By LAWRENCE M. WALDEN
This handy little box permits your direct reading of Megohms from the face of your DC voltmeter
---- THE MEG-O-DAPTER IS A COMPACT UNIT. Removing some screws and lifting
the front cover reveals the two circuit board-low voltage and high voltage.
Box sides have slots to hold printed-circuit boards in place.
WITH THE TIGHT ECONOMY, MANY OF US ARE EXTENDING THE life of home appliances, electrical tools, test equipment and salvaged parts. This can be done with considerable savings; but, is it safe? Many technicians depend on their volt-ohmmeters (VOM) even for insulation testing. For AC line-operated equipment, the potential used in the ohmmeter circuits are not high enough compared to Megohmmeters, or meggers, which have potentials from 500 to 1000-volts DC. Now, a simple adapter, we call it the Meg-O-Dapter, can be used with most analog multimeters without any modification to the meter circuits. This adapter provides an open-circuit output variable from zero to 1000-volts DC. Although these high potentials sound frightening, current levels are only 50 or 60 microamperes. Any charge on capacitances in the unit tested is dumped, or discharged, through normally closed contacts of the momentary-contact power switch.
Speaking of Megohms
Insulation resistance of an appliance may vary depending on age, moisture, corrosion, and dirt. Resistance will vary from several Megohms to infinity. Most low readings are caused by moisture and can be corrected by drying the appliance in a low-temperature oven. Heat will also cause lower readings so a second reading should be made after cool-down. In cases involving grease or carbon from brushes, washing the appliance with a cleaning solvent and re-drying can help. For harsh environments, insulating varnish can seal motor or transformer windings after cleaning and drying.
Three different model multimeters were tried requiring from 660 to 780 volts DC. This was the open- circuit voltage after zeroing. The first multimeter model was calibrated in Megohms multiplied by three; the second, calibrated in Megohms divided by two; and the third, calibrated directly in Megohms. All models were accurate over the entire scale with center-scale readings of 15 Megohms and full-scale reading of 2000 Megohms. The Meg-O-Dapter must be calibrated for each model but requires only one calibrating resistor.
All capacitors, excluding electrolytic types should show an almost infinite leakage resistance within their rated voltage. With the ZERO control on the Meg-O-Dapter adjust- able from zero to 1000- volts, DC, any selected voltage may be used. Resistance readings at reduced voltages will be incorrect but this is not important for insulation leakage tests. Small capacitors, even below 50 pF, will show a charge "kick" on the meter.
All capacitance, which includes wiring, motor windings, etc. can store high charges when there are no leakage cur- rents. For safety, a momentary contact power switch built into the Meg-O-Dapter is used to discharge, or dump, this capacitance.
The Meg-O-Dapter is powered by a 9-volt transistor battery and will operate between 6.5 and 10 volts with a current drain between 15 to 25 milliamperes. With only two controls, power ON /OFF and ZERO control, operation of the adapter is simple.
1. Plug output leads into the VOM and set to selected DC voltage range.
2. Short input leads depress power switch, and adjust ZERO control for zero reading on meter's ohms scale.
3. Release power switch and connect leads to circuit or component to be tested
4. Press power switch on for direct meter readings.
For lower voltage testing, controls are set as above except for ZERO set in step 2. With leads shorted, adjust ZERO control for pre-determined point on meter which will give the desired open-circuit potential. External capacitance, depending on value, will require time to reach full charge. This can range up to several seconds for large values.
---------- REMOVE THE BACK COVER and you'll discover that the transformer
and battery are mounted on it. Be very careful when laying out the parts
location before you drill holes-check clearances.
Circuit description
The schematic diagram for the Meg-O-Dapter (Fig. 1) is shown with the momentary contact pushbutton switch S I in the OFF position. This switch, a double-pole, double-throw type, is wired with S1-a in the normally-open position; and S1I-b, normally closed. In the off position, S1-b will discharge any capacitance across the input through R14, a 1000-ohm resistor.
When depressed, S1-b opens and S 1-a closes to supply power to transistors Q1, Q2, and IC1 and their associated circuits. IC1 , a 7555 timer, operates as an astable multi-vibrator producing a nine-volt squarewave output. This output connects to ZERO adjust potentiometer R4. The variable output from the wiper of R4 feeds through capacitor C3 to the base of Q1 . Emitter current from Q1 passes through R6 to the base of Q2 which drives transformer T1 . A high voltage develops across the secondary of T1 which feeds a voltage doubler composed of D1, D2., C5, and C6. This high voltage DC controlled by ZERO control R4, can be varied from zero to over 1000-volts DC. Three 10- Megohm bleed resistors, R7, R8, and R9 connect across the high voltage connected in series with a test jumper. For open-circuit voltage measurements, the jumper is removed and replaced by a microampere meter. The open-circuit voltage will equal the microamperes x 30 Megohms.
The positive high voltage also passes through resistors R10, R11, R 12, and calibration potentiometer R13 to the INPUT jack J1 . Any leakage current, from the circuit under test, returns to the negative INPUT jack J2 and is connected in series with the VOM before returning to the negative side of the high voltage source in Meg-O-Dapter.
For meter protection, and the reduction of resistors required in the calibration circuit, the VOM is set to a DC voltage range. Depending on the VOM selected, a range of 50 to 100 volts or more can be used. Once calibrated to a particular VOM, recalibration would be necessary for use with any other make or model.
FIG. 1--COMPLETE SCHEMATIC DIAGRAM for the Meg-O-Dapter clearly illustrates
the isolation be- tween the low- voltage circuit and the high- voltage circuit.
The 7555 CMOS integrated circuit is identical in function to the common 555
chip but draws very little current, greatly extending the useful life of
the transistor battery (B1) power supply. The Stancor transformer specified
for T1 in the Parts List should not be replaced or substituted. Switch S1
is a spring-return type shown in the schematic diagram in the at-rest position.
Check out transistor and diode connections.
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PARTS LIST FOR MEG-O-DAPTER
SEMICONDUCTORS
D1, D2--1000-PIV silicon diode rectifier, axial leads; SK3081/125, 1N4007, or equivalent
IC1--7555 CMOS timer
Q1- 2N1711 NPN silicon transistor
Q2- 2N3439 NPN silicon transistor
RESISTORS
All fixed resistors are 1/4-watt 10 %, composition types unless otherwise noted R1, R3, R6-1000-ohms
R2- 56,000-ohms
R4-- 10,000-ohms, 1/2-watt carbon potentiometer
R5- 10,000-ohms
R7, R8, R9- 10- Megohms, 5%
R10, R11, R12--Selected values, see text
R13--1-Megohm, multiple-turn trimmer potentiometer, PC mount R14-1000-ohms, 1-watt
CAPACITORS
C1--0.1-uF, 10% Mylar
C2--0.01-uF, 10% Mylar WVDC
C3--1-µF, 50-WVDC electrolytic
C4--100-uF, 25-WVDC, electrolytic
C5, C6--0.01-µF, 1000-WVDC disc ceramic
ADDITIONAL PARTS AND MATERIALS
B1--9-volt transistor battery
J1, J2--Banana-plug jack, one black, one red
J3, J4--Pin-plug jack, one red, one black
S1-DPDT, spring- return, pushbutton or toggle switch
T1-Stancor P8390 117/12-VAC, 150-mA secondary winding, power transformer Pomona 3301 aluminum case, knob, PC board materials, battery clamp, 8-pin DIP socket, hardware, wire, solder, etc.
==========
FIG. 2--LOW-VOLTAGE circuit board shown here holds most of the components
for this circuit. Board size is critical should you use the Pomona case specified.
Should you use a different size case, you may want to resort to a perfboard or wire-wrap techniques--it's up to you.
FIG. 3-HIGH-VOLTAGE circuit board is a bit tricky to wire because there's
not much room inside the Pomona case. Some parts are mounted on the foil
side, and others on the reverse side.
Resistor are axially mounted where space demands.
Construction and calibration
All parts and components are standard off-the-shelf items. Layout is not critical and can be made point to point on pert board. A model 3301 Pomona case was used measuring 4 1/2-inch long by 2 11/16-inch wide by 1 1/16-inch deep. Two circuit boards were used measuring 1 1/2-inch x 2 3/8-inch with all low-voltage components on one board and all high-voltage components mounted on a second board. Transformer T1 and battery B1 are both mounted on the bottom cover.
Pin jacks are used for the VOM output, J3 and J4, and banana jacks J1 and J2 used for the inputs and were mounted near the top of the top cover. Power ON /OFF switch S1 and ZERO control R4 were mounted on the top cover between the low-voltage board and T1. Due to the close tolerance of the above components, care should be used in their location.
The low-voltage PC board (Fig. 2) can be completed and the voltage doubler with the bleed resistors installed on the high-voltage PC board (Fig. 3). Connect a micro amp meter in place of the test jumper. See Fig. 4. Press power on and turn ZERO control, R4 to maximum. Output current should range from 30 to 35 microamperes which would equal 900-to 1050-volts DC open circuit.
For calibration, a 30 to 50 Megohm resistive source should be used. The exact value is not important for meter scale selection but accuracy will depend on the values selected. For example, 3 or 4 ten Megohm resistors may be series connected but they should be 1% rated.
The VOM to be used can now be connected to the circuit in the 50 to 100-volt DC range. Values for resistors R10, R11, R12, must be selected. These values will vary depending on the meter used. Begin with R10 at 4.7 Megohms, R11 at 2.2 Megohms and R12 at 1. Connect precision resistors to the input and adjust ZERO control R4 for the exact reading on the meter scale. On some models this reading can read directly in Megohms. On one model VOM, the readings were in Megohms times 3. One model produced readings in Megohms divided by 2. After adjusting the ZERO control to read the precision resistor value, short the input leads and adjust R13 for a zero meter reading on the ohms scale. If meter fails to reach zero, reduce the value of R11 and R12.
Should the meter go beyond zero, increase the values of R 11 and R12. These values are not critical as long as R13 is in adjustment range. It will be necessary to repeat this several times. Accuracy will depend on the calibration resistors and the meter itself.
With the VOM calibrated, open-circuit voltages can be selected. Connect the micro-amp-meter to the test jumper and set ZERO control R4 for the desired current. For example, 3.3 microamps through the 30-Megohm bleed resistors would equal 100 volts with the input open.
Now short the input leads and record meter readings in Megohms. This can be done for any value selected up to and including the zero set point. As mentioned above, resistance readings will be incorrect but are not important for insulation tests.
As with most ohmmeter circuits, the set points and zero adjust will depend on the battery voltage. For this reason, ZERO control R4 was not calibrated. With an average battery drain of 20 milliamperes and useful operation down to 6 1/2 volt, the life of B1 should be about normal shelf life.
As mentioned above, lower ranges could be calibrated and include a range switch, but for practical purposes, this does not seem necessary. What is important is that hazardous and deadly current leaks in appliances, electrical equipment and test instruments be detected by Meg-O-Dapter and not a human.
FIG. 4--PARTS-LOCATION DETAILS for both circuit boards. Note that the boards
are electrically isolated except for common transformer T1 and switch Si.
All parts are mounted on the foil side of the low-voltage PC board, whereas
both sides of the board are used to mount parts on the high-voltage PC board.
In the diagram both boards are shown foil up. Hole A was drilled by the author
to pass battery B1 leads. In your configuration in the Pamona box, one or
more others leads may T pass through hole A so drill a slightly larger hole
to cover any contingencies. Refer to Fig. 1 when connecting the external
leads to switch S1. Photo of high-voltage board is a bit different from drawing
below because of last minute changes. Use the diagram.
Adapted from: Radio-Electronics--Special Projects (Summer 1983, #7)
= = = =
Radio-Electronics--Special Projects (USA print magazine)
Also see: Scope Calibrator