Heat bed thru relay

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Re: Heat bed thru relay

Postby canadianavenger » Wed Mar 26, 2014 3:05 pm

You might be able to use them, but if they are unrated, I would steer away from them.

Good article on AC vs DC ratings on switches. [same would apply for relays]
http://www.eaa.org/sportaviationmag/200 ... itches.pdf
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Re: Heat bed thru relay

Postby fma » Wed Mar 26, 2014 3:06 pm

25A is Ok for 20A!

DC vs AC, see my post above.
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Re: Heat bed thru relay

Postby fma » Wed Mar 26, 2014 3:26 pm

canadianavenger wrote:Good article on AC vs DC ratings on switches. [same would apply for relays]
http://www.eaa.org/sportaviationmag/200 ... itches.pdf

Thanks for this link!

So, I was wrong, it is not a sqrt(2) factor... At least for relays; I think it is OK for Solid State Relays? Or Am I wrong too?
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Re: Heat bed thru relay

Postby cvoinescu » Wed Mar 26, 2014 3:30 pm

I would check the datasheet on switching DC with a solid-state relay. Some use triacs internally, so they can only switch AC. When used with DC, they will turn on, and stay on. The only way to turn off a triac in DC is to disconnect the load, or turn the supply off. I'm not saying all solid-state relays do that, but I think most do, especially at that price point.
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Re: Heat bed thru relay

Postby canadianavenger » Wed Mar 26, 2014 3:35 pm

fma wrote:25A is Ok for 20A!

DC vs AC, see my post above.


fma wrote:If your relay can switch 30A, it is OK for 20A.

Just check if it is for DC current, and not for AC current. In AC, average current is "peak current / sqrt(2)", if I remember. So, 30A for AC is only 21A for DC...


It's not as simple as converting the AC value to DC. [read the article I linked above]. Contact ratings for DC are usually a small fraction of their AC rating, grab any switch or relay that is rated for both [either the voltage or current is greatly reduced, far below the "average current" calculation]. Now you can, usually, perform the reverse calculation for going to AC from a DC rating, [and you'll have a fairly healthy margin of safety to boot] as the contact requirements for switching AC are much lower than that for DC.

[ edit: i see you picked up on the article while I was posting :) - as cvoinescu pointed out, SSR's are a different animal, and it would greatly depend on the internal design ]
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Re: Heat bed thru relay

Postby fma » Wed Mar 26, 2014 4:43 pm

Ok, so, it's not that simple!

@dave3d, what you can do is use the same design used on most board: using a big FET, and drive it from the board FET... You will have to screw it on something, and isolate legs, but...
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Re: Heat bed thru relay

Postby mattrsch » Thu Mar 27, 2014 8:09 pm

There are SSRs rated for DC for not too much money:
http://www.lightobject.com/40A-Solid-State-Relay-SSR-DC-In-DC-Out-P315.aspx
They also carry 100A versions for ~$23
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Re: Heat bed thru relay

Postby airkuld » Fri Mar 28, 2014 2:46 am

Thanks mattrsch! I ordered a 25amp plus a heatsink and should have it Monday. I'll report back on its operation.
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Re: Heat bed thru relay

Postby dave3d » Fri Mar 28, 2014 10:01 am

Yes, Thanks as well. I have also ordered a Solid State Relay. 40amp DC-DC.

http://www.ebay.co.uk/itm/1pcs-Soild-St ... 51a67183d5

............... but may take 4 or 5 weeks from China.
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Re: Heat bed thru relay

Postby airkuld » Tue Apr 01, 2014 3:43 pm

Ordered and received this relay from Lightobject:
http://www.lightobject.com/25A-Solid-St ... -P314.aspx
Relay was $9.95 I'm using gently used 12v hot swap server power supplies as they were cheap and offered 50 or so amps. I attached the heatsink to the top of the PSU and the relay to that. The 2 wires from the RAMPS board go to the input terminals on the SSR. One output lead from the PSU goes direct to the heated bed. The other goes to one of the output terminals of the SSR. The other output terminal goes on to the other heated bed lead. I put a 20 amp blade fuse in line from the SSR output terminal to the heated bed. Relay has a red LED that lights when its triggered. My bed heater is 12 inch by 12 inch and would never heat over 72 degrees before and now it easily hits 100. Amp draw while heating is from 16 to 18. I'm happy so far!

Regressedee mentioned a flyback diode. In searching for a wiring schematic, I did see mention of installing a diode across the load where the load in that case was a motor. As I understand it, the diode is to block a backfeed of energy from a collapsing magnetic field such as a coil or motor winding would provide when normal current flow is shutoff. Because there is no coil in the SSR and not much of a field built around the heated bed, do you think this is necessary?
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