Why is there a voltage drop when I connect my motor to Mosfet controlled by PWM signals?

Thread Starter

Adarsh Shanmugam

Joined Jul 27, 2018
9
Hi! I'm trying to make a mini quadcopter. To control the speed of the motor, I'm using a mosfet in 'Low-side' configuration controlled by PWM signals from an arduino pro mini. The mosfet I'm using is 'SQ2310ES', an n-channel mosfet. The motor is a coreless dc brushed one, operating on 3.7V. The motor has a resistance of 1 ohm roughly. The battery I'm using is a '3.7V Li-ion 780mah 25C'. The motor works perfectly when I connect it directly to the battery and the voltage across the terminals measure '3.7V'. However, when I include the mosfet in the circuit, the voltage drop across the motor measures only '0.5V' despite the arduino producing a full duty cycle PWM (5V). The battery voltage does not drop and remains at '3.7V'. Gate voltage doesn't change either (5V). When I disconnect the motor from the circuit, the voltage across the motor slot increases to 3.7V. The problem is when I connect the motor, the voltage drops to '0.5V'. What am I missing? I'm new to electronics and it would be great if someone could help me.
Here is the circuit:
WhatsApp Image 2018-07-28 at 10.43.18 AM.jpeg
Here is the datasheet of the mosfet:
https://www.vishay.com/docs/67036/sq2310es.pdf
 

crutschow

Joined Mar 14, 2008
25,109
That MOSFET is a logic-level device with a low Vgs threshold so I see no reason that it shouldn't be fully turned on.
Try connecting 5Vdc to the gate.
 

danadak

Joined Mar 10, 2018
4,057
What freq is PWM running at ? With a scope you see correct waveform at
gate of MOSFET, which at 100% duty cycle should be 5V ?

Regards, Dana.
 

Thread Starter

Adarsh Shanmugam

Joined Jul 27, 2018
9
That MOSFET is a logic-level device with a low Vgs threshold so I see no reason that it shouldn't be fully turned on.
Try connecting 5Vdc to the gate.
Hi. I tried applying 5Vdc with the help of a SMPS. It still doesn't work. So I'm kinda sure the problem is not with the PWM.

What freq is PWM running at ? With a scope you see correct waveform at
gate of MOSFET, which at 100% duty cycle should be 5V ?

Regards, Dana.
Hi. I don't have an oscilloscope but I read in the arduino website that the frequency of the PWM is approximately 490 Hz. I can't see the waveform but when I measure the voltage between the gate and the source, it measures 0.8 V while the motor is running.

Hi. I don't have an oscilloscope but I read in the arduino website that the frequency of the PWM is approximately 490 Hz. I can't see the waveform but when I measure the voltage between the gate and the source, it measures 0.8 V while the motor is running.
Update:
The gate voltage did drop I guess. The gate voltage remains as 0.8V whether the motor is running or not. This causes my drain source voltage to raise to a value of 3.09V. When I measure the voltage between PWM pin 10 and GND of arduino, it shows 4.59V. But only 0.8V drops across the gate and the source. So should I lose my 200 ohm resistor? The voltage across the resistor measures about 3.71V. Will removing the resistor solve my problem?
Thank you for your time.
 
Last edited by a moderator:

danadak

Joined Mar 10, 2018
4,057
Normally a R of 10 - 50 ohms is all that is used. To suppress L transients
when driving the gate.

Your measurement with DVM is actually measuring AC at Gate I would guess.

Free scope that might help -

You can start with a PC sound card based scope for free. Will give you basically
audio range scope, spectrum analyzer, and function generator all using your
PC sound card.


https://www.zeitnitz.eu/scope_en


http://www.zelscope.com/


http://www.ledametrix.com/oscope/


http://www.virtins.com/downloads.shtml


But first build a simple circuit to protect sound card inputs so you do not
ruin from transients, overvoltage. Google "protect sound card input".


For example http://makezine.com/projects/sound-card-oscilloscope/


Regards, Dana.
 

Thread Starter

Adarsh Shanmugam

Joined Jul 27, 2018
9
It should be next to nothing. Are you sure the resistor value is correct?
It's possible the FET is faulty (gate-source leaky) or fake. Do you have another FET you could try?
Hi. Yes, I'm pretty sure it is a 200 ohm resistor. I have a couple of FETS lying around. I'll try with them and update the status. Thanks!!!

Normally a R of 10 - 50 ohms is all that is used. To suppress L transients
when driving the gate.

Your measurement with DVM is actually measuring AC at Gate I would guess.

Free scope that might help -

You can start with a PC sound card based scope for free. Will give you basically
audio range scope, spectrum analyzer, and function generator all using your
PC sound card.


https://www.zeitnitz.eu/scope_en


http://www.zelscope.com/


http://www.ledametrix.com/oscope/


http://www.virtins.com/downloads.shtml


But first build a simple circuit to protect sound card inputs so you do not
ruin from transients, overvoltage. Google "protect sound card input".


For example http://makezine.com/projects/sound-card-oscilloscope/


Regards, Dana.
Hey! I don't understand that 'AC' part. Do you mean the square wave function generated by the arduino as AC? I'll try to build that oscilloscope and find the waveform. I'll update the progress soon. Thanks! :)
 
Last edited by a moderator:

ebp

Joined Feb 8, 2018
2,332
If you are testing with DC applied to the gate it doesn't matter if the resistor is 2 ohms or 200k ohms. The gate-source leakage current spec for the FET is 100 nA max, which would result in a voltage drop across a 200k resistor of only 20 mV, or 20 µV for 200 ohms. The actual gate leakage at moderate temperature is going to be vastly less than 100 nA.

If there is a voltage drop across the 200 ohm resistor of over 4 volts, the FET must be damaged.

===
A PWM signal is normally NOT AC - it is DC signal whose amplitude varies with time. AC means that there is an actual change in the polarity. The gate current when a PWM signal is applied has an actual AC component because the capacitance of the gate means current will flow into the gate to charge the capacitance when the PWM signal goes positive and out of the gate to discharge the capacitance when the PWM signal returns to zero (in and out here refer to "conventional current" which is considered to flow from positive to negative).

Sometimes the voltage applied to a FET's gate is truly AC. This typically would be the case if the gate were driven through a transformer.
 
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crutschow

Joined Mar 14, 2008
25,109
A PWM signal is normally NOT AC - it is DC signal whose amplitude varies with time. AC means that there is an actual change in the polarity.
I beg to differ.
It's a matter of semantics, but the generally used meaning of AC is that the voltage varies with time, not that it actually changes polarity with respect to ground.
Electricity is only defined as AC or DC.
DC doesn't vary with time. Any variation is AC.
In practice it's what an AC meter would read.
By your definition you would have to say that an AC meter doesn't always measure AC.

So a PWM signal is AC with a DC offset.

Running a signal through a capacitor doesn't convert it from DC to AC.
For an example a signal going through an AC amplifier is considered to be AC all though the amp, even if it is offset by the amplifiers DC bias as it goes through the amp.
 
Last edited:

Picbuster

Joined Dec 2, 2013
1,008
Hi! I'm trying to make a mini quadcopter. To control the speed of the motor, I'm using a mosfet in 'Low-side' configuration controlled by PWM signals from an arduino pro mini. The mosfet I'm using is 'SQ2310ES', an n-channel mosfet. The motor is a coreless dc brushed one, operating on 3.7V. The motor has a resistance of 1 ohm roughly. The battery I'm using is a '3.7V Li-ion 780mah 25C'. The motor works perfectly when I connect it directly to the battery and the voltage across the terminals measure '3.7V'. However, when I include the mosfet in the circuit, the voltage drop across the motor measures only '0.5V' despite the arduino producing a full duty cycle PWM (5V). The battery voltage does not drop and remains at '3.7V'. Gate voltage doesn't change either (5V). When I disconnect the motor from the circuit, the voltage across the motor slot increases to 3.7V. The problem is when I connect the motor, the voltage drops to '0.5V'. What am I missing? I'm new to electronics and it would be great if someone could help me.
Here is the circuit:
View attachment 157039
your motor running @ 3.7 V lower than the gate voltage no real problem but look at the data sheet to identify Vds minimum.

Picbuster

Here is the datasheet of the mosfet:
https://www.vishay.com/docs/67036/sq2310es.pdf
 

MaxHeadRoom

Joined Jul 18, 2013
20,875
A PWM signal is normally NOT AC - it is DC signal whose amplitude varies with time. AC means that there is an actual change in the polarity.
.
That has always my definition of the term, as in a service supply, the current changes (alternates) in direction through the conductor.
Max.
 

ebp

Joined Feb 8, 2018
2,332
As soon as you define AC to be anything that is time variant, DC becomes notional only and no longer exists in reality. You can no longer say that a primary cell or a voltage regulator or anything produces DC, except by special pleading. Change in direction of current flow is a true and identifiable physical phenomenon.

The definitions and explanations of AC and DC found on the web are all over the place - inconsistent and quite remarkably unhelpful. The fact that the terms conflate ideas of potential ("voltage") and current certainly doesn't improve matters.
 

crutschow

Joined Mar 14, 2008
25,109
As soon as you define AC to be anything that is time variant, DC becomes notional only
That is strictly true.
But in practice we take anything below an arbitrary frequency as DC, depending upon the situation.
For example at one extreme, I've seen the DC drift in op amp offset with time defined as very low frequency 1/f noise.
Usually we select some arbitrary frequency (normally unstated but usually in the μHz or lower region) as the point below which the voltage is considered DC, such as for a power supply or battery.
Change in direction of current flow is a true and identifiable physical phenomenon.
It certainly is.
That doesn't make it a good definition for "AC".
And typical AC measurement instruments pay no attention to that.
To me if an AC meter measures a voltage, or it looks like AC on a scope then that's an AC voltage.
To define it otherwise leads to a too restrictive definition of AC.
It can be AC with a DC offset but it's still AC (even if it doesn't reverse the current direction or go around zero volts).
The restricted definition of AC leads to the bizarre situation where there is AC going into an amplifier with no DC offset, but it's not AC inside the amp until it appears at the output with no DC offset.

Perhaps that is an engineering viewpoint, not an academic one, but it is certainly more practical.
 

danadak

Joined Mar 10, 2018
4,057
A PWM signal is normally NOT AC - it is DC signal whose amplitude varies with time.
Interesting point you made. I think of it as a measurement with respect to
a reference point.

So if reference is ground then I concur.

But if reference is Vdd/2, a gross approximation to CMOS threshold, then
polarity does change in the PWM example, but not independent of what it
is driving. Your point about C load. Reminds me no matter how much I have
learned I still know very little :)

Interesting discussion point.

Regards, Dana.
 

Thread Starter

Adarsh Shanmugam

Joined Jul 27, 2018
9
UPDDATE:
So, I tried using "P55NF06" mosfet in the same circuit because it had a minimum threshold gate voltage as 2V. There were some improvements but there is still some drop in voltage across the motor when I connect it.
However the improvements were:
1. The gate-source voltage doesn't drop and remains as 5V when duty cycle is full.
2. Voltage drop across the 200 ohm resistor has become negligible. (Approx 0V)
3. The drain-source voltage measures 0.18V
4. The voltage measure across the motor when connected to the circuit is 2.77V. So, that is definitely an improvement but still I'm missing out 1V approx. The battery voltage doesn't drop and is stable at 3.76V.

When I disconnect the motor, the voltage across the motor slot rises from 2.77 to 3.76V. The voltage across drain-source measures 0V. So, this must mean that the MOSFET is actually conducting perfectly until I connect the motor, right?

I feel like I'm close to solving this problem but still can't figure out why there is a drop in voltage across the motor even though the gate-source voltage is 5V. To make sure the problem is not with the PWM, I tested the circuit by applying 5Vdc across the gate and source with the help of a SMPS. The results remain the same. Is mosfet still the problem? Thanks in advance!

Here is the datasheet of the mosfet:
https://www.st.com/resource/en/datasheet/stp55nf06fp.pdf
 

crutschow

Joined Mar 14, 2008
25,109
3. The drain-source voltage measures 0.18V
4. The voltage measure across the motor when connected to the circuit is 2.77V. So, that is definitely an improvement but still I'm missing out 1V approx. The battery voltage doesn't drop and is stable at 3.76V.
So that means the MOSFET is working fine.
The problem appears to be a voltage drop somewhere from the battery to the MOSFET.
Measure the voltage drop between the battery and the motor, and between the motor and the MOSFET drain.

What wire size are you using for the connections?
 
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