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AT-6C ballast resistor?

 
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nuckolls.bob(at)aeroelect
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PostPosted: Wed Jul 24, 2019 7:28 am    Post subject: AT-6C ballast resistor? Reply with quote

At 11:19 AM 7/23/2019, you wrote:
Quote:
All,

Am looking for help.

We have a Harvard (AT-6C).

There is what allegedly is called a ballast resistor that sits in an externally accessible electrical box on the left hand side of the fuselage above the left wing leading edge. That box also contains the carbon pile rectified amongst other things.

Twice now, this ballast resistor has failed in a thermic way..... (replaced, and replacement failed after about 10 hours).

Do you still have a failed part? Can you share a picture?


Quote:
When this fails, on activating battery master, there is no power to the main 28v bus..... only power from battery to starter motor. 28v bus only becomes energised when generator kicks in. Battery does not charge.

Can anyone enlighten what this resistor is for? Rumour has it that it was only to protect old style avionics. If this is the case, would it be safe to remove it since we have modern avionics in the aircraft.

Do you have a wiring diagram for this airplane?
Have you availed yourself of the documentation
library on this aircraft available at:

https://tinyurl.com/y4bclrc2



Quote:
Problem us that we are having difficulty sourcing a further replacement, so the aircraft is unserviceable until a solution is found.

There are no doubt dozens of suitable
replacements. But without knowing the form,
fit and function of this part, making a
selection is impossible.


Quote:
Is there a modern replacement or mod that anyone knows of?

I'm led to believe its 125 ohm, but have no other information than that (and it's a 28v generator based system)

Does the failed part have any markings on it?



Bob . . .


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nuckolls.bob(at)aeroelect
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PostPosted: Wed Jul 24, 2019 11:23 am    Post subject: AT-6C ballast resistor? Reply with quote

At 10:50 AM 7/24/2019, you wrote:
Quote:
Hi Bob,


PART No: COIEK
125 Ohms
125 Watt


Hmmm . . . I seem to recall that COIEK is
a old-line resistor manufacturer . . .
[img]cid:.0[/img]

A resistor of that size would look about
like this?

28 volts applied to a 125 ohm resistor
dissipates only 6 watts. I'm having
trouble imagining how a resistor of
that size gets overheated in a 28v system.



Bob . . .


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ceengland7(at)gmail.com
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PostPosted: Wed Jul 24, 2019 11:36 am    Post subject: AT-6C ballast resistor? Reply with quote

The pic shows multiple taps; looks like some of the individual segments could easily be as low as around 20 ohms. Those are some really fat looking conductors attached to it, too.
Charlie
On Wed, Jul 24, 2019 at 2:28 PM Robert L. Nuckolls, III <nuckolls.bob(at)aeroelectric.com (nuckolls.bob(at)aeroelectric.com)> wrote:

Quote:
At 10:50 AM 7/24/2019, you wrote:
Quote:
Hi Bob,

 
PART No: COIEK
125 Ohms
125 Watt
 
 

  Hmmm . . . I seem to recall that COIEK is
  a old-line resistor manufacturer . . .
[img]cid:16c2574490be0346ea31[/img]

  A resistor of that size would look about
  like this?

  28 volts applied to a 125 ohm resistor
  dissipates only 6 watts. I'm having
  trouble imagining how a resistor of
  that size gets overheated in a 28v system.



  Bob . . .


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nuckolls.bob(at)aeroelect
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PostPosted: Wed Jul 24, 2019 12:25 pm    Post subject: AT-6C ballast resistor? Reply with quote

At 10:50 AM 7/24/2019, you wrote:
Quote:
Hi Bob,


PART No: COIEK
125 Ohms
125 Watt


Hmmm . . . I seem to recall that COIEK is
a old-line resistor manufacturer . . .
[img]cid:.0[/img]

A resistor of that size would look about
like this?

28 volts applied to a 125 ohm resistor
dissipates only 6 watts. I'm having
trouble imagining how a resistor of
that size gets overheated in a 28v system.



Bob . . .


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nuckolls.bob(at)aeroelect
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PostPosted: Wed Jul 24, 2019 5:58 pm    Post subject: AT-6C ballast resistor? Reply with quote

At 01:42 PM 7/24/2019, you wrote:
Quote:
Bob,

Here is the picture of the resistor....

Hmmm . . . that's in no way a 125 ohm
resistor. When they are wound with ribbon
wire on edge, it's more likely to be 12.5
ohms or even 1.25 ohms.

Will need to see the wiring diagram where
this device is incorporated before I can
be of much help.

If the last replacement lasted only a few
hours, then something is broke . . . need
to figure out what this resistor is supposed
to do.



Bob . . .


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nuckolls.bob(at)aeroelect
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PostPosted: Fri Jul 26, 2019 7:32 am    Post subject: AT-6C ballast resistor? Reply with quote

At 01:42 PM 7/24/2019, you wrote:
Quote:
Bob,

Here is the picture of the resistor....



There are companies that still make
this style of resistor: Ohmite and
TE Connectivity. The trick will be
to first identify specifications
for the failed part and then dig through
the catalog items for nearest if not
exact replacement.

Are the failed parts still on hand?
I've got a 4-wire ohmmeter that might
let me measure segments on either
side of the failure to deduce it's
original characteristics.



Bob . . .


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Eric M. Jones



Joined: 10 Jan 2006
Posts: 564
Location: Massachusetts

PostPosted: Sat Jul 27, 2019 6:49 am    Post subject: Re: AT-6C ballast resistor? Reply with quote

I want to add to what Bob N. said. The proper title is "carbon pile regulator." These use a variable resistance load composed of a pile of carbon disks (usually with an alignment hole in the center), compressed by a solenoid that pushes them together harder as the voltage increases.

Very common WWII regulator, and pretty reliable. There is a good site for these at: https://www.industrial-electronics.com/aircraft_6.html

And you can get more data if you Google: Images "carbon pile voltage regulator." You'll probably see yours.

As Bob alluded to, the resistor should not have failed...and I can surmise that either something is wrong with the system, or somebody put in the wrong part. There is lots of AT-6 information online.

BUT: It is entirely legal to put a substitute resistor. A schematic on your regulator is a very good idea.

I can't find the resistor manufacturer online. Perhaps Thomas Index of Manufacturers has them...but maybe the part info is hard to read.

Good luck,


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