Most multimeters can measure up to only 10 or - more rarely - 20 amps of current flow. Pump 20 amps through a typical multimeter and you'll get a blown fuse or maybe even a melted multimeter! But in many automotive applications, even 20 amps is too little.
In fact, 10 amps at a running car voltage of 13.8 volts equals just 138 watts, while even 20 amps is only 276 watts. That means it can be impossible to measure the current draw of all the lights with the headlight high beam on, you cannot measure starter motor current draw or maximum alternator charging, and the current flowing to a big car sound amp is also an unknown. Add to this the fact that the circuit in which the current flow is being measured always needs to be broken (ie the meter needs to be inserted into the circuit) and the practical difficulties of measuring high currents become immense.
However, there are devices available that allow the no-pain measurement of currents up to hundreds of amps. The circuit doesn't need to be broken and the meter won't get hot. It's also safe and easy. And what are these devices called? - current clamps.
Many current clamps can measure AC currents only, which is not useful in automotive applications. Those that measure DC (direct current) do so by using a Hall Effect sensor to measure the strength of the magnetic field produced in a ring of material that is temporarily placed around the current-carrying wire. The greater the amount of current flowing in the wire, the stronger the magnetic field that is produced in the ring. (Most current clamps that can measure DC can also measure AC, using a different technique where the clamp acts as part of a transformer.)
In addition to AC-only and AC/DC designs, current clamps are available as standalone accessories or built into multimeters.
When the clamp is a standalone design, it outputs a precise voltage per measured amp. For example, it might have an output of 1 millivolt per amp. This makes measuring the clamp's output easy - if the multimeter shows a measurement of 5 millivolts on its voltage scale when connected to the operating clamp, the current flowing in the wire is 5 amps. If the voltage displayed on the multimeter is 100 millivolts, the current flowing in the wire is 100 amps.
In designs where the clamp is built into a multimeter, measuring the current flow is as easy as selecting 'current' on the rotating selection knob of the meter. The value is then displayed in amps. Most of these meters can measure both AC and DC.
All sound very expensive? It's not - a simple kit for an AC/DC standalone current clamp is AUD$35 and multimeters with built-in AC/DC clamps start at about AUD$160.
When using a current clamp its jaws are opened, the clamp passed over the wire, and the jaws closed. The wire is then centred in the opening and the measurement made. Note that it's the individual conductor that is measured - not a cable containing both earth and power leads, for example.
Current clamps are not particularly good at accurately measuring very small currents. This is so for two reasons - firstly, if the output scale of the clamp is 1 millivolt per amp, a current flow of 0.5 amps is only 0.5 millivolts - a figure that is getting very low for many multimeters to accurately measure.
Secondly, because of the influence of stray magnetic fields, current clamps need to be zero'd before they can be used. That is, a knob on the clamp first needs to be turned until the current reading is zero - obviously, when there isn't any current flowing through a wire inside the jaws! In normal use, the clamp is zero'd with the clamp away from the wire that's to be measured. However, getting an absolutely precise zero can be a fiddly and time-consuming job, especially if the meter has good measuring resolution. Typically, the clamp might end up not being exactly zero'd, but instead might be showing on the meter the equivalent output voltage of up to 0.3 amps before measurement begins. If - say - 75 amps is being measured, 0.3 amps is a trivial amount. But it would be much more important in a measurement of only 2 amps...
For these reasons, current clamps are usually used for current measurements of about 5 amps and upwards. Note, though, that with very careful use, a good current clamp can measure tenths of an amp.
Another negative is that most current clamps have large jaws. This is so that firstly, they can be opened enough to slip over large diameter wires, and secondly, the magnetic core is less permanently magnetized by the current flows. The latter refers to the fact that in some designs, the core itself can start to become magnetized, giving a wrong output. (More on this in a moment.) Large diameter, specially constructed cores are less susceptible to this. The negative is that large jaws can make the clamp awkward to use in confined spaces.
So a current clamp probably won't replace the use of a conventional multimeter in all current-measuring situations, but it will allow the much easier and effective measurement of current on many occasions.
The Cheap Kit
The Australian electronics magazine Silicon Chip has developed an AC/DC current clamp design which is available in kit form (from retailers such as Jaycar Electronics - www.jaycar.com.au - where it's cat no KC-5368) for about AUD$35. This is extremely cheap, but the kit does have some significant drawbacks.
Most importantly, because it uses a small magnetic core that is not designed expressly for current clamp applications, the clamp gets magnetised fairly fast. This has two outcomes. Firstly, the clamp needs to be re-zero'd frequently and secondly, current cannot be monitored over a period, as the reading will become more and more in error as time passes. When measuring major current bursts (eg of a starter motor), the degree of magnetism is such that the clamp assembly needs to be de-magnetised by being reversed in its orientation on the wire and then having the same level of current pass through it.
However, if you need to use a current clamp just occasionally for a one-off measurement, it's ideal.
If building the electronic kit, be careful of a few points. Firstly, the Hall Effect leads are fragile where they enter the sensor, so they should not be bent close to the body of the sensor. Secondly, you may need to do some work with a hammer and a pair of pliers so that the jaws of the clamp (a modified battery clamp) always self-align. Finally, look carefully at how the PCB (printed circuit board) is mounted to the box lid and position both the switch and the pot at appropriate heights to mount the lid clear of the other components.
The specifications of this current clamp include a maximum DC amperage of 150 amps (900 if the core is demagnetized afterwards) and a resolution of 100 milliamps. However, our testing indicated that it would be preferable to use the clamp only when measuring currents above about 5 amps. Its output is 1 millivolt per amp.
The clamp adaptor has no indication of battery strength - nor even that the adaptor is switched on - so it is wise to turn it on only when it is being zero'd, make the measurement, then turn the adaptor off immediately after that.
The Expensive Clamp
Fluke is one of the premier names in electronic test equipment and as you'd expect, their i410 current clamp suffers from few of the drawbacks of the DIY kit clamp. However, it is about tens times as expensive! Its specifications include a current measurement range from 1- 400 amps (DC or AC) and an accuracy of plus/minus 3.5 per cent plus 0.5 amps.
It will work with any multimeter or as shown here, with a Fluke Scopemeter which also allows logging of current trends and allows the current waveform to be shown. Display devices that have the ability to sample quickly and record peaks can show some interesting data - here the recorded maximum reading is 0.32 kilo-amps - that's 320 amps. It was the peak current drawn by the starter motor when the car was started....
If you're often frustrated by the highest current reading that's possible on a normal multimeter, have a look at the option of a DC current clamp. It's a quick, easy and safe way of making high current measurements.