stan2004 posted:Do NOT try this at home.
Here's a DIY modification to the combo Voltmeter-Ammeter module willygee introduced to us at the beginning of this thread - less than $10 (free shipping) on eBay. As discussed earlier, I suggested there are 3 modifications required to make this self-powered combo meter better suited for O-gauge AC transformer monitoring.
Task 1. Reduce the operating voltage range so it runs at typical AC Accessory/Command voltages...or, say, 12V-20V AC rather than the 60-80V AC specified in the stock version.
Task 2. Increase the resolution 10x so that it read Voltages with 0.1V resolution (rather than 1V for the stock version)
Task 3. Add the decimal point to the LED display to indicate the Voltage is indicating 0.1V resolution
I made these modifications. First the proof of life video:
This is a very tedious modification and I don't wish it on anyone. But since I did it, I minds as well show what I did. This is just my 2 cents - no warranty, no refunds!
Task 1. The module indeed uses the "red capacitor" to drop the AC voltage. The stock unit has a 0.47uF/400V capacitor. I placed a 2.2uF/50V in parallel to reduce the capacitor's voltage drop at 60 Hz so that it can operate at O-gauge transformer voltages.
Task 2. The module has 1V resolution. There is a 2M Ohm resistor which is a main voltage dropping/divider resistor for the Voltage measuring function. I added a 100k Ohm resistor in parallel so there is less drop. The same trimmer/calibration trimpot (labeled VR) was used.
Task 3. The most tricky modification is almost cosmetic in that all it does is turn on the decimal point (DP) LED in the Voltage readout. This is not as "simple" as turning on a discrete LED. The LED display in the module is a so-called multiplexed display where timing is everything. You have to turn on the decimal point LED for the center Voltage digit only when the center Voltage digit is active which is essentially 1/6th of the time since there are 6 digits. So a circuit with a PNP transistor, diode, and 2 resistors must be spliced in to the existing circuit. This could have been effected with a line or two of code change in the software for the microcontroller chip but we have no access to this code nor the ability to re-program the chip. The before-after schematic is shown below.
The locations of where to tap/splice into the module are shown in the following 2 before-after photos; these are high-resolution shots so should be click-able to zoom in and see exactly where to mods are made:
All components were from my parts stash but $1-2 if purchased. Unfortunately a LOT of grief and tedium!
And while my interest is/was to monitor command/accessory voltages/currents, I understand some guys might want to measure down to 0V AC. Here's a hint. The dropped AC voltage goes through a bridge rectifier, into a zener diode that limits the DC voltage to about 11V DC, and into a 6203A voltage regulator IC with 4.4V DC output.
Stan
Thanks for sharing the information. It looks like the modified meter somewhat reads the same as the Z4000 meters. Did you have a chance to compare the V & A reading of the modified meter with a Fluke or something similar. Also, if we want to duplicate what you did, what was the ordering info for the meter you modified.
Thanks,
Bob D
