permitivity Posted September 26, 2006 Report Posted September 26, 2006 Hello, I have a really long 24VDC guard door circuit, and I went and calculated how much votlage drop there would be and whether there is still enough voltage at the end of the run to pull in a relay. Could someone look at the why I calculate this to see if it's right? Run: 1900ft of 18 guage wire: <0.00751ohm/feet * 1900ft = 14.3ohms> Relay: 24VDC coil, rated at 6.5W power consumption, so that's about 0.28A. So coil has 24V/.28A=86ohms Total resistance: 24V/ (14.3ohm+86ohm) = .24 amps. So I can expect .24amps on the wire, which is slightly less than the .28A needed to pull in the coil. Does that mean I wouldn't be able to use this relay, or might have to go 120VAC? How much wiggle room is there on these ratings? The relay doesn't list a minimum, simply the rated coil energy. Thanks, Quote
Snerkel Posted September 27, 2006 Report Posted September 27, 2006 You should have taken your math one step further, the amount of voltage across the relay will be a tad over 20v, check the relay rating for the minimum pull-in voltage, if this is <= 20v then you should be OK Quote
JesperMP Posted September 27, 2006 Report Posted September 27, 2006 What safety relay are you using ? Most modern safety relays have discrete inputs that draws very little current. Quote
Ken Moore Posted September 27, 2006 Report Posted September 27, 2006 Another possible solution: Most of the newer switching power supplies, can be adjusted up to 28 VDC. Buy a dedicated power supply just for this circuit, and adjust the voltage up until the relay functions as desired. Quote
ssommers Posted September 27, 2006 Report Posted September 27, 2006 Another possible solution: Change the wire size from 18 to 16 awg and drop the resistance to about .005 ohms/ft. Quote
permitivity Posted September 28, 2006 Author Report Posted September 28, 2006 Thanks for the replies: 1. I can not up the voltage of the power supply as it is being used for other things too. 2. The relay is a AB 700S relay. It has the smallest power consumption compared to other industrial relays. 3. I'm using a cable, so the gauge is not easily changed. 4. Weird, but the spec sheet on the relay doesn't list minimum pull in voltage - only power consumption. Is there anything wrong with my math? Quote
ssommers Posted September 28, 2006 Report Posted September 28, 2006 (edited) Just to make sure I've got the right spec sheet... Are you using a 700S-CF? If so, the Operating Voltage for 24VDC is: Pick-up = 16.8 to 30 VDC (see note 1 - 0.7...1.25 x Us) Drop Out = 14.4 to 2.4 VDC (0.1... 0.6 Us) I found this info right above the Coil Consumption rating. HTH! Susan Edited September 28, 2006 by ssommers Quote
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.