Electronics > Projects, Designs, and Technical Stuff
I fried my SSR........ HELP!!!
jwilson:
--- Quote from: duak on January 25, 2020, 05:31:49 am ---Jwilson, here's what I think:
1.) the SSRs are probably NFG. Last I checked, Crydom or other quality SSRs are quite a bit more expensive
2.) If the element has a metal case or tube, that can be grounded but you definitely do not want the resistance wire to connect or even slightly conduct to ground. Check this out!
3.) as wired, two SSRs in series is a good idea in case one fails.
4.) you are dealing with 30 amps here and anything smaller than 10 AWG conductors is marginal if not hazardous.
5.) each SSR is running at about 75% of capacity and will dissipate about 15-20 W. You seem to have a galvanized sheet steel enclosure probably about 0.030" thick. IMHO, this is nowhere near good enough as steel is not a good conductor of heat as is aluminum. It's just too thin to carry much heat away. I've used a rule of thumb of at least 1 square inch of 1/8" thick aluminum sheet for heat sinking. Where to put it is another question.
--- End quote ---
Bear with me here, so I do not want to ground the heating element?
The heating element is just coiled wire with 2 ends. I have 4 elements in total. Here's what I did.
Since I have 8 leads total, I wired 4 wires together for a + lead and 4 wires together for a - lead.
Should I ground the "-" leads to the chassis?
When I blew the SSR, I had power going to each set of leads and it blew the SSR.
I figured my mistake was that I didn't ground half of the leads.
What do you think?
THANK YOU!!!!!
jwilson:
--- Quote from: duak on January 25, 2020, 05:31:49 am ---Jwilson, here's what I think:
1.) the SSRs are probably NFG. Last I checked, Crydom or other quality SSRs are quite a bit more expensive
2.) If the element has a metal case or tube, that can be grounded but you definitely do not want the resistance wire to connect or even slightly conduct to ground. Check this out!
3.) as wired, two SSRs in series is a good idea in case one fails.
4.) you are dealing with 30 amps here and anything smaller than 10 AWG conductors is marginal if not hazardous.
5.) each SSR is running at about 75% of capacity and will dissipate about 15-20 W. You seem to have a galvanized sheet steel enclosure probably about 0.030" thick. IMHO, this is nowhere near good enough as steel is not a good conductor of heat as is aluminum. It's just too thin to carry much heat away. I've used a rule of thumb of at least 1 square inch of 1/8" thick aluminum sheet for heat sinking. Where to put it is another question.
--- End quote ---
Oh, and to address #5, you can't see it well in the pictures, but each SSR is connected to a heat sink per SSR. The heat sinks are aluminum and about 2"x2"x2" tall.
Thanks again!
calzap:
I agree that quality brand SSRs are the way to go. However, if the controller is slapped on the side of or anywhere near the kiln, passive cooling may not be enough. Advise fan-cooled heat sinks. And if possible, don't attach the controller to the kiln or place it above it.
My one experience with an Auber controller (for a smoker) was not good.
And, minor point, your power voltage is almost certainly 240 VAC, not 220 VAC
Mike in California
james_s:
For chassis wiring you can get away with #14 wire though I don't think I'd go smaller than #12. The ampacity is considerably higher than for power transmission where the length is greater and it may be enclosed within walls or covered by insulation.
https://www.powerstream.com/Wire_Size.htm
DBecker:
--- Quote from: james_s on January 25, 2020, 04:47:14 pm ---For chassis wiring you can get away with #14 wire though I don't think I'd go smaller than #12. The ampacity is considerably higher than for power transmission where the length is greater and it may be enclosed within walls or covered by insulation.
https://www.powerstream.com/Wire_Size.htm
--- End quote ---
That is correct: chassis wiring suspended in free air has much higher ampacity than supply wiring that is likely thermally insulated and bundled with other current-carrying conductors.
But for one-off prototypes and projects you can afford to be much more conservative and use thicker wire. That will compensate for inconsistent connections and act as additional heat sinking.
I agree with the other posters: eBay SSRs are often rated more like fuses than components. The stated current is where they very likely will fail, not a continuous operating current. A Chinese "40A" SSR is OK for 10 amps, but (generalizing here) unsuitable for 20 amps.
If you want reliable operation at a low price, use a single SSR to operate a 40 amp dual circuit contactor. Most will operate continuously at their rated current, survive surges and overloads if they aren't make- or break-current, and will survive hot conditions. Their main down-sides are size, slight noise, and a floor on power use.
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