Meter shunts / series resistors question

[Chris Wilson]

The circuit attached is an SWR bridge for part of a powerful LF (136kHz) TX. The meter used in the schematic is 0.25mA FSD and 300 ohms internal resistance, which I am ignoring for now. I want to use a 100uA meter instead, as that's what i have a few of. I can't quite get my head around whether the two 100k resistors are both in series in both positions of SW3 or not, and thus whether I need to change the two 100k resistors for 250k each. I think because one side of the meter is grounded the two 100k resistors are not effectively both in series, can someone clarify please? The meter is only for a relative reading of forward and reflected power, not needing to give absolute readings. The trimmer capacitor should null out reflected when transmitting into a dummy load. Thanks!

[Andy Watson]

I would say that they're not in series - the switch uses one at a time whilst grounding the other. Their effect on the sensing circuit is negligible due to the 68 R across the coil. But does it matter? If you're going to increase the sensitivity of the meter you need to increase both resistor values by the same amount.

You might want to look at the time constant of the two 22nF since the Reverse voltage appears to be monitored via the 47k and IC1b.

The easiest solution is to measure the resistance of your 100uA meter and shunt it with 2/3 of the resistance.

[Chris Wilson]

IC1B apparently monitors reflected power and if it becomes excessive shuts off drive to the power stages. But would changing the 100k resistor values to suit a more sensitive meter movement affect this, meaning those caps and the monitoring circuit would become an issue?

Thanks as always Andy!

[Andy Watson]

But would changing the 100k resistor values to suit a more sensitive meter movement affect this,

Yes it would make a difference - but not much. The two 100ks load the circuit by 50k (ignoring the contribution of meter). The resistance of the voltage source is 68 R (negligible) in series with the 4k7.  The latter loses about 10% with the 100ks, but only 4% with 250k loads. So, in very round figures it is going to be about 6% more sensitive.

I don't know if the increase in time constant is significant. I'm guessing that IC3 is a monostable that appears to have a relatively long time constant - so no effect there. The meter movement will be slower - but only by 10s of milliseconds - probably not noticeable.

[Chris Wilson]

Would changing the two 100k resistors for say a pair of 500k multi turn trimmers be a simple way out to allow some leeway, and set them equally, to suit FSD? Cheers Andy, and thanks for the explanation.

[Andy Watson]

I would shunt the meter - use a trimmer if you like.
Unless less you've changed the output of the transmitter I would suspect that 100k are designed to give the optimum range with a 250uA meter.

[Chris Wilson]

OK, will have a play with shunts then, thanks Andy.