DC relay for switching AC? - Page 1
Forum section: electronics / beginners

author: alah
author_rank: Contributor
timestamp: July 07, 2020, 05:33:13 pm
content: Hi ALL; would a DC relay switch AC as well as DC?

author: Zero999
author_rank: Super Contributor
timestamp: July 07, 2020, 05:36:27 pm
content: It depends on the relay. A machanical relay will be able to switch AC, as well as DC, but a solid state relay might only be able to switch one, or the other.

author: Seekonk
author_rank: Super Contributor
timestamp: July 07, 2020, 07:18:40 pm
content: DC relays are generally limited to about 30V DC. Their gap and material will not be suited to 120V AC . There are specialty expensive high voltage ones for several hundred volts. These would probably work for AC.

author: John B
author_rank: Frequent Contributor
timestamp: July 07, 2020, 11:12:26 pm
content: If it's a mechanical relay, you should also take the maximum DC rating and derate that by x0.707 to give you a max RMS voltage to account for the peak value of the sinewave.

author: alah
author_rank: Contributor
timestamp: July 07, 2020, 11:35:30 pm
content: Understood; so mine wont work Ill check on solid state relays, THANKs.

author: JustMeHere
author_rank: Frequent Contributor
timestamp: July 08, 2020, 06:03:52 am
content: Quote from: Zero999 on July 07, 2020, 05:36:27 pmWhy won't this $10 relay work?https://www.alliedelec.com/product/idec-corporation/rh2b-udc12v/70174865/?utm_source=google&utm_medium=cpc&adpos=&scid=scplp70174865&sc_intid=70174865&gclid=EAIaIQobChMIv_-eyv286gIV7giICR0z-QOZEAQYAyABEgLO0PD_BwE&gclsrc=aw.ds

author: tautech
author_rank: Super Contributor
timestamp: July 08, 2020, 07:45:17 am
content: Quote from: alah on July 07, 2020, 05:33:13 pmYes.Mechanical relays are specified by their coil ratings and contact ratings.A DC rated coil is not intended to be operated on AC and vice versa.AC is easier to switch as any contact arcing is extinguished at each zero crossing however DC will arc as contacts are opened and therefore DC contact ratings are always lower than AC.In some circumstances when controlling AC from digital circuits, DC relays can be cascaded to AC relays to provide additional isolation and keep say 3 phase 415VAC away from digital circuits.

author: Zero999
author_rank: Super Contributor
timestamp: July 08, 2020, 12:47:19 pm
content: Quote from: Seekonk on July 07, 2020, 07:18:40 pmGood point, it depends on the voltage, which the original poster hasn't mentioned.120VAC is too lower voltage to arc over anything, but there could be transients which will and an automotive relay won't have enough separation between the contacts and coil, to meet the safety standards for mains AC.Conversely AC relay won't be able to switch the same current and voltage, on DC, because the arc isn't extinguished, when the current falls to zero, as is the case for AC.Quote from: JustMeHere on July 08, 2020, 06:03:52 amYes, that relay will work.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 09, 2020, 07:38:22 pm
content: Download some datasheets for random relays and compare their specifications.You will quicly get an idea of the different voltage and current for the coils and the contacts.