Pro-tips exist to help you improve your performance at work. Sometimes, improving results is as simple as having the right tools.
Everyone taking a simple electronics class has made and built simple RLC circuits. You know what they can be used for and how they can be implemented, but let's talk about some real applications you might run into at a site.
Resistors
Imagine you have a discrete output card that uses a triac to send out 120VAC, like the 1756-OA8. Let's also imagine that you have a set of 120VAC interposing relays you want to use, like the Weidmuller 1123040000. You wire up the card from the 120VAC outputs to A1, then the neutral to A2 and volia- you are done! Everything looks great. Then, you go to testing.
The first relay seems to be fine. It opens when you command it. Heck, some might be. But I bet some of the relays will stay opened (or closed) regardless of your commands. What is happening?
If you take a look at the relay, it probably has a step down transformer on it. The coil is actually 60VAC and your drain current leaking out of your 1756-OA8 is keeping it energized. In this particular case, the relay itself has a LED for status, so you can see just a bit of a glow on the relays, even when they should be dim. What is the solution?
Usually, a 10kOhm drain resistor between A1 and A2 will do the trick. Adding it will immediately make the light on the relay disappear, which is a good sign. You can verify with a meter that the voltage is low. If it is below the threshold for the coil, your card will start working again. What happened? You gave the electrons a path to go away, de-energizing the coil.
If you are going to do this, make sure you have some fairly beefy resistors on hand. I like the 3W ones and try to keep some TE ROX3SJ10K nearby at all times for this purpose.
Capacitors
If you have a chattering relay circuit, you may want to take a look at adding in a simple RC filter. An on-delay or off-delay relay is a better long-term solution, but a band pass filter may well get you through the night.
Please note: Weidmuller makes relays with the step-down transformer but with a signal filter for this purpose. Do you want to hazard a guess on what that filter is?
Inductors
I am still looking forward to a use case for having these on hand. If you have one, let me know!
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