Help with PWM and SSR desperately needed

Help with PWM and SSR desperatly beeded

Hello all,

I have bought a PWM on ebay:

**broken link removed**

When I try to connect it to two SSR's only one works as of providing power to the load.
Both SSR's LED's are flashing simultainuously but only power out of one of the SSR.

If I disconnect one from the PWM the other works fine, and vise verca, just the two together can not work to trig both SSR's.

What is it that I dont understand? - can it be that one SSR sinks the power to such level the other SSR dont have stering power to 'pull' yet lightens the SSR led?

Please advise!

Re: Help with PWM and SSR desperatly beeded

Do you have an oscilloscope? Watch to see whether supply voltage drops at the moments when all devices turn on at the same time.

If you cannot determine which device is at fault, then you may need to explore whether a device is working properly, by testing it alone. You might find certain conditions cause it to perform in ways you do not expect.

- - - Updated - - -

Example, what is minimum supply voltage for the SSR?

What is minimum trigger voltage for the SSR?

Does trigger signal need to be in certain voltage range? Referenced to 0V gnd? Or referenced to supply +?

Is PWM frequency too fast for the SSR?

Does SSR need a minimum load to operate?

Etc.

Re: Help with PWM and SSR desperatly beeded

What kind of SSR? There are many, some (for AC apps)
are opto SCR / TRIAC and these might not share well,
bipolars tend to hog current. Or maybe you have some
kind of deal like first-to-fire gets it all and the second
hasn't got enough headroom left to catch.

An opto MOS type ought to share better (but lot-lot
differences might be significant especially if your drive
is marginal).

Re: Help with PWM and SSR desperatly beeded

I would guess the SSR has an optocoupler input but no current limiting resistor. Try connecting resistors (try 100 Ohms) in series with each optocoupler so it shares the current more equally between them.

I am a little concerned if these are standard SSRs because there is no point in driving them at frequencies higher than the AC, they latch on for the remainder of the half cycle (or full cycle in some cases). If you intend to use the PWM to control AC power through the SSR, I think you will find it difficult or impossible.

Brian.

Re: Help with PWM and SSR desperatly beeded

Solid state relay is just an umbrella term. There are different types, AC and DC relays. A type specification and description of used supply would help.

Re: Help with PWM and SSR desperatly beeded

These are the SSR's
**broken link removed**

I bought from different suppliers on ebay and no, I have n osciloscope.
As said the led turns on fine on both SSR's but only one pulls.

I run the PWM on low frequency- thanks for input

Re: Help with PWM and SSR desperatly beeded

They are intended for powercontrol of two water heaters on 2kW (240AC) each..

Re: Help with PWM and SSR desperatly beeded

betwixt said:

I would guess the SSR has an optocoupler input but no current limiting resistor. Try connecting resistors (try 100 Ohms) in series with each optocoupler so it shares the current more equally between them.

Brian.

Click to expand...

I tried to feed to SSR's via resistors and still same problem, I tried 68 ohm though.

PWM +
|
------ (R1) R68 ----- SSR1 +
|
------ (R2) R68 ----- SSR2 +


1) both SSR diods flashes about 1Hz
2) only one SSR 'pulls' and feed the load to heater element (240 load from 2 different fuses)
3 It is always the same SSR that 'pulls'
4 If I disconnect the SSR input - from the SSR that 'puls' the other SSR immediatly starts to work so both are working

What's next you think?
Increase the resistors?

Any other idea to why this fails?
I would really need this to work!

Thanks for support!

Re: Help with PWM and SSR desperatly beeded

Am I right to assume the AC sides of these SSR are wired independantly and the DC (control sides) share one common connection (GND or 0V on controller) and one control connection (PWM output on controller)?

There is a remote possibility that the AC current through one, if they share a common path, is disturbing the other but it's unlikely. The strange thing is the LEDs still working though as that would normally indicate the SSR was 'on'.

I can only guess there is insufficient current to operate both SSRs but still enough to make their LEDs light up. I sugest an experiment to isolate the problem, keep it wired so one SSR is responding and the other one isn't, then disconnect the DC control signal to the one that is NOT working and connect the SSR input directly to the controller supply. This should make the one you just rewired run constantly and the other one stay under PWM control. If that works, it would indicate the SSRs are fine and you just have insufficient drive current to their control inputs, that should be easy to fix but try the experiment first to confirm my suspicion.

A schematic of how you have it wired might help.

Brian.

Re: Help with PWM and SSR desperatly beeded

These SSRs don't need series resistors, the inputs can be simply parallel connected.

They are full wave switching triac/thyristor switches. A low frequent "PWM" signal, e.g. can be used to vary the output power, the control method is usually named packet switching rather than pwm.

It's not clear why you have problems to enable both SSR, the reason is possibly in the specific wiring of your load circuit. It may be incompatible with zero crossing operation of the SSR. Show the circuit.

Re: Help with PWM and SSR desperatly beeded

betwixt said:

.. I can only guess there is insufficient current to operate both SSRs but still enough to make their LEDs light up...

Click to expand...

Thanks guys (a lot!)

Tried different resistors and with 330 nothing 'pulled' and 249 Ohm the issues was the same as before (one pulled).
noticed in certain combinations that both SSR's failed to bull, especially when one SSR was connected direct to PWM and the other via resistor (just a panic test).

Either way I connected another DC power to PWM (14.4V) and by that powered the fan (computerfan) separatly.
Now - with the 249Ohem resistors both pulled, with 68bOhm only one and 330 Ohm none pulled.

The conclusion is that it seems to work and it was lack of power to PWNin combination with lack of resistors.
Issue seems solved but still dont understand..

Either way - I now realize that my device does not fulfill the purpose.
In my application I have 2 X 2.3kW heating elements for vort boiling, these are on different SSR's and fuses as said above.
I like to control them as when fully on they give 2.3kW but I can regulate down both to where the vort is boling nice - rolling but not pushing..

Now when my system is working can see that I can get only 50% of 2.3kW (at best) so even in full speed it seems that I get 50% loss - can only assume ut has to do with AC cycles..

Is there a better way to regulate heating power than PWM and SSR's?

Perhaps another thread..

Thanks a lot for help!

Re: Help with PWM and SSR desperatly beeded

That shouldn't happen, those SSRs should conduct AC so all the heating power is available. I use similar ones to control flow in large water pumping installations without any problems and apart from a small voltage drop across them, they work much as a mechanical relay would.

I'm coming to the conclusion, based on what you have described, that the problem is lack of VOLTAGE to operate the SSR. I suspect the PWM output isn't able to provide enough voltage while driving two loads. It can be fixed easily with an additional driver stage but before doing that, please try something simpler: try connecting a resistor of no less than 470 Ohms between the PWM output and the supply pin of the PWM board. It will give a little extra 'boost' to the voltage available to the SSR input.

You should note that the PWM board is not really suitable for precise power control to a heating element. Its output will not be synchronous the AC waveform so you can't use it for phase control (gives most precision) and it has no feedback input to maintain the temperature. The best you might achieve is setting it to 1Hz output and adjusting the pulse width to give a limited adjustment of average output power.

Brian.