Nuts and Volts - May/June 2018

voltage will not generate power.

Open a circuit and no power will be used in what used to be a circuit.

The Trick Question

In Figure 3, we have broken the circuit at the top. This is where we had 0.1 amps of current just before the break. The trick question is: “With the voltmeter leads across the break, what does the meter read?”

Hint: A good quality voltmeter will have a very high impedance. It will draw very little current when operating to minimize the effect that it has on the voltage being measured. In our case, the voltmeter is perfect and has an infinite impedance. Nary a charge shall pass; that is, current is zero.

Trains of Thought

If you are already sure of the answer, then this article is not for you. Some engineers and technicians will recognize immediately what is going on here. If you’re the least bit unsure, bear with me. This may be of some value.

Train A

“There is no current, so there must be no voltage. The meter would read zero.”

This train doesn’t get you there. This is the most common rationale that I have seen for this presentation.

The problem is that its premise is false. We can have voltage with no current. It’s called static electricity. Static electricity is electricity with no charge flow. Although batteries will have a small current flow and eventually self-discharge, you can buy a package of alkaline batteries that are a year old and still use them to provide a power source until they are depleted. Voltage can be present on the batteries without current flow all the time they are in the package.

Train B

“The meter completes the circuit, but I don’t know what the current would be.”

This train doesn’t get you there either. If the meter has an infinite impedance, then it really doesn’t complete the circuit. In this “circuit,” there is zero current flow. It’s essentially not a circuit.

Train C

“The resistor is still in the circuit. It must drop some of the voltage. However, without knowing the meter resistance, we can’t know for sure what it reads.”

Again, this train doesn’t get you there. The meter has an infinite resistance; therefore, the current through it and the resistor is zero.

Train D

“The meter resistance is far higher than the 100 ohm resistor. All the voltage from the voltage source must drop across the meter. I say 10 volts.”

Train D gets you there. We can have voltage without current. The meter has a very high impedance (much higher than the 100 ohm resistor). All the current is blocked by the meter. The voltage across the meter must be 10 volts. The entire supply voltage is dropped across the meter.

Reality

We can get a better feel for why this is right by setting the trick question up as a voltage divider as shown in Figure 4.

I also took the liberty of changing the meter impedance from infinite to a more real value. Here, we have a one million ohm resistor where the meter was placed. We can calculate

54 May/June 2018

During troubleshooting, if two devices are in series, we can sometimes tell the one that is bad by simply measuring the voltage across them. If the voltage is all across one device, then it must be preventing current and is possibly the bad device.