Hi, I'm switching an inductive load. Let's say a DC motor (20 A continuous current, 40 A peak surge) is going full on in one direction. I want to shut it off. Its power circuit is switched by the contacts of a very large electromechanical relay. Two questions: 1. First of all, I want to dissipate all of that magnetic energy safely without my control circuit facing an enormous back EMF. So I was thinking of putting a flyback diode in parallel with the load. Something like this: http://www.nteinc.com/specs/6000to6099/pdf/nte6013.pdf Is this a good idea? 2. I want to prevent arcing across the relay contacts. I came across this ridiculously old (1957) paper on the subject. Heh, it even predates the IEEE. http://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=01135909 I'm a bit confused by what they're saying. On the front page, they say that a certain voltage is required for arcing, because the contact material has to be vapourized and ionized. But a bit further down they contradict themselves and suggest that it is in fact dV/dt that matters more. Their first plot clearly shows that a set of contacts protected by a capacitor reaches a higher voltage than an unprotected set, however it rises much more gradually. So which is it??? Regardless of the answer, I know I need a capacitor across the relay contacts in order to give the current somewhere to go as the contacts are opening. However, I'm not sure how big it needs to be. Also, the paper suggests that a resistor should be put in series in order to reduce the discharge current of the capacitor that, when combined with the load current (that is re-established when the contacts are closed again) might tend to weld the contacts together. However, it also points out that resistors are not very good, and that diodes might be better. The paper doesn't talk about inductive loads at all. Anyway, my second question is actually two questions: How big a capacitor do I need, and how do I make this determination? Does anything need to be in series with it? Here is the relay I am thinking of using: http://www.kgtechnologies.net/products/100.html [Broken] I would appreciate any insights those of you who are experienced in such matters may have.