Upgrade for the Digital Insulation Meter
The Digital Insulation Meter described in June 2010 allows the insulation of mains-powered equipment to be tested with up to 1000VDC. But if the outputs are accidentally shorted, it can potentially be damaged; the 4.7kΩ 1W current-limiting resistor is the most likely casualty.
Ian Malcolm is this month's winner of a Peak Atlas Test Instrument!
But there’s a simple change which can be made that protects this resistor (and other components) and reduces the battery drain in case of a short circuit (or a near-short). Just one additional diode (D4, a 1N4148) is required.
If a lot of current is drawn from the output (as in the case of a short circuit between the output terminals), the current feedback signal to pin 1 (AN2) of microcontroller IC3 will go much higher than usual.
In the original design, op amp IC2b was unused. For the upgrade, it is used as a comparator. It compares the feedback voltage to the +3.2V reference voltage at Vref+ (pin 2 of IC3). Should the feedback voltage go above this level, IC2b’s output goes high and diode D4 becomes forward-biased, delivering current to the other feedback divider and so increasing the voltage at pin 5 of IC1, the MC34063 switchmode regulator.
This is the feedback voltage and so IC1 responds by lowering Q3’s duty cycle, reducing the regulated output voltage and so limiting the output current. This is the same response it would take if the output voltage were to go too high (which would also increase the feedback voltage).
The result is that the output current is limited to about 10mA under short-circuit conditions. This does not affect the meter reading as the voltage at AN2 is still above Vref+ and so the meter still shows 0MΩ.
If you want to modify the PCB, make cuts around pins 5 & 6 of IC2, to disconnect those pins from ground and then run thin wires between those pads and pins 1 & 2 of IC3. The 1N4148 can then be trimmed and soldered to pin 7 of IC2 and a flying lead run to IC1 on the separate board.