Questions on an auto fan circuit (741)

bassey

I have a presentation to do on this circuit



I know pretty much everything about it, the thermistor has a negative coefficient of resistance, so that when the temperature increases current flows into the 741, thus switching the transistor and activating the relay bringing on the fan.

My question is how does the feedback loop work with the 741 op amp being supplied from a single supply? I was told that in order to power the 741 op amp from a single supply a capacitor was needed to remove the DC bias that's required to make the supply all positive?

Michael Collins

Hey bassey. I don't really see a problem with the biasing. The op-amp is acting as a comparator: the [+] terminal is held at roughly 6 V by the two 10k resistors halving the total 12 V. The voltage at the [-] terminal changes as the thermistor changes resistance, as the temp goes up then the voltage here will go below 6 V, when this happens the [-] op-amp terminal voltage is less than the [+] terminal voltage so the output of the op-amp swings to its max voltage i.e. +12 V, ultimately turning the transistor on.

When the [-] op-amp terminal voltage goes above 6 V, now the op-amp output swings to the most negative voltage, 0 V and the transistor is off.

There's really no need for a capacitor near the op-amp. And there's no significant feedback really.

bassey

Thanks, that really helps to just think of the Op Amp as comparing the resistor/potentiometer with the thermistor. I just had feedback stuck in me head because thats all the lecturer goes on about.

Thanks again

DublinDilbert

This circuit is using positive feed back rather than the typical negative feedback your probably used to seeing with opamps. This gives hysteresis on the temperature input, which is quite desirable in this situation. For example fan will turn on @ 110C and will not turn off till temp sensor goes less than say 80C. This means the Fan isn't constantly cycling on / off.

This is used all the time in heating controls ect... Its also a good way of giving noise immunity, another name for this type of set up is a Schmidt trigger.

Michael Collins

Yeah there is a small amount of positive feedback which, as DublinDilbert says, will help prevent the fan from turning on and off rapidly around the transition point:

Above I said the threshold was around 6 V, in fact if the system is off, the threshold should be closer to 5.81 V, then as soon as the [-] voltage goes below this, the threshold shifts to 6.19 V, preventing any changes around the 5.81 V mark from having any effect. The opposite happens then when the fan turns off again.

2011

bassey wrote: »

I have a presentation to do on this circuit

Funny, I had to do a presentation on almost exactly the same circuit a few years ago in DIT

bassey

Mick Tully?

2011

A small man with a beard!
Fantastic lecturer too.

bassey

Ha yup that's the one. Yeah he's really good, too bad he's retiring at christmas.

Had the presentation this morning, it went fine. Thanks for all the help everyone