Thyristor voltage regulator - Page 1
Forum section: electronics / projects

author: Harvs
author_rank: Super Contributor
timestamp: August 29, 2013, 02:49:37 am
content: In the new Rigol lab supply video it look like they're using SCRs as the pre-regulator. Does anyone know of another supply (or anything else for that matter) that has available schematics which uses this technique? While the basic principle is quite straightforward, I'm just interested in having a look at some schematics of how it's been implemented in a real world device.Thanksedit: helps if you spell thyristor correctly.

author: ElectroIrradiator
author_rank: Frequent Contributor
timestamp: August 29, 2013, 09:43:32 am
content: I'm guessing Rigol isn't using those Triacs for an active voltage preregulator as such. I could be wrong, yet to me it looks like they are simply using them plus their associated optical Triacs/Diacs as neat 'solid state relays' to select taps on the transformer without using mechanical parts. I guess you could call this a preregulator, even if it only has two settings, being either fully 'on' or 'off'.The idea is used - among many other places - in the HP/Agilent E3631A triple rail PSU. Have a look at the service manual for that one to see more details.

author: Harvs
author_rank: Super Contributor
timestamp: August 29, 2013, 09:54:07 am
content: I see, silly me. I just assumed that for tap switching they would want more than two levels. The old tek supply on my bench uses 4 taps between 0-30V. But two levels is all the E3631A uses so it must be ok.

author: Unknown
author_rank: Unknown
timestamp: August 29, 2013, 02:48:00 pm
content: Less taps means just more dissipation and a lower efficiency. Tap switching may also allow for a larger current at lower ranges. There's nothing wrong with building a linear power supply with a fixed input voltage to the pass transistor(s), it will just need to dissipate a lot of heat when set to low voltages like 3.3 V (but who ever uses voltages that low? :p).Some of the medium power HP/Harisson power supplies used an SCR pre-regulator. An example of a power supply that used an SCR preregulator:HP 6286A service manualThe theory of operations contains a decent explanation of how it's implemented.Some additional references:http://powersupplyblog.tm.agilent.com/2013/05/more-on-early-power-supply-preregulator.htmlAgilent AN-90B (highly recommended, though not that much on SCR preregs)I don't think you'll find many modern designs that use an SCR pre-regulator. A switching pre-regulator is more efficient and eliminates the large mains transformer. The SCR might give a better transient response, though.

author: Marco
author_rank: Super Contributor
timestamp: August 29, 2013, 03:25:14 pm
content: Quote from: alm on August 29, 2013, 02:48:00 pmI don't see how. If the load needs more power outside of the small part of the cycle the triacs conduct you're out of luck ... only thing to do is wait.

author: Unknown
author_rank: Unknown
timestamp: August 29, 2013, 05:05:02 pm
content: My reasoning was that the filter caps will be larger, since they're designed to maintain adequate voltage for the pass element for close to 10ms (100 Hz cycle of rectified 50 Hz AC). But of course the regulation takes place on much longer time scales, so I might very well be wrong.

author: Harvs
author_rank: Super Contributor
timestamp: August 29, 2013, 11:41:53 pm
content: That's what I was looking for Alm, thanks!

author: Rufus
author_rank: Super Contributor
timestamp: August 29, 2013, 11:57:46 pm
content: Quote from: Harvs on August 29, 2013, 09:54:07 amOne switched tap can almost halve pass device maximum dissipation, to halve it again would take 3 switched taps. I can see heat sink quickly becoming more economical than additional taps, wiring, and switches.

author: edavid
author_rank: Super Contributor
timestamp: August 30, 2013, 12:20:01 am
content: There's a Jim Williams preregulator design in this app note:http://cds.linear.com/docs/en/application-note/an32f.pdf

author: anotherlin
author_rank: Frequent Contributor
timestamp: August 30, 2013, 01:04:39 pm
content: Quote from: alm on August 29, 2013, 05:05:02 pmIs there anything preventing using a big filter cap right after a switching regulator ?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: August 30, 2013, 01:18:19 pm
content: Quote from: anotherlin on August 30, 2013, 01:04:39 pmIt can cause stability problems.

author: AlfBaz
author_rank: Super Contributor
timestamp: August 30, 2013, 01:33:26 pm
content: The other week I was repairing an smps the 240/115V switch bypassed a triac in the 115V position. Pretty nifty I thought