DC MOTOR LOW BATTERY LED PROBLEM

Thread Starter

minte_7

Joined May 19, 2023
4
Hello everyone,

I have designed a circuit which is powered by a 3.7V LIPO battery. This supply voltage passes through a LDO (IC1 in the schematic) which regulates the voltage to 3V to feed a small DC motor (+3V_MOTOR).

In addition, it integrates a low battery detection circuit based on the TL431 (IC2), which activates a low battery LED (D1-LOW_BATT signal) when the battery voltage reaches 3V.

For activation of the DC motor, a button has to be pressed which causes the battery voltage to reach the LDO (BAT+_PULS signal) and then the motor. Therefore, only when this button is pressed, the battery voltage is measured and the LED is activated if necessary.

The general operation of the motor activation is correct. However, in the voltage range where the low battery LED should not activate I see a small flashing of the LED mainly when pressing and releasing.

I have thought that this effect could be due to the flyback effect during the shutdown of our dc motor. But as I mentioned before, this effect also occurs on activation.

Another idea that has occurred to me is that the battery has peak drops when this current is required to power the motor and that is when the low battery circuit detects these drops and activates the red LED for a few ms.

I'm attaching the schematic of the PCB.
 

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Sensacell

Joined Jun 19, 2012
3,441
Slow the response of the low_bat detector with a capacitor.

As the battery depletes, the internal impedance rises, motor start currents cause the voltage to droop for a few milliseconds.
Put a scope on the power line and have a look.
 

Thread Starter

minte_7

Joined May 19, 2023
4
Slow the response of the low_bat detector with a capacitor.

As the battery depletes, the internal impedance rises, motor start currents cause the voltage to droop for a few milliseconds.
Put a scope on the power line and have a look.
I have tried placing a 47uF capacitor between the motor pins, but it had no impact.

Maybe I should use a higher capacitance value? Or maybe it would have a greater impact to place the capacitor on the signal coming from the battery to the low battery detection circuit?
 

crutschow

Joined Mar 14, 2008
34,390
Adding a capacitor across the motor just increases the startup current required from the battery.
Do you not understand how capacitors work?

As Sensacell noted, put the cap at the low_bat detector (from Ref to ground on the TL431).
 
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