Uni Project Help!

[piggywig23]

Hi there

I'm after some help with a project I'm working on at university. I'm designing heated handlebars. I'm using two wirewound 50w 10R resistors in each end of the handlebar to act as the heat source. I've found that around 15v/2amps is enough to generate a sufficient amount of heat in these resistors.

My next step is to work out how to connect these two resistors to an 11.1v lipo battery so the handlebars can be powered on the go, as opposed to using a power pack which I'm using currently. Unfortunately electronics is not my strong point so I'd be very grateful for some guidence.

I'd like to know what circuitry I'd need to build to safely supply power from my lipo to the resistors, and how I'd control the current so I can control how much heat the resistors give out.

Thanks for any help!

 

[deanenp]

Hi

Im not an electrical engineer, nor do I know much about electronics, but, if you're using a Lipo battery, I know that you will have to have some sort of low voltage protection in place. Lipos are extremely dangerous (the pack will puff up and in some cases explode or burst into flames) when over discharged.

I used 11.1V 3S packs for my electric helicopters and I normally only drain them down to 30% capacity.

Deanen

 

[KrisBlueNZ]

There are two approaches I can see here.

The first is the approach used in simple heating appliances like bar heaters and electric blankets. Each heating element actually contains two (or sometimes more) resistive elements, and they are connected to the power source in different ways depending on which temperature setting is selected. This connection can be done with multi-position, multi-pole switches. For example:

Assume each handlebar heater contains two 11.1 ohm elements.
For LOW setting, connect the elements in series across the battery. P = 5.55W.
For MEDIUM setting, connect only one element across the battery. P = 11.1W.
For HIGH setting, connect both elements in parallel across the battery. P = 22.2W.

The alternative is to use PWM (pulse width modulation) with a single heating element, and vary the heat by controlling the proportion, or percentage, of the time that the element is powered. As long as the total cycle time of the PWM circuit is less than, say, a few seconds, the thermal inertia in the heater will smooth out the ON and OFF parts of the cycle and you will feel the average (mean) heating effect.

This method would normally use a MOSFET as the switching element. The PWM controller can be something as simple as a 555 timer IC, controlled from a muti-position switch or a potentiometer. In this application it would be best to run the two handlebars at different points in the cycle, to reduce the sudden heavy load current changes that the battery sees.

If you can describe your requirements IN DETAIL, I will draw up a circuit.

 

[Harald Kapp]

How big is your battery? A 2.2Ah (2200mAh) LiPo battery is already a big one.
At 2A this battery will last for 1hour, not counting the previous warnings about not discharging the battery too low.
And if that's 2A per handlebar, the total load on the battery would be 4A, draining it within 1/2 hour.

To get the maximum out of the battery follow Kris' advice and use a PWM regulator. If you don't want to build a controller yourself (Kris has given you an easy solution), you may "abuse" an electronic motor controller for RC cars for that purpose (but don't think that reversing the direction will cool the handlebars in summer ).

 

[(*steve*)]

Warming the handlebars is grossly inefficient. Unless what you mean is to wrap some heating element around the thermally insulated grips. Heaters inside gloves would be far better.

 

[piggywig23]

Really appreciate the help guys, thanks.

I'm using a 3 cell 11.1v 1300mAh Lipo.

I've attached two images to give you a better idea of what I'm working on. It's a project for my Industrial Design course by the way. The basic idea is that the wirewound resistor is encased in an aluminium tube which is then placed inside the handlebars so the heat is able to transfer quickly. So I'll have two of these components which both need combining into a circuit. The battery and circuitry will be attached to the stem of the bike.

I'd be very grateful if you could draw up the PWM circuit that you've described for me Kris. So I''ll be using a 3 cell 11.1v 1300mAh Lipo and two 10r 50w resistors (shown here: http://uk.rs-online.com/web/p/panel-mount-fixed-resistors/0158339/) - I could probably drop this to a 25 watt if necessary. So the other components I'd need would be a MOSFET and a 555 timer, is this correct?

 

[Harald Kapp]

Warming the handlebars is grossly inefficient

What Steve means is that the thermal energy from a heater within the handlebars will quickly be drawn away from the end of the handlebars (where it would be needed) to the center of the handlebar. From there it will quickly dissipate into the surrounding air.
You should construct the heater like this:
inner tube: handlebar
1st layer: thermal insulation
2nd layer: heater
3rd layer: thermally conductive but electrically insulating
4th layer: provides mechanical ruggedness (may be combined with 3rd layer)
That way the heat stays where it's neede: at the hands.