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Re: QMX T507 winding

 

Hi Evan, John, all

REALITY CHECK!

Somebody needed to measure this :-D?
  • QMX on 80m, power supply reads 12.24V and 87mA on RX.?
  • Dummy load: on TX: PSU reads 675mA, 'scope reads 42Vpp across dummy load, drain measures 33.6V peaks.?
  • OPEN (nothing at RF port): PSU reads 251mA, 'scope reads 38.40V peak on the drains.?
  • SHORT (shorted with a wire soldered across bottom of RF port): PSU reads 1211mA, 'scope reads 46.40V peak on the drain
  • Back to dummy load: same results as previously (i.e.: nothing fried).?
Conclusions:
  1. A model that shows an open load will breach the 60V rating?of the BS170s differs significantly from the practically observed reality
  2. An OPEN load type infinite SWR does not damage QMX either by high current or by high voltage
  3. A SHORT load causes high current, and presumably high power dissipation since ALL the power is being dissipated in QMX, which would lead to overheating and related probable failures.?
I'm not saying no type of SWR mismatch could cause excessively high voltages at the BS170s but if you're going to make a simulation to show a scenario where a particular SWR match causes a high voltage at the BS170 drains then I think the first reality check on the simulation should be for the simulation to reproduce this observed reality for the OPEN load case.?

The result is also good news because it is easier to detect and protect against a high current scenario; the high current itself is not the destructive factor since a single BS170 is rated 500mA continuous (1200mA pulsed) current; the more likely scenario would be failure due to overheating with excessive power consumption, which is not an instant process and therefore gives us some time to detect it and react. In contrast a pure excessive voltage destruction could occur faster than any detection and preventative measures. Therefore the fact that we do NOT see high voltage on open load is good news indeed.?

Finally: in case you think the band is the issue - I ran the same OPEN load test on all 5 bands wtih?drain peak voltage measurements as follows:
  • 80m: 38.4V
  • 60m: 50.4V
  • 40m: 36.0V
  • 30m: 42.8V
  • 20m: 39.2V
73 Hans G0UPL



On Sun, Aug 6, 2023 at 3:02?PM Evan Hand <elhandjr@...> wrote:
John,

An open load line would produce a very high voltage.? In this simulation, the 60-volt breakdown of the BS170s is the limiting factor (the measurement is on the Drains of Q9 and Q11).


Adding the Zeners changes that to the Zener breakdown.


I am not saying that this simulation is the best, but it gives an approximation that would support the open output causing excessive voltage on the BS170s.?

I plan to add 47-volt Zener diodes to my QMX when building them.

73
Evan
AC9TU

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