picking diode for relay - Page 1
Forum section: electronics / beginners

author: littlebill
author_rank: Regular Contributor
timestamp: April 28, 2015, 03:58:28 am
content: Hi,how do i pick a diode for the coil of a relay? does size matter? polarity? amperage going through relay? coil voltage?

author: LabSpokane
author_rank: Super Contributor
timestamp: April 28, 2015, 04:18:07 am
content: What kind of relay? AC or DC? What's the coil voltage? What's the turn-on and holding current?------Edit: if DC coil, and small, the 1n400x will do. If AC, then you need an RC snubber.

author: bookaboo
author_rank: Frequent Contributor
timestamp: April 28, 2015, 07:04:32 am
content: If you mean a "back emf" diode connected in reverse on a DC coil <24V then a 1N400x is usually a good choice.

author: Zero999
author_rank: Super Contributor
timestamp: April 28, 2015, 07:38:27 am
content: It's not critical. The diode's maximum reverse voltage should be greater than the voltage you're using and the peak forward current surge rating should be greater than the coil current. The 1N4007 will do for 99.99% of applications, as it has a maximum reverse voltage of 1000V and can stand 30A surges.http://www.diodes.com/datasheets/ds28002.pdf

author: LA7SJA
author_rank: Regular Contributor
timestamp: April 28, 2015, 07:53:00 am
content: On low energy relay I use 1N4148.ABSOLUTE MAXIMUM RATINGS (Tamb = 25 °C, unless otherwise specified)PARAMETER TEST CONDITION SYMBOL VALUE UNITRepetitive peak reverse voltage VRRM 100 VReverse voltage VR 75 VPeak forward surge current tp = 1 us IFSM 2 ARepetitive peak forward current IFRM 500 mAForward continuous current IF 300 mAAverage forward current VR = 0 IF(AV) 150 mAPower dissipationl = 4 mm, TL = 45 °C Ptot 440 mWl = 4 mm, TL ? 25 °C Ptot 500 mWJohan-Fredrik

author: littlebill
author_rank: Regular Contributor
timestamp: April 28, 2015, 02:03:40 pm
content: how do i determine the surge? its a 12v dc coil, but will be running a 3/4 horse power sump pump, its rated 30 amp ac on the load side.it uses 12v 56ma to hold the coil closed.is the coil voltage and current the only thing we are concerned with? i assume the load is not important.i will look at the data sheets shortly.does the diode have to wired a specific way across the coil? like the strip toward the negative etc?

author: w2aew
author_rank: Super Contributor
timestamp: April 28, 2015, 02:24:51 pm
content: Yes, you only have to concern yourself with the coil voltage/current. Sounds like a fairly low power coil, so a simple 1N4148 should do fine, or any 1N4001,2,3,4, etc. It should be wired so that it is reversed biased when the coil is turned on. Have you seen my video on the topic?

author: LabSpokane
author_rank: Super Contributor
timestamp: April 28, 2015, 03:09:02 pm
content: Quote from: w2aew on April 28, 2015, 02:24:51 pmThere's your answer.I asked about the size of the relay, because if this was a large motor contactor, you might need to do more than just the diode.You wondered about the contact (load) side of the relay. Hopefully someone with some industrial experience will chime in, but with larger loads, arc quenching can be an issue that must be addressed. For electrical substations, the switchgear can be filled with SF6 to assist in arc quenching. That is typically only a concern with *very* large loads.For small AC loads like you're working with, the relay itself is all you need.

author: littlebill
author_rank: Regular Contributor
timestamp: April 28, 2015, 05:36:37 pm
content: JQX-15F(T90) DC 12Vis the relay,i need to watch the below video as well, as i am not sure how to make sure i am "reverse biasing"the diode i plan to use was 1N4007, so it appears to be over kill, but i need to watch the video below. way too much learning going on here :-)thanks for the help everyone

author: LabSpokane
author_rank: Super Contributor
timestamp: April 28, 2015, 05:39:04 pm
content: Quote from: littlebill on April 28, 2015, 05:36:37 pmThe reverse biasing is easy. Just make sure the band of the diode is on the positive side of the coil and all will be well.

author: littlebill
author_rank: Regular Contributor
timestamp: April 28, 2015, 05:46:11 pm
content: thanks lab perfect,let me ask a question since you guys are so helpful,is there any reason why there are 6 posts instead of the normal 5? i know the NC contacts have a lower rating but i am a bit thrown why this has 6 posts

author: SeanB
author_rank: Super Contributor
timestamp: April 28, 2015, 06:19:05 pm
content: Just there for the relay to be attached to the PCB to survive more vibration. As it most likely will be in an automotive application you need all the support you can get so it does not fall off the board if you go down a rutted road. It also helps to reduce the resistance of the connection to the moving contact, but this does not do much.

author: Zero999
author_rank: Super Contributor
timestamp: April 28, 2015, 07:44:59 pm
content: Quote from: littlebill on April 28, 2015, 02:03:40 pmThe maximum current surge though the diode is equal to the coil current, which carries on flowing for a short period of time when the relay is turned off. The diode doesn't need to be rated to carry the coil current continuously, just for a short period of time after it's turned off.The current surge drawn by the load makes no difference to the diode but it's important to make sure the contacts can handle it.