PID Controller Enclosure Questions

[k1100t]

Hello, I'm in the middle of gathering bits to build a PID controller for my HLT; which uses one of Mr Lard's 3KW elements. I've pretty much got everything, except for the terminal connectors, as I bought the wrong WAGO ones (two, instead of three). Searching for old posts to see how others have built theirs, leads me to a couple of questions:

1. Do I need to put a fuse on the 240V input to my enclosure? I wasn't planning on it, now I'm paranoid that I need one, but have no idea what type to get; normal 13A, some sort of fast blow, or which bit to put it on; input to the enclosure, between the PID and SSR, or between the SSR and the element.

2. I bought some 240V 15A DPDT switches to kill the element, but keep the PID live to use as a temperature sensor, but am now confused about where to put them. I've read that the SSR's can leak, so do I put the switch before or after it?

Thanks,

Bob.

[themadhippy]

1. Do I need to put a fuse on the 240V input to my enclosure?
if your powering it via a 13A plug then no
2-after the ssr

[k1100t]

1. Do I need to put a fuse on the 240V input to my enclosure?
if your powering it via a 13A plug then no

Yes, the controller box will have a 13A fused plug on it. I'm assuming that's enough for the PID and element.

2-after the ssr

Good, I bought the right switches then...

Thanks!

Bob.

[k1100t]

One more question, if I may. I bought one of these as the enclosure -> http://uk.rs-online.com/web/p/instrument-cases/7733261/ It has aluminium front and back plates. So...

Do I mount the heat sink of the SSR directly to the back plate, or do I sandwich the back plate between the SSR and the heatsink...?

Thanks,

Bob.

[bobsbeer]

If you can I would sandwich the backplate between the SSR with the heatsink on the outside of the box. That will aid circulation around the heatsink. Don't forget to also ensure you have air holes in the cabinet. Regarding the switch point, I would put the switch BEFORE the SSR. When SSR's fail they generally fail to live. Having the switch before the SSR will isolate the SSR completely and cut the main power to the load (element). Putting AFTER the SSR, you will isolate the load, but the SSR will still be live.

[Fil]

OR if you make a couple of diy tabs I have used tin snipped sections of flattened out 15mm copper tube.
you can secure said tabs under the screwheads of the screws securing the heatsink to the ssr.

said tabs can then be screwed, bolted, wedged, or simply epoxied to the abs case allowing you to push the heatsink outside the box. a dab of heatsink compound is also useful to fill the tiny gaps between ssr and heatsink.

using a 3kw element you can expect significant heat to be generated, i recorded well over 60C on the 3rd vane down of a SSR/heatsink powering 3kw(2 x 1.5kw) of elements (25a ssr)

[k1100t]

If you can I would sandwich the backplate between the SSR with the heatsink on the outside of the box. That will aid circulation around the heatsink. Don't forget to also ensure you have air holes in the cabinet.

Do I still need holes if the heatsink is on the outside...? Would mean the box looses whatever splash/water proof nature it has, no?

Regarding the switch point, I would put the switch BEFORE the SSR. When SSR's fail they generally fail to live. Having the switch before the SSR will isolate the SSR completely and cut the main power to the load (element). Putting AFTER the SSR, you will isolate the load, but the SSR will still be live.

As they're DPDT switches, then both wires from the PID through the switch to the SSR? I thought that as they leak current, then having at least the neutral from the element on the switch would be preferable?

Thanks,

Bob.

[k1100t]

using a 3kw element you can expect significant heat to be generated, i recorded well over 60C on the 3rd vane down of a SSR/heatsink powering 3kw(2 x 1.5kw) of elements (25a ssr)

Looks like we have the same heatsink and it means things are easier inside, if it's on the outside.

Cheers,

Bob.

[bobsbeer]

Only the live cable goes to the SSR. The neutral cable goes from your element and back to the mains. Your isolation switch is only breaking the live cable and not the neutral, so you don't need the neutral wire connected, even if you have connections on the switch. There is no need to break the neutral wire.

Wiring sketch:

[themadhippy]

Your isolation switch is only breaking the live cable and not the neutral, so you don't need the neutral wire connected, even if you have connections on the switch

if the switch is for isolation purposes then it should break ALL current carrying conductors,and in the latest big book of electrical rules the neutral is classed as a current carrying conductor.

[chefgage]

Your isolation switch is only breaking the live cable and not the neutral, so you don't need the neutral wire connected, even if you have connections on the switch

if the switch is for isolation purposes then it should break ALL current carrying conductors,and in the latest big book of electrical rules the neutral is classed as a current carrying conductor.

You beat me to it

[k1100t]

Your isolation switch is only breaking the live cable and not the neutral, so you don't need the neutral wire connected, even if you have connections on the switch

if the switch is for isolation purposes then it should break ALL current carrying conductors,and in the latest big book of electrical rules the neutral is classed as a current carrying conductor.

So given this naive, and quite possible wrong, diagram. Where do I put the switch...? My natural inclination is a...



Thanks,

Bob.

[themadhippy]

yes A,make sure its the live(not + ) that goes through the ssr.If you put the switch at b youll be switching the low voltage and the circuit could still be live on the output

[k1100t]

yes A,make sure its the live(not + ) that goes through the ssr.If you put the switch at b youll be switching the low voltage and the circuit could still be live on the output

Just to be clear, in 13A plug parlance, it's the brown wire in the plug through the SSR...?

Thanks,

Bob.

[themadhippy]

yep brown is live