From: Robert Baer
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Subject: Re: Driving a coil-driven relay
References: <firstname.lastname@example.org> <3DF5A911.8F493BED@earthlink.net> <email@example.com>
Date: Wed, 11 Dec 2002 17:32:15 GMT
NNTP-Posting-Date: Wed, 11 Dec 2002 09:32:15 PST
Organization: EarthLink Inc. -- http://www.EarthLink.net
Michael Starks wrote:
> Hi, Robert. Thanks for the reponse. I have rechecked with the
> manufacturer, and this contactor is supposed to be driven by 24V DC.
> I should also mention that it is bi-stable, but normally-open (i.e.,
> the springs help you open it; you work against them to close it.)
> I have done further testing; I charged a bank of 88000uF to 26V and
> discharged it across the OPEN and then the CLOSE coils. In all cases,
> the operation was 100% reliable in both directions.
> Therefore, the problem lies with my driving circuit. There are only
> four ways the driving circuit differs from the directly discharging
> 1. The current flows through a MOSFET, which we have already proven is
> turning on and off fully.
> 2. The coils have blocking diodes across them.
> 3. Each coil is always connected to its driver circuit, rather than
> hanging free when its partner is being energized.
> 4. Four feet of 20 ga. wire between driver and contactor.
> I suspect that during the CLOSE cycle, current induced in the OPEN
> coil initially by transformer action and then induced by the motion of
> the core (and permitted to circulate through the coil by the blocking
> diode) is somehow preventing the mechanism from throwing closed all
> the way, thus ending us up in the "closed but stuck" limbo state.
> I plan more tests using the capacitors, this time including blocking
> diodes on one or both coils.
> Any ideas in light of this new information would be appreciated.
> Michael Starks
------------- SNIPped for brevity -------------
I am familiar with the mechanical design; there is only one maker that
has a design where the spring works mostly in the middle of movement,
allowing much faster operation and lower power.
In any event, i *still* would use that added air gap via the paper.
In fact, use a oscilloscope with both a voltage and current probe to
see what is happening.
Use a camera on a tripod to capture each single operation (2
And use a small compass or other equivalent magenetometer
(unmagnetized pin, even) to see about remaining magnetization.
BTW, *just* a diode across the coil may not be a good idea; the damn
thing could ring like crazy.
Instead, add a small resistor in series with the diode to help
dissipate energy (say 2 to 50 ohms).
Once that test setup is in place, then you can experiment with
variations on diode clamping, gap sizes, etc.