DRIVING COIL WITH RELAY

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DRIVING COIL WITH RELAY

Postby arivalagan13 » Mon Mar 04, 2013 8:34 am

Hello all,

One of my application needs to energize a coil(electromagnet...i wounded 1000 turns of AWG 26 coil in a cylindrical ferrite core)..

The idea is as follows:
Using an IR module as proximity sensor..If anything is near to its vicinity the output will go high(3.6 volts)..this output will drive the comparator to go high(i use LM311 for this purpose)...comparator circuit is similar to one found in OpAmp book Sergio Franco(3rd edition) page no.403,fig. 9.5(a) and i chose Rc be 4.7k...

The circuit is working fine...Problem occured as i used the comparator output to drive coil..

I did this to drive coil(like this: http://www.jaycar.com.au/images_uploaded/relaydrv.pdf )...used SL100 transistor for this...i used a 5V relay...

But things are not working the way they are expected....when i hook-up the circuit as explained nothing happens...

Am trying the relay only with LED and got negative response..if this works out i'll have to replace the LED with the actual requirement..i.e., coil as described above..

Kindly support in solving this issue..

Regards
Arivalagan M
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Re: DRIVING COIL WITH RELAY

Postby danadak » Mon Mar 04, 2013 4:41 pm

The following considerations might help -

1) To saturate the transistor you should drive it with a base current Ibase = Icollector/10.
So you need to know what current the relay needs to actuate.
2) To calc Rb = ( Vohpsoc - Vbe )/ Ibase.
3) Keep in mind Voh of PSOC is at a limited drive current, so use a Voh representative of the
Ibase you want.
4) To check circuit independent of your software just connect Rb to Vdd to see if you
get the relay current you want.
5) These circuits always miss one more resistor needed. It is from the transistor base to ground.
This is to absorb the leakage of the transistor, Icbo, and prevent transistor from turning on
when hot and PSOC output is tri-stated. Compute it Rleak = Vbe / Icbo, so use Vbe of ~ .5 V.
6) Use a transistor that can handle the power dissipation, compute Pdiss = Vcesat X Irelay.
Make sure you heatsink by going thru the thermal calculations.

Realy Drive.jpg
Realy Drive.jpg (11.42 KiB) Viewed 2975 times


Regards, Dana.
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Re: DRIVING COIL WITH RELAY

Postby arivalagan13 » Tue Mar 05, 2013 7:49 am

I tried the following:
Used transistor,SL100 , to drive the relay...the relay needs around 40mA..I drive the transistor with bench power supply,to verify the circuit...
Then i made the relay circuit to drive an LED as load...It works perfectly.....

But, the real load as i mentioned is coil....so i replaced the LED with the coil...but the coil was not energized..

Then i measured the amount of current being drawn by the coil...@ 5V it consumes nearly 1.7 Amperes...but the relay i use is capable of handling current is only up to 100mA...

So i decided to use a 12V relay which is capable of handling 10 + Amperes of current(only this is available in our nearby local market)....

Am i correct conceptually?

The question i have is in the transistor part...Now i thought of using 2N3055 in place of SL100..SO that it could the supply the necessary 1.7A current...
Now for testing purposes i will use bench power supply...
But, the transistor should be driven by a comparator...the moment i connect comparator output to transistor base(through resistor or straightaway to base) the comparator output voltage goes to ~0V...i think because of loading...

How do i solve this problem? Kindly support

Regards
Arivalagan M
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Re: DRIVING COIL WITH RELAY

Postby danadak » Tue Mar 05, 2013 8:32 am

If the ultimate load you are trying to drive is the coil, 1.7A, then either a darlington
transistor, or MOSFET would be more appropriate. Reason is if 2N3055, you will need
~ 170 mA into base to get it into saturation for a 1.7A collector current, way too much
for PSOC.

Prefered is a logic level MOSFET, with low Rdson, say .1 ohms or better. Since you are
driving a high L coil, you are not concerned with switching speed, MOSFET makes most
sense. At 1.7A, .1 ohm MOSFET, your will dissipate ~ 290 mW when it is on, so not much
heatsinking needed for most packages.

For a darlington, use Vbesat in your calculations for base R. Darlington will dissipate much
more power, so heatsink most likely needed.

The diode you strap across coil to handle turnoff transients, its current rating should be
>= coil current, and voltage 200 or greater.

Regards, Dana.
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Re: DRIVING COIL WITH RELAY

Postby arivalagan13 » Wed Mar 13, 2013 11:13 pm

I've solved the problem..
Thanks all..
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