Substituting a Mosfet for a transistor and relay.

[Ed.Kloonk]

Hi.

I have found an old article for driving a temp controlled pump. The circuit is pretty good the way it is but I need to make a few changes.

Firstly, it calls for a mosfet which would cost about 8 dollars at Jaycar. I want to swap that for a bc548, a high power relay with a coil resistance of 160 ohms with a 1A protection diode.

Except the mosfet, all of the parts above I have here and would prefer to use them if I can. I want to use a relay to both isolate the circuit from the pump and also there is a chance I may need to run a 240v pump in the future.

A further problem is I'd like the pump to stay on for an adjustable time period of 0-20 seconds. And this is where I'm stuck. I need help and suggestions for a simple circuit between the output of the comparator, the delay circuit between it and the base pin on the transistor. If I start adding capacitors, wont it stuff up the hysteresis? Can I somehow incorporate the unused 2nd comparator? How would you do it?

The problem is the pump would turn off too early since the water tank is quite far from the solar collector. I want it to keep running for a few seconds while the pipe is still hot.

Here is the original circuit:


And here is the article:
http://www.siliconchip.com.au/cms/A_30610/article.html

[onlooker]

Instead of doing the 20 secs, you can have a sensor3 at a pipe location near the water tank, and connect sensor3 + sensor2 to IC1b similar to IC1a; and then "OR" the outputs of IC1a and IC1b (e.g with 2 diodes). 

[Ed.Kloonk]

That's a good idea. Might just do that. Thanks.

 

[Simon]

how about connecting a signal transistor to the output and that can charge a capacitor with a resistor in parallel on the second comparator that will keep things going for another 20 seconds (I managed about 2 minutes with 100uF and 1MR by lowering the reference to 1/10 Vcc so 20 secs is a doddle).

Otherwise you could use a micro but that might be overkill.

[MartinX]

I would keep the mosfet solution, in that circuit you could use any mosfet capable of handling the current of the pump motor it does not have to be the expensive one in the schematic. It is possible to parallel a bunch of cheap IRF540 or what you happen to have at hand. Relays are always pretty bad at high DC currents especially if there is even higher start currents. It is easy to use an external relay for a 240V AC pump if the need occurs in the future.

[Simon]

Yes the mosfet will be the best solution particularly if your after low power, you can always put a relay on instead of the pump and connect the pump to the relay