Measuring Amps on output coils

[nathanj99]

Thanks everyone.

I will post details of my build later. Had to go to flipping work! I had another attempt this morning and I did start getting some readings. Although the numbers were jumping all over the place! I guess I need a analogue amp meter? I do seem to get higher amps (10mAmps) when the wheel is slow. If I speed it up I get around 1 mAmp. But it could just be down to the meter. Would the collapsing emf mess up the value shown on the dmm? Is there a better way to measure the amps? I had a look at capacitors but as yet have no idea what size I need.

Nath

[MarkE]

You should get one positive and one negative pulse from each passing magnet.  If your coil does not have a load, then you've got a basic generator and the peak voltage across the coil is determined by the number of turns, the strength of the magnets, the gap and the speed of the wheel.  If you pulse current from a battery or a power supply through the coil, and you don't do something to clamp the coil, or the signal through the meter, then you will get a big BEMF spike each time you interrupt the current.

Generally speaking, you will have an easier time if you use a shunt resistor for the current measurement.  Then you can insert any kind of needed filtering and/or protection between the current sense resistor and the DMM input.  It will also let you keep the wiring loops as small and tight in the circuit itself, which is important for pulse circuits.

[sm0ky2]

Thanks everyone.

I will post details of my build later. Had to go to flipping work! I had another attempt this morning and I did start getting some readings. Although the numbers were jumping all over the place! I guess I need a analogue amp meter? I do seem to get higher amps (10mAmps) when the wheel is slow. If I speed it up I get around 1 mAmp. But it could just be down to the meter. Would the collapsing emf mess up the value shown on the dmm? Is there a better way to measure the amps? I had a look at capacitors but as yet have no idea what size I need.

Nath

Hi Nathan,

When you generate electricity in this manner, alternating north and south poles, the current will come out as an A/C wave-form.
+ then -, respectively.
So you will need an A/C meter or a scope to get any kind of accurate reading.
some quick-response DC meters may show the result, but it flashes back and forth so its hard to see the read out.

another option may be to run the coil leads through a square-wave or full-wave rectifier
then you have a DC current to measure. This will be minus the losses through the diodes, of course.
which can also be measured with your DC meter then add the values up

hope this helps

[nathanj99]

You should get one positive and one negative pulse from each passing magnet.  If your coil does not have a load, then you've got a basic generator and the peak voltage across the coil is determined by the number of turns, the strength of the magnets, the gap and the speed of the wheel.  If you pulse current from a battery or a power supply through the coil, and you don't do something to clamp the coil, or the signal through the meter, then you will get a big BEMF spike each time you interrupt the current.

Generally speaking, you will have an easier time if you use a shunt resistor for the current measurement.  Then you can insert any kind of needed filtering and/or protection between the current sense resistor and the DMM input.  It will also let you keep the wiring loops as small and tight in the circuit itself, which is important for pulse circuits.

Could you explain to me why i get one positive and one negative pulse from each magnet please. What sort of shunt resister should i use?

So what i have at the moment it a 90cm wheel with 16 magnets around it. I was experimenting with a coil sat underneath the wheel. The distance between coil and magnets is about 3-5mm. I was messing about with a coil of 1.00mm wire, 2.2 ohms, 1066 turns. I have a coil of bifilar which i have not yet put the core in. That is 0.45mm 1619 turns.

Nath

[MarkE]

Could you explain to me why i get one positive and one negative pulse from each magnet please.

Because as the magnet approaches the coil, the flux will increase quickly in one direction, reach a maximum rate of change, and then the rate of change will fall to zero very close to where the magnet makes its closest approach.  That makes a voltage pulse in one direction.  As the magnet moves from its closest approach the reverse process occurs and a complementary shaped voltage pulse forms in both time and amplitude.

What sort of shunt resister should i use?

Use a resistor that has a low inductive impedance compared to the resistance at the signal frequencies you are measuring.  Your basic frequency with 16 magnets will be:  rpm*0.267.  I don't know how sharp your rise times will be, so let's just take a gross stab and say that they are  about 1/350th the magnet to magnet interval.  So, the signal frequency content will be significant up to around RPM*0.267*350/0.35 ~= rpm*270.  I am going to venture that you are staying under 2000rpm, so I expect the frequency content will be mostly under 500kHz.  You would like jwL @ 500kHz < 0.1*R_CSR.  So, a tabulation:

L < 0.314uH*R

R = 100 Ohms, L < 31uH, 5W resistor safely handles 170mA rms
R = 10 Ohms, L < 3.1uH, 5W resistor safely handles 0.5A rms
R = 1 Ohm, L < 310nH, 5W resistor safely handles 1.7A rms
R = 0.1 Ohm, L < 31nH, 5W resistor safely handles 5A rms

These resistors from Digikey have practical inductances down around 3nH, and they are affordable, handle a decent amount of power, and are 1%:  http://www.digikey.com/product-search/en?pv1=1323&pv1=1025&pv1=1028&pv1=103&pv1=2358&FV=fff40001%2Cfff80482%2C1c0002&k=wne&mnonly=0&newproducts=0&ColumnSort=0&page=1&quantity=0&ptm=0&fid=0&pageSize=25.

You should solder sense wires to each side of the resistor, right at the body.  You can tightly twist those wires together and take them away to your oscilloscope probe, or DMM.

So what i have at the moment it a 90cm wheel with 16 magnets around it. I was experimenting with a coil sat underneath the wheel. The distance between coil and magnets is about 3-5mm. I was messing about with a coil of 1.00mm wire, 2.2 ohms, 1066 turns. I have a coil of bifilar which i have not yet put the core in. That is 0.45mm 1619 turns.

Nath