributor
timestamp: August 02, 2016, 07:03:57 am
content: I'm just looking at what's available in this town (not that much) to run a 25A regulated battery charger to replace the C122D SCR in the old EA '97 Add on regulator for 12V battery chargers

author: tautech
author_rank: Super Contributor
timestamp: August 02, 2016, 08:42:16 am
content: Quote from: Australianstig on August 02, 2016, 07:03:57 amThis should do:http://www.jaycar.co.nz/c122e-500v-8a-scr/p/ZX7012You should have a Jaycar branch handy.

author: Australianstig
author_rank: Contributor
timestamp: August 06, 2016, 05:12:24 am
content: Aren't the C122D/E SCRs only rated for 8A or can I drive them that hard with a good heatsink as I'm going major overkill in that department with a Pentium II cooler for it and the rectifier

author: tautech
author_rank: Super Contributor
timestamp: August 06, 2016, 06:38:54 am
content: Quote from: Australianstig on August 06, 2016, 05:12:24 amNZ you'll need to look for a 35A unit, RS or Element14 should have what you need. Pretty sure I'm using that same circuit in a charger I built years back. Can you post up the schematic ?

author: wraper
author_rank: Unknown
timestamp: August 06, 2016, 06:55:17 am
content: Quote from: Australianstig on August 06, 2016, 05:12:24 amQuoteHow it turns out that you need toreplacethe SCR, but magically also need one with multiple time higher current rating? Isn't that SCR on primary side of transformer by the chance?

author: tautech
author_rank: Super Contributor
timestamp: August 06, 2016, 07:15:58 am
content: Quote from: wraper on August 06, 2016, 06:55:17 amIf the OP posts up the schematic I think we'll see he's looking for a bigger current rated SCR as substitute more so than a replacement.

author: Australianstig
author_rank: Contributor
timestamp: August 06, 2016, 11:50:43 am
content: The old EA add on regulator for battery chargers, I'm going for a transformer with a 25A secondary instead of a wimpy Arlec4 charger

author: oldway
author_rank: Super Contributor
timestamp: August 06, 2016, 01:21:13 pm
content: I recomend LITTELFUSE S6070WTP THYRISTOR,70A,600V,TO-218Xhttp://au.element14.com/littelfuse/s6070wtp/thyristor-70a-600v-to-218x/dp/1869366

author: Australianstig
author_rank: Contributor
timestamp: August 12, 2016, 03:37:05 am
content: That gives me the idea of this one as my cooling solution for it and the rectifier is more than up to the task by a long shothttp://au.element14.com/littelfuse/s8025r/scr-thyristor-25a-800v-to-220ab/dp/1605731?MER=bn_level5_4NP_Echo_1Plus the 25A is for heavier loading when needed but not continuous

author: TerraHertz
author_rank: Super Contributor
timestamp: August 13, 2016, 01:00:54 am
content: In your charger, is the triac before or after the transformer? If it's before, you don't need that huge current rating, assuming you are running it from a 10A power point. I get the feeling from your description that it is after the transformer, so takes the full output current. Not a clever design, but if that's what you have...Would these do? Toshiba SM16JZ41http://www.datasheets360.com/pdf/-3482752358628465745VDRM 600V, IT(RMS) 16A.Just had a rummage through my crate of triacs, and I have three of these you can have. PM me your postal address if you want them. I'm in Sydney; sounds like you're in a small town somewhere?Btw, in upgrading to a much bigger than original triac, check that the circuit can supply the required gate drive for the larger device.

author: Australianstig
author_rank: Contributor
timestamp: August 18, 2016, 11:24:25 am
content: Even one of those might do it considering the cooling I have for it. Plus it's not going to be carrying 25A continuous but may be called on for up to around 30 minutes anyway

author: oldway
author_rank: Super Contributor
timestamp: August 20, 2016, 06:37:22 pm
content: Semiconductors and their cooler reach very quickly the steady state temperature (less than 5 minutes) and it does not matter that it's not going to be carrying 25A continuous but may be called on for up to around 30 minutes anyway.For the bigger transformer, I remember you that transformers for battery charger must have a higher short circuit voltage than that of the usual transformers. (20% instead of 7%)NB: See definition of "short circuit voltage" on Internet if necessary.This is achieved by windings primary and secondary coils side by side.

author: Zero999
author_rank: Super Contributor
timestamp: August 20, 2016, 08:10:24 pm
content: I couldn't find any reference to short circuit voltage. Don't you mean short circuit current?The short circuit voltage will be close to zero. I'd also expect a battery charging transformer to have a lower short circuit current because it will have a high leakage inductance to limit the current.

author: oldway
author_rank: Super Contributor
timestamp: August 20, 2016, 08:28:41 pm
content: How to measure this short circuit voltage ? (I see that it is also named short circuit impedance in english)An adjustable autotransformer (variac) is connected to the transformer under test.A voltmeter is connected to the transformer's primary winding.An ammeter is connected in series with the transformer's short circuited secondary winding.Starting from 0 V, the primary voltage is increased until secondary current reach it's rated value. (full load current)The short circuit impedance (ou voltage) can be calculated by dividing such primary voltage by the rated primary voltageA short circuit voltage (or impedance) of 20% means that the short circuit current will be 5 x the rated secondary current with rated primary voltage.

author: Zero999
author_rank: Super Contributor
timestamp: August 20, 2016, 08:41:47 pm
content: Oh I see, it was a translation error. Please don't use the term short circuit voltage. It makes no sense in English.Short circuit impedance is the correct term and yes, it's used to calculate the short circuit current.http://www.cooperindustries.com/content/dam/public/bussmann/Electrical/Resources/solution-center/technical_library/BUS_Ele_Tech_Lib_Electrical_Formulas.pdfYes, you're right, the short circuit impedance should be higher on a battery charging transformer as it will have a higher leakage inductance to limit the current.

author: oldway
author_rank: Super Contributor
timestamp: August 20, 2016, 08:51:07 pm
content: In french, it's "la tension de court-circuit"....It seems it can't be simply translated....sorryIt's always very difficult to participate in a forum in a language that we not perfectly dominates.This is why my interventions are rare and very short.

author: Zero999
author_rank: Super Contributor
timestamp: August 20, 2016, 11:26:48 pm
content: Quote from: oldway on August 20, 2016, 08:51:07 pmNo need to apologise. Sorry if I sounded a little harsh. Your English is much better than my French.