Discussion:
Controlling inrush current
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Steve B
2014-03-20 18:39:49 UTC
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hello

I am trying to get a handle on how inrush current can be alleviated when
arrays of LED lights are switched from one point.
It occurs to me that a solid state relay (SSR) would be useful in this
application. A module allowing for a turn-on switching voltage of ~230V
and incorporating zero-crossing topology would, seemingly, solve a lot
of the problems associated with inrush as well as being easy to plumb
in.

Can anybody on this group advise me if this is a viable approach or
otherwise?

thanks

Steve


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Peter Jason
2014-04-01 01:29:22 UTC
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On Thu, 20 Mar 2014 18:39:49 -0000, Steve B
Post by Steve B
hello
I am trying to get a handle on how inrush current can be alleviated when
arrays of LED lights are switched from one point.
It occurs to me that a solid state relay (SSR) would be useful in this
application. A module allowing for a turn-on switching voltage of ~230V
and incorporating zero-crossing topology would, seemingly, solve a lot
of the problems associated with inrush as well as being easy to plumb
in.
Can anybody on this group advise me if this is a viable approach or
otherwise?
thanks
Steve
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Maybe a "thermistor"
Andrew Gabriel
2014-04-01 11:15:18 UTC
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Post by Peter Jason
On Thu, 20 Mar 2014 18:39:49 -0000, Steve B
Post by Steve B
hello
I am trying to get a handle on how inrush current can be alleviated when
arrays of LED lights are switched from one point.
It occurs to me that a solid state relay (SSR) would be useful in this
application. A module allowing for a turn-on switching voltage of ~230V
and incorporating zero-crossing topology would, seemingly, solve a lot
of the problems associated with inrush as well as being easy to plumb
in.
Can anybody on this group advise me if this is a viable approach or
otherwise?
thanks
Steve
Maybe a "thermistor"
That can work, with some provisos...

It won't work if you switch the lights off and on quickly, as the
thermister will already be hot.

You need to mount it in some way which
a) allows it to be hot, safely,
b) allows it to cool quickly at switch-off,
c) allows it to fail (blow up) safely.

Zero-crossing switching won't help much - the inrush is charging
the DC resovoir capacitor from nothing to full charge in the first
1/4 mains cycle. Some PSU's have a resistor to spread this over
a few cycles (similar idea to the thermister, but with different
pros/cons).

But to echo Steve B's point, does it really matter?
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Andrew Gabriel
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2014-04-01 05:55:10 UTC
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Post by Steve B
I am trying to get a handle on how inrush current can be alleviated
when arrays of LED lights are switched from one point.
Keep in mind that incandescent lamps are also pretty much a dead short
when cold. If your application involves retrofitting LED for
incandescent, you might not *have* to make any changes. But maybe
switching on the LED lamps is causing flicker on other circuits or
other problems.
Post by Steve B
A module allowing for a turn-on switching voltage of ~230V and
incorporating zero-crossing topology would, seemingly, solve a lot of
the problems associated with inrush as well as being easy to plumb in.
I am not an electrician and this is not electrical code advice. Having
said that... It could be. A few points to keep in mind:

SSRs dissipate a little heat when switched on - the data sheet for the
relay will say. You may need to mount it on a heat sink. Usually there
is a metal plate on the bottom of the SSR, which is isolated from the
line (mains) and control voltages, so you don't need to worry about
insulating the heat sink. If it's in a big enough steel electrical box,
bolting it to the box itself may be sufficient.

SSRs never turn completely "off". There will be a few mA of leakage
through the SSR all the time. It is probably a good idea (and may be
required by local electrical code) to install a plain old mechanical
toggle switch in series with the SSR and lamps. This way, if someone
needs to work on the lamps, they can flip the toggle switch and be sure
that the whole thing is off, and they won't get a tingle when working on
it.

It probably helps if the SSR you get is approved by some safety agency.
In the US, it's pretty easy to buy UL/CSA listed parts. For Yurp, you
probably want something listed with TUeV or BS or similar.

If you select an SSR with a low-voltage control circuit (like 24 V AC),
you need a small transformer to provide this voltage. You can also use
cheaper cable on the control circuit and more easily install multiple
switches, if you like.

Some SSRs are available with LEDs to indicate when they are "on". This
might be handy if the SSR is installed where you can't see the main
LED lamps. Or, install a local pilot light near the relay - but
beware the leakage current.

I am not an electrician and this is not electrical code advice.

Matt Roberds
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