djk Posted October 21, 2004 Share Posted October 21, 2004 "The capacitor is placed in parallel with the switch contacts, and the idea is that the capacitor opposes any change in voltage" Sorry, no. When the switch is closed the cap is shorted out. When the switch is opened the cap begins to charge and the load current now flows through the cap. When the cap is charged it acts like an open circuit. The cap simply gives time for the switch contacts to open far enough that the inductive kick back from the load does not arc across the switch contacts. This is the same way the points and condenser worked in your pre-electronic ignition car. If your condenser was bad you got noise on your radio and your points burned. Most Japanese receivers in the 70s had a 0.1µF 600V film cap with a 100 ohm resistor in series with it, this was in parallel with the switch. The size of the required cap depends on large part on the current in the circuit, and how fast the switch contacts can open. Quote Link to comment Share on other sites More sharing options...
RIklipsch Posted October 21, 2004 Share Posted October 21, 2004 ---------------- On 10/21/2004 6:30:28 AM djk wrote: When the switch is opened the cap begins to charge and the load current now flows through the cap. ---------------- sorry, no. No current ever flows through the cap,(except leakage current) unless it's faulty. Current may flow in the *circuit*, but never through the cap itself. A better place to install the supression capacitor is in parallel with the power transformer primary. IT will absorb the counter-EMF created by the inductive kick-back of the trans primary. I do not like to place capacitors in parallel with a switch, as AC voltage is present across the power trans primary. While the current is very low,(leakage current) it can still give you a "tingle" should you come into contact with it. When I turn off a power switch, I want complete isolation from the AC mains. -R.Inman Quote Link to comment Share on other sites More sharing options...
djk Posted October 23, 2004 Share Posted October 23, 2004 What a pedant! Do you obsess about conventional vs electron flow too? I prefer a 150V MOV to a cap across the primary. Quote Link to comment Share on other sites More sharing options...
RIklipsch Posted October 23, 2004 Share Posted October 23, 2004 ---------------- On 10/23/2004 3:38:47 AM djk wrote: What a pedant! Do you obsess about conventional vs electron flow too? I prefer a 150V MOV to a cap across the primary. ---------------- No, I don't obsess about conventional VS. electron flow, I could care less! MOV would work great, *for a while*. It can only take so many "hits" before it goes open though. I guess its life is a function of the current rating on the primary, and how much counter-EMF is "reflected" back on the MOV at turn-on. I'm no MOV specialist, and really can't comment on lifespan. 1000V ceramic disc will get the job done and last longer. If you look at tube products from the 50's and 60's, they all used these ceramic caps, and they are still good (I never had to replace one). -Ryan Quote Link to comment Share on other sites More sharing options...
Erik Mandaville Posted October 23, 2004 Share Posted October 23, 2004 Would you two please play nicely! Quote Link to comment Share on other sites More sharing options...
djk Posted October 24, 2004 Share Posted October 24, 2004 "MOV would work great, *for a while*. It can only take so many "hits" before it goes open though. I guess its life is a function of the current rating on the primary, and how much counter-EMF is "reflected" back on the MOV at turn-on. I'm no MOV specialist, and really can't comment on lifespan." MOVs have energy ratings. They need to be selected for the application. Back EMF is finite and can be measured. The failure mode you describe is from line surges, which can be infinite. Your cap will offer no protection either to a big enough surge on the line. A big enough surge on the line can destroy your equipment even if the switch is 'off'. Quote Link to comment Share on other sites More sharing options...
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