I have a motor controller that runs off the same power as that supplied to the motor. Is there an accurate way to find the over all current the control circuit is taking from the supply? I have doubts that my multi-meter is accurate.

 

I have a motor controller that runs off the same power as that supplied to the motor. Is there an accurate way to find the over all current the control circuit is taking from the supply? I have doubts that my multi-meter is accurate.

How did you expect to measure something without a meter? If you doubt the current setting on your meter put a small resistor in series with it all and measure the voltage across the resistor then calculate the current.

 

There are several ways. It depends on what kind of controller and your application.

Do you have a print? Do you understand electronics? Have you worked around your power before?

 

I was thinking (or not thinking) that a resistor in series with the whole thing changes something.

The circuit causes a small drop in the supply when I hook it up. I was thinking that just reflects poorly on my supply.

Ahhh....I always look at current as flowing from + to ground. If I think of as flowing from ground up then the drop at the supply is the drop across the circuit. (not meaningless) so can I just divide the drop by the unloaded supply voltage?

 

I was thinking (or not thinking) that a resistor in series with the whole thing changes something.

The circuit causes a small drop in the supply when I hook it up. I was thinking that just reflects poorly on my supply.

Ahhh....I always look at current as flowing from + to ground. If I think of as flowing from ground up then the drop at the supply is the drop across the circuit. (not meaningless) so can I just divide the drop by the unloaded supply voltage?

It does. That's why you use as small a resistor as possible. I can't suggest a value without seeing your circuit or having a ball park idea of how much current is flowing. It doesn't matter which way you imagine current flowing. Measure the voltage across the resistor and divide it by the value of the resistor to get the current flowing through the resistor.

 

There are several ways. It depends on what kind of controller and your application.

That covers it.Depending on the setup there is likely a half dozen methods to measure total current. You don't mention anything like motor rating, AC or DC motor? Controller make and model?

Ron

 

Is the supply drop useful as a measure of current draw? Is the whole thing not equivalent to a single resistor?

I am concerned about the draw from the control circuit alone regardless of the motor load.

The circuit is attached. I "designed it" except for the key components (the op amp layout and the use of a constant current source to linearize the resistor string). It is a high side linear motor controller. Controlled by a magnet/slider that sweeps past an array of Hall switches. any and all comments on the circuit are welcome I am learning more and looking to improve it. My questions mostly pertain to the op amp layout. However, the High side configuration must remain. so don't suggest low side pwm. I have a pwm version, also high side, posted in another thread. No improvement suggestions yet.

 

If you do not anticipate high currents there are simple solutions like the ACS712 current sensor. I like the versions like this already mounted on a board with an operational amplifier. You could measure Itotal and Imotor and subtract the motor from total and get the controller current. If you want to get fancy there are solutions like this. Finally you can use a current shunt which is along the lines of the series resistor mentioned. Current shunts are common and generally you can find 10 amp 100 mV, 50 Amp 100 mV and about any range you want. Lately I have seen plenty of ampmeters with shunts on Amazon from China for really low cost.
If this was to be a quick one time test I would just use an amp meter for under 10 Amps like my Fluke DMM or any general purpose DMM. If this will be long term and part of the system I would look at some of the other options.

Nice circuit by the way, brings back memories of quadrill pads.

Ron

 

Is the supply drop useful as a measure of current draw? Is the whole thing not equivalent to a single resistor?

I am concerned about the draw from the control circuit alone regardless of the motor load.

The circuit is attached. I "designed it" except for the key components (the op amp layout and the use of a constant current source to linearize the resistor string). It is a high side linear motor controller. Controlled by a magnet/slider that sweeps past an array of Hall switches. any and all comments on the circuit are welcome I am learning more and looking to improve it. My questions mostly pertain to the op amp layout. However, the High side configuration must remain. so don't suggest low side pwm. I have a pwm version, also high side, posted in another thread. No improvement suggestions yet.

Is the fuse removable? That would be a good place to temporarily connect an Amp meter or the suggested resistor to monitor current. How much current does the Amp meter measure? How much current do you expect the motor to draw?
Assuming say 5 Amps, a 0.1 Ohm resistor would develop 0.5 V across it. It would be close enough to confirm or discredit your Amp meter reading. How small a resistor do you have available?
If no such resistor is available use a diode, cathode towards ground, and measure the voltage across it. Compare the voltage across the diode to the forward voltage graph on the data sheet to see what the current is. The forward voltage drop will vary slightly with the current.

 

Control current = fuse current - ground leg motor current.

 

Monitoring is not needed. And I do have a fractional ohm resistor or two salvaged from some amplifiers. I am understanding. I will revisit my klein meter soon.

BUT lets clear up my rephrased original question: is the supply drop useful as a measure of current draw? If the whole thing is equivalent to a single resistor (Thevein) then that supply drop is the drop across that single resistor? Or does the supply drop represent the source R?

 

Maybe.....could be.......depends..........WHAT is your real question?

Is it a secret?

 

I'm asking - is the supply drop useful as a measure of current draw? If the whole thing is equivalent to a single resistor (Thevein) then that supply drop is the drop across that single resistor? Or does the supply drop represent the source R?

 

dlatch.........it depends.........they make power supplies that don't drop.

It depends on what you have and the application.

One does not use the power supply voltage drop to measure current.

One use the voltage drop across the current transducer, to determine current.

It's not the voltage drop of supply. it's voltage drop across component.

 

dlatch......Connect a 1.5 volt battery to a 1000 ohm resistor. To measure supply voltage, we put the positive meter lead on + side of resistor, and negative lead on - side of resistor.

This is the SUPPLY voltage..........NOT the voltage drop of resistor.

Reverse the meter leads. This is the resistor voltage drop. This is the voltage you use to measure current.

If you add the supply voltage, to the circuit voltage drops (resistor in this case).........it will equal zero.

 

If this is just for occasional monitoring, then why not get the Clamp on attachment for your multi-meter, if it takes one.
The current is going to vary at different times dependent on motor load variation.
Max.

 

You have not mentioned what you think the current is? Maximum motor current? The reason I ask is because using a current shunt (series resistance) is not as simple as it looks. What if your motor current is 10 Amps? Maximum of 10 Amps. So you place a 0.1 Ohm resistor in there as a shunt. The voltage drop across the shunt is 1.0 volt at 10 amps and that is 1.0 less volt your motor will see. You started with 13.8 Volts and you have the drops across the FETs. Your shunt resistor will also be dissipating power which needs to be figured in choosing the shunt resistor. In this example you are looking at a 10 watt resistor and even that is a poor margin.

Typical current shunts are in the 10 milli-ohm and 1 milli-ohm range and use amplification. For example a 10 Amp 100 mV shunt is a 10 milli-ohm shunt, a 50 Amp 50 mV shunt is a 1.0 milli-ohm shunt.

If this is just for occasional monitoring, then why not get the Clamp on attachment for your multi-meter, if it takes one.
The current is going to vary at different times dependent on motor load variation.
Max.

That's what I would be considering since this won't be permanent. If you want the total current I would do, as suggested, measure at the fuse.

Ron

 

I used a ceramic 3 prong unit salvaged from the output stage of an audio amplifier. It says 0.22 ohms on it. I measure 66mV across it when it's in series with the power input, (motor connected but not running. So we can say my control circuit draws about 300 mA.

I want to know the current draw as way of comparing variations of the circuit. If the only improvement I make is less bias current then that is still something.

I feared this question was too basic to even ask but sometimes it's easier to post a question than to think about it.