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Subject: Re: Zener Diode
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NNTP-Posting-Date: Tue, 10 Sep 2002 11:02:58 EDT
Date: Tue, 10 Sep 2002 10:03:42 -0500
How about using a transistor switch and connecting the zener
to the base, once the zener starts to conduct it provides a
bias to the transistor base. The transistor then could switch
the source across the relay coil. Would this work, if so
what would be an appropriate transistor and what other
components would I need.
"John Popelish" wrote in message
> Redd wrote:
> > 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:
> John Popelish
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