murray@sun13.scri.fsu.edu (John Murray) (04/01/91)
Howdy, folks. I'm in the process of doing some cheap-and-cheezy design work as the preliminary stage of a hobbyist project, and I'd like to get some typical characteristics of mechanical relays. Application: High-current (peaks may be around 40V, 20A or more) switching of the magnetics on a coil gun... :-) Why relays, and not SCR's or something? I've got some relays sitting around unused, (take-outs from surplussed equipment) and swallowing the cost of a couple of pounds of magnet wire is going to be tough enough by itself. Besides, it's more challenging this way! And think of the great noises! (open-frame style relays..) So, specifically, I'd appreciate any information on typical switching times of relays. I assume times would be in the milliseconds range, is this about right? Also, which is typically faster, turning on (energizing the coil) or turning off? I know this info is entirely dependent on the specific relay, I'm just looking for ballpark figures (so far..) (Anybody have any handy hints about driving relay coils with phototransistor/led pairs as detectors?) -- *Standard Disclaimers Apply*| ---Get Out Of HELL Free!--- John R. Murray |The bearer of this card is entitled to forgive murray@vsjrm.scri.fsu.edu |Himself of all Sins, Errors and Transgressions. Supercomputer Research Inst.| -- D. Owen Rowley
mcovingt@athena.cs.uga.edu (Michael A. Covington) (04/01/91)
A nice way to measure relay switching time is to hook the relay up as a buzzer, i.e., use a normally closed pair of contacts in series with the coil, and apply power. It will switch itself on and off at a particular frequency (typically 500 Hz), which gives you a measure of the switching time. You can also see how switching time varies with voltage. Most relays will work at considerably less than their rated voltage. Also, the voltage necessary to hold the relay in the energized position is less than the voltage needed to pull in the contacts from the un-energized position. -- ------------------------------------------------------- Michael A. Covington | Artificial Intelligence Programs The University of Georgia | Athens, GA 30602 U.S.A. -------------------------------------------------------
rainer@boulder.Colorado.EDU (Rainer Malzbender) (04/01/91)
In article <2640@sun13.scri.fsu.edu> murray@sun13.scri.fsu.edu (John Murray) writes: > >Application: High-current (peaks may be around 40V, 20A or more) switching >of the magnetics on a coil gun... :-) > Hmm...will that be enough ? (Don't feel like doing a Jackson problem right now :-). For reference, the power requirements of the second Princeton-MIT mass driver design (a = 5000 m/s^2, vmax = 112 m/s, bucket mass = 1kg)) were V = 668 volts, Imax = 6315 amps, di/dt = 58.6 amps/microsecond. Run that through your average mechanical relay! In contrast the first one had an acceleration of 33 g's and used mechanical switches to trigger the coils, so maybe it's doable. I know there's been a magnetic-weapon discussion here before, and I don't want to reopen it, but the Germans had trouble with electromagnetic cannon during WWII due to melting projectiles. I don't know anything about coil guns, but their efficiency must be a lot worse than mass drivers, which have pretty good coupling between drive and bucket coils. Maybe you could write a Popular Electronics article for the DIY crowd once you build it ?? -- Rainer Malzbender Save a dinosaur - buy DEC. Dept. of Physics (303)492-6829 U. of Colorado, Boulder rainer@boulder.colorado.edu 128.138.240.246
murray@sun13.scri.fsu.edu (John Murray) (04/02/91)
In article <1991Apr1.004857.17286@colorado.edu> rainer@boulder.Colorado.EDU (Rainer Malzbender) writes: >Hmm...will that be enough ? (Don't feel like doing a Jackson problem >right now :-). For reference, the power requirements of the second >Princeton-MIT mass driver design (a = 5000 m/s^2, vmax = 112 m/s, bucket >mass = 1kg)) were V = 668 volts, Imax = 6315 amps, di/dt = >58.6 amps/microsecond. Run that through your average mechanical relay! Well, the only reason I'm using 40V is my 28V (adjustable 20-40V) 10A (continuous) heavy duty lab power supply. Built for NASA, you could throw it off a bridge with impunity. I thought about a railgun, (better for low voltages) but was discouraged by the sliding-contact problem. >In contrast the first one had an acceleration of 33 g's and used mechanical >switches to trigger the coils, so maybe it's doable. I know there's been >a magnetic-weapon discussion here before, and I don't want to reopen it, >but the Germans had trouble with electromagnetic cannon during >WWII due to melting projectiles. I don't know anything about coil guns, >but their efficiency must be a lot worse than mass drivers, which have >pretty good coupling between drive and bucket coils. Efficiency is not what I'm worried about. This is pretty much an "Amaze Your Friends!" (and amuse myself) project. Also, I guess I could just build half of a mass driver and toss the bucket out of the thing - technically still a coilgun, I guess, since I wouldn't bother decellerating the bucket. Thanks for the hint about how Survival Research Labs did their 200mph molten-iron thrower ;-) >Rainer Malzbender Save a dinosaur - buy DEC. >Dept. of Physics (303)492-6829 -- *Standard Disclaimers Apply*| ---Get Out Of HELL Free!--- John R. Murray |The bearer of this card is entitled to forgive murray@vsjrm.scri.fsu.edu |Himself of all Sins, Errors and Transgressions. Supercomputer Research Inst.| -- D. Owen Rowley