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:

 

hi m,
Welcome to AAC.
I would increase C2 from 10uF to say 100uF or 220uF, also C1 to 10uF.
E

 

Welcome to AAC!

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.

Sounds like the battery voltage is dropping too low under load. You could use a buck/boost regulator.

 

Where did you get the 18650s? There are a lot of counterfeit ones out there that contain a tiny battery and a bunch of rice!

Test them at a draw of 2A and measure the terminal voltage.

Bob

 

Where did you get the 18650s? There are a lot of counterfeit ones out there that contain a tiny battery and a bunch of rice!

Test them at a draw of 2A and measure the terminal voltage.

Bob

Hello Bob,
Thank you for your answer. I bought batteries froma genuine shop and tested before use. Regardless, I tried to power the circuit via Lab Bench Power S. and the problem continues

 

Welcome to AAC!

Sounds like the battery voltage is dropping too low under load. You could use a buck/boost regulator.

Hello Dennis, batteries are capable of 20A current draw. Same batteries as used in E-Cigarettes. Also tried with a Lab Bench Power S. same problem unfortunately

 

Well, then, something else is wrong. Is the heater heating? Is it possible your MOSFET is wired wrong and is shorting the battery?

Bob

 

batteries are capable of 20A current draw.

For how long? Measure the input and output of the voltage regulator under your low conditions.

 

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:

In your schematic, pin31 of U2 is labeled "Led." I can find no matching "Led" label anywhere else in the schematic. Where is pin31 supposed to connect? Viewing the layout I find that pin 31 appears to go directly to a via to who-knows-where; since we don't have an image of the other side of the board, we cannot trace pin31 further. Surely that via does not connect pin31 to the ground plane? Is there an LED & current limiting resistor hidden on the backside of the board?

Does the problem persist with the heating element disconnected?