Small DC Voltage regulator

[dano191]

I built a headlamp for mountain biking that uses a 20 watt MR16 light bulb. It is great except that it kills batteries quickly. I would like to be able to turn down the brightness to save the batteries when I don't need the brightness. I now have a simple toggle switch. Is there something about the same size that would work?

 

[Blueteeth]

Hi,

Well bulbs are pretty inefficient as far as power -> light output, this is why LED's seem to be everywhere now days (a lot less heat conversion).

I don't think a voltage regulator would be any good for your application, since a standard linear regulator (such as the 7805 series) disappates the difference between input and output as heat, in order to maintain a constant voltage output. That is, with 7-12V in..and 5v out, with 0.5A going through it, the output is 5V @ 0.5A, but the input is 7-12V also at 0.5A. This means, with a 7v input, the regulator 'wastes' 7-5v = 2V, multiplied by current, 0.5A = 1 watt of power just to hold the voltage output.

That said, there are switch mode power supply modules. The 'module' approach would save you having to design/build one (bad option for a novice) and these can be up to 90% efficient. These would convert your battery voltage, to the desired bulb voltage, with little loss. So, if your bulb is 12V, and your battery is 18V, it could step this down to 12V, drawing less current form the battery.

Probably the best option for you would be to use pulse wdth modulation.
You can google it, but basically, it turns the power to the bulb on and off very quickly (>100Hz) and its the exact method used in household dimmer switches

By changing the 'on time' and 'off time' you can determine how much of the total time the bulb is powered. For example, 10% on, 90% off, means that your bulb is at 10% brightness, also it is only taking power 10% of the time, so theoretically, it should last 10 times longer with the same battery.

You can off course vary the mark/space ratio of the PWM signal to find the optimum compromise between 'brightness' and 'battery life'.

I googled your bulb, and it says '12v', so I can only assume you are using a 12v battery. What capacity would this be?

If you're after long battery life but cannot afford the weight/size requirements of a larger capacity storage, then consider converting your headlamp to LED's. More efficient, and with all the 'power LED's around today, not particularly expensive. There are also regulator 'modules' available than can efficiently convert your 12V battery voltage to the required LED voltage. Or connect a strong of LED's in series/parallel...so many ways to do it.

Hope this helps.

Blueteeth

 

[HarveyH42]

Here's a link to a bunch of MR16 LED bulbs, up to 8 watts. It's a good company, ordered from them many times. Usually 10-12 days from ordering to delivery.

**broken link removed**

 

[Hero999]

Is it halogen?

I wouldn't recommend dimming halogens as it shortens their life.

What about upgrading to an LED light?

6W of LED power should produce as much light asthe halogen and be easy to dim.

 

[dano191]

The MR 16 is a halogen bulb. I use 14.4 Volt 2 Amp Hour Bosch cordless drill batteries. Is there a LED that has comparable brightness? You need a very bright bulb for mountain biking. I was hoping to be able to use a 35 watt bulb and dim it when I don't need it so bright, for climbing, etc. Do standard dimmer switches need line voltage? Bulb life isn't important as the light only will get used 10-15 times per year at 2 hours per ride. I need 2 batteries per ride for a 2 hour plus ride. I would like to be able to get away with one battery.

Thanks for all your help

 

[Blueteeth]

The MR 16 is a halogen bulb. I use 14.4 Volt 2 Amp Hour Bosch cordless drill batteries. Is there a LED that has comparable brightness?

I would have thought so, now days LED's can reach 6 Watts and beyond. But considering they are much more efficient than halogen's the LEDs replacement for the MR16's would be comparable in brightness, but require lower power -> longer battery life. Because they are designed as replacements, not alternatives, I imagine that any manufacturers/sellers website would include additional information comparinbg the two..brightness, viewing angle etc...

You need a very bright bulb for mountain biking. I was hoping to be able to use a 35 watt bulb and dim it when I don't need it so bright, for climbing, etc. Do standard dimmer switches need line voltage?

As far as I know, they do. I think you can get dimmer switches for lower voltage bulbs used in home lighting systems but I haven't seen any. If you are indeed electronics savy, a simple PWM dimmer using comparators, a 555, or even a micro shouldn't be too difficult, there are plenty of example circuits on google, many with handy stripboard layouts and detailed explaination.

Whether you choose to use your standard halogen bulb or LED's, either way you would still need a device to switch it on and off at PWM frequencies. This should be a power MOSFET, or possilby a pwoer transistor. MOSFET's generally have a much lower on resistance so they waste less power. Not only does this mean you get a slightly longer battery life, but also they don't get as hot as standard power bipolar transistors...so you own't need a massive heatsink. (LED drivers have this built in as part of the power supply).

Bulb life isn't important as the light only will get used 10-15 times per year at 2 hours per ride. I need 2 batteries per ride for a 2 hour plus ride. I would like to be able to get away with one battery.

I'm sure that could be done.

I know it was mentioned that dimming would reduce halogens life, and I assumed as much, but it won't be a drastic reduction.

Now, I know it seems like the peeps here are pushing the LED idea, and of course if you already have some bulbs, and just need an 'add on' then perhaps the PWM dimming would be a good idea. However, they are coming into fashion for a reason, especially for portable applications: efficiency.

These websites seems to give some half decent figures as to the reduction in power LED's could make:

**broken link removed**
**broken link removed**
**broken link removed**

As you can see, it depends on viewing angle (the more focused the beam, the brighter they say it is, but a narrow beam might not suit your application). But generally it seems you're saving a at least 60% power for the same brightness/viewing angle. And yep, that means 60% more battery life. Some even claim to use 5W to replace a 30W bulb (not sure I beileve that) so there thats 1/6th the power, or 6 times longer batt life.

YOu may have a tough time finding one that woudl be an idea replacement as your currepnt setup as far as power output/beam angle goes, but they don't seem to be too expensive. Their life is rated in thousands of hours, so I guess unless you fall off your bike onto a rock it'll last forever.

You mentioned your current setup lasts for over 2 hours, but requires two batteries. So lets say each battery provides 70 minutes of life. (for a 2 hour 20 minute ride). For a 60% increase in power (thats the minimum) using an LED replacement, each battery would last
70 mins + 60% = 1.6*70 = 112mins (hour 50 mins). Two batteries: 224mins (3 hour 40).

As I said, thats a minimum saving, some bulbs appear to be more efficient, meaning I tihnk you can get away with just one battery for 130mins.
Or hell, even get some higher capacity batteries

Hope that gives some food for thought.

Blueteeth