Riley & Associates, Inc.

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Monorail Control Products

Triac Leakage

Most all devices which use a Triac use a snubber to protect the Triac from voltage spikes which are generated by inductive loads.  When designing a Triac output there is a tradeoff between how expensive the Triac is and how large the snubber must be to protect the Triac.  

Newer Allen Bradley PLC output cards (and perhaps other vendors as well) have higher leakage than older models and in some cases they may require additional load to bleed off the leakage caused by the snubber circuit on their cards.  While it is possible to disable their snubber circuits, we do not recommend this because it will void their warranty and also because doing so will create an inventory problem in the plant in that outputs so modified will no longer be 

There is some leakage caused by the snubber circuit and if the leakage is a problem and the load is not inductive you may disable the snubber on our JNF-078 boards by cutting the leads of R6 and R7.  

Alternately, you may add load to the output by installing a resistor across the load.  Some experimentation will be necessary to determine how much load is necessary to bleed the leakage from the snubber off.

The test is done with a full wave signal applied to the load.  Do not try to do the test with half-wave signals.

You will need the following:

A meter which will read RMS.
Two 10K 5 watt resistors.
Two 5K 10 watt resistors.
Two 2.5K 20 watt resistors.

Start with a 10K 5 watt resistor (dissipates 1.6 watts), then try a 5K 10 watt (dissipates 3.2 watts), 2.5K 20 watt (dissipates 6.4 watts).    After adding each resistor, one at a time, measure the voltage across the load.  It should be less than 55 volts with the Triac off.  If one of the above values does not do the trick then use them in combination.  

Do not put more load on the circuit than necessary to achieve 55 volts when off, or more load than necessary to keep the half wave decoder board from falsely turning on.  Doing so will result in much more heat being generated in the enclosure.

Power resistors are designed to run very hot.  To keep the surface temperature down we always de-rate them.  This is why the above units are only being run at 40% of their rated wattage.  Even at 40% they will burn unprotected skin ! ! 

Remember that the heat from a bank of these will build up in an enclosure!  Size the enclosure accordingly, or put the resistors in a small auxiliary enclosure.  Consider venting the enclosure.

When installing these resistors in banks it is recommended that they be suspended between two terminal strips and that the leads NOT be cut shorter since the long lead dissipates heat and also keeps the terminal blocks at a lower temperature.  In other words space the terminal strips far enough apart that the resistors can be installed with a full lead and no bends in the lead.


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Page last revised 7-24-2001
Riley Home Page


Ronald J. Riley
1323 West Cook Road
Grand Blanc, MI 48439
Phone (810) 655-8830 - Fax (810) 655-8832

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