Why resistor inline with Transponder

[Retz]

So I'm in the middle of an upgrade from factory installed King to some Garmin goodies.  My factory KT76a power runs through a large Dale resistor between source power and KT76a....why?

 

im installing a GTX330ES, there isn't anything in the wiring diagram about a resistor.  Can I remove the resistor and wire power straight to the 330?

 

thanks

Joe

[milotron]

If you have a 28V system, which I suspect, this might be a voltage divider resistor as the KT76a you have is 14V rated.

 

Just a guess.

 

iain

[Guitarmaster]

If you have a 28V system, which I suspect, this might be a voltage divider resistor as the KT76a you have is 14V rated.
 
Just a guess.
 
iain

This ^^^ or it could be part of a noise filter.

Sent from my XT1585 using Tapatalk

[Jerry 5TJ]

You can delete the resistor.  The GTX33 is compatible with 14-30 volts. 

[Stanton R]

You should remove the resistor. If you leave it in it may drop the voltage to far and the 330 might not work properly 

[thinwing]

Classic voltage divider shown to wire a 14 v unit to 28 v aircraft...remove

[Andy36]

Well it was probably the cheapest, hottest, likely  heaviest solution 30 yrs ago and relatively high reliability. Today however with units that run from 10 -30V it is a very bad thing to leave in line with the 14 or 28v power supply. Modern transponders use high efficiency, typically~90% switching power supplies to convert whatever voltage is applied, to what they need. Apart from the extra weight, an in line resistor will generate heat all the time, it will drop the voltage to the transponder so the switching power supply will draw more current to maintain internal regulation, efficiency will go down a lot, the alternator will need to generate more power and it will apply a higher load on the engine.  It will of course warm the cabin a little quicker

[PT20J]

BK never made 28V version of of the KT 74 and 76. Just remove it and include it when you update W&B.

Edited January 31, 2020 by PT20J

[carusoam]

Welcome aboard new Andy!

Great first post.

Brings up an interesting question...

I swapped out a KT76A for a KT76C one day... had too.

Does a 76C come in dual 14/28V should I look for a resistor in line...

Naturally, I have a 28V system that was adjusted specifically for the KT... 

Best regards,

-a-

[mhrivnak]

I found the same last night in my 1990 MSE while tracing and removing old wiring as part of a new engine monitor install. Paperwork says that a KT76A was removed in 2011 (prior to my ownership), so I assume that shop just neglected to remove this too. It's hard to see and get to, mounted behind the center console (where apparently vacuums can't reach...), not that that's any excuse to leave it in.

Thanks for the thread confirming what it was for.

[carusoam]

Looks like the big resistor is riveted in place...

Great pic, mhrivnak!

Best regards,

-a-

[0TreeLemur]

Wow, if I read correctly, that is a 55 Watt resistor.  Electricity waster.  Good riddance.

[carusoam]

Essentially,

The resistor is used to split a river of electrons in half...  a 24V river, split into two identical 12V rivers...

• one half go through the transponder...
• The other half drains to ground...
• The resistor is sized to match the resistance of the avionics box...

You end up wasting the same amount of energy of two transponders this way... and dumping all the heat inside the cabin...

PP thoughts only, not an avionics guru or electrical engineer...

Best regards,

-a-