LM 393 switching soft

[Terminal Crazy]

Hi guys, hope someone can help.

I've got a LM393 on a 12v supply measuring a thermistor. I've got it
switching around the right temperature but have a grey area where the
output ramps up brightening the LED until it switches fully on.

How do i increase the feed back so the output switches from 0 to 1 ?

TIA

 

[Terminal Crazy]

You'll need to introduce some hysteresis by connecting a large-valued
resistor form the output to the + input.

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 ?

Got a schematic?

I'll have a play with this first thanks.

 

[default]

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.
--

 

[Eeyore]

i.e. some small positive feedback.

by connecting a large-valued
resistor form the output to the + input.

Typically. Cs can help too.

Got a schematic?

Always a good place to start.

Graham

 

[Eeyore]

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 ?

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

due to the hugely increased level of spam please make the obvious adjustment to my
email address

 

[Eeyore]

This is an op amp?

No. An LM393 is a dual comparator

Graham

due to the hugely increased level of spam please make the obvious adjustment
to my email address

 

[Michael Black]

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 ?

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

 

[default]

No. An LM393 is a dual comparator

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

 

[Michael Black]

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

 

[Terminal Crazy]

Got a schematic?

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

12v
____________________________________________________________
| | | |
| | | |
| / | |
| \ / _____________ |
| /10K \ | / |
| \ |\ /10k | \4K7|
| / 2 | \ \ | / |
| |-------------| - \ / | \ C|\
| | | \ | | /---B| | 2N3906
| / |LM393 \_|______| _ E|/
/ \ | / | _______|
\ /10K 3 | / | | |_______
/<----\----/\/\/\---| + / | / | |
\ / 47k | | / | 4K7 \ r / | d
/ | | |/ | / e \ _|_ i
| | | 470k | __\__ l / /_\ o
| | |-----\/\/\/\--| led \ / a \ | d
___|_____|______________________________________\_/______y_|___| e
0v

I've got hysteresis set at 12 / ( 470K / 47K) = 1.2V which appears ok for
now.

The relay doesn't buzz now around the switching point but
I'm still getting the LED to illuminate just before the relay energises,
any suggestion please.

TIA

 

[Nobody]

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.

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

 

[Terminal Crazy]

. If you're using a PNP, like you show, you've got the emitter and
collector connections reversed and the diode really isn't doing
anything.

Try this:

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

 

[Eeyore]

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

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.

But usually much slower.

Graham

 

[Eeyore]

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

And indeed some can reverse their output if overdriven.

Graham

 

[Eeyore]

The circuit was of the internet

There really is some utter rubbish out there.

Graham

due to the hugely increased level of spam please make the obvious adjustment to my
email address

 

[default]

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

Presumably for on/off, the standard is lower than for linear. But
they make both with the same pin outs and I've subbed linear for
comparators with good results. Maybe the output dissipation is a
little better with a comparator.

--

 

[default]

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

Today?

The old 709 or 701's would sometimes latch up if you drove them to the
supply rail (particularly if they were operated single supply - and/or
with a high impedance power source)

The 324's seem to be immune to this.
--