KT-78 Transponder info

tonycondon

tonycondon

Gastons CRO (Chief Dinner Reservation Officer)
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Tony
Does anyone have a manual or other sort of spec sheet for the KT-78? It's a low power (150 watt) version of the KT-76. Thanks!
 
mostly power draw and if it will run on a 12V battery or if 14V is a hard requirement.
 
Tony:

What we routinely call "12v" is actually a nominal 14v system (13.8v). A "14v" device should work fine on a good "12v" battery, but you should determine the current draw, as many transponders are current pigs, and could deplete your storage battery VERY quickly.
 
thanks spike. i've got a couple 12V 7.5 amp-hr sealed lead acid batteries. Im thinking that i would run my new Microair Radio (courtesy of fine PoA people) and electric variometer on one of the batteries, and try to find a transponder to run on the other battery. maybe ill have to get a beefier battery but that only takes money.
 
I cannot find any current specs for the KT76 or the KT78 when I looked. I did find a Narco transponder ATC-165 which did indicate a max current draw of 1.75Amps for a 250W power output. The DC input voltage is 11-32VDC.

A Big SWAG would be that the KT78, being less than half that for power output, should be less than 1.75A and able to run for some time on a 12VDC 7.5Ah battery.
 
tonycondon said:
thanks spike. i've got a couple 12V 7.5 amp-hr sealed lead acid batteries. Im thinking that i would run my new Microair Radio (courtesy of fine PoA people) and electric variometer on one of the batteries, and try to find a transponder to run on the other battery. maybe ill have to get a beefier battery but that only takes money.
Click to expand...

I run two 10 or 12AH Sealed Lead Acids in my HP16T. With a little shopping around these can usually be purchased for less than $30. I have purchased them from www.gotbatteries.com but have found I can usually find them at comparable prices locally.

The spec, beside current draw, that you would be interested in is minimum voltage. This is what makes the Becker and Dittel Radio's so popular in sailplanes in that the will continue to function down to about 10 volts. The reason I am running so much battery is that I often do 4 to 6 hour flights above 14,000 feet. The batteries get cold and the voltage drops. I finally traded in my Terra 760D radio for Becker last year so this is not as much of an issue. The Terra would not transmit after a few hours at these altitudes. Of course unless you are planing on coming out West you probably won't be flying at these altitudes.

Brian
 
brcase said:
I run two 10 or 12AH Sealed Lead Acids in my HP16T. With a little shopping around these can usually be purchased for less than $30. I have purchased them from www.gotbatteries.com but have found I can usually find them at comparable prices locally.

The spec, beside current draw, that you would be interested in is minimum voltage. This is what makes the Becker and Dittel Radio's so popular in sailplanes in that the will continue to function down to about 10 volts. The reason I am running so much battery is that I often do 4 to 6 hour flights above 14,000 feet. The batteries get cold and the voltage drops. I finally traded in my Terra 760D radio for Becker last year so this is not as much of an issue. The Terra would not transmit after a few hours at these altitudes. Of course unless you are planing on coming out West you probably won't be flying at these altitudes.

Brian
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brian,

what else do you run on the battery? that seems like a lot of battery for just a radio. Ive used these 7.5 amp hr batteries for quite a while in my glider, but i was only running a handheld radio with it.

And I dont have any plans for much flying above 10K or more than 4-5 hrs in the Cherokee. I just don't want to get waxed by the millions of airplanes flying on the East side of Wichita.
 
SCCutler said:
Tony:

What we routinely call "12v" is actually a nominal 14v system (13.8v). A "14v" device should work fine on a good "12v" battery, but you should determine the current draw, as many transponders are current pigs, and could deplete your storage battery VERY quickly.
Click to expand...

I don't know where you get the idea that transponders are "current pigs" but none of the ones I've seen specs for pulled more than a few amps. Granted the solid state ones could be designed to draw a tenth of that but there's little incentive to go below 1 amp in aircraft avionics that aren't intended to work in gliders. Also FWIW, the power output of the transponder as virtually no relationship to the power required to run it because the duty cycle on the transmission is very small.

Tony, I doubt that a KT-78 draws less power than a Garmin GTX-327 which unlike most transponders has no cavity tube which wastes power on the heater. The spec's on a GTX327 call out 15 watts as the nominal power required and it will operate with an input voltage between 11 and 33 volts. I think this would be a good choice for a glider unless you could find one specifically designed for battery power.

Also if I were you I'd work out a way to get a battery setup providing at least 13-14 volts. Your 12 volt battery will put out something like 12.6 volts when fully charged but to get the full rated energy out of it you have to let the voltage drop to about 11 and that may be problematic for your comm radio or some transponders. If the comm radio can work with 14-28 volts, I'd recommend you go with two 12v batts in series. As long as the radios use switching supplies (allmost all the new stuff does and chances are very good that any radio that automatically adjusts to 14 and 28 volts has this) the power input will be fairly constant over the allowable voltage range. In that case the batteries will last as long wired in series as they would in parallel.
 
If I could afford to buy a new transponder I would go for a Becker or Microair which draw about .5 amps. They are designed specifically for gliders. But Im not to the point where I have 2K laying around, yet. The radio cannot handle more than 16 volts.

However, I can get 2 volt batteries to hook in series to my 12 volt battery. That would probably be a good solution for powering the transponder.
 
tonycondon said:
If I could afford to buy a new transponder I would go for a Becker or Microair which draw about .5 amps. They are designed specifically for gliders. But Im not to the point where I have 2K laying around, yet. The radio cannot handle more than 16 volts.

However, I can get 2 volt batteries to hook in series to my 12 volt battery. That would probably be a good solution for powering the transponder.
Click to expand...
Only if you are looking to up the voltage.

If you put two same votlage batteries in parallel then you get he same voltage but higher capacity.

For example 2 12VDC batts in series is 24VDC while two 12VDC batts in parallel is still 12VDC but at twice the capacity (assuming they both have the same capacity to begin with).

Your problem with using the 2VDC battery to get to 14VDC is that for it to work the 2VDC batt has to have the same capacity as the 12VDC battery.
 
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yea scott. i dont remember much from the section in freshmen physics that we spent on electricity, but i do remember that much. my comment was only based on the idea that the transponder really needs 14 V to operate, so I better line the batteries up and give it what it needs.
 
gismo said:
I don't know where you get the idea that transponders are "current pigs" but none of the ones I've seen specs for pulled more than a few amps. ...
Click to expand...

...come to think of it, neither do I! :mad2:
 
tonycondon said:
yea scott. i dont remember much from the section in freshmen physics that we spent on electricity, but i do remember that much. my comment was only based on the idea that the transponder really needs 14 V to operate, so I better line the batteries up and give it what it needs.
Click to expand...
Go read spike's post again. Your transponder should have a voltage range spec. I'll bet that something like 10 or 11 VDC is the low end and that the 14VDC spec is the nominal when based on an electrical system that is under charge i.e. 13.6VDC.

What you need to figure out is that low end voltage and then calculcate a discharge curve for the battery. When the votlages goes below that low point specification, that is the life of your battery for that system. I used to do this all the time when I was first starting out as an engineer. I still ahv emy lab books filled with the stuff and the simple HPIB code to automate it all on a test bench. Shoot I still even have the constant current sink I designed and made so that we simulate varying loads.
 
yea scott, i know. thats the point of the first post :)
 
tonycondon said:
yea scott. i dont remember much from the section in freshmen physics that we spent on electricity, but i do remember that much. my comment was only based on the idea that the transponder really needs 14 V to operate, so I better line the batteries up and give it what it needs.
Click to expand...

The only trouble with adding an extra 2v cell (besides finding one with the same AH rating) is that most standard chargers won't work with that, which is why I recommended going with two 12v batteries in series assuming your transceiver will work with 24v. Chances are the second 12v battery will cost little if any more than a 2v battery and this way you are also assured that they will have the same capacity.

A used GTX327 can probably be found for around $1000-1200. You could also go with a GTX320 and I would expect the power requirement to be about the same as the 327. You will need an encoder as well (I happen to have a spare one you can have real cheap).

http://www.avionicslist.com/garmintransponder.php

[FONT=arial,verdana]GTX320 (like new) $875, TDR950 $525, AT150 $650, AT50A $495, RT359A $495, will exchange, call Tom (704)393-7553

[/FONT][FONT=arial,verdana]Garmin GTX-320 transponder in excellent condition! Unit comes with the tray and connectors. $925. 850-478-7132 aviati_b@bellsouth.net


[/FONT]
 
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gismo said:
The only trouble with adding an extra 2v cell (besides finding one with the same AH rating) is that most standard chargers won't work with that, which is why I recommended going with two 12v batteries in series assuming your transceiver will work with 24v. Chances are the second 12v battery will cost little if any more than a 2v battery and this way you are also assured that they will have the same capacity.

A used GTX327 can probably be found for around $1000-1200. You could also go with a GTX320 and I would expect the power requirement to be about the same as the 327. You will need an encoder as well (I happen to have a spare one you can have real cheap).
Click to expand...

if this king transponder works out, i can get setup for at least half the cost of a garmin transponder, if not less. of course there is some benefit to having a new, more reliable, transponder. but then there is always the issue of spending about 1/5 of the value of the glider on one piece of equipment for it.
 
Hi Tony,

Since I was struggling with radio issues at the time and I was suspicious of low voltage being the issue I measured my power requirements a few years ago and still have the numbers.

The no load voltage on the battery was 13.3 Volts. It had probably just came off of the charger a short time before and was a new battery.

EW Model D Logger = 20ma
Garmin 12XL GPS = 100ma
Cambridge CNav& Vario = 200ma
Gear Horn = 10ma
Terra Radio LED ON/Rec = 330ma
Terra Radio LED ON/Xmit = 2360ma
Terra Radio LED OFF/Rec = 230ma
Terra Radio LED OFF/Xmit = 2380ma

Total when Receiving = 550 ma (minimum)
Total when Transmitting = 2680ma (minimum)

I have since added a Compaq 1550 PDA and a Borgelt B40 Vario, I haven't measured the power draw on them.

I admit I have a lot battery capacity, I usually run the tadio on one battery and everything else on the other. The problem is I could only use less than 1/2 of the capacity of the battery due to the high minimum voltage requirement of the Terra Radio.

I bought a used Becker radio last year and I could probably use smaller batteries now, but it is nice not to have to worry about running out of power.

In response the suggestion to use a 14 volt battery, I have done this as have a few other glider pilots in the area. It works well for a short period of time (one season) unless you build a custom battery. Just adding a 2volt to a 12 volt as I did didn't seem to work very well even though the AH ratings were matched. Also you likely will have to build a custom battery charger for it as well. The best way to do it is to buy seven 2 volt cells and build a custom battery, however this gets expensive. Also a fully charged 7 cell battery can approach 15 volts. You need to make sure all you electronics can handle this.

Brian
 
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