Design Project - Circuit Help!

[piggywig23]

Hi guys,

I'm currently doing a major project for my final year studying Industrial Design and Technology.

I'm designing heated handlebars for Cyclists. At the moment I'm working on a few rough prototypes but I'm struggling on the circuitry as electronics is not my strong point.

I basically want to power up a heated bed through a Lipo battery. So far I've been powering it via a power pack. So I need to regulate the voltage and current but I have no idea how.

My Lipo battery is 3 Cell, 11.1v, 1300mAh.

So I want to regulate the voltage to a smooth 11.1v and limit the current to 1.31a, as this creates a decent amount of heat. To do this is it just a case of using a current limiting resistor, or is it more complicated than this?

Hope this actually makes sense, any help would be much appreciated!

 

[CocaCola]

Consider PWM control of the heat, it's the most efficient, and you are going to need this with such a small capacity battery if you want more than a few minutes of run time...

A current limiting resistor is only moving the heat source from one location to another, aka instead of heating the grips you are now heating the current limiting resistor...

 

[KrisBlueNZ]

What are the characteristics of the handlebar heater elements? I'm going to assume they are resistive - made from resistive wire or something equivalent. If it's a Peltier heater, this answer doesn't apply.

If the heaters are resistive, then the current they draw will be roughly proportional to the voltage you apply. The amount of heat generated will be proportional to the power dissipated, which is equal to the applied voltage multiplied by the current. In your case, 11.1V x 1.31A = 14.5 watts.

Therefore you need to regulate them by adjusting the voltage you apply. When we talk of "regulating current", this actually operates by regulating the applied voltage in response to the measured current. If the load (heater) tries to draw too much current, the voltage is reduced, and this causes the current to drop. In other words, you can't regulate the current independently of the voltage. The voltage is the factor that is regulated; reducing the voltage will reduce the current (for any normal load, anyway!).

Now I wonder why you feel you need to regulate the voltage. Your LiPo battery's terminal voltage will not vary greatly over the range from fully charged to fully discharged. Therefore the current, and the power dissipation, and therefore the heat output, will not vary much either.

If you really need to regulate the voltage/current/power fed into the heating elements, so it's exactly constant over the full range of battery terminal voltages, I agree with CocaCola - use PWM. You connect a MOSFET between the battery and the load, and switch the MOSFET ON and OFF using a signal whose duty cycle is controlled according to the input (battery) voltage. If the input voltage is exactly right, you leave the MOSFET ON all of the time, i.e. 100% duty cycle. If the input voltage rises, you reduce the duty cycle, so that the load sees the correct average voltage. This requires some kind of control circuit - a small microcontroller, or a discrete circuit, to generate the PWM signal.

Anyway have a think about all of this.