trying to use a transistor controlled by ntc 15k to switch on a relay

Thread Starter

roadey_carl

Joined Jun 5, 2009
126
Hey folks, Im trying to get a transistor to switch on a relay. the transistor is controlled by a 15k ntc. It works ok but as the risistance slowley decreases it triggers the coil in the relay very slowley so the contacts have a high resistance between them untill the ntc havs a small enough resistance to alow the foward voltage on the transistor to be full flow rate. Is there any way I can get it so the relay just works as on/off when it gets to a certian risistance? maybe some kind of regulator?

Im a bit of a newbie with electronics so cheers for any help!:)

Carl
 

hgmjr

Joined Jan 28, 2005
9,029
Can you post a schematic of the circuit you are using so that we can have a look?

hgmjr
 

kkazem

Joined Jul 23, 2009
160
Hi,
What you need is to dump the transistor ckt and replace it with a single-supply comparator circuit with hysteresis to make the relay snap-on. These ICs are inexpensive and easy to use. I recommend the LM393CN. Also, unless you want to blow-up your transistor switch, you need a diode (a run-of-the-mill 1N4004 or similar will do fine) across the relay coil to prevent the coil inductance from generating a voltage that will breakdown your transistor and it works the same if you use a comparator. If you a google search for comparator relay driver circuits you'll find many. Also, the TI website (www.ti.com) is another great source of application notes on op-amp and comparator designs. You may not even need the hysteresis (positive feedback), but it only adds 2 or 3 small resistors to the ckt.
Regards,
Kamran Kazem
 

Thread Starter

roadey_carl

Joined Jun 5, 2009
126
Thanks alot kkazem, you are a true legend!! I have no idea what hystereresis but I will have a read up about it and see if I'll find it any use to me!

Thanks again thats been a real help :D
 

Jony130

Joined Feb 17, 2009
4,969
You can use one of those classic Schmitt trigger diagrams.
 

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Thread Starter

roadey_carl

Joined Jun 5, 2009
126
Hey, I have fallen in love with the LM393N, what an awsome chip!! I just wonderd.... what is the "inverting" and "Non-inverting" all about? I've made the circuit I needed to make, but was just curious about what that ment?
 
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