This is actually an improved version of the Digital Megohm & Leakage Current Meter we described in the October 2009 issue of SILICON CHIP.
Our original design had a distinctly mixed reception from some of our readers. It could be summed up as “OK but ….”
Fig.1: in this block diagram, the two sections of the circuit can be clearly identified. On the left is the power supply, consisting of a regulated 5V plus a high-voltage supply. On the right is the metering and display unit. These can be seen in the two separate PC boards below.
The first “but” was that it would not deliver the nominal test voltage of 1000V or 500V DC into the minimum load resistance of one megohm, as specified in the relevant Australian Standard, ie, AS/NZS 3760:2003.
The reason for this drawback was largely because we had set the internal current limit too low and partly because the DC-DC converter could not deliver the current required, even if the current limiting resistor had been removed.
Furthermore, some readers pointed out that the test voltage of 500V DC was too high for testing insulation of equipment with EMI suppression and MOVs (metal oxide varistors). These devices should be tested at no more than 250V DC.
Faced with that criticism, all we could do was to revise the design so that (a) the inbuilt DC-DC converter can deliver the full test voltage into a 1MΩ resistor and (b) provide the additional test voltage of 250V DC. In fact, the new circuit can deliver the test voltage of 250V or 500V into a load of 100kΩ, if required, for the testing of portable RCDs (residual current devices).
The physical presentation of the new meter is also quite similar to the original except that it now has a 3-position switch to select the test voltages of 250V, 500V or 1000V DC. Apart from the redesigned inverter section, the revised meter now has two current ranges instead of one, under the control of a PIC microcontroller.
As before, the Digital Insulation Meter is easy to build, with most of the major components mounted directly on two small PC boards. These fit snugly inside a compact UB1 size jiffy box, along with a 6xAA battery holder used to supply the meter’s power.
It can be built up in a few hours and for an outlay much lower than commercially available electronic megohm meters.
So to summarise, it can now test at 250V, 500V or 1000V and can measure leakage currents from below 1μA to above 6mA. As well, it can measure insulation resistance from below 1MΩ up to 999MΩ.
How it works
The block diagram of Fig.1 shows the arrangement of the new meter with its somewhat more complex DC-DC converter. This is on the left-hand side.
The metering section, on the right side of the diagram, is used to measure any leakage current which flows between the test terminals and from this it calculates the external resistance connected between them (knowing the test voltage in use).
In more detail, the DC-DC converter converts the 9V DC from the battery into AC, so it can be stepped up to a few hundred volts using an auto-transformer. The resulting high voltage AC is then rectified using ultra-fast diode D3 to produce the test voltage of 250V, 500V or 1000V DC.
We use negative feedback to control the converter’s operation and maintain its output voltage at the correct level. The feedback uses a voltage divider (RD1 and RD2) to feed a small proportion of the high voltage DC output back to one input of a comparator inside IC1, where it is compared with an internal 1.25V reference voltage.
The output of the comparator is then used to control the operation of the DC-DC converter, turning it on when the output voltage is below the correct level and turning it off again when the output voltage reaches the correct level.
Inside our Mk II Insulation meter. The
PC board in the bottom of the box is the high voltage generator; the board "hanging" from the front panel handles the metering and display tasks.
The basic voltage divider using RD1 and RD2 alone is used to set the high voltage level to 250V, with multi-turn trimpot VR1. To change the test voltage level to 500V or 1000V, switch S1 is used to connect RD3 or RD4 in parallel with RD2, increasing the division ratio of the divider and hence increasing the output voltage maintained by the feedback loop.
Note that the converter generates the test voltage only when TEST button switch S2 is pressed and held down. As soon as the button is released, the converter stops and the high voltage leaks away via RD1 and RD2/RD3/RD4. This is both a safety feature and a simple way to achieve maximum battery life.
Referring back to Fig.1, the meter section uses a shunt resistor connected between the negative test terminal and ground to sense any leakage current IL which may flow between the test terminals. It is the voltage across this resistor which we measure, to determine the leakage current. The effective shunt resistance is switched between 100Ω and 10kΩ to give the meter two measurement ranges. The switching is done using relay RLY1, under the control of the PIC microcontroller (IC3) inside the metering circuit.