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From: Joseph Legris
X-Mailer: Mozilla 4.77 [en]C-SYMPA (Win95; U)
Subject: Re: Engineering Assistance Needed
Date: Thu, 12 Sep 2002 13:25:41 -0400
NNTP-Posting-Date: Thu, 12 Sep 2002 13:19:57 EDT
Organization: Bell Sympatico
Ed Edmondson wrote:
> Can you provide me with a little engineering help? I tried the news groups and
> had no success. I want to use a LM358 and a 10K Ohm (@ 25 degrees C)to turn on
> a resistive load. I have a basic circuit which appears to work however I can't
> get the output to actuate a small 5 volt relay. I can get it to turn on a LED
> with a 2N3906, a 1K Ohm base resistor, and a LED current limiting resistor.
> Is it possible to use an IRF530/531 instead of a relay? If so how can I
> implement the design? My resistive load is about 1.5 amps @ 12 volts and the
> control circuitry is run from 5 volts.
> Circuit Description....
> LM358 dual op amp. Power is 5 volts to pin 8 and ground to pin 4. I have a 10K
> Ohm thermistor connected in series with a 10K Ohm resistor. The tap between
> these two resistors is conected to pin 2 (minus input). The thermistor is
> connected to 5 volts and the resistor is grounded (a voltage divider).
> Then I have two 10K Ohm resistors in series with a 10K pot in the middle to set
> the trip point. The pot wiper is connected to pin 3 (Plus input). One end of
> the 10K resistor is connected to 5 volts and the opposite end to ground. I have
> a 10 meg Ohm resistor between pins 3 (plus input) and pin 1 (output).
> The output of pin 1 is fed into pin 5 (plus input) and pin 6 (minus input)is
> connected to pin 7 (output). This is a voltage follower?
> I can feed the output (pin 7 or pin 1) into the base of a 2N3906 through a 1K
> resistor. The collector is connected to ground. The emitter is connected to an
> LED in series with a current limiting resistor to plus 5 volts.
> I tried to actuate a relay with the same circuit used on the LED. For some
> reason it will not turn the relay on?? I am so close but it seems miles away.
> Should I use a relay or power MOSFET to power the 12 volt @ 1.5 amp maximum
> resistive load.
> The only indicators I planned on were a red LED when the load is powered and a
> yellow LED when the load is off. It would also be nice to have a green LED when
> the unit is idle (not on or off).
> Since my stroke last tear I have a problem designing anything. However, I can
> build anything if provided with a schematic.
> Ed Edmondson
> Alamosa, CO
You haven't said how much voltage and current your relay requires to
operate, but I doubt that you are getting enough voltage on the output.
There are 2 reasons for this:
1) You are using the PNP output transistor as an emitter follower. In
this configuration, the voltage at the emitter can never go less than
roughly 0.7 volts above ground. That limits the output voltage that
appears across the relay to 4.3 volts maximum.
2) It gets worse. The LM358 claims to have outputs that go to ground,
but ONLY for very small output currents (much less that 1 mA). Otherwise
the output voltage never gets below roughly 1 Volt. That further limits
the output voltage that appears across the relay to roughly 3.3 Volts
- Use the IRF530 to drive the load directly as you mentioned. You can
connect its gate directly to the LM358 but it is better to place a 1K
series resistor there to prevent the LM358 from oscillating into the
high capacitance of the FET's gate. You'll have to interchange your
connections at pins 2 and 3 of the LM358 to invert the action because
the FET turns ON when the gate is above 3 volts or so. Leave the 10Meg
ohm resistor on pin 3. Put a pullup resistor of around 10K between the
FET's gate and 5Volts just to ensure that it turns completely on.
- Get a relay that will operate reliably at 4.5 Volts and 100mA or less.
Use the transistor in Common Emitter mode, with the emitter connected to
+5Volts, a 1K base resistor as before. Add a 2.2K pullup resistor from
the base to +5Volts to make sure it turns off.
You might consider using an LM393 comparator instead - it has open
collector outputs that are easier to use in such an application.
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