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nuckolls.bob(at)aeroelect
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 Posted: Sat Sep 29, 2012 11:49 am Post subject: Crowbar OVM adjustment/test fixture |
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The crowbar ovm has been in production for OBAM
aircraft since 1985 and has been incorporated in to
light airplanes in about a dozen configurations.
I'm getting an increasing number of queries for a
means by which they can be tested either on an airplane
or on the bench. Here's the fixture I have used here
for some years.
http://tinyurl.com/8e552q9
A 6v relay is wired to work like a buzzer (coil
in series with N.C. contacts). A 10 ohm resistor
tied across the coil to increase it's apparent
'draw' to make sure the crowbar scr latches
well.
In the airplane, you open the field supply breaker
and switch and clip the fixture across the ovm
to be tested. Increase applied voltage until the
relay begins to 'buzz' and then back the voltage
off until it clicks at about one per second rate.
THAT voltage is the current calibration point for
the OVM. On the bench, I set the power supply for
16.3 volts and adjust the calibration pot for
1 click per second. This is MUCH more precise
and convenient than the technique described in
the DIY crowbar literature.
Bob . . .
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klehman(at)albedo.net
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| Posted: Sat Sep 29, 2012 4:21 pm Post subject: Crowbar OVM adjustment/test fixture |
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This might work with just a heavy automotive light bulb instead of the
relay and resistor. Perhaps a brake light or maybe a headlight. The bulb
might well drop insignificant voltage prior to tripping. It would have
to be heavy enough to keep the scr conducting so that it stayed lighted
when the ovm activates.
Ken
On 29/09/2012 3:48 PM, Robert L. Nuckolls, III wrote:
| Quote: |
<nuckolls.bob(at)aeroelectric.com>
The crowbar ovm has been in production for OBAM
aircraft since 1985 and has been incorporated in to
light airplanes in about a dozen configurations.
I'm getting an increasing number of queries for a
means by which they can be tested either on an airplane
or on the bench. Here's the fixture I have used here
for some years.
http://tinyurl.com/8e552q9
A 6v relay is wired to work like a buzzer (coil
in series with N.C. contacts). A 10 ohm resistor
tied across the coil to increase it's apparent
'draw' to make sure the crowbar scr latches
well.
In the airplane, you open the field supply breaker
and switch and clip the fixture across the ovm
to be tested. Increase applied voltage until the
relay begins to 'buzz' and then back the voltage
off until it clicks at about one per second rate.
THAT voltage is the current calibration point for
the OVM. On the bench, I set the power supply for
16.3 volts and adjust the calibration pot for
1 click per second. This is MUCH more precise
and convenient than the technique described in
the DIY crowbar literature.
Bob . . .
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nuckollsr
Joined: 24 Mar 2009
Posts: 95
Location: Medicine Lodge, KS
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| Posted: Sat Sep 29, 2012 6:59 pm Post subject: Re: Crowbar OVM adjustment/test fixture |
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Sure. This is what I've shown in the DIY crowbar instructions . . . but the supply needs to be manually reset after each trip. An adjustment procedure or QA check on the bench is tedious. This fixture resets the ovm each time the contacts open. By seeking that once-per-second sweet spot, the test establishes the near infinite trip time setpoint by simply listening.
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