I was hoping to get some advice as a beginner. I have the following circuit where the important bit is shown at


(The solder bridge active on the pcb in the one from sns to com)

This is a simple wifi operated blind / hbridge controller using the MAX14871 https://datasheets.maximintegrated.com/en/ds/MAX14871.pdf

I have a sense resistor that sets the maximum allowed current to around 1.4 A (I have a 70 mOhm resistor) and using the data sheet the max current is calculated with


In my firmware when I receive a fault through pin 10 going low I disable the hbridge by setting pin 11 (enable pin) high and setting PWM ping to 0.

Now I have this connected to a 12v dc motor with gearbox (10 ohms) and it mostly works (I can physically try to stall it and then the max1487 will throw a fault which is probably "over temp" as I don't see the current getting close to going over 1A with a multi-meter)

The major problem is I have another very similar motor identical in appearance but with no markings that measures 9 ohms. I thought it was the same type but I get very different behaviour.

This one causes a fault immediately every time I enable the hbridge. I don't see the motor move at all. I had assumed this is due to in rush current as it could not reach over temp that quick ? But I have reduced the voltage to 6v and it behaves the same which is surprising.

I have tried to measure the voltage across the motor when I enabled the hbridge but I don't see large voltage spikes

The following pic is with a supply of 6V

enter image description here

My best guess at the moment is that it is noise across the sense resistor caused by the motor ?

The voltage across the sense resistor is shown below.

enter image description here

Here I see a spike of 216 mv

The datasheet does say

"SNS Input Leakage Current ISNS_LKG VSNS = ±250mV"

Which I don't understand and hope someone can clarify ?

Maybe a capacitor across the motor here would help here but not sure what value.

Thanks for any advice

  • \$\begingroup\$ It’s quite possible that the motor is different internally- there are a few very common case designs but the number of turns of wire is varied to optimise for different supply voltages and motor speeds. \$\endgroup\$
    – Frog
    Sep 13, 2021 at 19:35
  • 3
    \$\begingroup\$ Tip: You'll get more interest in your question if you embed the images in your post rather than expect users to follow links to understand your question. \$\endgroup\$
    – Transistor
    Sep 13, 2021 at 19:42

2 Answers 2


The motor follows some basic physical rules:

$$T_{mot}=J\cdot\alpha + T_{LOAD}$$ Where T is Torque, J is the moment of inertia, Alpha is the angular acceleration. The current is proportional to the torque, if not limited then the stall current is \$I_a=\dfrac{V_a}{R_a}\$. If your PWM frequency is too low and duty ratio (DT) is high then it will start with stall current.

In order to prevent this, you should ramp the DT from 0 to max DT.

From the 2nd screenshot you can see that current is increasing (almost linear line) until the overcurrent is detected. $$I_L=\dfrac{1}{L}\int V_L dt$$

You should do PWM on the motor, not turn on 100%.

I can physically try to stall it and then the max1487 will throw a fault which is probably "over temp" as I don't see the current getting close to going over 1A with a multi-meter

That's too bad. You have an unused protection. At 12V and 10 ohms ,you get a stall current of 1.2A and you have a limit on 1.4A, a motor is not supposed to work with a stall current, rather with nominal current. So this limit shall be at least half lower.

  • \$\begingroup\$ Thanks. I am now ramping up the PWM which seems to work. It was also due to having my pwm set to 70% duty and that wasn't enough. I will replace the sense resistor to reduce the current limit when I can face those digkey delivery prices again. PS Do you know of a link that describes the derivation of the stall current Ia=VaRa ? I would like to read more of the theory \$\endgroup\$ Sep 13, 2021 at 22:24

Voltage Spike on Shunt Resistor?

large voltage spike over shunt resistor even at no load

What i see is that when a Mosfet gets turned on or off i see a spike over the shunt resistor, even with no load attached.

This long thread concludes that the capacitance in the H-bridge is causing a voltage spike on the sense resistor even with no load (motor) attached. However inductive switching means the inductance and stray inductance contribute to voltage spikes and diodes are used to reduce voltage spikes.

So maybe the second motor is causing a voltage spike on the sense resistor in your application?

MAX 14871 Datasheet


Connect a sense resistor (RSENSE) between COM and GND to monitor the motor current during operation. Select RSENSE such that the voltage at COM created by motor current flowing through the sense resistor is limited to within 250mV relative to GND (-250mV ≤ VCOM ≤ +250mV).


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