So today I decided to open my E3611A to find out what's on the display board. I just found the expected (pair of) TC7107CKW but also a TSC9491 1.22V voltage reference; and no other chips. Much to my surprise though, the 7107 was configured for 2V full-scale; not the 200mV that I'd assumed! I confirmed this by checking the voltage between 7107 V+ref and V-ref pins which showed 1.00V; not 100mV.
This makes sense given, in the E3610A, the 1:1 current divider R12=0R/R11=open and 10:1 voltage divider R18=100K/R17=900K. With a 2V full-scale meter, 3A looks like "3.00", and 15V (max for the supply) looks like "15.00". I'm assuming (uh oh) my E3611A display board is virtually identical to the E3610A or E3612A in all but which decimal digits are enabled. My E3611A has a 100:1 voltage divider and thus has only 100mV voltmeter resolution.
Given what Kleinstein said about working with small voltages, we can avoid them by choosing 2V full-scale panel meters like the Murata DMS-20PC-1 (last digit changed). However, if choosing a generic panel meter like the "5135" that I proposed, it likely won't be possible to change the reference voltage to 1V. And the most you could do is modify it to 200mV full-scale (by removing the RA resistor). Actually, I'm not so sure anymore what the reference might be on some of these DC5V, 20V, 50V, etc panel meters.
Furthermore, the set current (CREF R20 wiper) is negative vs actual current (CMETER) but the way the E361xA series power supplies skirts this is by dropping the "+1" left digit, which you won't be able to do with a purchased panel meter. Also, I believe the only reason negative voltmeter readings aren't shown is due to the floating supply allowing IN-LO and IN-HI being tied to R18/R17 center point and +S/GND/+OUTPUT post, respectively. If you drop the isolated display supply requirement, I believe an inverting amp is necessary since meter ground will be tied to +S/GND/+OUTPUT post.
Unfortunately, with a 2V full-scale panel meter, the CMETER divider would be R12=0R/R11=open/no populate, which means we won't have an opportunity to fit a calibration pot. As such R2 (0.1R) would have to be very accurate or at the least slightly higher, which would at least give us the opportunity to put a high-value resistor in parallel to adjust it down to exactly 0.1R.
The brain hurts from too many trade-offs to consider.
@KleinsteinIt's true that the LM324 unit A buffer isn't needed as CREF is already driven by opamp U4A, thus CREF can be tied to the first resistor in the inverting amp. However, I believe VMETER (R18/R17 divider center) requires a buffer (unit C) into its inverting amp, otherwise the transformation won't be correct.
Even-though the LM324 (powered by a +/-12V supply) would be working with small voltages (<30mV), the op amp inputs never come close to the rails. If there is (crossover) distortion close to 0V maybe another op amp like the TL074 would be more appropriate.
Attached you will find another wiring revision for 2V full-scale panel meters (vs for 200mV FS meters in my previous post).
Changes are:
- R12=0R/R11=open/no populate, the entire R2 Vdrop goes directly to the panel meter.
- Addition of a high-value resistor in parallel with R2 for the option to lower R2 to exactly 0.1R; can't increase it.
- CREF inverting amp is now at a modest 20:1; changed to 100K (left resistor).
- R18=100K/R17=1M is now at a modest 11:1; changed R18.
- VMETER inverting amp remains 1:1 except for the calibration pot raised to 2.2K.