Problem in Relay Drive through  MCU - Page 1
Forum section: electronics / projects

author: amar
author_rank: Contributor
timestamp: March 27, 2013, 11:33:35 am
content: hii am using ULN2803 to drive relay(omron).coil resistance is 1100 ohm Voltage +24v current 30mA.i gave input to ULN2803 +24v through 2k2 ohm resistancebut relay is not energies can anyone suggest where is the problem.

author: c4757p
author_rank: Super Contributor
timestamp: March 27, 2013, 11:37:42 am
content: Please give a schematic showing how it's hooked up.

author: hammil
author_rank: Regular Contributor
timestamp: March 27, 2013, 12:14:13 pm
content: The coil resistance is usually set for the relay to be powered directly from the supply rail, with very little ESR. Try removing the series resistor and see what happens.24 / (1100 + 2200) ~ 7mA24 / 1100 ~ 21mA

author: amar
author_rank: Contributor
timestamp: March 27, 2013, 01:04:35 pm
content: i have post the schematic please suggest me where is the problem

author: c4757p
author_rank: Super Contributor
timestamp: March 27, 2013, 01:27:30 pm
content: Er... that's not really a CQ1-12V, right?Quote from: hammil on March 27, 2013, 12:14:13 pmThe series resistance is where it belongs, into the base of the Darlington pair. The chip has an integrated base resistor, but it's a bit small for 24V.I don't see anything wrong with it. Are you sure it's actually connected like that?One thing I think is worth noting: You are using a Darlington array with integrated series resistors and clamp diodes, and using your own external resistors and clamp diodes. It would be cheaper to just use a PN2222 or similar. You don't need a Darlington for 21mA, and you already have the other external parts.On that note, try it. Do it with a simple transistor and see if it works that way. You'll want to use a base resistor around 22k though, 2.2k is a bit small for that. (Edit: 22k is fine for 24V, but if you're going to interface to a MCU like the title suggests, you'll want something smaller.)

author: richcj10
author_rank: Unknown
timestamp: March 28, 2013, 04:48:35 am
content: Make sure you have enough gain.

author: Bloch
author_rank: Unknown
timestamp: March 28, 2013, 06:54:16 am
content: Quote from: amar on March 27, 2013, 11:33:35 amEasy the ULN2803 can sink 500mA.Quote from: amar on March 27, 2013, 11:33:35 amWhy ?from datasheetInput voltage (see Note 1) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30 VNOTES: 1. All voltage values, unless otherwise noted, are with respect to the emitter/substrate terminal GNDAnd the diode on the relay is unnecessary it is already in the ULN2008

author: ivan747
author_rank: Super Contributor
timestamp: March 29, 2013, 01:17:43 am
content: Quote from: amar on March 27, 2013, 01:04:35 pmAre you actually controlling the ULN2803 from 24V? When I first used that chip I placed 2.2k resistor in series with all the transistor bases. At 5V it wouldn't turn on my relays. You don't need D1 if you tie pin 10 to the 24V rail. If you look at the datasheet closely, there are 8 flyback diodes inside this chip.I would try placing 5V right between pins 1 and 9 and see if the relay toggles. There shouldn't be any problems.