heating element

[Remur123]

I am trying to reduce the current going through a heating element. The resistance of the heating element is 2.5 ohms. I am using a 5v 2A battery and at the moment it is pulling 2.1A. How do I reduce the current going through the heating element? I've been thinking about just using a PWM(pulse width modulator) but was wondering if there are other options.

 

[Pommie]

Why are you trying to reduce it? Do you think the battery can only supply 2A?

Mike.

 

[Nigel Goodwin]

I agree with Pommie, why do you want to reduce it? - at the moment it seems perfect, 5V and 2.5 ohms is 2 amps anyway, a slightly high reading is well within tolerance.

However, if there's actually any reason to do it?, a simple series resistor would do the job - of a suitable resistance and wattage.

If you wanted to make it variable power, THEN you could consider PWM.

 

[Remur123]

at the moment the heating element is getting hotter than i would like. i am trying to find a way to be able to vary the current so i can change the temperature of the heating element.

 

[Remur123]

putting a resistor in series with the heating element will only cause the resistor to get hot also.

 

[Nigel Goodwin]

putting a resistor in series with the heating element will only cause the resistor to get hot also.

Which is why I specified of a suitable wattage - but while that gets hot, the element gets less hot - It's the standard and simple solution.

As I said, if you want it variable, construct a PWM circuit to power it, or if you just want one reduced power level, a simple resistor will do - which historically is how the fan speed controller in car heaters used to work (and is why they had switched speeds, and not variable ones).

 

[Diver300]

putting a resistor in series with the heating element will only cause the resistor to get hot also.

Also, if you put a resistor in series with the heating element, the overall power is reduced. The resistor in series will never create more than 1/4 of the heat of the heater on its own.

The 2.5 Ohm resistor will take 2 A from 5 V and create 10 W of power.

If you put another 2.5 Ohm resistor is series, the overall resistance will be 5 Ohms, so only 1 A will flow and there will be 2.5 W of heat in each resistor. That is the maximum power you can get in the series resistor.

I guess you will only want to cut the power down a bit. For example, if you put 1 Ohm in series, the overall will be 3.5 Ohms, so the current will be 1.43 A, and there will be just over 2 W of heat from the 1 Ohm resistor and just over 5 W in the heating element.

Heating is very often controlled by PWM, and it can usually be very slow due to the thermal inertia of the heating element and the item being heated. It is common to have a 10 second cycle, or 0.1 Hz but small heaters may need to go faster. It's a long way from the 20 kHz or more that is often used for motors.

 

[gophert]

Remur123
I agree with you that adding a resistor to a heater circuit is stupid because, no matter the wattage of the resistor, the voltage drop across that resistor will be converted to more heat that you are trying to avoid.

the two options are, as you suggest,
1) PWM (or some other modulation method) of the supply.
2) DC/DC conversion to drop the voltage to drop the power at the heating element.