Turning on 2 fets from one opamp output

[bryan1]

G'day Guy's,
I went and changed the logic output on my shunt reg so the IRF540 will be on now until the preset voltage is reached but now the red led I use to show the set point is the opposite that I want.

I was thinking I could just use a 2N7000 setup as NOT gate to pwer the red led so now my question has arose about using the logic 1 off the opamp to switch on 2 fets. The opamp is fed by a 9V1 zener so thats enough voltage to turn on the fet in the current configuration but would there be any problems in doing it this way????

Regards Bryan

 

[MikeMl]

At what frequency are you switching the FETs? You can parallel gates of FETs as long as the driver can charge/discharge the gate capacitance(s) at a rate appropriate for the switching frequency.

 

[bryan1]

Hi Mike,
The frequency will be 10 seconds as that is the delay set for when the set point is reached, anyway that is the current setup and I'll increase that delay as the intended operation is to open circuit solar panels when batteries are fully charged. When I get some time hopefully tonight I'll draw up a quick schematic and post it here.

Regards Bryan

 

[MikeMl]

At 10sec per cycle, not likely to be a problem. Even a whimpy opamp can charge/discharge the gate capacitance of several FETs...

 

[Mr RB]

It's a good idea too to use a comparator for on/off type setpoint switching, or if you must use the opamp then to add some positive feedback from the output to the + input so its output "snaps" from off->on instead of hovering in a linear zone inbetween.

 

[bryan1]

Hi Roman,
The 2nd opamp in the LM358 is used as a comparator and goes high when the setpoint is reached and also turns on the led, now I've switched pins 5 and 6 the opamp is set to 1 until the setpoint is reached then turns off. The idea of using the 2N7000 is just as a NOT gate so the led will turn on when the set point is reached (ie: the opamp output goes low).

The old setup is designed to switch on a dumpload then 10 seconds later see if the voltage is still above the setpoint. Now the design is to open circuit the solar panels so no dumpload is needed and it would be handy for the red led to switch on when this occurs. In the new design I'll also be putting in a pot so the delay time can be adjusted to suit the user.

Regards Bryan

 

[Mr RB]

Open circuiting the solar panels greatly increases their heat dissipation and reduces their life, it is a lot better to short the panels to ground as a method of regulation.

 

[bryan1]

G'day Guy's,
Finally found some free time to draw up a schematic of what I want to do with this shunt reg mod. The voltage divider is there mainly to power the SSR and the RED LED using the second 2N7000 as a NOT gate. The other 2N7000 is used as the negative input for the SSR.

Now does everything seem in order or is that schematic designed to let out that magic smoke.........





Regards Bryan

 

[MikeMl]

What voltages are the LM358's supply pins (Vcc, Vss) tied to?

 

[bryan1]

Hi Mike,
The circuit runs off a 9v1 zener and has the circuit has run in my shed for years with no problems, thats why I have only showed on the schematic from the second opamp output. A mate asked me to make one up with a different configuration so thats why I'm changing the circuit.

Regards Bryan

 

[MikeMl]

Ok, a LM358 running off 9V and gnd will swing its output pin from near gnd to about 7.5V, which is more than enough to switch on/off two NFets. You dont need the series 1K.

 

[bryan1]

Hi Mike,
Thanks for that mate, now with that 2nd FET to light the LED is that connected right ????

 

[MikeMl]

Hi Mike,
Thanks for that mate, now with that 2nd FET to light the LED is that connected right ????

I just looked at your circuit again, and I see that the LED is basically telling you that the SSR is driven. Is that right? If so, why not just wire the LED in series with SSR? The SSR usually has a built-in current-limiting resistor in-series with the photo emitter such that the SSR turns on if it has > 3V across it. Since you are starting with ~12V, why not just put the indicator LED in-series with the SSR input? The forward drop of the LED (~2.2V?) will subtract from 12V, leaving 9.8V across the SSR. The same current will flow in the SSR as the LED.