Re: Using SSR to switch transformer
- From: Arlet Ottens <usenet+5@xxxxxxxxxx>
- Date: Mon, 24 Mar 2008 17:36:05 +0100
panteltje@xxxxxxxxx wrote:
On 17 mrt, 20:10, Arlet Ottens <usene...@xxxxxxxxxx> wrote:Hello,
I'm considering using an SSR to switch the primary side of a
transformer. The transformer is a toroid, 230V -> 27V. The secondary
side is fixed to a 1 Ohm nichrome wire heating element.
I've read some articles that say switching on at zero crossings causes a
high inrush current, and that it's best to switch on at peak voltage.
If I follow this advice, are there any other issues I need to look out
for ?
What kind of (surge) current capabilities does the SSR need to have for
reliable operation ? (DC resistance of primary side is about 1 Ohm)
My original idea was to switch the secondary side using a pair of SCRs,
but at that current it would require a fairly decent heat sink. If I can
switch the primary side, the dissipation would be reduced considerably.
It is nearly impossible to answer unless you mention what you think
is the secundary current.
Main point with semiconductor switching in the primary is the fact
that the slightest
imbalance will cause a DC component, so DC current to flow.
Also may well need snubbers etc.
Consider a normal relay :-)
If you want proportional control, secondary is what I personally would
prefer,
a resistive load is simple, could be SCR's or a bridge rectifier
followed by some MOSFET,
perhaps with PWM.
It will need a housing anyways, metal housing makes good heatsink.
After reading the various bits of advice, I decided to order a big SSR (50A continuous) to build a prototype.
I'm using a small MCU to produce the control signal for the SSR, with a simple zero crossing detector for synchronization. The current in the primary coil is measured by a current probe on the scope.
After some experimenting, I now have:
- Initial turn-on done using a soft start mechanism, as suggested by Fred Bloggs, but instead of using a secondary module, the MCU is doing this.
- When the core is fully magnetized, the MCU keeps track of the direction of the magnetic field.
- Further proportional control is done by switching at zero crossings, but only when core is magnetized in favorable direction.
Even without a snubber, switching at voltage peaks only produces small transients (*much* less than switching a 100W light bulb). Inrush current during soft start phase is only slightly higher than normal. Once the core is magnetized, and the MCU switches with alternating polarity, currents are the same as steady-state currents.
Significant DC currents are easily avoided, and small ones that I added for testing don't seem to have much effect on the performance.
The SSR has no heat sink and only gets slightly warm. Much better than switching on secondary side which produces more than 25W of heat. The soft start mechanism is a nice bonus.
.
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