Friday, December 10, 2010

A threaded discussion I liked.

Khan Alim 7 Dec 10 12:24 PM MST

Discuss how solid state devices can work as mechanical relays.

Nicholas Clay 8 Dec 10 8:08 PM MST

In a normal mechanical relay a coil is charged which physically moves component to make a contact. There are downsides to these types of devices, one that i know of is called arcing, this happens when too much current is applied, there is also the fact that mech relays have a shorter life and make a clicking noise.
Non mechanical relays or solid state relays normally use photo coupling, this is done by illuminating an LED that acts as the bias for a photodiode which controls the actual switching.

Jerome Munroe 10 Dec 10 9:19 PM MST

In a solid state relay the photo diode and led physically separate the triggering signal from the power signal. These have all the advantages that Nick mentioned. They are even becoming viable in high power situations where mechanical relays were thought to be a necessity. There is one thing that has to be considered when using a solid state relay. It is that even the most efficient ones have a voltage drop across them. This is a lot like the voltage drop from the collector to the emmiter of a transistor in saturation. In most cases this can be thought of as being directly connected, but there is about a .2 volt drop present.

Jerome Munroe 10 Dec 10 10:56 PM MST

I was just thinking about another disadvantage of using a mechanical relay. There is a delay from applying power till when the switch would flip over to the N.C. connection. This might not be a problem in a lot of analog circuits that could just wait until the switch flipped over, and then would not know the difference. But in a system where the times between millionths of a second are noticed and measured, these things matter. In these cases, the speed of the silicon in a S.S. relay and the speed of the silicon in a Microcontroller are a much better match.

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