tomw wrote:
I am using a relay with 120VAC coil as a "charger relay" - it turns off 12V
when AC is connected to the charger so the car cannot be moved.
J1772 does not have neutral so the coil must be driven by 240VAC for it to
work with J1772. The relay coil is rated at 3.5 VA, 1700 Ohm, 1.44W
dissipation. I connected a 4kOhm 10W power resistor in series with the
coil and get 144VAC across the coil, about 20% higher than rated voltage.
Specs say up to 110% rated coil voltage. Instead of running about 29 mA
through the coil I will be running about 35 and dissipating about 2 W rather
than 1.44W. Think this will shorten the coil life significantly? Should I
purchase a 5.8k 10W power resistor?
There are many ways to deal with this issue. It's been discussed before,
so see if you can find in in the archives.
A relay coil is an inductive load; therefore, a simple resistor in
series won't give you the expected voltage drop. The resistor will have
to be a higher value (as you found) so that half the supply voltage
appears across the relay coil.
If you're going to buy a resistor, buy one with a sliding tap that can
be adjusted with a screwdriver. Then you can trim it for exactly 120vac
on the coil.
Another method is to put a 4 watt 120v night light bulb in series with
the coil. Light bulbs act like constant current source. 4w / 120v =
0.033a, so the bulb will only allow 33ma to the relay coil over a wide
range of supply voltages. That is within the 10% tolerance specified for
the relay's coil.
If you don't want the heat from a resistor, you can also use an inductor
or capacitor in series to limit the coil voltage to 120vac. You'll have
to experiment, because the inductance of the coil is unknown, so we
can't calculate the inductor or capacitor value directly.
You can also get a tiny transformer with dual 120/120 vac primaries.
Ignore the secondaries, and connect the primaries in series across the
240vac supply. Connect the relay coil between the center tap and one
end. The transformer is being used as a 240/120v autotransformer.
Or, get a different relay with a 240vac coil.
Or get a second identical relay with a 120vac coil, and wire the two
coils in series.
--
In life’s great adventures, keep in mind thy cause.
Be neither moved by critics, nor multitude’s applause.
-- Nasreddin
--
Lee A. Hart, http://www.sunrise-ev.com/LeesEVs.htm
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