I have designed a circuit to keep the heating element at a constant temperature. The digital part of the circuit is being feed from a 5V LDO which is powered by two 18650 batteries in series and the heating element is directly connected to the output batteries. Also heating element wire(approximately 2ohm) is controlled with an N channel low side mosfet by the microcontroller.

What i noticed is that the LED and the seven segment display(connected to 5V output of LDO) seems to flicker when controller toggles the mosfet because of the voltage drop on the supply. Is there anything i can do to isolate these rails so that current draw for the load does not affect the 5V components?

The highest supply voltage for the system is 8.4V and currently the load roughly uses 2A. The problem is becomes more significant when the supply voltage gets lower. I don't think that any inrush current situation occurs because voltage continues to stay below 5V at the output of LDO when the load is on after few seconds. Also i tried with both batteries(which i am sure that can supply a lot more than 2A) and a lab bench power supply with no luck. I read that bulk capacitors can fix instant voltage drops but can they prevent constant voltage drop? If so where should i place these bulk capacitors?

Here is my schematic and pcb: Schematic


  • \$\begingroup\$ Using a switching regulator would help but not fully fix the problem. Can you use a 3s2p battery pack instead of just a 3s pack? \$\endgroup\$ – Hearth Oct 13 at 21:44
  • \$\begingroup\$ "seems to flicker when controller toggles the mosfet" - does it only flicker when turning on, or both turning on and off? What happens if you power everything but the heater from another supply? "voltage continues to stay below 5V at the output of LDO when the load is on after few seconds." what are the exact voltage readings at input and output of the regulator, with heater off and on? \$\endgroup\$ – Bruce Abbott Oct 13 at 22:05
  • \$\begingroup\$ Could you change the control software to give a soft start over , say, 500 msec? \$\endgroup\$ – Peter Jennings Oct 13 at 22:17
  • \$\begingroup\$ @Hearth I am using 2S battery pack as a power supply but as i stated i tried a lab bench power supply and it shows the same behaviour. \$\endgroup\$ – Berkays Oct 14 at 17:26
  • \$\begingroup\$ @BruceAbbott Voltage drops when the load is on so the display gets dimmer while the load is active. It returns to normal state when MCU toggles off the mosfet. It works as intended if the heater is supplied from another power supply. With the heater on while battery is full(8.4V) the LDO input gets around 4.5V. The input of LDO drops to 3V at lower supply voltages. The output also suffers from the drop around 3-4V depending on the input. \$\endgroup\$ – Berkays Oct 14 at 17:33

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