Moving back to Stuart's issues, (Thanks for your reply BTW Sargon). I was just raising a question about his current readings back in post #24. If these readings are correct then something's very wrong. At standstill, the total coil currents should be 4*3.32A*2/3 = 8.85A NOT the 3.615A Stuart records. HOWEVER if Mariss's statement that a chopper drive "draws" current at 20KHz from the PS is correct then I agree with Sargon's comment about using a multimeter to try to read this is not going to give useful results. So are Stuart's motors being current starved - who knows? IF IF IF there was a cap in there we could read the steady DC between the PS and the cap and get a more meaningful reading but I've learned here that I was wrong and that apparantly caps on switched supplies is not advised so..... just glad I use purpose built unregulated power supplies with hunky caps (and after tossing a coin - no dc fuses
).
To figure this out lets take a look at how multimeters (which would be the same as a typical in-line ammeter) work, as well as DC clamp meters. The theory of operation will give us a good clue about what will work in this situation. Time to redeem myself from my earlier stupidity!
For a DC ammeter (or multimeter in DC current mode), the idea is to use a shunt to measure the current. Basically, we have two paths for the current to flow through - one main bypass to move most of the current around the metering device, and a low current shunt to measure the current. This method is actually going to measure the voltage drop across a shunt resistance, and using the value of the shunt, calculate what the current should be. It's important to note that here
we are actually measuring DC voltage. In addition to this, there are various ways the internal circuitry can be arranged - the shunt configuration differs from meter to meter so results in this situation will not be same with all meters, but in any case it should not be considered accurate. AC ammeters are built different, measuring an alternating magnetic field by using an iron core with input and output coils. This will not respond to a constant magnetic field, and in turn will not respond to constant current (DC). In short, you're not going to get a good reading on a combination AC/DC signal using either of these measuring techniques.
What will work is a DC clamp meter. These devices will detect the current by making use of the Hall effect. Essentially this type of meter will measure the
strength of the magnetic field by creating current in a conductor within a chip that is arranged such that the conductor is at right angles to the magnetic field. The voltage generated by this Hall effect is directly proportional to the strength of the magnetic field and thus current can easily be calculated. It is important to note that this device is not dependant on a reversing magnetic field. It is only looking at the strength of the field. In addition, it will still respond to changes in the magnetic field (caused by an AC component or changes in the DC current) instantly, and therefore will also be able to measure any complex AC that is on the line, or in our case a fluctuating DC signal.
In short, if you want to measure the actual current draw your best bet will not be a multimeter or oscilloscope, but a DC clamp meter. This will give you, by far, the most accurate measurement. That being said, there is always more than 1 way to skin a cat, and there are likely other methods that would work, but this would be the easiest and should be very accurate.