The automotive electrical system is a harsh environment. Normal voltages range from 9 to 15 volts, and even greater extremes. A jump start from a tow truck often pushes 24 volts down the wires. Stopping the engine can generates a spike up to 130 volts. And if a battery cable comes undone, the load dump can send 40 amps looking for somewhere to go.

Automotive equipment, like the factory or after-market radio, is designed for this. But consumer electronics, especially the "cost minimized" imports, can easily be blown out of the water, or sunk by cumulative damage.

Think about a cell phone charger, an old CD player plugged into the cigarette lighter, or some other device that came with a 12 volt "wall wart" power supply, which you want to run off the 12 volts in your vehicle. It needs protection!

In my case, the radio went missing, and I had a cheap amplifier intended to drive some bookshelf speakers. So I decided to move it to the car and hook up a MP3 player for (indefinite) temporary use.

Then I remembered how tough a car's electrical system can be, and built this little protection system using a HITFET BTS117 chip designed exactly for protecting things connected to a car battery.

Step 1: The Amp (a Diversion)

To hook this thing into my car, I bought a car wiring harness connector. It's a grey rectangle.

These are vehicle model dependent, plug into your car's wiring harness, are have a bunch of loose wires hanging out. There is a second connector that fits into your aftermarket radio and also has a bunch of loose wires hanging out. The installer matches the wires from each connector according to a table of which wire is for what, crimps them together, and slaps your radio into the dash.

I my case, I have push button connectors and screw connectors on the surge protector board to match up to the wiring harness connector. Part of the beauty of my rigging is the coax DC power connector and board edge connectors let me install all the wiring with only a mini-screwdriver and my thumb.

And since the board is floating around inside the dash behind the amp, I cut the end off a plastic medicine bottle to put around the board to protect it and electrically insulate it from anything inside the dash. It's "flow-through" for cooling. I drilled 1/8" holes in the bottle and threaded cable ties to hold it on.

A double ended phone cord is also plugged into the back of the amp and brought out to hook into my phone or mp3 player.

Step 2: The Circuit

The BTS117 is a CMOS chip that started life as a hunky MOSFET transistor, so it comes in a TO-220-3 package, suitable for connection to a heat sink. Then the designers used it as an on/off switch and added all sorts of protection for reversed battery leads, over voltage, over current, short circuit, overheating, and anything else they could realize.

CMOS is static sensitive (even with the modern builtin protections), so I decided to handle the chip just once. When all was ready, I put on my wrist strap, grounded myself and the board, then inserted the chip into the 3-pin connector. And of course, if the chip ever gets blown, it's a trivial replacement job.

The BTS117 control lead should be kept under 10 V. I put in a divider network of two 10K resistors to cut the car voltage in half. The values aren't really important. The control line needs almost no current so high resistor values are OK. 2 volts is enough voltage, so the divider could be 70-30 instead of 50-50.

The BTS117 manufacturer marketing materials talk about minimizing the need for heat sinks. It has always run stone cold for me, but then I've never run the full 7 amps though the chip. I'm not worrying about a heat sink until I find some appliance that makes it start running warm. (BTW, you can get higher amperage versions: BTS133, BTS141, BTS149.)

But just in case I ever add a heat sink, and to provide a trivial bit of assistance to air flow cooling, I've mounted the chip tab side up from the board.

BTW, the tab is connected to pin 2, which is NOT GROUND. It may well be close to ground (as this is a low side switch), but resist temptation to fasten it to your chassis.

The control line is a low current "on/off" switch for the high current device. It's no problem to connect it to the power being switched, so when the ignition switch comes on, the control line comes up along with the power for the device. (The chip actually delays turning on the device until power has stabilized on both lines.)

(The photograph of the chip is actually a BSP 75, a similar member of the family.)

Step 3: Surge Protector Board Parts

The board itself is 6 lane PCB tinned prototype board I had in my box, sufficient width for the 5 pin connector.

The two edge connectors were also in my box. A 5-pin is needed for the power/control in and out. A 3-pin is used to hold the BTS117.

The BTS117BKSA1 is about $2.10 plus $3.50 shipping (USD) from Digikey. (5-pin, 3-pin, 4.7K resistor at Digikey)

The two resistors are about 5K, again from my parts box. Your values can be off an order of magnitude.

A real help to assembly is using a tooth pick to apply a small drop of colored nail polish in front of each connector pin.

The two connectors will be placed back-sides facing, so the colors are in reversed order on the two.

Note that the yellow side of the 3-pin connector is pin 1 of the BTS117, which means the BTS117 is upside down.

Low sides (battery and device) are on the outsides (Black, Blue) of the 5-pin. High sides (Red and White) are inside from the low sides. Yellow is an on/off control line in the middle.

So far, I haven't needed a heat sink.

Step 4: Assembly

If you did the nail polish trip, then everything is color coded.

Except for the diagonal resistor, each jumper wire ends up directly beside a connector pin. Because of the size of the connector, their holes won't be adjacent. There will be 1 hole between the wire and the connector pin. Cut the jumper leads extra long to bridge this distance.

The resistors go in first, because the jumpers need to cross over the top of a resistor.

The jumpers go in last, and the long tails are parked beside and wrapping a bit around the connector pins. Finally, take the leads from the diagonal resistor and cross them over to the row with the connector pin where they will attach.

The diagonal resistor is special, in that each end is folded over into an adjacent row to reach the proper connector pin. Look carefully at the next step to see how two wires and a connector are all globbed together.

Step 5: Soldering

This is straight-forward. Start with a piece of painter's tape to hold the jumpers and resistors down, then turn the board over.

Tack the center pin on each connector. Then press the connector firmly into the board and reheat the tack pin. When the solder reflows, you will probably feel the connector set down even firmer against the board. Then finish the connector pins.

Wrap the leads around the connector pins, making sure everything under the tape is still flush against the board.

Solder things down. A steel dress maker's pin (or aluminum nail) laid across a wire that doesn't want to stay against a connector pin will overcome its reluctance and be easy to work off the joint after it cools.

Finally, check you work and then clip the excess lead ends.

Step 6: BTS117 Into Socket

I clipped the leads when I put it in the socket. If I ever have to remove it and attach a heat sink, I will probably wish that I hadn't since the BTS117 might have to move up a bit for the heat sink to clear the socket. Ah well, live and learn!

Remember the BTS117 is a static sensitive device. Wear your wrist strap/ground everything before handing it.

Step 7: Using the Surge Protector

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Step 8: Extra Bonus -- Other Output Voltages

Here's an easy way to get 2 to 30 volts up to 2+ amps.

Put a cheap DC-DC Buck/Boost Power Supply between the surge suppressor and a device that needs something other than 12 volts. Dial in the desired voltage with a screwdriver, and the surge suppressor will protect both the voltage converter and your device(s).