Wiring bed heater to relay

ronnien59 · 8 Dec 2016, 23:58

Can i wire the heater output to A1 and A2 of the relay to start heat on AC heating bed? I want to make sure of this before it before I try. It is a 12VDC relay with 1 NO and 1NC. I want to wire two in for redundancy.

ronnien59 · 9 Dec 2016, 02:27

So the 12 to 24v relay did not work. I had another 3 to 32v SSR-25 that worked fine. I will see if i can get something with a smaller amperage. I will put two in line to have a little safety in case one fails.

gzcwnk · 9 Dec 2016, 02:42

So the bed power is AC or DC? and at what voltage?

Most cheap SSRs are AC. If you got it off ebay or aliexpress and paid about $5US for it its almost certianly an AC SSR even if the label says DC. Running an AC SSR is DC '"mode" is a bad idea.

You will be switching a lot a mechanical relay is a bad idea IMHO.

If you want a mains voltage heated bed or AC then an SSR is a good idea.

ronnien59 · 9 Dec 2016, 03:02

I have an 110VAC bed and I am using 3 to 32VDC control 24-380VAC SSR-25 DA. Would like to use the SSR15DA also DC control AC out.

dc42 · 9 Dec 2016, 06:43

I agree that using a mechanical relay is not a good solution: but if you do want to use one, make sure you connect a flyback diode in parallel with the relay coil.

I'm not sure that wiring two AC SSRs in series will work, because they might not both detect zero crossing at the same time.

gzcwnk · 9 Dec 2016, 06:58

"3 to 32VDC control 24-380VAC SSR-25 DA.
DC control AC out"

I'll admit im no electronics engineer, but this sounds perfect to me, exactly what i will do except I have a 40amp SSR and 240V AC. I m not aware that de-rating is an issue, means less heat I believe and hence more reliable.

ronnien59 · 9 Dec 2016, 11:02

@dc42:

I agree that using a mechanical relay is not a good solution: but if you do want to use one, make sure you connect a flyback diode in parallel with the relay coil.

I'm not sure that wiring two AC SSRs in series will work, because they might not both detect zero crossing at the same time.

do you have a schematic of how to install a flyback diode and size?

dc42 · 9 Dec 2016, 14:23

See e.g. http://www.douglaskrantz.com/ElecFlybackDiode.html. Note, the diode will also slow down the action of the relay releasing, which may lead to more arcing of the contacts. The diode rating should be at least as high as the relay coil current. A 1A diode such as 1N4001 should be more than adequate.

ronnien59 · 10 Dec 2016, 00:36

@dc42:

See e.g. http://www.douglaskrantz.com/ElecFlybackDiode.html. Note, the diode will also slow down the action of the relay releasing, which may lead to more arcing of the contacts. The diode rating should be at least as high as the relay coil current. A 1A diode such as 1N4001 should be more than adequate.

It is funny that you pointed to the very website I went to earlier. I have talked with my electrical engineer at work and we have a diode terminal block that will fit the bill. I plan to use it on the positive lead.
Thanks again for all your help