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From: John Popelish
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X-Mailer: Mozilla 4.7 [en] (Win98; U)
Subject: Re: Zener Diode
Date: Mon, 09 Sep 2002 23:39:20 GMT
NNTP-Posting-Date: Mon, 09 Sep 2002 19:39:20 EDT
> I wanted to energize a 12 volt relay coil only when my
> supply voltage rises to 10 volts or higher so I place
> a 10 volt zener diode in series with the relay coil.
> The relay will not energize when the zener turns on.
> the voltage across the zener is 10 volts but only 2
> volts across the relay coil. The relay coil only needs
> about 35 milliamps to turn on, after that the contacts
> will latch the supply voltage across the coil.
> What am I doing wrong and how can this be accomplished.
The problem with using a series zener to make the coil inoperative
below the zener voltage is that once the zener begins ot conduct, you
still need an additional voltage (called the pull in voltage, for
pretty obvious reasons) to get enough current passing through the
zener and coil to activate the relay.
You will have better luck separating the voltage decision from the
application of voltage to the relay coil. You will also need a relay
that definitely has a pull in voltage less than 10 volts, unless you
want deal with a voltage booster. If your 12 volt relay pulls in
reliably at a little less than 10 volts, so good. if it does not, it
would probably be better to use a 5 or 6 volt relay, and regulate the
12 volts down to that when the relay is on.
The turn on decision would be precise and easy if you use a comparator
to decide when the supply voltage was equal to or greater than 10
volts, and should also stay on if the voltage falls a bit below the
turn on point, to make it impossible for the relay to chatter if the
supply was to hold at exactly 10 volts.
A comparator is a device that compares two voltages and whose output
indicates which is more positive. Create a reference voltage with a
resistor and a zener diode (say, 2k2 1/4 watt resistor and a 5.6 volt
1/4 watt zener. Then create a divider with two resistors that
produces 5.6 volts when the supply is at 10 volts (say, a 75k to the
10 volt supply and a 100k to ground with a 5k pot between them to
allow for a little adjustment). Connect the zener to the - input and
the wiper of the divider to the + input, and connect the output of the
comparator to the gate of a mosfet big enough to carry the relay coil
current to ground. you can also tie the output back to the + input
through a 1 meg resistor to give the decision some snap. You will
also need a gate pull up resistor on the output (say, 10k, 1/4 watt),
and a diode across the relay coil, to keep it from spiking the mosfet
when the current is turned off.
If the supply is noisy, you might also connect a .1 uf capacitor
between the + and - inputs, to ignore the noise.
Here is a data sheet for a common dual comparator:
and a whole list of them:
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