Controlling a relay with a PIC microcontroller

rtarbell · 12th October 2006, 12:57 AM

I am controlling a relay switch with a microcontroller. The relay coil is rated at 5V, and the microcontroller is rated at 5V (maximum output voltage per pin). One pin on a PIC microcontroller is rated to source or sink 20mA max. The coil resistance is 125 ohms and the coil rated current is 40mA, so I don't *think* the microcontroller itself will be suitable to directly control the relay.

I also have the option of buffering the microcontroller output by using a BJT (3904) as a switch (the coil would be in series with the collector of the BJT), so more current can be sourced for the relay coil. If I do this, then the relay would only get about 4.3V for activation (5V supply - .7V Vbe drop). Is this a problem for reliable operation, considering it is rated at 5V control voltage?

I'm just looking for a simple (and cheap) solution!

xplod1236 · 12th October 2006, 01:09 AM

Should be fine.

Just curious, which uc are you using?

rtarbell · 12th October 2006, 02:11 AM

I am using the 10F222 which is one of the smallest of the small in terms of PIC microcontrollers. Now that I have the datasheet in front of me, I see that the max source/sink current per pin is 25mA, not 20mA.

Thanks for your reply! I will try a direct connection between uC and the relay.

xplod1236 · 12th October 2006, 02:13 AM

Sorry, I meant you should be fine using the transistor (4.3 v across the realy). I didn't read your post carefully enough. I was going to mention paralleling 2 or 3 pins, but I just saw that your uc only has 4 i/o pins, so I think that is out of the question.

Which language are you programming it in?

So many edits....
125 ohm resistance will pull 20 mA from 5v. So either way, it should work.

jackinnj · 12th October 2006, 02:20 AM

use a Darlington configuration on the switching transistor...

billr · 12th October 2006, 02:23 AM

hi.

I use the ULN2003. its a 16 pin DIL device, with 7 darlington drivers in it, excellent for simple connections to relays, of course if you only have one relay to drive it is overkill, I have about 14 to do, so it works out fine for me.

YMMV.

kind regards

bill

Leolabs · 12th October 2006, 06:52 AM

Quote:

Originally posted by billr
hi.

I use the ULN2003. its a 16 pin DIL device, with 7 darlington drivers in it, excellent for simple connections to relays, of course if you only have one relay to drive it is overkill, I have about 14 to do, so it works out fine for me.

YMMV.

kind regards

bill

Yes.The ULN2003 is a good stuff.I have been using it with an ADC driving some DC12V relays without problem.Beside,it is cheap and easily available.

peranders · 12th October 2006, 07:29 AM

Using 5 V relay is not very smart, 12 V or more is better and you must use a transistors, plain one, darlington or a small mosfet like BS170. Don't forget to add a doide across the relay coil, otherwise you will break the transistor, eventually. Cathode towards "+".

Worth noticing is that the ULN driver has built-in freewheeeling diodes.

It is not recommendable at all to drive a relay directly from an MCU output.

RussB · 12th October 2006, 07:43 AM

Check the relay data sheet for the minimum operating voltage, these are normally expressed as 'pull-in' and holding voltage. If the pull-in is within 4.3V, then you should be fine. Don't forget a diode accross the relay coil to suppress back-emf when the relay releases.

cpemma · 12th October 2006, 09:29 PM

With relay on the 2N3904 collector, emitter to ground, drop is only about 200mV; it's V_CE(sat) not V_BE.

Most 6V relays will switch OK with 4.8V, though level-shifting to a 12V coil is no problem with an NPN switcher.

You will need a base resistor to limit the PIC current, say 470R for about 5mA, and an inverse diode across the coil. The ULN2803A is an 18-pin 8-channel darlington device with both diodes and base resistors built-in.

12th October 2006, 12:57 AM	#1
rtarbell diyAudio Member Join Date: Sep 2005	Controlling a relay with a PIC microcontroller I am controlling a relay switch with a microcontroller. The relay coil is rated at 5V, and the microcontroller is rated at 5V (maximum output voltage per pin). One pin on a PIC microcontroller is rated to source or sink 20mA max. The coil resistance is 125 ohms and the coil rated current is 40mA, so I don't think the microcontroller itself will be suitable to directly control the relay. I also have the option of buffering the microcontroller output by using a BJT (3904) as a switch (the coil would be in series with the collector of the BJT), so more current can be sourced for the relay coil. If I do this, then the relay would only get about 4.3V for activation (5V supply - .7V Vbe drop). Is this a problem for reliable operation, considering it is rated at 5V control voltage? I'm just looking for a simple (and cheap) solution!

12th October 2006, 07:29 AM	#8
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	Using 5 V relay is not very smart, 12 V or more is better and you must use a transistors, plain one, darlington or a small mosfet like BS170. Don't forget to add a doide across the relay coil, otherwise you will break the transistor, eventually. Cathode towards "+". Worth noticing is that the ULN driver has built-in freewheeeling diodes. It is not recommendable at all to drive a relay directly from an MCU output. __________________ /Per-Anders (my first name) or P-A as my friends call me

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12th October 2006, 01:09 AM	#2
xplod1236 diyAudio Member Join Date: Dec 2003 Location: On the couch	Should be fine. Just curious, which uc are you using?

12th October 2006, 02:11 AM	#3
rtarbell diyAudio Member Join Date: Sep 2005	I am using the 10F222 which is one of the smallest of the small in terms of PIC microcontrollers. Now that I have the datasheet in front of me, I see that the max source/sink current per pin is 25mA, not 20mA. Thanks for your reply! I will try a direct connection between uC and the relay.

12th October 2006, 02:13 AM	#4
xplod1236 diyAudio Member Join Date: Dec 2003 Location: On the couch	Sorry, I meant you should be fine using the transistor (4.3 v across the realy). I didn't read your post carefully enough. I was going to mention paralleling 2 or 3 pins, but I just saw that your uc only has 4 i/o pins, so I think that is out of the question. Which language are you programming it in? So many edits.... 125 ohm resistance will pull 20 mA from 5v. So either way, it should work.

12th October 2006, 02:20 AM	#5
jackinnj diyAudio Member Join Date: Apr 2002 Location: Llanddewi Brefi, NJ	use a Darlington configuration on the switching transistor...

12th October 2006, 02:23 AM	#6
billr diyAudio Member Join Date: Dec 2003 Location: new zealand	hi. I use the ULN2003. its a 16 pin DIL device, with 7 darlington drivers in it, excellent for simple connections to relays, of course if you only have one relay to drive it is overkill, I have about 14 to do, so it works out fine for me. YMMV. kind regards bill

12th October 2006, 07:43 AM	#9
RussB diyAudio Member Join Date: Oct 2006	Check the relay data sheet for the minimum operating voltage, these are normally expressed as 'pull-in' and holding voltage. If the pull-in is within 4.3V, then you should be fine. Don't forget a diode accross the relay coil to suppress back-emf when the relay releases.

12th October 2006, 09:29 PM	#10
cpemma diyAudio Member Join Date: Jul 2003 Location: S Yorkshire OK	With relay on the 2N3904 collector, emitter to ground, drop is only about 200mV; it's V_CE(sat) not V_BE. Most 6V relays will switch OK with 4.8V, though level-shifting to a 12V coil is no problem with an NPN switcher. You will need a base resistor to limit the PIC current, say 470R for about 5mA, and an inverse diode across the coil. The ULN2803A is an 18-pin 8-channel darlington device with both diodes and base resistors built-in.

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