The stall current is proportional to the supply voltage, its just the supply voltage divided by the winding
resistance.
So measure the resistance between the terminals.
Or clamp the motor spindle and measure the current (but very quickly, motors tend to cook rapidly on
stall).
Whoops; this is awkward. Looks like I was measuring DC current with an AC clamp meter. I just thought current was current; didn't know there were different kinds.
At 5v, stall current is 1.45A. At 12V, stall current is 1.05A.
Does that sound right? Shouldn't stall current be going up with voltage? I'm testing it through a buck converter, if that makes a difference?
When I first saw this example I assumed it was for a car with a 12v system. Can you explain what this is for? Maybe you don't have enough power... more description would help greatly.
wolframore:
When I first saw this example I assumed it was for a car with a 12v system. Can you explain what this is for? Maybe you don't have enough power... more description would help greatly.
It is a small motor that opens a small door when it gets light out. For a chicken coop, to be exact.
The one thing I'm worse at than actually building circuits, is drawing them. But, I might as well get some practice in. Here is a circuit, and motor, if that gives a bit more info:
polymorph:
You cannot measure DC current with the motor supplied with PWM with an AC clamp meter, or a DC clamp meter, or a DC current meter in series.
The current will be a complex mix of AC and DC, and will depend on inductance, frequency, and frequency content.
That's actually very good to know. It never occurred to me that PWM would screw up results. Thanks!
You must change you Motor Drive Mosfet. The way it is drawn you must bring the gate above 12V to turn in on.
You can connect the source to ground and the drain to the motor (-) the the motor (+) to the wall adapter.
You should also add a diode across the motor to protect the MosFet.
For strictly schematics I suggest you look at a program called TinyCAD. It is very easy and intuitive.
JohnRob:
You must change you Motor Drive Mosfet. The way it is drawn you must bring the gate above 12V to turn in on.
You can connect the source to ground and the drain to the motor (-) the the motor (+) to the wall adapter.
You should also add a diode across the motor to protect the MosFet.
For strictly schematics I suggest you look at a program called TinyCAD. It is very easy and intuitive.
Any way to adjust the above circuit to get an output proportional to the potentiometer? For example, 0V pot = 0V Source, 2.5V pot = ~6V Source, 5V pot = ~12V Source? Basically, what you said in your comment, but without the motor?
polymorph:
You cannot measure DC current with the motor supplied with PWM with an AC clamp meter, or a DC clamp meter, or a DC current meter in series.
The current will be a complex mix of AC and DC, and will depend on inductance, frequency, and frequency content.
A DC ammeter will read the DC component of a current quite happily, meters integrate/average.
Furthermore the current is likely to be close to DC in a PWM'd motor, the large amount of inductance smooths out
the current waveform.
Any way to adjust the above circuit to get an output proportional to the potentiometer? For example, 0V pot = 0V Source, 2.5V pot = ~6V Source, 5V pot = ~12V Source? Basically, what you said in your comment, but without the motor?
No
The Mosfet can be driven linearly (i.e. with an output variable to input) but not with your circuit. You would need a more complex circuit to control the MosFet.
BTW typically one would turn the MosFet on and off real fast. The %age of on time vs off time will cause the motor to vary in a way I think you are looking for.
JohnRob:
BTW typically one would turn the MosFet on and off real fast. The %age of on time vs off time will cause the motor to vary in a way I think you are looking for.
Which is what pin 3 would be doing, in the picture of post #23, if I connect the mosfet like you said in this post?:
JohnRob:
You can connect the source to ground and the drain to the motor (-) the the motor (+) to the wall adapter.
You should also add a diode across the motor to protect the MosFet.
Also, I got TinyCAD, seems like the most user friendly circuit maker I've used thus far. Any good Arduino library imports for it? Stock, i believe it only came with a Arduino Mega schematic.
Edit:
Also, I was under the impression since it's a logic level mosfet (IRF520), I would be able to fully turn it on with 5V, I.e. Arduino pins.
And, is a resistor needed from pin to gate? Seems to be common practice, but another user on the forum "debunked" it, saying it's not actually necessary.
And... (sorry), on the mosfet data sheet, what are the important stats too look at? Specifically, what exactly tells me the voltage needed to fully "open" the mosfet from D -> S? I was under the impression it was VGS, but since it's 2 - 4 on this specific one, and doesn't fully open (you said I need 12V to fully open it), how can I tell if a mosfet will get fully opened by an Arduino pin?
toxicxarrow:
Also, I was under the impression since it's a logic level mosfet (IRF520), I would be able to fully turn it on with 5V, I.e. Arduino pins.
Oops! An IRF520 is specifically not a logic-level MOSFET.
Which is what pin 3 would be doing, in the picture of post #23, if I connect the mosfet like you said in this post?:
For post #23 schematic you would have to put the pot input to an analog input, measure the voltage on the pot and vary the PWM by that voltage.
You MUST have a logic level MosFet. The specification to look at is Vgs, it must be well below 5V.
Reason: the specification of Vgs is measured when the MosFet is just starting to turn on. The output of the Arduino is somewhere in the mid 4's. You can usually see a graph of different Vgs values and the current they support.
Paul__B:
Oops! An IRF520 is specifically not a logic-level MOSFET.
An IRL520 is.
JohnRob:
For post #23 schematic you would have to put the pot input to an analog input, measure the voltage on the pot and vary the PWM by that voltage.
You MUST have a logic level MosFet. The specification to look at is Vgs, it must be well below 5V.
Reason: the specification of Vgs is measured when the MosFet is just starting to turn on. The output of the Arduino is somewhere in the mid 4's. You can usually see a graph of different Vgs values and the current they support.
Sorry, I did, in fact, mean "IRL520". I accidentally typed the wrong letter there. I DO, have the IRL, yet it it only passes 3.15V.
Pic to prove I have both:
Edit:
Looking at the specs, VGS is min: 1, max: 2, so why doesn't that output 12V from S if I have 12V going to D? Aren't I fully opening it up, with just 1-2V?
Edit:
Looking at the specs, VGS is min: 1, max: 2, so why doesn't that output 12V from S if I have 12V going to D? Aren't I fully opening it up, with just 1-2V?
First to make thing easier, the state of a MosFet (used as a switch) is either ON or OFF.
ON means the MosFet is conducting and the Drain voltage in nearly the same as the Source voltage. Similar to a mechanical switch being closed.
OFF means the MosFet is NOT conducting and it will be similar to an open mechanical switch.
Why it won't work in your application:
When the MosFet is trying to be turned ON by the Arduino, the Gate voltage will have to be 4 volts higher that the source (i.e. Vgs) That would mean the Source is 4 volts less than the Arduino output of ~4.5V.
So then the source will be a approximately 1/2 volt.
JohnRob:
First to make thing easier, the state of a MosFet (used as a switch) is either ON or OFF.
ON means the MosFet is conducting and the Drain voltage in nearly the same as the Source voltage. Similar to a mechanical switch being closed.
Ohh, that make a bit more sense. That's what you were talking about using PWM, rapidly switching it on and off which controls the speed. I'm assuming that's why using a pot didn't work as a variable speed controller...
JohnRob: Why it won't work in your application:
When the MosFet is trying to be turned ON by the Arduino, the Gate voltage will have to be 4 volts higher that the source (i.e. Vgs) That would mean the Source is 4 volts less than the Arduino output of ~4.5V.
So then the source will be a approximately 1/2 volt.
But isn't source 0V, until switching ON the mosfet, after which it passes on the voltage from D? And if Arduino pins can output 5V, doesn't that make them at least 4V higher?
What about a (IRL540L)? Is that even more suited for Arduino projects?
toxicxarrow:
But isn't source 0V, until switching ON the mosfet, after which it passes on the voltage from D? And if Arduino pins can output 5V, doesn't that make them at least 4V higher?
Vgs is the voltage at which the FET turns on - or begins to. This means that the source will always be at least that much less than the gate voltage. If in that circuit, the FET by turning on, "pulls up" the source, it will necessarily pull it up Vgs less than the gate voltage, otherwise it can not be turned on.
Paul__B:
Vgs is the voltage at which the FET turns on - or begins to. This means that the source will always be at least that much less than the gate voltage. If in that circuit, the FET by turning on, "pulls up" the source, it will necessarily pull it up Vgs less than the gate voltage, otherwise it can not be turned on.
Interesting; thanks.
Is that why most circuit I see using mesfets use this "pull up" method?
toxicxarrow:
Is that why most circuit I see using mesfets use this "pull up" method?
I do not know about mesfets or misfits, but this is the reason why most MOSFET circuits do not use a "pull-up" or common drain/ source follower arrangement.
Having the source grounded means you apply the full control voltage to the gate to turn it on fully and the drain is then pulled down to ground to control the load.
