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Old 05-25-2010, 02:49 AM   #1
Jan 2010
Roswell, GA
Posts: 6

It has been a long time coming but I am finally putting together a controller and building a RIM unit. I have gotten everything put together but think I may have picked up the wrong SSRs. I currently have a Watlow 988 which the following output specs:
"Output Options
Solid-state relay, 0.5A @ 24V (ac) min., 253V (ac)
max., opto-isolated, burst fire. With or without contact

I also picked up a couple OPTO22 SSRs which has the following specs:

Nominal AC Line Voltage
Nominal Current Rating (Amps)
1 cycle Surge (Amps) Peak
Nominal Signal Input Resistance (Ohms)
Signal Pick-up Voltage
85 VAC (280 allowed)
Signal Drop-out Voltage
10 VAC
Peak Repetitive Voltage Maximum
Maximum Output Voltage Drop
1.6 volts
Off-State Leakage (mA) Maximum
7 mA
Operating Voltage Range (Volts AC)
I2t Rating t=8.3 (ms)
Isolation Voltage
?jc* (C/Watt)
Dissipation (Watts/Amp)

I am planning to use the SSR to control a LD 5500w/240v element running on 120v drawing ~12 amps.

My question is if the controller can switch the SSR since the signal voltage exceeds the output voltage?

Any info would greatly be appreciated.

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Old 05-25-2010, 03:41 AM   #2
Senior Member
Dec 2006
Turner, Oregon, Oregon most of the time now
Posts: 2,305
Liked 54 Times on 47 Posts

After looking at the controller it would seem that the open collector DC control would be used with a small dc power supply (wall wart) to fire the Opto 22 SSR's. It looks like the built in SSR is meant for AC loads as there is no DC load information for that output, and the OPTO 22 SSR's probably would not appreciate AC input to DC control side of devices.

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Old 05-25-2010, 03:36 PM   #3
May 2010
Temecula, CA
Posts: 1

The Opto 22 SSR you picked switches AC to the load and it is controlled by an AC voltage, so you are OK there. Your load won't draw a lot of current, so the relay should also be fine on that account.

The possible problem is that the controller specifies a Minimum amount of load current; it is intended to drive the coil of a mechanical contactor directly. I'm afraid that when the controller is Off, the 'leakage' current through the output 'contacts' of the Watlow will be enough to keep the Opto 22 SSR turned on.

If that happens, you can solve the problem by placing an additional load across the SSR control terminals to make it 'appear' to require more current. Place something like (close to in value) a .1 microfarad, non-polarized capacitor with a voltage rating of at least 300 volts between terminals 3 and 4 of the Opto 22 relay, and everything should be good.

Good Luck!

Dr. Tom (Opto 22 Sr. Tech Advisor)

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Old 05-26-2010, 02:26 AM   #4
Jan 2010
Roswell, GA
Posts: 6

Dr Tom,

Thanks for the input sounds like I still might have a chance. In fact that was one thing that was confusing me, the circut seemed to always be on.

If one were interested in buying said ....
Originally Posted by DrTom View Post
a .1 microfarad, non-polarized capacitor with a voltage rating of at least 300 volts
...where would one shop? Are these close to what you referred?

Dayton DFFC-0.10 0.10uF 400V By-Pass Capacitor

Dayton DFFC-0.22 0.22uF 400V By-Pass Capacitor

I realize these are not polarized, but the polized ones I found were only up to 100V

2.2uF 100V Non-Polarized Capacitor

Thanks for your help. I may get this working despite me forgetting all my electrical learnings as limited as they may be.

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