SCR Speed Controller for Mains.  Are these any good ? - Page 1
Forum section: electronics / beginners

author: PinheadBE
author_rank: Regular Contributor
timestamp: November 08, 2023, 01:25:19 pm
content: Hi,I wonder if that item can be used to power devices with a classic power transformer at their input ?And what about Switching power supplies ?https://www.aliexpress.com/item/1005003774681066.html

author: CaptDon
author_rank: Super Contributor
timestamp: November 08, 2023, 11:26:17 pm
content: NEVER for a switching power supply!!! Some SMPS units with no undervoltage lockout will indeed self destruct at low input voltage. As for use with a transformer this is hit and miss. The SCR controllers designed for ceiling fan motor control seem to usually work o.k., other low budget low quality controllers will either self destruct or burn the transformer when operating below about 80% conduction angle due to an unsymetrical upper and lower half waveform which tends to make the transformer think it has some D.C. bias and goes into hard saturation with skyrocketing current draw.

author: PinheadBE
author_rank: Regular Contributor
timestamp: November 09, 2023, 01:06:39 am
content: Thanks for that answer.My problem is I have an AV amplifier which is designed to work with 220 V.But we now have 240 V in our sockets, here in Europe, and it results with a bit too much heat on the regulation of the (linear) power supply of the amplifier, because the secondary voltage is also a bit too high. (+- 28 V instead of +- 24 VDC after the bridge rectifier, which is, after that, stabilized to +- 15 V.... That's a lot of heat !)I thought that maybe such a cheap "voltage controller" could help by lowering the mains to 220 V.I've already tried to insert two diodes in the + and - lines, but that's not enough to significantly lower the heat dissipation of the regulating transistors, and due to the availabe space, I cannot insert more diodes.I'll check the waveform for any unbalance in the two halves of the (pseudo) sinus, after having connected the whole thing on my isolation transformer, of course....

author: BrokenYugo
author_rank: Super Contributor
timestamp: November 09, 2023, 01:31:23 am
content: Look into making a "bucking transformer", e.g.https://sound-au.com/articles/buck-xfmr.htm.

author: Xena E
author_rank: Frequent Contributor
timestamp: November 09, 2023, 02:02:52 am
content: Excellent suggestion from Broken Yugo.Use a traditional line transformer with a 20Volt secondary.Connect the primary across the incoming supply and the secondary in series with the supply to the unit.**You do need to confirm that the phasing is correct to buck the supply rather than boost it: test before connecting the load **The transformer only needs rating to 10% of the maximum load of your application.

author: IanB
author_rank: Super Contributor
timestamp: November 09, 2023, 02:11:48 am
content: I second the buck transformer idea. Only trouble is, you might have to make an enclosure for it.Alternatively, if you are lucky, the existing transformer might be a toroidal type. In which case, you could add a buck winding to that transformer. Simply wind a few turns of ordinary insulated wire around the core until you get about 4 V out of it. Then connect this additional winding in series with the existing secondary winding, in reverse polarity, so it drops the voltage down from 28 V to 24 V. If the transformer is a center-tapped 24-0-24 type, then you will have to do this twice, once for each side.