1) I've redrawn the switching around K4 a number of times, then stared at it forever, but I keep doing it over thinking it's wrong. I'm sure it's wrong.
2) Does M1 need a shunt? The meter is already what I need it to be, but whats the safest way to implement it?
3) Is my thinking around M2 right? (5ma 100mv FS meter, but want 50ma... so 2ohm shunt instead of the built in 20ohm? Then if I want a 500ma scale, add a 1k in series with + also?)
4) Is R12 needed, and if so how do I figure out the right value?
Thanks for reading, and thanks to all I've learned from on here while lurking.
2) M1 looks fine if it measures what you want, 750mA, no need for anything I think, I see you already have protected the meters.
3) For M1, if you already measured the stock meter and it reads 5mA full scale, is 20 Ohms, and uses 0.1 Volt, then instead of 2 Ohms, add 2.22 Ohms. This is because with the 20 ohms of the existing meter across the 2.22 Ohm added shunt, it will pass 5mA through the meter and 45mA through the shunt, giving 50mA for sull scale. ("2.22" as close as possible, really 2.222222...)
For a 500mA full scale indication with the same "stock meter that reads 5mA full scale, has 20 Ohms, and uses 0.1 Volt", a shunt of 0.2020 Ohm can be used. Perhaps a good quality 0.2 Ohm resistor will be close enough.
If a shunt is very low resistance, a few feet of small wire like #30 wadded up zigzag (non-inductively) can be used to make such a low value shunt across the meter. A milliohmmeter is useful. 1FT of #30 AWG copper wire is 0.103 Ohms, so 1.96FT should do it, refer to your regional wire table for Ohms per distance of the small wire.
I don't understand where you are saying about adding a 1K resistor.
4.) R12 does not seem to be needed.
With K4 de-energized, the 'standby' current through the standby bias resistor R19 will not go through M1. Should the non-relay end of R19 which now goes to B- be moved to pin 4 of K4 (plate meter +) instead? The slight indication might be useful one way or another.
