LM 393 switching soft

  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

--
Mitch

terminal_crazy@sand-hill.freeserve.co.uk
www.sand-hill.freeserve.co.uk/terminal_crazy

 On Thu, 04 Jun 2009 18:56:12 +0100, Terminal Crazy


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

Got a schematic?

JF  

 
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.

--
Terminal_Crazy

Mitch - 1995 Z28 LT1 M6          terminal_crazy@sand-hill.freeserve.co.uk
Lancashire England          http://www.sand-hill.freeserve.co.uk/terminal_crazy/

 On Thu, 04 Jun 2009 19:47:27 +0100, Terminal Crazy


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.
--

 

default wrote:


No. An LM393 is a dual comparator

Graham

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