Last winter I installed an alternator on my Tecumseh powered snow blower. I did design and build a circuit for the voltage regulator. I also purchased an LED flood light and mounted it to the blower. Here is the long awaited documentation and finishing of last years project!
TruckStar 12-24 Volt LED Flood Light - Clear, Round, 4in., Model# 1492115:
Claims a meager 1.06A of draw at 14.3VDC. Through 6x 3 Watt LED's it promises an impressive 1350 Lumens. I really like the quality of the unit. It's a solid metal case that's fully sealed. It's also quite slim, unlike a traditional flood light. I paid $59.99. It's regular price is $69.99. I have seen it for as low as $54.99 depending on the sale.
Voltage Regulator
Opted to go with the TI LM2677/8/9 5A Voltage regulator.
http://www.ti.com/product/lm2677
It was the highest power voltage regulator I could find in a TO220 package. It's also sufficient power for my use. I used TI's special software (Power Stage Designer) to calculate the values of the inductor, capacitors, and resistors I needed for my specific requirements. I especially like that it's a switching regulator, not a linear regulator, so it's more efficient. TI claims up to 92% efficiency. This is the type of chip which could be used in high end computer power supplies.
Here is a generic example provided by TI:
This example gives 5V with a maximum current of 5A.
Using TI's online WebBench tool I input my requirements of 30-40V input and 24V, 5A output. It yielded this circuit:
To run the WebBench tool for yourself, go to the component page here:
http://www.ti.com/product/lm2677
On the right hand side you input your requirements and then click "Open Design". You will need to create a TI account and log in if you don't already have one (it's free).
Formerly I used TI's older software called "LM267X Made Simple" which is a download and install. It lets you choose your chip and also picks out the components you need. Both tools are nice because they generate a BOM for you.
With this schematic I made the following PCB layout:
And the isolation routing for my CNC router:
There are some problems with the above PCB. Something is wrong with the Tantalum capacitor because I popped two of them almost immediately, however the circuit seemed to run fine without them. I'm not sure of the cause for this as of yet. I also screwed up my routing for the 3 resistors (I wanted to use 2 in series to get the correct resistance). Once I corrected the resistors, I was able to run my LED spot light with this circuit. I am currently in the process of making a 2nd revision of the circuit to address these two problems as well an incorporate a board mounted heatsink.
Quick BOM:
- U1 - LM2877 Switching Voltage Regulator - Sample
- L1 - Inductor 50uH 5A - Digikey #553-1121-ND
- Cin - Aluminum 1000uF 63V 20% - Digikey #P11280-ND
- Cout - Tantalum 47uF 20V 10% - Digikey #478-6065-ND
- Cb - Tracon 2A104K - Already Had
- D2 - Rectifier Bridge 10A 100V - Digikey #GBU1001DI-ND (not shown in TI schematic)
- D1 - STPS2045CT - Power Schottky Rectifier 2x10A 45V- Recycled from old computer PSU
- R1 - 1K
- R2 - 10K
Heatsink Selection:
The switching regulator is very efficient when compared to a traditional linear voltage regulator like the LM7805. The lowest efficiency it claims to get it better than 94%. This means when serving 5A of current at 24V (120W) it will need to dissipate ~7.2W. However, my light only pulls around 0.5A so we're talking much less. Never the less, we need to have a heatsink on our regulator.
How to calculate resistors:
See pages 15 & 16 of the datasheet:
http://www.ti.com/lit/ds/symlink/lm2677.pdf
1K is recommended for R1
Using the formula provided, to get 14.4V one should use a 10.9K resistor for R2.
If I have made an mistakes above, please let me know and I will correct them. I hope this helps some people.
