Precision multimeter calibration reference

Step 8: Operation notes

Picture of Operation notes
I thought I'd better put in some notes on how the reference works, for the benefit of those who are just getting started in electronics. If I've got anything wrong, I'd be grateful of anyone pointing it out. I realise there's some over-simplification here - but I wanted to cater for everybody.

The unit is designed for battery operation, because batteries provide a nice clean, smooth source of power.

Reverse protection
I found this circuit on the internet. This is a P-type MOSFET is connected with it's source and drain the opposite way to normal applications. When the battery is connected correctly, the transistor's intrinsic diode conducts the first bit of current, allowing it to be biased on and into saturation, providing a very low resistance path. If the battery is connected the wrong way, the transistor is biased off and no current can flow.

For those who don't know, a standard crowbar protection circuit works by using a diode to conduct reverse battery current, which blows the fuse to prevent damage. The disadvantages of this circuit are it's response time and reliability. I could have used a series diode, but didn't want to sacrifice the 0.7 of a volt which it would drop, although a shottky or germanium diode would present a lower voltage drop of 0.1 to 0.2 volts, the mosfet is still better.

Low battery warning and power light
It took me quite a lot of head scratching to work out the battery warning light - and since discovered I'd re-invented a well known design.  With a good battery, there is sufficient difference in voltage between the base and emitter of Q3 for it to conduct and keep Q2 biased off, which keeps Q5 on to drive the power LED. As the battery voltage approaches the the zener voltage (D2 is to offset the base-emitter drop of Q3), a point is reached where Q3 turns off, allowing Q2 to turn on and light the battery warning LED, and turning Q5 off. The voltage drop across R1 is negligible in normal operation, however when the battery warning LED starts to conduct the voltage drop increases, reducing the voltage at Q3 emitter, turning it off sharply. This also introduces a bit of hysteresis (if for some reason the battery voltage rises a little, the warning light stays on).

For this, I initially built an emitter follower regulator, a very basic form of that used in traditional linear power supplies - all it does is amplify the current from the zener's feed resistor. Precise regulation is not important as it's function is merely to stabilise the supply to the reference chip as the battery decays, and protect it from an over-voltage power supply. The two capacitors are to clean up any noise generated by the zener and various transistors.

This design is not good for battery powered applications because the base-emitter voltage drop means the voltage at the emitter will always be 0.7V less than the voltage at the base, meaning the battery has to be at least this amount higher than the desired output voltage. Using the low-dropout arrangement with a P-type, low Rds(on) mosfet eliminates this problem as a decrease in gate voltage increases the voltage at the drain, the output. You can use a PNP transistor instead, with its collector connected to the output, and emitter as the input, but while still better than an emitter follower, this isn't as good as the mosfet. Since the transistor is providing inversion, the error feedback is connected to the op-amp's non-inverting input.

Extra current capability
The transistor and resistor which provide this are controlled by the amount of current the reference chip is drawing. When the current drawn by the reference chip reaches about 3.5mA, the voltage across R10 increases to around 0.7V and it starts to conduct and supply the majority of the current. Intersil specify this resistor as 200 ohms, and the circuit to supply up to 50mA.

Output divider
The potential divider is calculated from the need to divide the output of the reference by at least 26. The values were chosen to provide the lowest resistance possible across the meter probes in order to minimise errors introduced by the meter's internal resistance, without exceeding the maximum rated current for the reference chip (although this is moot, with the presence of Q5). The voltage across the resistors is given by Ohm's law and the formula for series resistance:

Rtotal = R1 + R2 + R4 + R5 ..... + Rn
I = V/R, R = V/I, V = I*R

So, Rtotal = Rx + Ry = 806 + 29.4 = 835.4
I = 5/835.4 = 0.0059851
V across Ry = I * Ry= 0.0059851 * 29.4 = 0.1759636

Rounding this figure up gives 0.176 with an error of +0.0000364, or 176mV including an error of +36.4µV

Adjustable output
This is provided by a potentiometer buffered using an op-amp. The inverting input of the op-amp is connected to it's output to create 100% negative feedback, so that the output will follow exactly the voltage at the non-inverting input. A multi-turn potentiometer would be better, but these are expensive so I've used the following: One potentiometer is the main, "coarse" control, the other is connected as a variable resistor and acts as a "fine" control by changing the total resistance in the circuit by about 1/10th of the value of the coarse control. A better circuit is to use a dual gang potentiometer for the "fine" control, as shown here using Circuit C (but without R1). The op-amp is chosen by being what I happened to have - it was that or an NE5532, and the LM358 seemed to be more suitable. The output goes sufficiently close to ground to not register a voltage on my meter. This should not be considered a stable output, and the design certainly doesn't do justice to the precision the reference chip can provide.

Range switch
The switch is connected so that two of it's poles complement each other, one is connected to the 3 alternative outputs, with the first position left unconnected, and the other pole connected in series with the battery, with the positions corresponding to the outputs are connected together, and the first position left unconnected.
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