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Amp load question


PTK

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I was doing an electrical load analysis of my 14V system and came across something strange. My GTN750 puts out 10 Watts min transmit power. This calculates to 0.7 amp draw when transmitting. My KX165 otoh transmit specs list 8.5 amps which calculates to 119 Watts! 

10W vs. 119W! Am I missing something?

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I don't know a lot about avionics but I presume one is measuring output, probably from a certain distance. The other is measuring the power requirement from the avionics bus to produce that output. I'm sure the efficiency is far less than 100%. 

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You might be comparing apples and oranges.  You are probably comparing transmit power on one and total power on the other.  Even though the GTN transmits 10W of signal, it is also using electricity for calculations and the display screen.  I'm sure the power just to keep the screen lit up is quite a bit.

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Sort of like buying a piece of property...

The useful living area is an important measurement, but it is a much smaller area than the total area of the property, Or the total area of the house...

If you add another floor to the house, the efficiency improves a good amount.  And really becomes dissociated from the area of the property...

There is so much electricity being used by the device, just from going from 12V supplied, transforming down to the 5V(?) being used...

All the excess power is be given off as heat. Do you have any BK radios left in the panel? They are hot to the touch.  Lots of excess electricity being used to generate such heat...

awaiting @Piloto's  expert opinion...

Best regards,

-a-

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I got the figures from the respective websites. I expected them to be similar. Certainly didn't expect the KX165 consumption to be 14 times that of the GTN!

I just want to be sure I'm reading it correctly.

https://buy.garmin.com/en-US/US/p/67886#specs

http://www.bendixking.com/Products/Navigation-Communication/Navigation-Communication/KX-165

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Peter, I'm not sure what you are comparing.  Mostly because I'm having difficulty finding comparable data on the websites...

I believe you are looking for the power requirements for the installation to properly size a circuit breaker.

Radio specs usually tout the strength of their transmitters and receivers measured in watts. Both Garmin and BK give transmit power at a standard 10watts with an option of 16watts. 16watts would be good when traveling outside of NJ...

Transmit power is generally 10X the receive power.  Both systems may be using some power at the same time...

I would bet there are rules of thumb to follow.  These made a lot of sense back in the day...

I would also bet there is an install manual that spells it out clearly...

Garmin supplies a lot of manuals on line, but I was not able to easily find the install manual for the GTN...

Best regards,

-a-

 

Edited by carusoam
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Anthony, I'm comparing (I think!) power consumption. Amp draw which peaks while transmitting.

Garmin shows 10W transmit which translates to 0.7 amps. King does not show watts but shows draw while transmitting to be 8.5 amps, which calculates to 119W!

So the 165 draws 12 times as much?!

http://www.bendixking.com/Products/Navigation-Communication/Navigation-Communication/KX-165

https://buy.garmin.com/en-US/US/p/67886#specs

 

I thought it's a typographical error on King site. But their new nstall manual shows same thing!

IMG_1453.PNG

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P=IV

watts=Amps X volts

13.75 X 8.5  = a lot of heat

It looks like the KX 165 is actually a bit stronger than a KX155 when it comes to transmitting... the chart doesn't seam to be laid out properly...  which data goes to which radio?

Great discussion...:)

Best regards,

-a-

Edited by carusoam
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1 hour ago, carusoam said:

Sort of like buying a piece of property...

The useful living area is an important measurement, but it is a much smaller area than the total area of the property, Or the total area of the house...

If you add another floor to the house, the efficiency improves a good amount.  And really becomes dissociated from the area of the property...

There is so much electricity being used by the device, just from going from 12V supplied, transforming down to the 5V(?) being used...

All the excess power is be given off as heat. Do you have any BK radios left in the panel? They are hot to the touch.  Lots of excess electricity being used to generate such heat...

awaiting @Piloto's  expert opinion...

Best regards,

-a-

A lot of this heat is due to the old gas discharge type of display. Besides the heat generated by the display itself the power supply to drive them had to raise it to high voltage generating even more heat. The high voltage itself presented a risk of arcing thus burning a hole on the PCB. This why all these PCBs were seal coated. Later units use LEDs displays that makes them less hotter.

José

Edited by Piloto
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FWIW,

There are a few different ways to make an AM transmitter. The easiest way is to use high level modulation. This requires the power supply to the final RF amplifier to vary with the modulation (speech) This takes a powerful audio amplifier just to power the RF amplifier. Simple but very inefficient. The other way is low level modulation with a linear amplifier. this uses a very weak modulation signal just after the first oscillator stage and then the modulated signal is amplified. This is a bit more complicated but much more efficient. I would think with modern class D PWM amplifiers the whole process happens on a magic chip that has been optimized to the Nth degree and is close to 100 percent efficient.

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