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Need help with Solid State Relay

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Saz43

Fri Apr 22 2011, 05:02AM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

I bought some of these SSRs:

I'm using them in a typical application to switch a capacitive load with 115VAC. The only problem is, I have no idea how to drive one of these. I thought it was as simple as applying a voltage across pins 3 and 4 at the appropriate current, but this has no effect.

Thanks for any help!

Adam Munich

Fri Apr 22 2011, 06:26AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Basically you are lighting up an LED inside the relay and that triggers the mosfets or whatever they are using in that one. Did you fry the LED with too much current?

Sulaiman

Fri Apr 22 2011, 08:37AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Test the device out of circuit,
drive the diode with 10mA minimum, 100 mA maximim
(e.g. 9v battery + 560 Ohm = (9-1.2)/560 = 16.6 mA)
a 9v battery connected to the diode the wrong way around could damage the diode as the maximum reverse input voltage is 5V.
Externally it still measures like a diode but the output will not change....

so, two warnings, too much forward current or reverse voltage may kill the diode.

Saz43

Fri Apr 22 2011, 02:22PM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

Thanks for the replies. I applied 12V to the gate with proper polarity and 1 1k resistor (12mA) and again with a 150 resistor (80mA). I also tried directly connecting the gate to the output of a LM311 comparator, which can only source 6mA. SOMETHING I did was wrong, cus now when I use my multimeter to diode test the gate, it gives a different number than the other 3 identical relays I bought which I haven't used yet (and a non-zero number in the reverse direction). Hmmmmm........

radiotech

Sun Apr 24 2011, 06:40AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

Please show a schematic of the circuit you are switching.

doctor electrons

Sun Apr 24 2011, 06:34PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

Some of those ssr's are load dependent. I had an engineer at work who was having an extremely hard time trying to
figure out why his relays were not working on his control board. Turns out, the load was not resistive enough to make
it operate properly. This may not be the problem in your case, i don't know for sure, hook one up on the bench and
try switching a few different less capacitive loads. Your solution may simply be adding a resistor in series with the output
of the form a. I have also seen some ssr's that require a voltage present at the contact in order to switch. I guess the
voltage is somehow used internally to operate the device. Apart from the control voltage that is. (The dc in this case,
or what would be the coil on a conventional relay)

Steve Conner

Mon Apr 25 2011, 07:04AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

That is true for the usual kind of SSR which has a triac or pair of SCRs inside. It needs a minimum voltage and current to latch it, and looks like a couple of diode drops.

But these are supposedly MOSFET ones, which should work for any voltage AC or DC, and look just like regular switch contacts. Unless you already blew the LED.

One thing to bear in mind is that MOSFET SSRs are very slow compared to directly driven MOSFETs. The LED shines on a tiny stack of solar cells that charge the FET gates, and the current is minuscule. If you're driving one with pulses, the result will be nothing like what you were expecting. I don't know if this has anything to do with your problem, though.

doctor electrons

Mon Apr 25 2011, 10:46PM

Registered Member #2390 Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381

Steve raises a good point! If you were to drive a capacitive load with a pulse at the mosfets gate you could run into time issues.
If the pulse were to stop before the capacitor has a chance to reach its respective charge, the gate would open before it would allow
an output. (at its rated voltage anyway.) Sounds like it would be an even bigger problem with the type you have. If Steve says theyre slow,
i believe him! Keep in mind the time involved could be in the uS, but that can sometimes be enough to cause a problem.
If the ssr is in your circuit when you test it, maybe you dont have the proper voltage or current at it? Something to check out.
Please do let everyone know what you find out, i know it will be tickling my brain until you let us know what happens!!
(what i mean by output above is that the gate would open before the cap reaches its rated voltage, perhaps causing improper operation
of the components downstream. I dont know exactly what your circuit looks like.)

Saz43

Tue Apr 26 2011, 05:33AM

Registered Member #1525 Joined: Mon Jun 09 2008, 12:16AM
Location: America
Posts: 294

My circuit is simply a voltage doubler, C2 is the load (200uF lytic). The SSR just sits between ground and D1.

You're right, the SSR I'm using is VERY slow, measured in ms in fact! But that's okay, since I'm driving it at about 15Hz maximum.

You all offered valid theories as to what was wrong, thanks for the input! But I'm going to chalk this one up to Circuit Gremlins. I did the same thing again today with a fresh but identical SSR out-of-circuit, and it worked perfectly. Put it back into the circuit, and it works perfectly. The SSR gate is driven by the output of an LM311 comparator, with 12V supply and a 1k pull up resistor. The output goes through a 1k gate resistor and an indicator LED and then into the gate of the SSR. Now it switches my voltage multiplier on and off without a problem.

Only problem now is the capacitor is charged up by the isolated AC supply and has no common reference to logic ground, so I have no idea how to get a comparator to read its voltage in order to control the turning on and off.

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