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Heatsink for solid state relais?

Posted by Tomsand 
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Tomsand
Heatsink for solid state relais?
November 14, 2015 06:38PM
Hi,
I'll using a Fotek SSR-25DA solid state relais to power my 230 AC heatbed.
Do I have to use a heatsink as well to keep it cool? Read that a heatsink is needed when using DC, but I'm unsure if that's the case for AC as well.
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tjnamtiw
Re: Heatsink for solid state relais?
November 14, 2015 10:05PM
Solid state relays have to be derated if they don't have a heat sink. Look up the specs for your particular one and it should tell you how much current it can handle without a heat sink.
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the_digital_dentist
Re: Heatsink for solid state relais?
November 14, 2015 10:53PM
I use a Crydom D1225 SSR to switch AC power to a 450W heater without a heatsink. It doesn't even get warm.



Ultra MegaMax Dominator 3D printer: [drmrehorst.blogspot.com]
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Dust
Re: Heatsink for solid state relais?
November 15, 2015 01:49AM
cheap clone SSR generally show them selves by getting overly hot...
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dc42
Re: Heatsink for solid state relais?
November 15, 2015 03:26AM
I use a SSR-10DA to switch a 230V 350W heater. My printer uses an aluminium tray to support the electronics so I bolted the SSR to the tray so it conducts the heat away (see link in my signature). But you probably don't need any heatsink at all. Even if your heater power is 500W, that's only a little over 2A @ 230V. The voltage drop of the SSR should be no more than 2V. So the power dissipated will be around 4W, which is not much for a package of that size.


Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
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Srek
Re: Heatsink for solid state relais?
November 15, 2015 05:51AM
A good SSR for AC will switch while the sine curve is at 0 (zerocrossing). Due to this there is no load at switch time. Cooling is not neccesary.

[www.bonkers.de]
[merlin-hotend.de]
[www.hackerspace-ffm.de]
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tjnamtiw
Re: Heatsink for solid state relais?
November 15, 2015 11:43AM
Quote
Srek
A good SSR for AC will switch while the sine curve is at 0 (zerocrossing). Due to this there is no load at switch time. Cooling is not neccesary.

You are only talking about the switching on and off of the relay but you don't address the conduction of power through the SSR. A typical SSR absorbs 1 to 2% of the power it is passing so at some point if you draw enough power, the juncture will approach its failure point.
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Srek
Re: Heatsink for solid state relais?
November 15, 2015 03:02PM
Quote
tjnamtiw
Quote
Srek
A good SSR for AC will switch while the sine curve is at 0 (zerocrossing). Due to this there is no load at switch time. Cooling is not neccesary.

You are only talking about the switching on and off of the relay but you don't address the conduction of power through the SSR. A typical SSR absorbs 1 to 2% of the power it is passing so at some point if you draw enough power, the juncture will approach its failure point.
That's correct. The mentioned SSR though can handle up to 25 A, which is more than 10 times what most heatbeds only ever go up.

[www.bonkers.de]
[merlin-hotend.de]
[www.hackerspace-ffm.de]
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tjnamtiw
Re: Heatsink for solid state relais?
November 15, 2015 03:32PM
Quote
Srek
Quote
tjnamtiw
Quote
Srek
A good SSR for AC will switch while the sine curve is at 0 (zerocrossing). Due to this there is no load at switch time. Cooling is not neccesary.

You are only talking about the switching on and off of the relay but you don't address the conduction of power through the SSR. A typical SSR absorbs 1 to 2% of the power it is passing so at some point if you draw enough power, the juncture will approach its failure point.
That's correct. The mentioned SSR though can handle up to 25 A, which is more than 10 times what most heatbeds only ever go up.

That is also correct. smiling smiley
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dc42
Re: Heatsink for solid state relais?
November 15, 2015 03:37PM
Quote
tjnamtiw
Quote
Srek
A good SSR for AC will switch while the sine curve is at 0 (zerocrossing). Due to this there is no load at switch time. Cooling is not neccesary.

You are only talking about the switching on and off of the relay but you don't address the conduction of power through the SSR. A typical SSR absorbs 1 to 2% of the power it is passing so at some point if you draw enough power, the juncture will approach its failure point.

The percentage static power that the SSR absorbs depends on the voltage drop of the SSR compared to the power supply. Some examples:

1. Typical DC-AC SSR (e.g. SSR-25DA) dropping 1.6V and switching a 230V bed heater: about 0.7%.

2. Cheap DC-DC SSR (e.g. SSR-40DA) dropping 2V and driving a 24V bed heater: about 8%. This is why you should never use a cheap DC-DC SSR to drive a bed heater.

3. Good DC-DC SSR (e.g. Crydom DC100D40) dropping 0.14V @ 20A and driving a 24V bed heater: about 0.6%..


Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
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ElmoC
Re: Heatsink for solid state relais?
November 15, 2015 04:10PM
Quote
dc42
3. Good DC-DC SSR (e.g. Crydom DC100D40) dropping 0.14V @ 20A and driving a 24V bed heater: about 0.6%..

Given the Crydom unit costs almost $100 USD, I think I am glad I decided to go with an AC powered heater The "typical" DC-AC unit can be picked up for $5 (and hopefully isn't some cheap clone). I have seen some alerts about un-certified Fotek SSRs being labeled with false certification. I had one of those and it burned up right after turning it on and pulling only about 1A (was rated for 25A).
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