Solid State Relay question.

[high compression ii]

2nd terminal to ground

Well--That would mean the supply out of the SSR would be going to earth....

Assuming Live to pin 1, pin 2 should go to the element, and then other pin of the element to Neutral.

[julianf]

You need to look at the post in context : )

[therecklessengineer]

Put one meter probe on one SSR terminal, other meter probe on the other terminal

If it reads 0 (or close to) then it's conducting. If it reads 240, it's not.

[julesandtash]

I'm pretty sure that my SSRs (and they are not cheap Chinese ones) still show mains voltage at the output terminal even when they are not conducting.

However, there is just about no current flow.
Ie, if there is 240v live connected to one terminal on the load side and the other is not connected to any load then the terminal will read 240V with a digital meter. However, if a load is connected (heater, pump etc) between that terminal and neutral then the voltage dissapears and only reappears when the SSR is conducting.

I think that, because they are semiconductor devices and not a true isolating relay, there is a very small leakage current even when they are off which shows as a voltage when tested with a very high impedance digital meter between the SSR and neutral.

This is why I have contactors between the live supply and the SSRs. The contactor only supplies power to the SSR terminals when all the interlock logic and control switches permit it. Then the PID actually controls it via the SSR.

I suspect that this is what your mate is seeing if they have no load connected to the SSR and would explain why two have given exactly the same result.
If you like I will measure mine tommorow with the load connected and disconnected and with the SSR energised & not energised

[julianf]

I did consider this, but then, for some reason, discarded the idea...   Im feeling like that may be the bottom of it.

[high compression ii]

You need to look at the post in context : )

Ooops!--Bugger, Seem to do that a lot these days!

--IF he was to replace the heater with a 100W bulb--That would take the measuring errors out of the question...

[julianf]

Yep, i sent him that suggestion last night.  He said he would report back his findings.

Thank you.

[julesandtash]

Ok, here are my findings.
All taken on the Potter & Brumfield 40A SSR which controls the processor heater on my plant

3KW heater connected to SSR and control input on :-
Supply terminal to neutral 233.1V
Load terminal to neutral 228.9V

3KW heater connected to SSR and control input off:
Supply terminal to neutral 234.2V
Load terminal to neutral 0.2v

Heater now disconnected from load terminal of SSR - control input on
Supply terminal to neutral 234.2V
Load terminal to neutral 234.2V

Heater now disconnected from load terminals of SSR - control input off
Supply terminal to neutral 234.2V
Load terminal to neutral 234.0V

Clearly this shows that, with no load, the Load terminal will be at full supply voltage IF no load is connected to it no matter what the control input is doing
But, with a load connected, it behaves as expected.
Effectively it controls the current flow but does not isolate the circuit from mains voltage.

This is a good reason to make sure that the supply is isolated before playing with anything downstream of an SSR. I suspect that the current flow through it, even when off would be enough to give a nasty shock.


This article from Omron states that the minimum load current for AC output SSRs is 100mA to ensure reliable switching which equates to around 24Watts at mains voltage.
https://www.ia.omron.com/support/faq/answer/18/faq02152/index.html

This one is a pretty good read too if you want to know lots about SSRs. It would appear that the snubber circuit (inside the SSR) is the cause of the supply voltage at the load terminal with no load.

http://docs-europe.electrocomponents.com/webdocs/0d82/0900766b80d82ad3.pdf

[Julian]

Jules, good find, some fantastic information in that second reference (not that I understand much/any of it!).

We don't have an SSR wiki page at the moment, so I've started one here ... http://www.biopowered.co.uk/wiki/SSR_-_solid_state_relay  and put a link to the pdf.

If anyone with a better understanding of electronics than myself wants to have a bash at the text, I'll be happy to help with page format and any diagrams that need drawing.

[high compression ii]

Julian--Ive bunged a bit down for that page, feel free to potch with it as you see fit...

[Tony]

Very useful info there. I didn't know about voltages on the output with no load.

I presume mains spikes can make them fail short circuit too.

Just needs a bit of formatting and a photo and that page is good to go.

[high compression ii]

Surprisingly SSR's are pretty robust against spikes due to the snubber-network they contain.

I once severely spiked one--to the extent I burned a flame pattern on a 13A plug and socket,--nice big flash and bang too, but the attached (El-Cheapo) SSR was just fine!

[Julian]

Julian--Ive bunged a bit down for that page, feel free to potch with it as you see fit...

Thanks Alistair,  I'll have a crack at it as soon as I can ... I think I now have three pages which need completing!

[therecklessengineer]

3KW heater connected to SSR and control input off:
Supply terminal to neutral 234.2V
Load terminal to neutral 234.0V

Errrm...this shows that the heater is powered up in this state. The load terminal should be at zero here.

Seeing as the calculations for heatsink sizing are fresh in my head (I've been sizing up one for 3 x 40A triacs at work) I'll add them to the SSR page when I have a second.

[Julian]

That would be great, thanks.