[AMRadio] Relay Voltage ID

JAMES HANLON knjhanlon at msn.com
Sat Jul 4 16:08:01 EDT 2009


It sounds like you could use this relay to fill in for the non-working contacts on your existing relay.  You just need to find out what operating current it needs and then configure the circuit in the 32V2 to feed it properly.

I say "current" because relays, being electromagnetically driven, are physically operated by the number of "ampere turns" in their magnets.  Of course Ohms Law still applies, so you can measure the operating voltage too.

What you want to do is to measure the voltage necessary to cause the relay to close.  Ideally you would do it with a variable power supply, connecting it across the relay coil and bringing it up slowly until the relay closed.  This would be the "just operate voltage."  Normally one runs a relay a bit higher in voltage/current than the "just operate" point, 1.5 to 2 times being typical.  So a 48 volt relay would have a just operate voltage in the 24 to 30 volt range.  

A variable supply can be made quite simply.  A Variac feeding a half-wave silicon diode rectifier with a filter capacitor across the output would be fine.  Even a selection of decent power rated resistors in series with the 120 volt line into the rectifier/filter would give you a few trial points - but watch out not to electrocute yourself of course.  

If you can't come up with a variable supply, you can always just see if the relay will work with rectified and filtered DC from the 120 volt line on it.  If it works, measure the voltage across the relay coil at that point and use that as your target voltage when you put it into the rig.  

You could even borrow some current from the +240 volt supply in the 32V2, putting some trial resistors in series with the relay coil.  A series resistor in the 10 to 20 K range would be a good place to start.  

If you wind up with 110 VDC across the relay coil for its normal operating condition, you would be dissipating just slightly more than 1 watt in the coil which should be no problem for it at all.  That would correspond to an operating current of 9.4 ma.  

You told me that your existing K301 relay has a 3K coil resistance, and it operates on a nominal 48 volts or on 16 ma.  

If you wanted to go easy on the +240 volt supply in your 32V3, you could revise the relay feed resistors into a series-parallel arrangement.  I'll send you a copy of my calculation page, but it amounts to Using the existing 5K, 10 watt R309 and acquiring an additional 3.9K, 3 watt resistor.  You will wind up pulling virtually the same current from the +240 supply to drive  both relays.

Jim, W8KGI

Date: Fri, 3 Jul 2009 14:40:36 -0700 (PDT)
From: Robert Lawson <w4rl at bellsouth.net<mailto:w4rl at bellsouth.net>>
Subject: [AMRadio] Relay Voltage ID
To: johnson at mailman.qth.net<mailto:johnson at mailman.qth.net>, AMRadio Reflector
<AMRadio at mailman.qth.net<mailto:AMRadio at mailman.qth.net>>, samrc at mail.wdavis.net<mailto:samrc at mail.wdavis.net>
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I've a Potter & Brumfield KR-2349-1 double pole single throw open frame relay.? I am trying to find out the AC or DC voltage requirements as they are not marked on the coil.? The coil measures 11.7 Kohms.? Any help would be appreciated.? I've already done a Google search with no results on this particular relay.? Thanks in advance.

73 and fly our Flag,

Robert W4RL Pensacola FL

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