Contact resistance of relay.? - Page 1
Forum section: electronics / beginners

author: Faringdon
author_rank: Super Contributor
timestamp: August 10, 2021, 09:24:36 am
content: Hi,Do you know what is the contact resistance of this relay...?...AHES3292...https://www.panasonic-electric-works.com/cps/rde/xbcr/pew_eu_en/ds_hes_en.pdfThere are figures on page 3 relating to 6VDC with 1A and 20A current but........we need contact resistance at the following conditions...1....16Arms and 240VAC2...32Arms and 240VAC..do you know what is contact resistance in those cases?

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 10, 2021, 09:36:58 am
content: I don't know but I think we may guess that it is somewhere close to 20 ADC rated value. I would be glad to hear from somebody more familiar with relays.The manufacturer states a rated value for a DC current because it is very easy to measure. But I guess it has to be about the same for AC.

author: GlennSprigg
author_rank: Super Contributor
timestamp: August 10, 2021, 12:06:37 pm
content: What you seek is difficult to simplyextrapolateas such, and requires'actual' testing!Generally, such relays etc have aMaxcontact resistanceof100-mOhms, which is not an'actual'resistance! And this is generally tested using6Vand1A! This makes calculations simple, as ifE = IR,and'I'is1, thenR = E!!, therefore, say an actual20-mVdrop across the contact means the contact resistance is20-mOhms!You can see such an example by reading the short description/info in my link below from Panasonic...https://na.industrial.panasonic.com/blog/how-measure-relay-contact-resistanceI'm personally curious as to your application/needs for such specific/detailed info, that this would be critical, given the 'Max' value stated?

author: Gyro
author_rank: Super Contributor
timestamp: August 10, 2021, 04:55:18 pm
content: Quote from: GlennSprigg on August 10, 2021, 12:06:37 pmIt relates to his other EV charger relay thread...https://www.eevblog.com/forum/renewable-energy/relay-for-electric-car-charger/.

author: Faringdon
author_rank: Super Contributor
timestamp: August 10, 2021, 07:08:46 pm
content: Thanks, i must say it amazes me that a 35A ACrms relay datasheet does not give a contact resistance in terms of {contact RMS voltage}/{35Arms}.That is, give that value of contact resistance at the relays maximum rated current, that allows you to calculate the power dissipated in the contacts when at max current.Surely any user of a relay needs to contact the power dissipated in the contacts at the maximum rated current?Page 3 of the datasheet seems to be suggesting that the contact resistance changes with the current, ie, from 1AA to 20A the contact resistance appears to go from 100milliohms to 3 milliohms?....resistances usually get higher the hotter they get...and you'd expect the contacts to get hotter with 20A than 1A?

author: Vovk_Z
author_rank: Super Contributor
timestamp: August 11, 2021, 09:58:10 am
content: Quote from: Faringdon on August 10, 2021, 07:08:46 pmThis is quite a common situation. Sometimes the manufacturer gives typical values, sometimes not. So you, as a designer, have to buy a typical part and make a little investigation.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: August 11, 2021, 11:38:29 am
content: Contact resistance of a relay is not an important design factor.It also varies with current, contact wear and relay to relay, and possibly also with planet alignment and moon phases.If you want to properly design around it, then make sure your circuit does not depend on the value of that contact resistance. For example, if you want to switch between different current shunts, then do not measure the voltage drop over both a current shunt and the contact resistance, but only measure the voltage drop over the current shunt itself ( And use Kelvin connections too).