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From: Joseph Legris
X-Mailer: Mozilla 4.77 [en]C-SYMPA (Win95; U)
Subject: Re: Engineering Assistance Needed
References: <3D80CE15.7AA8F912@xympatico.ca> <firstname.lastname@example.org>
Date: Thu, 12 Sep 2002 19:07:31 -0400
NNTP-Posting-Date: Thu, 12 Sep 2002 19:01:45 EDT
Organization: Bell Sympatico
Ed Edmondson wrote:
> Still no luck...... I swapped pins 2 and 3 on the comparator. I wired the op
> amp output through a 1,000 ohm resistor to the gate. I added a 10,000 ohm
> resistor from plus 5 volts to the gate. I wired the source to ground and the
> drain through an LED and limiting resistor to plus 5 volts to test the
> configuration. With 1.697 volts at the wiper of the pot I have 4.06 volts on
> the gate and the LED is on. With 4.88 volts at the wiper of the pot I have
> 2.692 volts on the gate and the LED glows slightly. With 2.689 volts on the
> wiper of the pot I have 4.02 volts on the gate and the LED comes on to full
> brightness. With this configuration the LED or relay won't turn off. The relay
> is a NAIS model TQ2-4.5v DPDT with a coil resistance of 150 ohms. Since the FET
> didn't turn completely off I didn't try the actual load.
> I have double checked the wiring on the plug board and there are no wiring
> errors. I have change the 530 to insure it wasn't the FET. I tried another
> manufacturer of FET and nothing changes.
> Why is something so simple unable to work?
> Ed Edmondson
> Alamosa, CO
Disconnect the FET but leave the 1K and 10K resistors in the circuit and
measure the voltages as you did before at the point where the gate was.
Also measure the voltages at both pins 2 and 3 each time.
The 10 Meg ohm resistor is still on pin 3, right?
It is odd that the gate voltage only goes down to 2.6 Volts or so.
Assuming the voltage at the output of the op-amp is roughly 1 Volt when
it is low, you should be getting about 1.4 volts at the gate. Are you
sure that you have a 1K and a 10K resistor on the gate? Maybe they are
both 1K or 10K.
Another possibility us that the circuit is oscillating. That is why I
asked you to remove the FET when making the above measurements. I assume
you are using a digital voltmeter to make the measurements -
oscillations will certainly give odd readings. If you can, use an
oscilloscope to look at the signals.
The relay you have chosen looks good. If you can't make progress with
the FET, give the relay a try with the common emitter output transistor.
Don't forget to put pins 2 and 3 back to the original connections. And
put a reversed diode across the relay coil or the transistor will get
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