Output Style triac to SSR

  • Thread starter Hugo Magallanes Gonzalez
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Hugo Magallanes Gonzalez

I have a problem in output module from GE Fanuc, I need energize a single phase Eurotherm SSR, the output module have output circuit style Triac, the SSR work with 100Vac and output module work with 115Vac, the problem is that with the output circuit disabled (output OFF) the SSR is enabled (go to ON state), can I put a device that avoids this problem?, the output module is IC693MDL330. Thank you. Hugo Magallanes

Steve Bailey

Try reducing the supply voltage for the output circuit group. The rated range for that module is 85 to 264 volts. The leakage current is 3 mA at 120 volts. The minimum recommended load current is 100 mA. If the SSR is less than this, try a resistor in series with it. Make sure the resistor is high enough wattage. All of this information is from the I/O module Specifications manual GFK-0898.

Todd Goodman

Hi Hugo. I have worked in another facility where this problem has occurred. The problem is not with your Triac Output Card, it is with the SSR. Eurotherm SSR's trigger on very low voltage so it is necessary to put a dropping resistor across the control side of the SSR. You will have to measure the voltage across the SSR and calculate the resistance value so that it is less than 2 or 3 volts. I don't remember exactly what the value was, but I am thinking it was 1k or 2k ohms. I wish you the best!!
If I understand the problem, it seems leakage current through the PLC output TRIAC is enough to turn on the Eurotherm SSC even when the PLC is in the off state. This is a very common problem when interfacing TRIAC outputs to relatively high impedance inputs on SSCs, and other similar equipment. There are two tried and true work-arounds: 1. Add an interposing relay. In this case, it would be whacky to add a mechanical component with a high but fixed number of operational cycles into this application. 2. The favored option is to add a burden or 'swamper' resistor in parallel with the load sized in resistance to a value capable of bringing down the voltage resulting from the leakage current, but not so low as to create undue heat generation. For instance, here's a sample of burden resistors and current flows and wattages resulting from a 120V source. 3.3K ohms - 36 mA - 4.4W (use 5W resistor) - lots of heat generated 10K - 12 ma - 1.44W - (use 2 watt resistor) 20K - 6 ma - 0.72W (use 1 watt resistor) - works in just about any situation 33K - 3.6 ma - .44W (use 1/2 watt resistor) 47K - 0.26 ma - 0.31W (use 1/2 watt resistor) - might not swamp the leakage enough for proper operation How much resistance is required depends on the amount of TRIAC leakage. I prefer to place a resistance decade box across the load (in this case, the Eurotherm SSC) with the PLC output in its off-state, and measuring the voltage acoss it with a meter while switching in progressively less resistance. Eurotherm probably specifies a minimum turn-on voltage for the SSC - likely somewhere between 30 and 50 VAC - so I'd find out what resistance was necessary to drop the voltage to about half that value (in order to give some 'headroom'). Generally, newer generation TRIACs have less leakage than those produced, say, 20 years ago, and I've been able to get away with 47K resistors in some applications, although almost any present generation device should be sufficiently burdened using a 20K resistor. The tradeoff in using lower value resistors is greater heat generation, which might become a concern if there are a large number of output points that must be burdened. For instance, if there were 16 identical output points burdened using 20K resistors then an additional 11.5 watts of heat would be generated, but using 3300 ohm resistors would yield a whopping 70 watts of extra heat! The other consideration relates to how much current the PLC board can source - (16) 3300 burden resistors would draw almost 580 ma through the board (depending on how the common circuits are arranged) while 20K resistors would add only about 100 ma above that to turn on the SSCs.

Jeffrey W. Eggenberger

Try running your output to a relay, and have the relay contacts switch your device. Jeffrey W. Eggenberger Electrician: Industrial, Commercial, and Residential
Hugo This problem is very common on AC output modules due to the use of an RC (Resistor/Capacitor) bypass across the triac. With AC - the capacitor conducts a small amount of current, your SSR only requires a small amount of current to activate - and therefore stays on. One solution would be to add an extra load resistor at the SSR - You should carefully consider the safety aspects to this - if your extra load fails - the SSR WILL TURN ON. In many applications this is not a desirable action. Another solution is to add extra an series resistor to reduce the leakage current to an amount that is below the "switch-on current" of the SSR. You could measure the existing leakage current (on several different outputs), then bench test a few SSRs to find their switch-on current. I would suggest that you aim for a leakage current that is below 1/4 of the switch-on current of the SSR. A more elegant solution is to use a DC output module driving a DC input SSR. You will not have any leakage current problems with the DC modules. SSRs (20amp..60amp) are available for under $50. regards, Phil McKay TRESCO INTERNATIONAL (AUST) PTY LTD Silvan, Vic, Australia [email protected]