Biopowered - vegetable oil and biodiesel forum
Biodiesel => Biodiesel equipment => Topic started by: julianf on July 12, 2013, 04:52:15 PM
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Im trying to remotly diagnose a friends system.
He has the standard PID / SSR setup.
With the system wired as id expect it to be, the load output of the SSR is 240v all the time.
The LED on the PID and SSR pulses away, but the terminal on the SSR remains at 240v.
If the SSR is removed from the system (isolated completely) and checked with a multimeter, it shows no continuity across the load terminals.
If a 240v feed is connected to one of the load terminals, sure enough, the other terminal goes to 240v also!?!? (this is with no control lines connected at all - just the one 240v feed to one of the load terminals)
My conclusion was - SSR is knackered.
...so he purchased another one. Which behaves in an identical way : /
Either he has purchased two duff relays, or im missing somthing that should be obvious?
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Did you measure the controlling voltage pulses from the PID to the SSR? What is the switching specification for the SSR?
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Did you measure the controlling voltage pulses from the PID to the SSR? What is the switching specification for the SSR?
Did not measure the PID output.
The SSR is, i assume, normally closed. He just bought it, cheap from china, off ebay.
Ive just looked on google image search - im 99% certain its this one -
(http://img.alibaba.com/wsphoto/v0/491841119/5PCS-80A-SSR-SOLID-STATE-RELAY-DC-AC-free-shipping.jpg)
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...basically, if no connections are made, there is no continuity between terminals 1 and 2
if one single connection is made to terminal 1, of 240v ac, then 240v ac also shows at terminal 2.
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That SSR Pictured--Is 'contacts' Normally Open, so there should be No power flow with no signal/supply to the 'control' connections.
Has the system ever worked...?
I ask, as maybe somethow--he's sending 240V down to the control of the SSR and destroying the control-circuits inside.
Alternatively--he's using a 'Neon Screwdriver' which is illuminating due to the normal leakage of the internal Triacs of the SSR with no load connected....
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When I've played with SSRs I found that you couldn't measure continuity through terms 1 & 2, irrespective of supply on terms 3 & 4.
If you rig up terms 1 & 2 in a 240v circuit and something like a car battery on terms 3 & 4, it should work OK.
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When I've played with SSRs I found that you couldn't measure continuity through terms 1 & 2, irrespective of supply on terms 3 & 4.
If you rig up terms 1 & 2 in a 240v circuit and something like a car battery on terms 3 & 4, it should work OK.
Yup--Thats exactly how they work--Cant measure with a normal meter as there's four semiconductor junctions in a Triac, so you need around 3V at least across them to allow current flow....Meters--even on 'Semi' test usually are 1.5V or maybe up to 2V and aint enough to forward-bias the triacs
You can use something as small as a PP3 9V Battery on the Control input to turn the SSR 'ON' Any supply of DC between 3V and 32V can be used as the control-volts to an SSR
--There are Some SSR's that use 240V ac as the control volts--but Not the type pictured, and they are less common....
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Sorry - i had a blip in concentration, and said that i assumed the relay was NC, when i ment to say NO.
...but, without a control line, its closed. Indeed, with a control line its closed also (however, the LED on the relay itself blinks as you would expect).
Fair point about not being able to measure continuity with a multimeter, however, thats ancillary to the real problem - the relay (both of them) seem to be closed all the time, regardless of control line.
The system has never worked - its a new setup.
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When I've played with SSRs I found that you couldn't measure continuity through terms 1 & 2, irrespective of supply on terms 3 & 4.
If you rig up terms 1 & 2 in a 240v circuit and something like a car battery on terms 3 & 4, it should work OK.
Yup--Thats exactly how they work--Cant measure with a normal meter as there's four semiconductor junctions in a Triac, so you need around 3V at least across them to allow current flow....Meters--even on 'Semi' test usually are 1.5V or maybe up to 2V and aint enough to forward-bias the triacs
You can use something as small as a PP3 9V Battery on the Control input to turn the SSR 'ON' Any supply of DC between 3V and 32V can be used as the control-volts to an SSR
--There are Some SSR's that use 240V ac as the control volts--but Not the type pictured, and they are less common....
Hey, I can do electronics me!
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Oh - one last thing - he purchased the SSR and the PID / Thermocouple as a package off ebay.
So, id assume, the SSR is 12v control.
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Wish I Bloody Could!
As to the problem. Seems to me, either he has somehow damaged the SSR's or he's using a neon-screwdriver to test for continuity.
Can you ask him to replace the heater with a lamp-bulb,--say 100W 240V to confirm that its actually faulty...?
--I wonder, if he's switching the Neutral side with the SSR instead of the Live--giving the impression its not switching--the way he's measuring it.....
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Test was a mutimeter on the terminal, not a screwdriver.
It was me who originally wired the unit, but i passed it over to him before even a plug was put on it (ie untested).
It would be embarrasing if ive made a pigs ear of it, but, hey, its possible!
He lives an hour and a half away, hence remote dianostics.
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This might be an idiot question, but you're not measuring the voltage across it are you? With one multimeter lead on one terminal and one on the other?
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2nd terminal to ground
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ps.
i dont mind any suggestions - i mean there's somthing stopping it from working!
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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.
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You need to look at the post in context : )
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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.
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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
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I did consider this, but then, for some reason, discarded the idea... Im feeling like that may be the bottom of it.
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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...
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Yep, i sent him that suggestion last night. He said he would report back his findings.
Thank you.
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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
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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.
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Julian--Ive bunged a bit down for that page, feel free to potch with it as you see fit...
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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.
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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!
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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!
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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.
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That would be great, thanks.
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Sorry James - that was me being dump and not reading my scribbled piece of paper properly. I have now corrected it.
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Added a small bit on selecting the right size of heatsink.