12 volt differential controller [RE-wrenches]

John Raynes john at raynes.com
Wed Oct 15 07:49:58 PDT 2003


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For anyone that is interested in developing this further, there are neat 
little IC packages available now that wrap this whole circuit into a single 
chip with built in reference, and open drain or open collector outputs 
suitable for driving relays directly.  You still may need a resistor or two 
to tweak levels and hysteresis, but the overall temperature performance is 
much better than the old classic LM series op-amps (which are still great 
parts, by the way, I still use them regularly).

The main drawback is that the newer parts typically aren't available at 
Radio Shack, but you can order single or small quantities of them at 
reasonable prices from Digi-Key and Mouser.

I don't have a lot of time available to research into the specifics of this 
at the moment, but if you're interested, contact me off-list and I can 
certainly help point you in that direction.  In a couple weeks when things 
settle I could probably help someone work out a circuit if they needed it.

John Raynes
RE Solar
Torrey, UT
john at raynes.com
435-425-2110


At 08:28 PM 10/14/03 -0700, you wrote:
>hi Mark
>I put together a simple op amp comparator "schmitt
>trigger" circuit from radio shack parts and the
>circuit board.  It seems to work.  I used the LM324 op
>amp, and 10 k resistors, and a 100 K positive feed
>back resistor.  I hooked it up to a 12 VDC power
>supply.   give me the temperature range you would like
>and I will tell you how it works, by doing some
>calculations.  Send me your Fax number and I will fax
>a drawing.  This is not a refined circuit and cost
>about $8 or so. This is not a refined circuit, and the
>differencial will vary woth temperature.  The
>selection of the feed back resistor determines the
>switching hysterisis.   Please note I am not a circuit
>designer and only took a couple of design coursed at
>school, about 42 years ago.  So I was never good and
>now am forgetfull.  If some one else who has a young
>brain would like to help. please do so.
>
>Daryl
>PS the LM324 can only source 40 mills, to drive a
>relay you should use an emmetter follower and free
>wheeling diode.

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