AC Coils
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Posts: 46,084
Hi David,
Unfortunately a rectifier across the coil will do
nothing more than short both polarities of the
applied AC power.
One trick for controlling AC coils is that one can
power them with DC, provided you do a bit homework
first. The AC reactance of the coil is what prevents
excessive current through the DC measured resistance of
the coil. By doing some ohms law calculations one can
treat an AC coil like a DC coil by placing a resistor
in series with the coil to limit current flow and then
add a standard diode across the coil, for flyback. Just
remember to use power as part of the calculation.
IF you have a variable power supply one can vary the
voltage to find out the pull in voltage for a coil and
then measure the current draw. It is not untypical for
a 24VAC coil to pull in on 5 volts DC.
The other option is to use an SSR rated for inductive
loads.
Sincerely,
Ron
> Message: 19
> Date: Thu, 3 Apr 2003 21:52:38 -0800
> From: "Dave" <dokagaki@w...>
> Subject: a/c coil transient suppression
>
> Hi folks,
>
> I'm new to the group and to stamps, so thanks in
advance.
>
> Here's the situation: I've got a bs2 controlling a
5vdc relay which switches
> a 24vac sprinkler valve controller. The power supply
for the whole setup is
> a 24vac transformer; I'm using a bridge and regulator
to cut it down to 5vdc
> for the electronics (with lots of filter caps) and it
also powers the valve
> selenoid directly via the relay. I've got a
rectifier across the relay coil
> to clamp spikes, but I have a feeling I need
something across the selenoid
> (something called a snubber, perhaps?) to block
spikes generated by it
> during switching. The system behaves somewhat
irratically when the selenoid
> is switched on and off (scope -- set for a/c,
2v/div -- seems to show
> occasional spikes during the switching), it behaves
fine without the
> selenoid, and also with the selenoid if I put a cap
across it (probably not
> a great idea since it's driven by a/c).
>
> I'm not that familiar with a/c circuit transient
suppression, and would
> appreciate any insights.
>
> thanks,
>
>
> d...
>
>
*******************************************************
*******************
> * David Okagaki
dokagaki@w...
> *
Unfortunately a rectifier across the coil will do
nothing more than short both polarities of the
applied AC power.
One trick for controlling AC coils is that one can
power them with DC, provided you do a bit homework
first. The AC reactance of the coil is what prevents
excessive current through the DC measured resistance of
the coil. By doing some ohms law calculations one can
treat an AC coil like a DC coil by placing a resistor
in series with the coil to limit current flow and then
add a standard diode across the coil, for flyback. Just
remember to use power as part of the calculation.
IF you have a variable power supply one can vary the
voltage to find out the pull in voltage for a coil and
then measure the current draw. It is not untypical for
a 24VAC coil to pull in on 5 volts DC.
The other option is to use an SSR rated for inductive
loads.
Sincerely,
Ron
> Message: 19
> Date: Thu, 3 Apr 2003 21:52:38 -0800
> From: "Dave" <dokagaki@w...>
> Subject: a/c coil transient suppression
>
> Hi folks,
>
> I'm new to the group and to stamps, so thanks in
advance.
>
> Here's the situation: I've got a bs2 controlling a
5vdc relay which switches
> a 24vac sprinkler valve controller. The power supply
for the whole setup is
> a 24vac transformer; I'm using a bridge and regulator
to cut it down to 5vdc
> for the electronics (with lots of filter caps) and it
also powers the valve
> selenoid directly via the relay. I've got a
rectifier across the relay coil
> to clamp spikes, but I have a feeling I need
something across the selenoid
> (something called a snubber, perhaps?) to block
spikes generated by it
> during switching. The system behaves somewhat
irratically when the selenoid
> is switched on and off (scope -- set for a/c,
2v/div -- seems to show
> occasional spikes during the switching), it behaves
fine without the
> selenoid, and also with the selenoid if I put a cap
across it (probably not
> a great idea since it's driven by a/c).
>
> I'm not that familiar with a/c circuit transient
suppression, and would
> appreciate any insights.
>
> thanks,
>
>
> d...
>
>
*******************************************************
*******************
> * David Okagaki
dokagaki@w...
> *