Thanks for the info everyone. I have been spending a lot of time googling about Triac based SSRs. I didn't find much information about stacking SSRs in parallel. Though I already knew MOSFETS are fine in this mode.
By what Jon (or John?) says my spindle switcher is going to go off like a bunch of Greyhill firecrackers as each SSR is blown in turn! (That's if they blow open circuit, which my googling says most SCR/Triac based ones blow short circuit.) So I am shelving it. I will use a regular relay probably. I really don't want to use a rotozip or home depot quality trim router as the run out on most of these things are horid. Eventually I will spend the dollars for a real spindle, probably used or refurbished off ebay. If I had a good spindle head I could probably make the controller. I used to make digital servo controllers, these replaced the electronics in hobby RC servos for robots using I2C for communication. My website at www.colinmackenzie.net (very outdated) has details. Mike Thompson then took my design and revamped it into openservo (www.openservo.com), and it now has an active forum of developers. These open servos are great for small scale robotics. Colin > John Kasunich wrote: >> Colin MacKenzie wrote: >>> I have a simple relay board I made using >>> grayhill solid state relays. These relays are opto-isolated. >>> Because my >>> relays are 3A 140v rated and my rotozip is 5am motor, I used 3 of >>> them >>> in parallel to comfortably run the rotozip with lots of amp >>> headroom. >> >> >> Not sure solid state relays can be paralleled or not. I bet if you >> measured the current through each one you'd find it is NOT evenly >> distributed. Might even be flowing through only one of them. A 3A >> relay will handle "5A" at least for a while, and under light load the >> rotozip probably draws less - maybe even less than 3A. So you >> might be >> running on one relay right not and not know it. > Colin, > > When paralleling transistors you have current sharing problems, > and need to use ballast resistors. Assuming these SSRs are > SCR/Triac type, due to the nature of those devices, I suspect > one relay is ALWAYS carrying all the current. Unless you use > significant ballast resistors in series, I can almost guarantee > that one relay has the lowest on voltage, and the other two drop > out as soon as that one turns on. You are living on "borrowed > time", and one of those relays will almost certainly short out > eventually. I'm just amplifying what John is saying here. > > If in doubt, do a little google searching about paralleling > SSR's, I'm sure anyone who knows says it won't work. > > Jon > ------------------------------------------------------------------------- Check out the new SourceForge.net Marketplace. It's the best place to buy or sell services for just about anything Open Source. http://ad.doubleclick.net/clk;164216239;13503038;w?http://sf.net/marketplace _______________________________________________ Emc-users mailing list [email protected] https://lists.sourceforge.net/lists/listinfo/emc-users
