LM 393 switching soft

thanks

I'd tried feedback to the +ve side of the sensor(on the -ve input) . Presumably feeding back to the +ve input would raise it towards the +ve rail ?

I'll have a play with this first thanks.

This is an op amp? A smidgen of positive feedback is what it takes. Output of the amp to the + input (with very high value resistor) actual value depends on resistors in the sensor and reference but usually 100Ks to meg ohms.

Frequently a resistor in series with a pot to ground on the output will be used to take a small voltage off the output and gives an adjustable hysteresis to feed back to the non inverting input. Cheap and dirty is a high value resistor from out to non-inverting.

i.e. some small positive feedback.

Typically. Cs can help too.

Always a good place to start.

Graham

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You need to know what you're doing to get it right, especially if this is to be a volume manufactured item. 'Fiddling' with components is not the way. It's calculable and you should understand exactly the effect you're introducing.

Graham

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No. An LM393 is a dual comparator

Graham

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Feedback to the inverting input will change the gain of the comparator from "infinite" to some finite value. That's not what you want.

This is about hysterisis, adding "snap" to the changeover of the comparator. Hence some positive feedback, ie a resistor from the output to the non-inverting input, means that when the input goes above the value on the other input of the opamp (that other input sets the level at which the comparator will switch) the comparator output will go "high" and that resistor will add some voltage to the input. The input thus sees more voltage than the incoming voltage, and thus the comparator is "more" on.

Michael

Only real difference is the open collector out versus push-pull out ?

Maybe, it there may be some specific shaping of the circuitry to make it better for comparator use.

Any op-amp can be used as a comparator, it is the same general circuitry.

Michael

I've had a play and read a bit more about them and got this layout now...

Perhaps a capacitor in parallel with the LED?

That might delay the turn-on of the LED until the relay engages. You'd have to experiment with various values of cap, though...

Are you really using and 2N3906? If so then you have it connected incorrectly. The 2N3906 is a PNP transistor. You need to either:

or:

The 2N3906 might work (I have never tried it) in your current arrangement but if so then it will work very poorly. The resulting very low gain from using the transistor 'backwards' might explain the results that you are seeing.

Dan

Some op-amps don't like being driven into saturation.

thanks Dan & John. I'm just measuring the thermister atm temp vs resistance.

The circuit was of the internet (one i've got stashed as notes) but didn't really explain enough re feedback etc.

The transister pinout is now confusing.. also from the same set of circuits.

The Phillips specsheet shows pinout as

TO-92 c | \\ b | | e | / but doesn't state view(ie above / below)

My Electronic catalogue shows it as b | \\ c | | e | / Viewed from BELOW

Can anyone confirm orientation. I'll try it now with the 2N3904.

Thanks

OK I.ve' now got this working again...

B C C rather than B E E can anyone confirm pinouts for 2N3904 & 2N3906 ?

LED still glows before relay energises ( Not really a problem though as I cm mount an LED OFF the relay output )

Thanks

Yes, many comparators including this one are open-collector. It tends to be useful.

But usually much slower.

Graham

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And indeed some can reverse their output if overdriven.

Graham

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