microammeter DVM add-on, part 2
Here is the schematic of my version of the microammeter add-on circuit for the Metex ME-11 DVM
 |
| schematic with slight changes |
I didn't have any LF351 op amps so used a TL081 which was a popular op-amp with JFET input transistors. The TL081 JFET inputs are very high impedance, the input bias currents are typically 1 pA so nearly all of the current flows only through the current sensing resistors connected to either pin 2 or 3.
When the bypass button is pressed the DVM will indicate the current up to 4 mA and then after the startup current dies down the button can be released and the lower current will be measured using either of the two ranges. The 10 k ohm resistor was included to allow reading higher currents like the 7 uA from the LM2936 regulator. Using a separate switch for bypass allows the circuit to ignore any complications from shorting versus non-shorting switching. The error is 0.1 percent for 100 ohms in parallel with 100 k ohms. The error is 1 percent for 100 ohms in parallel with 10 k ohms.
The circuit is the keyer test fixture, the current being measured is just the keyer chip.
 |
| The Metex alone measuring active current of .64 mA, note the lit LED upper left |
 | ||
| The Metex alone measure sleep current of zero mA, LED is off |
 | |
| The microammeter with a 100 ohm sense resistor measuring the active current, LED on |
 | |
| The microammeter with a 100 ohm sense resistor measuring the sleep current, LED off |
 |
| Microammeter with 100 k ohm sense resistor measuring 0.6 uA of sleep current, LED off |
Note that the Metex alone readings and the microammeter readings aren't quite the same - some analysis is required to understand them. The Metex alone measuring a sleep current of zero would indicate to me that the reading is below .5 uA. The microammeter is reading a sleep current of .601 uA so if the readings were consistent the Metex alone should indicate a .001 mA sleep current reading (.6 uA rounded up). One thing that I see lacking is decoupling caps on the power supply pins. Also note that the active currents vary a lot during a message play, so the active readings shown here are literally snapshots.
More to follow.
Best Regards,
Chuck, WB9KZY