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From: Robert Baer
X-Mailer: Mozilla 4.75 [en] (Win98; U)
Subject: Re: Driving a coil-driven relay
Date: Tue, 10 Dec 2002 08:41:43 GMT
NNTP-Posting-Date: Tue, 10 Dec 2002 00:41:43 PST
Organization: EarthLink Inc. -- http://www.EarthLink.net
Michael Starks wrote:
> Hi all,
> I am having fits trying to troubleshoot a circuit. Perhaps some ideas
> will help.
> The circuit is designed to drive the Open and Close coils on a 800A DC
> contactor. This thing is bi-stable and has a permanent magnet inside.
> When you hit the open coil with 24V (requiring about 300mA in
> steady-state), it opens. When you hit the (oppositely-wound) close
> coil with 24V (requiring about 2A in steady state), it closes.
> I can fire this contactor both directions using a +/-12V 1A brick,
> which has small filter caps (4400 uF), by touching its leads to the
> coil terminals. It manages to close the thing, although movement of
> the mechanism is slow. It can easily open it. Movement occurs 100%
> of the time in both directions. The same supply elevated to +/-15V
> 800mA opens and closes the device robustly.
> The problematic driving circuit charges large banks of caps to 28V
> through diodes and a current limiting resistor. It is then fired for
> 250ms by a monostable, which turns on a MOSFET and shorts the
> capacitors across the appropriate coil. We use 8800uF for the Open
> coil and 88000uF for the Close coil.
> I have discovered through a lot of testing that the Open circuit is
> 100% reliable if you always close the contactor by manually resetting
> it (pushing it closed). If you ever close it using the Close circuit,
> something funny happens inside and the Open circuit cannot open it,
> despite the fact that it did close and latch. The only thing that
> will open it is +/-15V from the brick. Clearly my problem lies with
> the close cycle.
> I know that I have sufficient capacity (hence current), as the DC
> current draws for Open and Close are steady for the entire pulse. I
> know that I have sufficient voltage, because the circuit drives the
> coils at 28V, while the brick works with just 24V, even when doing a
> halfway job on the closure. I also tried upping the circuit voltages
> to 30V, without any change. I know the pulse lengths are sufficient,
> because I tried different ones, and the contactor specs say 150ms.
> I don't really expect any specific ideas from you without your seeing
> the circuit, but I do have a general question:
> It seems like I should be prone to oscillation if I connect capacitors
> across an inductive coil. I don't see any oscillations in my voltage
> or current scope traces, however. Voltage immediately jumps to equal
> the cap voltage, and the current increases exponentially toward
> steady-state. Could there still be oscillation causing a problem
> What is the difference between connecting the coil to a +/-15V brick
> that clearly shouldn't hold it's voltage up (insufficient maximum
> current), and connecting it to a bank of capacitors that are pretty
> stiff? The presence of the inline regulator must cause the coil to
> see some of the full-wave rectification in the brick, and probably
> also acts as a sort of variable resistor. Could this be causing
> damping the natural oscillation of the coil/filter cap combination,
> something I don't have in my driver circuit?
> Any ideas as to where to go with this? I have the current; I have the
> voltage; I have the pulse lengths; yet the close cycles still don't
> close it properly.
> Thanks for any thoughts. I've got 3 days into this thing and all I've
> done is eliminate possibilities. Still no answers.
> Michael Starks
Your problem is that all contactors (that i know of) are rated to be
driven by AC voltage, and you are using DC.
The nominal magnetization of the iron core by using AC is low, and
only rarely high.
And the AC applied for the other operation will quickly de-magenetize
If you put a piece of paper between the fixed core and the moving
part, the minimum gap will be about 0.003 (thickness of a piece of
paper), and will decrease the magnetic attraction you create during the
magenetizing (energizing) phase.
The closing speed for any given drive will not change, the opening
reliability and speed will increase.
BTW, if you insist on using DC, lower that voltage or clamp to 15
volts (ie acts like current source due to limiting resistor but voltage
swing diode clamped to max of 15V; the diode also helps decrease ringing
on the coil and may increase driver reliability).
Now, if you used 24VAC drive like is specified by the maker, perhaps
the problems might go away......
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