MO-1 PRECISION MILLIOHMETER - Well, some of the jobs I do at home are to repair audio amplifiers from some friends and some heavy metal bands that are friends of my friends, etc. The tough part, when repairing solid-state ones is to measure and choose good resistors for the power outputs (0,47ohm, 0,22ohm, 0,15ohm, etc), and match them with a reasonable difference. I had made some tests and the difference in the resistors values, for good balance, must be kept under 5%. And there are some cases when the equipment has resistors with none value assigned, and you need to measure it, for replacement and to adjust quiescent current, etc.

For lower values of resistance, it is hard to measure with the common Wheatstone Bridge, because if the resistor is not VERY near the bridge, the leads resistance adds to the value of the resistor, and even if you adjust the ZERO, including the wire leads, the result sometimes make you a bit confused because it changes when you move the position of the lead.

My personal experience showed that the better way to do it is by making a know current run through the resistor and measure the voltage drop across it. Another interesting aspect is that you can make the current on these high power resistors be high enough to warm it, and see if the resistance changes with the temperature (believe me I had cases that the damned amplifier had a problem when hot, and guess what: the resistors of the high current branch changed their ohmic value after a few minutes of heating).

Basically the milliohmeter MO-1 is a constant current generator and a voltmeter.

It has 6 plugs:

– Four (2 black & red pairs) are used for 4 wire measurements, when the resistor is mounted, or when I can’t put it directly into the milliohmeter. One black & red pair is the current source and the other pair is the voltage sense;

– Two of them are gold-plated and I use it for direct readings. Of course, internally they are interconnected, so even when using the gold-plated ones the measurement still being done in the same way as the 4-wire leads.

You can se by the pictures that I used huge wires to connect the plugs, in an attempt to reduce the resistance internally. It don’t make not much sense at all, since the principle of the measurement is by voltage drop, the current  is held constant despite of the load, and the input impedance of the voltmeter is extremely higher than the resistor being measured, so there is no difference in using smaller wires than those I used.


The schematic of the current source and the entire unit can be seen here.


         I have bought on Santa Ifigênia, an Omron digital indicator, and 6 digits. It was sold “as is”, totally dead, I paid something about USD3,00 on it. Firstly I have bought it just to use its box and display, but just a blown fuse and a little transistor brought the unit back to life. It is programmable, and easy to use. Its programmable decimal point allowed me to use with 100mA current, and the reading is direct on the panel in Ohms.

I had already preset the Voltmeter to move the decimal point to the right, multiplying the value by a factor of 10. In this manner, 1.0000V reads 10.000V. The reason is because with the use of 100mA current the reading for a 1-0hm resistor in a normal voltmeter (without scale correction) is 1ohm x 100mA = 0.1V. 

         The current source is as simple as possible, using a 0,5% bandgap reference TL431BCLP for monitoring the current, and limiting the current to 100mA. The equations of this circuit are given below, if you want to change it or use in other application whatsoever. (Reference: Texas Instruments Data Book – Power Supply Circuits, 1996, page 3-108, fig. 27). 


        A tap in P1 monitors the voltage drop across R2, and it is held in 2.495V, and the T1 base driving provided by R1 is reduced if the load “ask” for more current, maintaining the current in the adjusted value.  

        The power input come from a 12+12VDC transformer, rectified D1 and D2 and filtered by C1, L1 and C2. Across the output are 4 diodes D3-D7, to limit the voltage in the input of the Voltmeter, avoiding it to be damaged. When not measuring, if the current switch is pushed, the instrument shows a flashing 23.148 (the decimal point is changed, actually it shows 2.3418), indication that the limit of 1.9999 volts is reached on its input.


         Operation – impossible to be easier:    


         The versatility of the Omron module is such that I can compare two resistors by using the “Offset Zero”. I put one reference resistor on the plugs (or leads) and press “Null”. Now all values will be compared with this reference, and the display shows only the difference between the reference and the other resistors under test.

         Other usage I gave to the instrument is to check the Forward drop voltage in diodes and other bipolar transistors. As these devices do not follow Ohm’s law, I have to divide the reading by 10, or just change the Decimal point on the Omron’s menu. Just note that the current is 100mA and some devices can be permanently damaged at this current. Reduce the current in most cases, just for safety.