Using Recycled Hex Mosfets for High Powered Halide running lights
| Foreword | Monitor Assimilation | Monitor PCB Recovery | Copper Collecting | Halide Lights and HexMosFets | Page 5 | Page 6 | Page 7 |
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My Car got an "owie", the lady in the big truck said she thought she had just bumped me a little as she was backing up. Well it sure left a big dent in the front hood and ICBC darn near wrote my little Ford Escort right into the scrap yard. It came to the tune of $1800 for that little bump. Anyway to make a long story short, when I bought the car it had one side of the bunper caved in and these neat looking halide running lights dangling down on burned out 22 gauge wire. A young fellow had the car before me and didn't have a great ability at the electrical side of technology, and he was a delivery driver and missed a turn somewhere and hit a stump or something putting a big crack in the right side of the bumper. At any rate I decided to take this oportunity to fix up and repaint the bumper and put those neat halide running lights together.
I won't get into the details of the body work or of fixing that plastic piece of %$&!$ bumper. But the details on getting those lights working is interesting. First off each lamp draws 4 amps so two of them draw 8 amps. This is a lot of current. When they are running you can't touch the plastic case as it will burn you that is how hot they run. You would have to make sure you ran good 16 gauge automotive wire and had a big heavy duty switch inside on the car dash. At this current any butt connectors used would have to be very well done or arching would occur and the wire would burn inside eventually becoming inoperative. Also a big fire hazard exists if you don't so this high current wiring correctly. So I figured I would built a solid state switch and then I could use a low current switch in the dash and have a nice little led light to tell me if things are OK or not under the hood. A relay to handle this current is fairly expensive and the contacts are prone to get arched at this high current. I would only have to run heavy wire to the Solid State Switch and a fuse would give me protection against a fire hazard. So a Solid State switch it was going to be. A high current switch can be quite expensive, I saw some PIAA switches for those offroad Truck Metal Halide Lamps run at $500 - $800.
I had just stocked up on Mosfets as I was doing some DC Motor Controls so I had a batch brand new IRF840's. They had great specs Vds of 500 Volts. Great for a car that only has a 12 V Battery. Ids On of 8 amps current a little touch and go for the two lamps as they each draw 4 amps; but at a buck apiece I could use one for each lamp. And a Rds on of 0.85 ohmn seemed small enough. But when I tested it out on the bench I could only get 10 Volts across the Lamp. I wanted the full 12 Volts across the lamp for the maximum brightness. Thinking about it the seemingly low on resistance at a higher current that the lamp is drawing means 0.85 ohms times 4 amps =3.4 Volts (I had a good battery 13.4 Volts). Mosfets can easily be paralled not like bipolar transistors. So when I paralled 4 of them I got reasonably close to my goal. This would have been Ok but it was extra wiring 8 Mosfets and a lot of overkill not to mention $8. So then I went through my stock of Mosfets looking for the lowest Rdson. Most were close to an ohmn and even some like the IRF810 was 1.5 ohmn. Finally in my recysling collection I found the IRFZ34N. This little devil had a lower Vds of 55 Volts but had a whopping Id of 29A and the lowest Rds I have ever seen 0.040 ohmns. This little devil was perfect and it was in my recycling collection! I tested it out and it performed perfect not even getting warm after 10 minites of burn time.
Notice I also have a great stock of Heatsinks from my recycling project. There isn't much room in the engine compartment of that little escort So I picked out a heatsink that was smaller and could bolt into this little area that I wanted in the engine compartment.
The terminal board was the only thing that was not part of my recycled parts bin. It came from Princess Auto. With this arrangement all the wires can get a solid connection to the terminal board. I also put two little leds on the backside of the heatsink just to show when the Hexfets are on. I will mount a little L shaped piece of plastic over the whole lot to give it some weather protection and protect it from me throwing a wrench in the wrong spot. It is going in a tight spot in the engine compartment and will be quite shielded in its final position.
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