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QCX+ Resistor hot


K3YV
 

Hi everyone. I find that R48 (270 ohms) on the readout display board is quite hot. With a 13.8v source, I measure 8.57v across the resistor. This is 0.27 watts dissipation. That is rather marginal for a 1/4 watt resistor. I checked the resistor, it is indeed 270 ohms. Do I have something misassembled, or is this normal? Thanks all.


Alan G4ZFQ
 

I find that R48 (270 ohms) on the readout display board is quite hot. With a 13.8v source, I measure 8.57v across the resistor. This is 0.27 watts dissipation.
It feeds the backlight.
And, indeed you would expect about that dissipation. Seems a lot but must be normal.

73 Alan G4ZFQ


Gary Bernard
 

Very hot on mine also.

On Sep 12, 2020, at 4:22 AM, Alan G4ZFQ <alan4alan@...> wrote:



I find that R48 (270 ohms) on the readout display board is quite hot. With a 13.8v source, I measure 8.57v across the resistor. This is 0.27 watts dissipation.
It feeds the backlight.
And, indeed you would expect about that dissipation. Seems a lot but must be normal.

73 Alan G4ZFQ



ajparent1/KB1GMX
 

You can increase the value for a less bright display.  I went for approximately
3mA into the backlight and is still very visible in my room.  To get to about
3ma I use a 2.7K resistor. I tried 1K and it was still very bright to me.

Outdoors I'd try 470 ohms for moderately bright conditions.

The display  on memory is up to 30mA (very bright) and the backlight
can be run on less than 1ma easily.  If you don't need bright
and run on a battery that will save power.

Allison
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@CurtisM
 

Woody

Yep noticed same thing when I finished my qcx+ a week ago. If you like the brightness, lift one side and put another 270 ohms in series, to split the dissipation and retain much of the backlight. Congrats on your qst cover award for leaky antenna switch, I need to go back and look at that. 73

Curt wb8yyy


K3YV
 

Hi Curt. Thank you. The cover plaque was fun and really appreciated. I am changing the resistor value to adjust the brightness and solve the dissipation problem. Lots of room for two resistors. 73


Ian VA7ITM
 

With 11,000+ QCX(+) kits sold, I assume R48 running hot is not a problem? However, I also assume various solutions to remove any concern include replacing R48 with 270R 0.5W instead of 270R 0.25W, add another resistor in series as mentioned above, or maybe add a 4K7 pot in series with R48 which would allow you to manually adjust the display brightness?

I also assume adding a small LDR circuit to vary the brightness automatically would be overkill?

Maybe Hans will replace the blue display in the future with the yellow/green displays he plans to use in the QCX Mini, as these are still readable with the backlight turned off.

Ian


geoff M0ORE
 

4k7( four thousand, seven hundred Ohms ) pot is series with 470R,  How much visible adjustment do you think you might get?

On 13/09/2020 19:51, Ian VA7ITM wrote:
With 11,000+ QCX(+) kits sold, I assume R48 running hot is not a problem? However, I also assume various solutions to remove any concern include replacing R48 with 270R 0.5W instead of 270R 0.25W, add another resistor in series as mentioned above, or maybe add a 4K7 pot in series with R48 which would allow you to manually adjust the display brightness?

I also assume adding a small LDR circuit to vary the brightness automatically would be overkill?

Maybe Hans will replace the blue display in the future with the yellow/green displays he plans to use in the QCX Mini, as these are still readable with the backlight turned off.

Ian


Alan G4ZFQ
 

4k7( four thousand, seven hundred Ohms ) pot is series with 470R,  How much visible adjustment do you think you might get?
On a similar display I have a 4K7 pot and 680R in series. At maximum resistance it is quite visible inside in "average" daylight, plenty bright enough in the dark.

73 Alan G4ZFQ