SCR control of an inductive load is not so cut and dried simple as some
would like to think. The gates of SCR's need to be pulse fired for reliable
phase control. Commercial units have pulse train burst applied to the gates
through a pulse transformer. The phase angle of the burst is derived any
number of ways which can be very precise and repeatable.
But, all that said, use a buck boost solution and save yourself a headache.
Paul K0UYA
-----Original Message-----
From: Manfred Mornhinweg
Sent: Tuesday, August 16, 2016 7:36 PM
To: amps@contesting.com
Subject: Re: [Amps] SCR to adjust 240v to 200v
Jim,
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.
Manfred
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