Posted by
Bob McGraw - K4TAX on
Feb 10, 2020; 7:30pm
URL: http://elecraft.85.s1.nabble.com/QRO-KPA500-Failure-tp7658244p7658355.html
Oh but contraire. I agree that lower dissipation typically reduces
MTBF. Therefore, never plug in your KPA500, leave it in the box and it
should last a very long time.
Sorry folks, I just couldn't resist.
I did not do extensive measurements, but the ones I did show the following:
450 watts out, 819 watts in, for a delta of 369 watts, I presume for
heat or 55% efficiency
210 watts out, 576 watts in, for a delta of 366 watts, I presume for
heat or 37% efficiency {I do work 30M that has a power limit of 200 watts}
Note the difference in heat dissipation between 450 watts out and 210
watts out is only 33 watts but is 240 watts difference in RF output.
Based on these numbers, there is not a lot of difference in heat to be
dissipated in the upper half of the power rating.
100 watts out, 393 watts in, for a delta of 293 watts, I presume for
heat or 25% efficiency.
73
Bob, K4TAX
On 2/10/2020 1:16 PM, Doug Turnbull wrote:
> Thank you Andy,
> Bob, if maximum power dissipation occurs at 50% max output power then the greatest amount of heat should be generated for the shack at half max output power. Power dissipation is in the form of heat not RF. The subject is interesting for a number of reasons perhaps most importantly to me is that of reliability. I believe the chance of failure is reduced as one lowers dissipation while staying within all other component ratings.
>
> 73 Doug EI2CN
>
> -----Original Message-----
> From:
[hidden email] <
[hidden email]> On Behalf Of Bob McGraw K4TAX
> Sent: Monday 10 February 2020 18:47
> To: Andy Durbin <
[hidden email]>
> Cc:
[hidden email]
> Subject: Re: [Elecraft] QRO - KPA500 Failure
>
> Unless one is concerned about power consumption, the efficiency factor should be really of no concern. Total heat output will always be greater at maximum output and less at lower power. Nice for warming the shack on a Winter day.
>
>
> Bob, K4TAX
>
>
> Sent from my iPhone
>
>> On Feb 10, 2020, at 12:18 PM, Andy Durbin <
[hidden email]> wrote:
>>
>>
>> "This is probably what folks are thinking of when they assume maximum dissipation occurs at less than full output."
>>
>> A possible source of error in my data is that the dummy load is cool at the start of the power ramp and hot by the time peak power is reached. I recorded LP-100A data during each test run and can see some change in the load characteristics. I did not attempt to compensate for this when plotting the data. I simply reported values of R and X that were about average for each test run.
>>
>> One way to find out if the change in load is significant would be to start the power ramp at maximum power rather than minimum power.
>>
>> Some time after I ran those tests I expanded the capability of my Arduino station controller to add a page that displays PA voltage, PA current, PA DC input, PA dissipation, RF Output, and efficiency. A snapshot of the displayed data can be sent to the controller's logger output.
>>
>> I just ran a test which had a brief transmission close to full power and another brief transmission at half power. This test was performed at 14.01 MHz using a dummy load. Here are the data:
>>
>> 26:36:36.672 KPA Mon frozen
>> PA voltage=60.6
>> PA current=14.1
>> PA input=854
>> RF out=495
>> PA diss=359
>> PA effic=57
>>
>> 26:37:32.437 KPA Mon frozen
>> PA voltage=64.6
>> PA current=10.0
>> PA input=646
>> RF out=250
>> PA diss=396
>> PA effic=38
>>
>> Dummy load heating would not have been significant factor in this test but the results still show higher PA dissipation at 250 W out than at 495 W out.
>>
>> It's really simple to produce a similar data set and there are lots of KPA500 owners here. If you think my data are invalid please share your test data. I don't mind being wrong as long as I learn something new.
>>
>> 73,
>> Andy, k3wyc
>>
>>
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