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Originally Posted by osiyo

Has to be, actually.

Sometimes folks don't realize that in this usage (things wired as in the schematic in prior post) a high impedance coil relay is needed, since they just use matching replacement from manufacturer, or one bought using same part number and supply house provided proper replacement.

But it's simple Ohm's law stuff.

Let assume high impedence lockout relay with 230 ohm coil. And compressor relay with normal coil, with a resistance of 10 ohm. (numbers are imaginary, just being used for example purpose) Control circuit is 24VAC.

In series, that's 240 ohms. 24VAC supply. Amps would equal 0.1. Thus, 23 volts would be dropped across lockout relay coil, and only one volt across compressor relay coil. Lockout will have plenty juice to pull in and stay in, compressor relay isn't even gonna chatter much less pull in.

That's if any of the safeties are tripped.

If they aren't, electrons like path of least resistance. Taking normal control path, with only compressor relay coil in the path (10 ohms), is a lot easier than taking the combined 240 ohms path.

BTW, high impedance relays are also often used in ground fault protection circuits.

I logged in just wanna say thank you to OSIYO for your very detailed and easy-to-understand explanation. Thank you.

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Originally posted by duke of earl
definitely right about the high impedance relay.

Has to be, actually.

Sometimes folks don't realize that in this usage (things wired as in the schematic in prior post) a high impedance coil relay is needed, since they just use matching replacement from manufacturer, or one bought using same part number and supply house provided proper replacement.

But it's simple Ohm's law stuff.

Let assume high impedence lockout relay with 230 ohm coil. And compressor relay with normal coil, with a resistance of 10 ohm. (numbers are imaginary, just being used for example purpose) Control circuit is 24VAC.

In series, that's 240 ohms. 24VAC supply. Amps would equal 0.1. Thus, 23 volts would be dropped across lockout relay coil, and only one volt across compressor relay coil. Lockout will have plenty juice to pull in and stay in, compressor relay isn't even gonna chatter much less pull in.

That's if any of the safeties are tripped.

If they aren't, electrons like path of least resistance. Taking normal control path, with only compressor relay coil in the path (10 ohms), is a lot easier than taking the combined 240 ohms path.

BTW, high impedance relays are also often used in ground fault protection circuits.

definitely right about the high impedance relay.

Originally posted by lenb
Thought it might be worth mentioning that you need to use a relay designed for a lockout circuit, with a high impedance coil.
If you use just any standard relay, it might not work right. Like this one:
http://www.white-rodgers.com/pdfs/04...Cat_pg_123.pdf

This is true,

although I have never had a problem , as long as I made sure the relay coil was tighter than the contactor coil,,but Len is right here, there is a chance that it will not work with normal relays and contactors,

Thought it might be worth mentioning that you need to use a relay designed for a lockout circuit, with a high impedance coil.
If you use just any standard relay, it might not work right. Like this one:
http://www.white-rodgers.com/pdfs/04...Cat_pg_123.pdf

No problem hope it helps,

If you use this on 24 volts, try to use a pilot duty relay,,,

Thank you TitleLess. My memory has been jogged. Been awhile since I had to set one of these up from scratch. My company is in a small city, so I can't afford to specialize--i.e. I wind up wearing a lot of hats--a little refrig, some control work, some heating, some hydronics, some air conditioning, etc.etc.--then when you own the business you are also the boss, employer, occasional bookkeeper, occasional salesman, etc.--the head gets stretched a bit & it is easy to forget a few things........

The lochout circuit stats at 1TB3-5

Ignore lines, 1TB3-4 and -6

Just caught the different voltages, I'll adjust it.

No adjustment necessary if you are using 24 volts contactor,

[Edited by TitleLess on 02-05-2006 at 11:53 AM]

grainger sells all kinds of nifty timers.

Not fully understanding what you want. Do you want it to lock-out on any safty until someone manual hits a reset button? Or do you want it to reset automatically after a determined amount of time?

575V 3PH Compressor
Using 24 volts for controls/contactor/overloads/safeties
120V for crankcase heater only

If noone posts one soon I'll dig up a Trane schematic and cut it out of that, what voltages are you working with ?

Looking for a simple way to set up a lockout/reset on a standalone 20T Carlyle compressor to avoid short cycling on safety controls. Can figure it out myself, but don't need to reinvent the wheel...........Thx