add a resistor - Page 1
Forum section: electronics / projects

author: kjn4685
author_rank: Regular Contributor
timestamp: August 03, 2015, 08:43:03 pm
content: Hi guysMy question is can you add a resistor to a relay coil. I have a relay with a resistance of 58 ohms and I need 200 ohms or more.

author: barry14
author_rank: Regular Contributor
timestamp: August 03, 2015, 09:13:14 pm
content: You can put anything in series with the relay coil that you want but understand that the voltage drop across the resistor must be considered in your design. Also the power dissipated in the resistor must also be allowed for. It might be easier to adapt the voltage source driving the relay to the actual voltage required by the relay. That would be more efficient.

author: AG6QR
author_rank: Frequent Contributor
timestamp: August 03, 2015, 09:15:16 pm
content: Yes, you can add a resistor. Whether the relay will work afterwards is another question.The relay will have a data sheet. It will tell how much voltage and/or current is required across the relay coil to move the contacts. If you want reliable operations, you can't get around that.If you add about 142 ohms in series with your 58 ohm relay coil, you'll get the 200 ohms you need, but you'll need to supply roughly four times the voltage to the series combination than you would have needed to supply to the relay by itself. That's probably not what you wanted.

author: kjn4685
author_rank: Regular Contributor
timestamp: August 04, 2015, 01:25:19 am
content: Thank you guysI will try what you have suggested.

author: retrolefty
author_rank: Super Contributor
timestamp: August 04, 2015, 02:00:01 am
content: Quote from: kjn4685 on August 03, 2015, 08:43:03 pmJust keep in mind that a relay coil is a current operated device so while you can certainly add series resistance the coil must still 'see' current within it's minimum and maximum operating current rating from it's datasheet.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 04, 2015, 05:27:36 am
content: Why do you need a resistor?

author: kjn4685
author_rank: Regular Contributor
timestamp: August 04, 2015, 06:19:11 pm
content: The chip I have on the datasheet says that the relay needs a 200ohm resistance to work properly. That was the only relay that I had.

author: retrolefty
author_rank: Super Contributor
timestamp: August 04, 2015, 09:37:52 pm
content: Quote from: kjn4685 on August 04, 2015, 06:19:11 pmWhich chip? Which datasheet? With the info given you simply most likely can't use the relay you have, unless you wire a transistor switch between your 'chip' and you relay.

author: kjn4685
author_rank: Regular Contributor
timestamp: August 04, 2015, 11:11:30 pm
content: If you look at one of my first post you will see what chip I am using,but if not the chip is sis2.

author: kjn4685
author_rank: Regular Contributor
timestamp: August 04, 2015, 11:35:37 pm
content: I am sorry retrolefty I posted it at another post. Like I said the chip is sis2 it is a programmable ir chip with 8 pins.

author: ludzinc
author_rank: Unknown
timestamp: August 05, 2015, 12:01:34 am
content: I took a look at the SIS-2 datasheet. Others can find it here:https://www.sparkfun.com/datasheets/Sensors/Infrared/SIS-2.pdfIt's states the I/O 'max current' is 25mA. So I'm guessing you get 200R by dividing 5V by 25mA.This is a bad plan.Use the I/O pin to drive a transistor, and use the transistor to switch the relay. Chose a transistor beefy enough to sink your relay current. Don't forget the flyback diode.As for your original idea, adding series resistance to the coil will stop the relay form operating.

author: DTJ
author_rank: Super Contributor
timestamp: August 05, 2015, 02:41:11 am
content: I think you should try something like this:You could use lots of different type of NPN or PNP transistor depending on when you want the relay to operate. The diode across the relay coil is important for protecting the sis chip you are using and ensuring reliable operation. Make sure you have some bypass caps 100nF or similar from your power pin on the sis chip to ground.http://www.electronics-tutorials.ws/articles/switch5.gif?81223bhttp://www.wired.com/wp-content/uploads/blogs/geekdad/wp-content/uploads/2012/09/relay_driver.jpgEdit - added more info.

author: AG6QR
author_rank: Frequent Contributor
timestamp: August 05, 2015, 05:44:27 am
content: You're getting good info. Let the chip switch a transistor, and let the transistor drive the relay.To see why adding a resistor won't work, look at the data sheet of the relay. I can see that your SIS-2 chip is rated at about 5V. If you put a 142 ohm resistor in series with a 58 ohm relay, and then apply 5V to the series combination, then yes, your chip will see a 200 ohm load, and that load will draw 25mA.But the relay's coil will see about 1.4V and 25 mA. Does relay's data sheet say it will trip using 25mA at 1.4V? I suspect not, but do check.Using the transistor as a switch to drive the relay will allow you to give the relay a full 5V, or whatever your power supply rail can supply. For example, it's easy enough to drive a 12V relay with a 5V chip if you have a 12V supply rail available and you let a suitable transistor do the switching.

author: digsys
author_rank: Unknown
timestamp: August 05, 2015, 06:14:36 am
content: +1 for using a transistor or other device ... Rule of law !! Never try to switch directly out of an expensive / complex / etc device !Always go though something you can sacrifice and is very easy to replace IF you blow stuff up. It's a HECK of a lot easier to replacetransistors, as you experiment, than a MPU, PLC, MB etc etc EVEN IF you work out that you have enough current on that I/O.If you work in a "dangerous" environment ie Motor vehicle, I'd even use ISOLATION on I/Os. ie optos etcIt just isn't worth the risk of blowing an / few embedded outputs -

author: kjn4685
author_rank: Regular Contributor
timestamp: August 05, 2015, 07:26:30 pm
content: I have a 2n2222a transistor switching the relay.