Lee Hart wrote:
The simplest way to get a short motor starting delay is to add a small relay ... Make sense?
Michael K Johnson wrote:
Yes. Thanks! That's a much smaller capacitor than trying to debounce across both the contactor coils, which was what I was vaguely thinking of last night. I ordered 8 of the 4 relays I needed for my zener-based battery DoD indicators: http://www.excesssolutions.com/cgi-bin/category.cgi?item=ES3735 on the theory that I'd find other uses for them... Since it's a 48V relay I should be able to skip the series resister to the relay coil.
These look like acceptable relays for the application. I doubt that your 48v contactor coils will draw over 0.6 amps.
Note that the R in series with the relay coil for the time delay function is still necessary. This R, together with the Capacitor across the relay coil is what provides the RC time constant to delay the relay pull-in.
Relays are normally guaranteed to pull in at 75% of rated voltage; that's 36v for these 48v coil relays. So you can still have a resistor in series that drops 10 volts. That will give the capacitor something to work with.
I'd be more inclined to use a small relay with a 24vdc coil, so the resistor could be larger, and the capacitor smaller.
One other thing: When you're using relays as analog devices with slow coil ramp-up and ramp-down voltages, the contacts open and close slower. So you want to derate the contacts; use them at (say) half their rated voltage and current to give yourself a safety margin.
The SB175 is rated 280A continuous. If I damage the contacts pulling it apart because it is pulling more than 175A I'll be happy that I pulled it apart.
Indeed! Note that they make a nice arc-n-spark show when you pull them apart at high current and voltage. Don't have anything flammable in the path. You also don't want to slowly "tease" the contacts apart, or it can arc for an extended period and damage the contacts.
Anderson build an emergency disconnect with these contacts. It had a big red button that you pressed down (hard) with the palm of your hand to compress a spring and push the contacts together. When you pushed the button to release, the spring opened the contacts *FAST* to minimize arcing.
-- "Obsolete" means nothing more than "the salesmen would prefer you buy something else". -- Dave McGuire -- Lee A. Hart, http://www.sunrise-ev.com/LeesEVs.htm _______________________________________________ UNSUBSCRIBE: http://www.evdl.org/help/index.html#usub http://lists.evdl.org/listinfo.cgi/ev-evdl.org For EV drag racing discussion, please use NEDRA (http://groups.yahoo.com/group/NEDRA)
