A low resistance range extension unit for the Avo 7

[Phil G4SPZ]

If you restore Avometers it is necessary to measure the value of shunts, and sometimes make new ones, the lowest of which is 0.004 ohms as used in the 15/30 amp range on the Avo Model D. Granted, these hefty copper straps don’t get burned out very often, but it’s more common to find burned shunts on other models of Avometer. This simple low resistance range extension unit will enable them all to be measured, and will allow replacement shunts to be constructed from brass strip or copper wire by actual measurement of the resistance.

Perhaps I should first explain why this circuit has been designed to suit the old Avo Model 7 and Model 40, and not more popular models. The simple reason is that, crucially, these meters have a variable shunt in the form of the Q-knob, which renders this simple circuit independent of battery voltage. The principle is similar to that used in Avo’s contemporary Resistance Range Extension Unit, but I have increased the test current which makes this design ten times more sensitive.

The unknown resistance is connected in a series circuit with a 1.5 volt alkaline or 2 volt cell sealed lead acid cell, typical of what any pre-war service engineer would have had to hand. The 1-ohm resistors limit the test current to around 1 Ampere. The 0.1 ohm 1% resistor develops a voltage drop in the region of 100mV which is used to standardise the meter, using the Q-knob to set the reading to FSD on the Model 7 switched to its 2mA DC range. On the Model 40, use the Q-knob to set the reading to 100mV on the 120mV DC range. The meter prods are then moved and the voltage drop across the unknown resistance is measured, which provides a direct indication of its value as a percentage of 0.1 ohm. This is of course a linear scale, as we are measuring voltage rather than resistance.

Note that the meter must be set up using the Q-knob for each measurement. It is advisable to disconnect the battery as soon as the reading is taken; a typical alkaline D cell has a capacity of at least 8-10 Ah, although capacity reduces rapidly with increasing discharge current, but a good quality cell should still last a very long time in this application.

I haven’t provided any constructional details as the circuit is so simple. My version was built on a tag strip, but works just as well as a lash-up on the bench. The only critical component is the 0.1 ohm 1% resistor which determines the accuracy. It’s advisable to use short, stout wiring, and to connect the meter prods directly across the terminals of the unknown resistor to remove the effect of wiring resistance. To measure higher unknown resistances, between 0.01 and 1.0 ohm, simply calibrate the meter to FSD using the voltage drop across one of the 1 ohm resistors using the meter on its 1 volt or 1.2 volt DC range. Resistance values above 1.0 ohm can be measured quite accurately using the meter's built-in 1,000 ohms range.

To avoid testing the Avometer's cut-out unnecessarily violently, disconnect the battery before disconnecting the resistance under test!

[Ed_Dinning]

Hi Phil, I think there may be problems if you use Brass or Copper strip for the shunts as these have quite large positive temperature coefficients.
Best materials for these shunts is one of the low temp coefficient materials such as Nichrome or Constantan of a suitable size. Note that it does not always solder well and may need to be clamped. If anyone needs any I have a range of sizes an can supply for postage.

Ed

[Phil G4SPZ]

Hi Ed, good point. I'm doing a few tests on bits of wire I have lying around. But thank you very much for the generous offer of nichrome or constantan, much appreciated

Phil