Comparing the relative percentage change of two voltage outputs.

[Chris Wilson]

I have two separate variable voltage signals, 0 to 12V DC. Signal A and signal B. I want to read  both signals and if A ever rises more than say 10% higher than B an output goes high (or low, it doesn't really matter). I would much prefer this to be all analogue CMOS the stuff running a single 12 v supply. The circuit creating the voltages must not be unduly loaded. What are my options and please consider I am a real novice. It's to measure the outputs of an SWR bridge and at different operating power levels, and hence different voltages from the bridge, still be able to trip a no TX circuit if reflected power gets too high. Thanks. I can post a schematic of the circuit including the bridge if required.

[Ian.M]

Assuming they share a common ground, two fairly high resistance potential dividers, with ratios differing by 10% using precision resistors,  feeding a CMOS comparator with rail-to-rail inputs.   You can easily get the resistive loading on the signal source above 1Meg.  If you need hysteresis,  use feedback via a resistor from the output to a low resistance tap at the bottom of the +ve input's divider, as that lets you apply a small amount of feedback without needing a multi-meg feedback resistor.

[Chris Wilson]

Hi Ian, thanks for the reply. I attach the schematic below, I have taken signals at the pink starred points to HEF4001's configured as set / rest flip flops, in the case of the first measuring point, which goes Low on loss of drive signal, I have put it through an inverter. This works fine and a separate  LED is lit, and stays lit if drive signal is lost, PA current rises too high, or reflected antenna power rises too high. This improves on the shown circuit which just lit the one LED no matter which of the 3 protection circuits was triggered, so you didn't immediately know why it had tripped.The simple reflected volatge measurement shown  is OK'ish at full power where a mismatch can return over 5V to the flip flop and it's then triggered, but a t low power, using a reduced PA FET voltage, not enough reflected voltage is seen by the HEF2001 SR flip flops (not shown) to trigger it, even with no antenna connected. Hence the need to compare the percent difference between forward and reflected voltage from the bridge.

Complicated to explain concisely, I hope you can follow my rambling?

Some hysteresis would be good to stop shuffling. Can you see any issues with your kind suggestion based on the above and the attached schematic? Thanks again Ian!