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Re: [Amps] SCR to adjust 240v to 200v

Subject: Re: [Amps] SCR to adjust 240v to 200v
From: Manfred Mornhinweg <>
Date: Wed, 17 Aug 2016 00:36:44 +0000
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So, I was just asking if anyone on Amps had tried using an SCR to control a large choke-input power supply ?

I haven't done it myself, but I have worked with equipment from the 1970s that worked in this way. I had no reliability problem with that equipment.

The principle is simply controlling the conduction phase angle with those SCRs, just like a dimmer. The choke-input filter extracts the average value. Note that the equipment I worked with had the SCRs directly drive the choke and capacitor, NOT a transformer. On the other hand, I don't see why it shouldn't work with a transformer in between, using two antiphase SCRs, but it becomes critical that the conduction times on both sides are identical. Otherwise the transformer will saturate due to flux walking. Instead when two SCRs are used with two plain diodes in a controlled bridge that feeds the filter, asymmetric conduction doesn't cause big trouble.

I agree with those who warn that interference could become a big problem. Unlike MOSFETs or bipolar transistors, the switching speed of SCRs cannot be controlled. Once triggered, they switch on very fast, and that creates a very considerable amount of RF energy. You will have to control that noise purely with filters and possibly some shielding, and it may not be easy.

Considering the tremendous advantage in terms of size and weight, and even in cost, I think that you should try it. But if you cannot get the noise under control, in the end you might have to resort to the external bucking transformer anyway!

but also might damage the power supply choke by applying non-sinusoidal power

I don't see why or how it should damage the choke. That choke conducts mainly a DC, with some ripple on it. The fast rise time every time an SCR switches on shouldn't cause any trouble. And if it does, the trouble will be mostly to the SCRs, which might have to conduct a capacitive inrush pulse every time, if the choke has a high interwinding capacitance. But I doubt that would really scare the SCRs, and if it does, a small additional inductor (basically an RF type) with a resistor in parallel will fix that.


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