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Science Forum Index » Electronics Forum » Need current sensing relay...
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| Earl Kiosterud... |
 Posted: Mon Jun 02, 2008 8:30 pm |
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Hi folks,
I'm looking for a relay I can put in series with a TV set to sense when it's on. The TV
uses between 175 and 240 Watts, depending mostly on the backlight setting -- it's an LDC
TV). So the current will be roughly 1.5 to 2 Amps AC. The contacts need to handle 1/2 A at
120 Volts AC. I know I could wire up something solid state, with a series resistor and some
circuitry driving a triac, but I'm looking for something simple and quick. Any ideas?
Thanks.
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Earl |
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| Eric Sears... |
| Posted: Tue Jun 03, 2008 9:39 pm |
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On Tue, 03 Jun 2008 01:30:32 GMT, "Earl Kiosterud"
<someone at (no spam) nowhere.com> wrote:
Quote: Hi folks,
I'm looking for a relay I can put in series with a TV set to sense when it's on. The TV
uses between 175 and 240 Watts, depending mostly on the backlight setting -- it's an LDC
TV). So the current will be roughly 1.5 to 2 Amps AC. The contacts need to handle 1/2 A at
120 Volts AC. I know I could wire up something solid state, with a series resistor and some
circuitry driving a triac, but I'm looking for something simple and quick. Any ideas?
Thanks.
--
Earl
Firstly, I presume you meant LCD (Liquid Crystal Display)? not LDC
Presumably you want to me able to trigger some other circuit whenever
the TV is on.
Two ideas
1. Use a small solar cell close to the screen to sense the light and
trigger a relay (probably this is a bit messy, but it avoids any
direct connection to the wiring).
2. The best way I have seen something similar done was with a sense
circuit to trigger an inverter "on" when a load was connected.
Basically you get two strings of three diodes each (able to handle say
6 amps in this case - for a safety margin, and probably 400piv). These
strings are connected back to back, and then put in series with the
TV. When the TV is on, you will probably find about 1.8 - 2.5 v ac
across the string. This can be used to turn on a low voltage relay
(after rectification with a single diode and smoothing cap) or even to
switch a reed relay with a coil of wire wrapped around it (I have done
this).
REMEMBER!!!! - the diodes are at grid voltage!!!! Be careful!!!
These are suggestions only. Your responsibility.
Eric Sears. |
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| Eric Sears... |
| Posted: Wed Jun 04, 2008 6:23 am |
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These
Quote: strings are connected back to back, and then put in series with the
TV.
Sorry - this is may not be clear. I mean that the two strings are
connected in parallel, but in reverse order.
An alternate is to use three diodes in one direction and one in the
other. This will result in slight (but probably unimportant)
inbalance, and you should probably have enough voltage to operate a
relay without need for any extra diodes.
But remember that if any diode fails, you could end up with some large
voltages that potentially destroy any attached relays!
Eric Sears |
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| Earl Kiosterud... |
| Posted: Wed Jun 04, 2008 9:05 am |
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Eric,
Thanks. But a relay whose operating current was roughly that of the TV would drop only the
Voltage it needs. It wouldn't need the diodes and other stuff. I'm hoping. I could still
add the diodes so that when the TV power was higher (the screen is turned up brighter), it'd
limit the Voltage across the relay. The relay would need to be just sensitive enough to
operate with the lower current of the TV.
Back in the days of turntables, there were boxes into which you plugged your turntable,
which had a relay whose coil was in series with the turntable. The contacts operated
another circuit to power other gear when the turntable was running. This would be similar,
but since there's quite a bit more current available (how much current would a turntable
draw?), the voltage drop ought to be really low. I guess it's just a matter of getting the
right coil impedance, given the power the coil needs.
Hey, lemme ask a similar qestion. Anyone know of a small relay whose coil is easily
rewindable? I could rewind it with whatever gauge wire would give the optimum results. The
relay need be a somewhat sensitive one (not big heavy contacts with a big fat spring etc).
--
Earl
"Eric Sears" <phoneme at (no spam) 025379386.for.email.address> wrote in message
news:48467980.988865 at (no spam) news.clear.net.nz...
Quote: These
strings are connected back to back, and then put in series with the
TV.
Sorry - this is may not be clear. I mean that the two strings are
connected in parallel, but in reverse order.
An alternate is to use three diodes in one direction and one in the
other. This will result in slight (but probably unimportant)
inbalance, and you should probably have enough voltage to operate a
relay without need for any extra diodes.
But remember that if any diode fails, you could end up with some large
voltages that potentially destroy any attached relays!
Eric Sears |
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| Eric Sears... |
| Posted: Wed Jun 04, 2008 4:56 pm |
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Guest
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On Wed, 04 Jun 2008 14:05:47 GMT, "Earl Kiosterud"
<someone at (no spam) nowhere.com> wrote:
Quote: Eric,
Thanks. But a relay whose operating current was roughly that of the TV would drop only the
Voltage it needs. It wouldn't need the diodes and other stuff. I'm hoping. I could still
add the diodes so that when the TV power was higher (the screen is turned up brighter), it'd
limit the Voltage across the relay. The relay would need to be just sensitive enough to
operate with the lower current of the TV.
Back in the days of turntables, there were boxes into which you plugged your turntable,
which had a relay whose coil was in series with the turntable. The contacts operated
another circuit to power other gear when the turntable was running. This would be similar,
but since there's quite a bit more current available (how much current would a turntable
draw?), the voltage drop ought to be really low. I guess it's just a matter of getting the
right coil impedance, given the power the coil needs.
Hey, lemme ask a similar qestion. Anyone know of a small relay whose coil is easily
rewindable? I could rewind it with whatever gauge wire would give the optimum results. The
relay need be a somewhat sensitive one (not big heavy contacts with a big fat spring etc).
Well I guess you could do it this way, though relays operated on ac
tend to "chatter".
Its just a bit of ohms law I think.
A 12v, 100ma coil (that's fairly heavy) uses 1.2watts. So if you want
a coil that passes 2 amps (ie 20 times the current), then it will drop
about 0.12v at the same power rating.
R = V / I = 0.12 / 2 = .06 ohms.
You are probably looking at about a metre of 20 gauge wire maybe, used
for the coil (just a guess).
You chances of finding such a relay ready-made are pretty remote I
think.
The good thing is that the TV will limit the current to about 2 amps -
but remember its ac, not dc. That's why I would use the diodes and a
smoothing cap. There is a way of putting a bridge rectifier in between
the relay and the ac voltage so that the relay sees dc (which can be
smoothed).
Why not experiment with a homemade coil on the relay, using 12v dc and
a 25 watt bulb (to limit the current to 2 amps). This will prove if
you have it right, before you put it in circuit with the TV.
Eric Sears. |
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