Wattmeter re ranging
 

This has come into my hands from a university clear-out.

There are two connections for the current coils , series and parallel, to give X1 and X2 scale readings but I was wondering if I could use a current transformer for ac measurements to provide a divide by 10 scaling factor. That is I would put 10 times the 500mA test current through the current coils to give a divide by 10 , 0 to 120 Watts on the X1 setting.

I have some donor toroid’s to experiment with but before I go too far does the above sound reasonable. I was thinking of copper tape overwound by the low current side turns but I am not sure how many turns to use to ensure good 10:1 linearity? Unless anyone has a 10:1 current transformer that will do 10 Amps on the high current side.

I have yet to cross check it but by design would this meter reading be rms as it states dc and ac on the scale? I seem to remember that the magnetic force is averaging and not rms on a meter movement but this has emf and current coils and no magnet so I am trying to work out if that compensates to give rms. ?

Failing the above it looks really smart sat on the shelf.
Pete

Re: Watmeter re ranging
 

It multiplies the circuit current by its voltage which gives true instantaneous power, but if the circuit is running on AC, the power value will fluctuate more rapidly than the pointer can follow, so the pointer will show the true mean power.

'RMS power' is a bit of loose terminology that's very common. It's mostly harmless, but this is exactly the situation where it causes confusion. It's simply mean power that does the work, and that's in watts. RMS values only make sense for voltage or for current and RMS is the equivalent DC voltage (or current) that gives you the same mean power.

The power meter has TWO coils and no permanent magnet. One coil provides the fixed field... connectedd across a shunt resistor, sensing the circuit current. The moving coil is driven by a small current from a burden resistor, sensing the circuit voltage.

The torque on the moving coil is proportional to the fixed field strength (circuit current) times the moving coil current (circuit voltage) and the coil moves against the spring proportionally to the torque. Volts times amps equals watts, and voila! the meter produces a deflection proportional to the power in the circuit!

David

Re: Watmeter re ranging
 

Thanks, I have now looked at the coils in new light. It is only in these old instruments that you can see the first principles in action. It also neatly shows what happens if the current and voltage are out of phase, that is only the phase overlaps will have force to deflect the coil and move the pointer.
That also explains the lower limit of 25 Hz which below I am guessing there will be significant "wobble" on the pointer.
Pete

Re: Watmeter re ranging
 

I've always believed that the use of that phrase 'RMS Power' arose when manufacturers of cheap 'Hi-Fi' started to quote things like 'peak power', 'music power', etc. If only certain people would get this simple notion clearly established in their heads - 'electrical power' is a measure of 'real' power', measured in watts; power that heats things up; makes things move, etc. - there would be a lot less confusion. I suppose that to certain sections of the Audio World, terms such as 'phase', and 'volt-amps' mean very little - if anything. :-)

Al.

Re: Wattmeter re ranging
 

Hi Pete, these instruments were often used with current transformers to extend their range and you may be able to pick one up.
However, unless it is designed to work with your particular instrument you may find accuracy is compromised.

Ed

Re: Wattmeter re ranging
 

As Radio Wrangler says, it will give true power. Volts x Amps, averaged.

You can indeed use a current transformer to alter scaling, but design of a CT depends on accuracy needed and the load imposed by the wattmeter's current coils. It probably won't be very much: you could get a cheap toroidal transformer from RS of about 30VA, wind 100 turns for connection to the wattmeter; wind 10 turns for connection to your own circuit, and be reasonably confident of a 10x scaling achievement.

If you wanted to be a bity more certain, I'd get a 1kW radiant electric fire which will be virtually pure resistance, connect AC voltmeter and ammeter, multiply the readings, and compare this against what the wattmeter says.

Re: Wattmeter re ranging
 

From memory RMS (allways understood it as root mean square) arose to prominence in the 60's when terms like PMPO (peak music power output) were bandied around as they obviously gave a far higher reading by makers of music equipment. I understood that the rms figure was the average power being put out by an amp with an input signal of 1 kHz sine wave 1 volt peak to peak. However others may correct me over this - my experience was in guitar amps where it was a handy measure/nonsense eliminator.