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light switch/relay box build

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jclaudii

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Good morning; Long time away here.

I am trying to build a "relay box" that simply separates the load (tow trailer lights) from the vehicle's circuits. In some circles, especially in the Chrysler/Dodge world this is called a TIPM "saver". TIPM stands for Totally Integrated Power Module and is the power distribution "brain" of most modern cars, but have a few quirks in the Dodge/Chrysler world. It is basically a "smart" fuse box. One of those quirks is shorted trailer wires can and do cause serious damage to this part that is in the 500-1k range. The trailer tow wires, headlights, blinkers, etc ARE NOT protected by relay's or fuses but load sensing circuits that can and do burn out if limits are breached. I have also heard there is some logic to this TIPM and that they will "lock out" the defective circuit and it can be re enabled at the dealership, but can only be done less than 3 or 4 times. In fact there were Service Bulletins that bumped the "lock out" amperage from ~15 amps to 17.5 amps.

Anyways to prevent any damage, this box I want to build basically just isolates the trailer wires from the tow vehicle's circuits. Many of these boxes were built with off the shelf standard 12v relays that one can get from any auto store or Walmart but I was thinking there may be a bit cheaper way without the mechanical aspect of the relay. Basically what I was thinking was building this:
The box is fed power directly from the battery and is fused at the battery. The box then splits power out to the Left Turn, Right turn, Running, Backup, and a fused/circuit breaker output for electric brakes and 12v aux.

Now I was thinking, how hard would it be to use NPN transistors instead of relays, NPN's are CHEAP and a 15 amp NPN can be had for under 2 bucks each. My question is how would I wire this up? Can the NPN take the 13 volts directly or do I need a current limiting resistor and bring the voltage down or at least the current?
For example: http://www.jameco.com/webapp/wcs/st...Id=10001&ddkey=http:StoreCatalogDrillDownView

How would I go about using something like a 10-15 amp NPN for this setup? I am also thinking it would be super handy to have some LED's in the box as well to let one know the circuit is active and working. Here is an exmaple of what I am looking for but it is done with relays. http://www.dieseltruckresource.com/forums/1st-gen-ram-all-topics-93/trailer-wiring-upgrade-305370/

Thanks for the advice!
 

MikeMl

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Most Helpful Member
Since the lamps in the trailer are grounded to the frame, you have to do the switching in the high-side. That requires either a PNP or PMOS device. Since a high-side P-type switch requires a low-going control signal (base or gate), then you need an inverter between the high-going signal out of the vehicle and the high-side switch.

Here is what that takes:

26..png



Note that the current required to operate R4 [I(R4), green trace] is coming from the vehicle battery; not through the simulated flasher. By using a PMOS device that has a low Ron for M1 (instead of a PNP transistor), the power dissipation in M1 (red trace) is tiny, so that it doesn't require a heat sink unlike a the big heatsink a PNP transistor would require.
 

MikeMl

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Most Helpful Member
...If you really wanted to isolate the systems, you could use an opto isolator...
Not really needed because the towing vehicle and the trailer share a common ground through the trailer hitch, and the vehicle battery powers the trailer lights, anyway.
 

KeepItSimpleStupid

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I think some trailers actually have a battery, so that the brakes activate if the trailer should disconnect.

Your right, isolation is not necessarily a requirement and that's what I tried to say.
 

AnalogKid

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An NPN does need a base current limiting resistor. A power MOSFET does not, but I recommend sticking with bipolar parts in a car; it is not an electrically "friendly" environment. You might consider power darlington transistors, something in the TIP12x line. On my trailer the lights are grounded and you switch the +12 to them. If yours is the same, you would need PNP transistors.

ak
 

MikeMl

Well-Known Member
Most Helpful Member
I think some trailers actually have a battery, so that the brakes activate if the trailer should disconnect...
I have a Lance travel trailer. It has two big deep-cycle Lead-Acid batteries to run lights, fans, appliances (fridge, furnace, and hot-water heater use electronic controls), and enterainment systems. The house batteries are used to power the electric brakes on the trailer axles in the event of a break-a-way, but all of the running lights (brake, turn, clearance) are powered by the towing vehicle. During braking, the towing vehicle supplies PWM proportional to de-acceleration to the trailer's electric wheel brakes.
 

jclaudii

Member
Thank you for this info. I'll explain a few more facts to set a few things straight about vehicle towing.
The light system is powered by the tow vehicle battery 12+ and ground system. I am not doing anything special for tow brakes at the moment besides a bit lower amperage fuse as protection for the brakes. The whole idea is to run a separate power wire to a box that controls these circuits so the only load the TIPM on the truck sees is a few amps at most to turn on the transistors/mosfets/relays/etc!

I did think about optio-isolators but I did know know of any that could handle the amperage required. I would think max 7.5 to 10 amps per circuit Left, Right, Backup, Running. Run a 50 amp circuit breaker to the "distribution box" as I am calling it from the tow vehicle battery. I completely missed the fact that NPN's were negatively controlled and I don't see how it would work easily unless I follow your idea MikeMI. I may need some help in following exactly what you said. I love the idea of a system that is fully digital with no mechanical parts.
 

MikeMl

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Most Helpful Member
NPNs or NMOS devices used as saturated switches would normally be placed between the load and ground (low-side switch). Since the existing lamps are grounded inside the lamp holders, then you have to do the switching in the high side.
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
The opto's aren't strictly necessary. OPTOMOS relays would work fine (Overkill). These take about 10 mA to the LED and can give you an isolated DC switch out. That would drive your bulb driver. That protects the bulb, LED bulb or whatever. But they would have independent grounds and be in the pulling vehicle.

There are lots of options for high drivers switches at ST. You can break out the status info as well.
 
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