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Any EE's up in here? Homemade PDM?

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  • Any EE's up in here? Homemade PDM?

    I've been looking into PDM's and love the idea but the price is not really feasible for a enthusiast level car.

    The electronics hobbyists have been going to town and there are now microcontroller kits that make it very easy to create some really cool custom devices that before required a lot more effort to build from scratch.

    I've been thinking that it would be possible to use an Arduino or similar kit to make a home made open source PDM. They include software libraries and integrated USB programmers making it easy to get circuits working quickly.

    There is also a now a Arduino add on (called a 'shield') that has a built in CAN controller.

    The idea would be to use the Arduino to program the switching and power management of solid state relays to replace and simplify all wiring in a the car. To start a proof of concept could be done on a stripped down race only harness on the track car so less circuits are needed.

    Motec PDM example:

    This could make the electrical system more reliable and reduce weight significantly if a CAN bus was incorporated.

    On of the limitation seems to be Arduino's limited number of inputs (14 digital I/O pins, 6 of which can produce PWM signals, and 6 analog inputs.)

    Here is a list of projects that have used an Arduino -

    with the low cost (Arduino's cost between $20 - $50) I gusstimate that a working PDM could be made for $300 (including all the solid state relays and circuit breakers but probably not wiring for the car circuits) or less in parts.

    Anyone with some EE experience care to comment?
    Last edited by Umai Kakudo; 03-28-2009, 10:21 AM.
    The Nerd shall inherit the podium for knowledge is power.

  • #2
    My $0.02 (and yeah, I'm an EE....)

    As with any project, time= money. The PDM looks like a good idea to me, but only for those who have the time and/or money to put into it. There will be some SERIOUS programming for all of that to be done from scratch. It can be done on a simple 1MHz processor (I usually use an Atmel 40 pin 8051 microprocessor since it has many more I/O channels and costs only a few bucks more, but newer processors integrate ethernet/usb for easier programming and communication)

    The big issue would be doing the programming. The wiring and hardware side would be easy.
    You're still going to have a ton of wiring to do though if you want to run this on a street car... You have all of the window motors, door locks, headlights, blinkers, wipers, brake lights, hazards, A/C, and then of the engine start/run functions to deal with.
    Then throw in the switches for all of the above.

    there's a ton of inputs that would need to be handled simultaneously and programmed to work with each other. (what happens to the computer when you're trying to open a window while the air is blowing and you've got your wipers and turn signal on?). The software has to be able to multitask for that kind of stuff.

    It's all easily doable hardware-wise once you do the wiring, but the software will take a huge amount of time to program properly.

    Considering the hassles of programming, it looks like the price of 2450 euros for the MoteC PDM32 is cheap assuming you have the software to set it up.

    Cliff's Notes simply says it's not 'economically feasible' for a street car. pro level race car? sure. 1once a month club racer or street car? I would rather put my time and money elsewhere.

    All that said, more power to ya if you want to try it... Just realize how much of that bite you're going to have to chew!
    Originally posted by SoSideways
    I don't care what color they are as long as they are LONG AND HARD.
    '04 G35 Sedan 6MT- The DD
    '96 240SX- The Track Toy


    • #3
      depends on what level you take it to. if you get crazy like matt93se was talking about then it's ridiculous. If you narrow the scope to say a race car (no window motors, turn signals, etc) then it's not so bad. Or if you narrow the scope by leveraging some of the existing street car infrastructure (ie use factory wiring for power windows and lighting) by only redoing the distribution of say engine (including fuel and ecu) then its probably not all that bad.

      Only the intentionally basic Arduino's lack some form of networking functionality which could easily be used for expansion of I/O. It's really not hard to do the master with multiple slaves thing, which could work with how you segment the power distribution (between say fuel, ecu, cooling fans) using 1 arduino per group.

      Leveraging all the library's and help regarding circuits, I wouldnt think it would take someone that long, or that much money to build something nice. Then do a write-up and share the information like everyone else is doing

      Personally, I think you're nuts if you're trying to do this just because you want to save money on a PDM. But if you're into electronics / your car then I'm sure it wouldn't be too hard and very rewarding. I mean you're programming it so you can make it do whatever nutty / cool things you could add a switch so that your lights flash in some special sequence, you could build in some sick alarm functions, you could implement your own automatic climate's really endless.


      • #4
        I'm not a big electrical guy - PDM = ?
        '18 Chevrolet Volt - Electric fun hatch for DD duty!

        DefSport Koni Sleeve and Spring Perch Buy!!!


        • #5
          Power Distribution Module.
          My translation is that essentially he's trying to get rid of the shoeboxes full of relays and wiring under the hood and run with a solid state solution that has no moving parts or contacts to wear out.

          Sounds good in theory to me, but I don't see it being used on a street car with much success. There would just be too much work and hassle involved in getting it working.
          Branching off the circuits like Floodo was mentioning will simplify the programming a LOT, but then you add cost and hardware complexity to it. Software is 'free' in that it doesn't cost anything to add a few more lines of code assuming the memory and outputs are available.
          Daisy-chaining these things adds quite a bit in hardware every time you add another module, and the cost skyrockets when you add complete modules like that. Maybe there's another solution that will work as well for cheaper?

          (Then again, the 16 year old relays in my Maxima are still going strong.. never had a single wiring problem in that car other than the door window switch and lock timer needing resoldered every 5 years or so.)
          Originally posted by SoSideways
          I don't care what color they are as long as they are LONG AND HARD.
          '04 G35 Sedan 6MT- The DD
          '96 240SX- The Track Toy


          • #6
            I was thinking of using it for a stripped out race car with a few things like lights, heater, and wipers with some datalogging and video toys.

            Motec summarizes the function and benefits as such:
            MoTeC Power Distribution Modules (PDM) are designed to replace conventional relays, fuses and circuit breakers by providing electronically switched power to the various electrical systems in the vehicle. This simplifies wiring and switch requirements, while increasing reliability.

            Each PDM output is over-current protected and can be controlled via a combination of switch inputs, CAN messages and logic functions. In addition to performing simple functions such as flashing indicator lamps, the logic functions can be used to selectively turn off systems during low battery voltage or engine starting.

            The PDMs also provide full diagnostic information, including output currents and error status that can be monitored on a PC or transmitted via CAN to a display or logging device.

            For race teams looking for the ultimate edge they offer extreme reliability, the ability to control and diagnose power issues through software, and a reduction in the amount of wiring and therefore weight.

            Typically, a sedan constructor would use 2 PDM modules, one in front and one in back connected only by the main power lead and a pair of CAN wires. This allows for short, minimal wire runs which can save a lot of weight in wire and simplify troubleshooting.

            Imagine only running 3 wires for your electrical system through the middle of your car!

            With a DIY project you likely would not get the level of reliability and sophistication in the software available from a commercially developed product.

            With OEM's are still using good old fashioned, fuses, circuit breakers, and relays (but increasingly controlled by CAN modules) they are still the most cost effective way to go.

            Here are some other PDU Suppliers


            They start at 2K for a 15 channel and 5K+ for 32 channel units.

            That is a lot of speed if put into other parts of the car for a low buck privateer.

            The lowest cost consumer grade system I've found is $800 to start from ISIS Power


            It is not a true PDU because it uses fuses but it does simplify wiring through controlling power distributions using multiplexed distributed components and a software programming interface.
            Last edited by Umai Kakudo; 03-31-2009, 05:46 PM.
            The Nerd shall inherit the podium for knowledge is power.


            • #7
              Ummm, there's nothing that says a PDM has to be solid state relays. You can accomplish the same thing by using mechanical relays mounted on the distribution board instead.

              I assume, that in order to limit yourself to 3 wires, you are going to have a control module attached directly to the control board, that is communicating to the power distribution board via the CAN bus? So, you have two processors just in those two modules.

              IMO, this solution is overkill for a vehicle with only 3 or 4 switched loads in it. You're going to have a minimal amount of wiring anyway. I can definitely see an OEM manufacturer moving to this, based on the idea that a solid state relay is cheaper in bulk than a mechanical relay, and saves a good bit of material when manufacturing 100K vehicles.

              Again, if you're doing this for the neat factor and just for the opportunity to program, then yes it's a cool project. I think a custom datalogger / gauge / warning notification project would be more fun and more involved.


              • #8
                This project is beyond my current skill set and too much effort for little gain.

                I think I'll end up going with a custom harness that uses good old circuit breakers and relays in a bespoke enclosure.

                Although here is a good cost/benefit analysis of using the Motec unit vs wiring up your own fused system.


                Take for instance the PDM 15 I recommeneded. US $1900 retail.

                To wire this up might take 4 hours. For the sake of simplicity let's assume 15 switch inputs and 15 outputs. You will have to strip 30 wires, terminate them, put them in the connector. At even cheap shop labor rates that might be around $320.

                Now, imaging you're doing the same thing with relays and breakers. Let's assume you're starting with the same 30 wires. I want you to go through the labor of terminating 15 switch inputs and 15 high current outputs with the proper terminals - some ring terminals, some spades. And these kinds of terminals really should be crimped AND soldered. Now go build a "bus bar" to mount all your breakers to. There are 2 wires per breaker. Then figure the 4 wires per relay. How about the cost of low-end (non-ETA) breakers. How about $30 a piece? Using Bosch Automotive relays, you're looking at around $600 in hardware.

                Now how about time? In my experience, building a 15 channel relay/fusing panel is about a 2-day job. That's 16 hours of labor. And if you were me, you'd spend 2 hours generating proper diagrams. So now you're at $2040 in parts and labor. I guarantee you the time and money you save with the very first failure that MOTEC's diagnostics allow you to easily fix will save you even more money. . . .
                Last edited by Umai Kakudo; 10-23-2009, 12:53 PM.
                The Nerd shall inherit the podium for knowledge is power.


                • #9
                  Better to realize it now than once you've bought the parts!
                  I have several "projects" in the garage of stuff I purchased and ran out of time/skill/desire to finish.
                  Originally posted by SoSideways
                  I don't care what color they are as long as they are LONG AND HARD.
                  '04 G35 Sedan 6MT- The DD
                  '96 240SX- The Track Toy


                  • #10
                    For sure. Now if anyone else want's to figure it out I'll beta test it for them.
                    The Nerd shall inherit the podium for knowledge is power.


                    • #11
                      Update: Example of working system

                      Here is a guy who built a setup for a 24 hours of Lemons race.

                      It features basic telemetry with GPRS wireless modem to post data to their webserver where the data can be displayed in many different charts and graphs.



                      • Lap times
                      • Vehicle speed
                      • Brake pedal position
                      • Accelerator pedal position
                      • Lateral and longitudinal forces
                      • Vehicle position on the track via GPS

                      See link for full pictures and system diagrams.

                      He also hooked up an active spoiler that was activated based on the telemetry data and his custom programming.

                      Future plans include adding CAN and a few other things.
                      The Nerd shall inherit the podium for knowledge is power.


                      • #12
                        Here is an excellent article on PDM's, how they work, and the Motec units in particular.


               has a lot of great DIY info especially for electronic stuff.

                        Here is an example of ROI for pro team using a PDM


                        n Spring 2009 the Synchro Honda was competing in a BRITCAR round at Northamptonshire’s Rockingham Speedway circuit. After 60 mins of hard racing they had fought their way to second in their Production class, but as the race went on they experienced loss of engine power. The cause? Fuel had begun dissolving some of the foam present in the structure of the fuel tank. This partially dissolved foam sludge was blocking the fuel pump filter causing erratic fuel delivery and, with only three laps to go, the fuel pump finally tripped out.

                        The fuel line was so blocked that the PDM’s current threshold settings were exceeded, triggering it to shut down the pump to preserve the wiring and prevent a current induced fire. This temporary shut down allowed the foam bits to drop to the bottom of the tank. The PDM then automatically restarted the fuel pump, as it had been programmed to do, and they carried on racing. By this means, with over 15 shutdown and restarts over the last 3 laps, Synchro was able to limp home over the line and retain their second place podium position.
                        Rather than being frustrated about their fuel problems, team manager Alyn James was thrilled that their investment in MoTeC’s power management technology had paid off. “Had it not been for the intelligence built into the PDM module’s programming, and if we’d used old-fashioned fuses and relays, we would have just burnt out the fuel pump and ground to a halt, losing the entire race and valuable points.”

                        He went on to say, “Instead, the MoTeC PDM saved our race and, more importantly, its diagnostics helped us to see that it was the fuel pump current that was the problem. We were able to quickly diagnose the cause of the fault and fix it.

                        “The PDM saved our wiring, our race and points and showed what advances in electrical control can be made with such a great product. It has been a brilliant bit of equipment and has saved us from retiring from races over 5 times”.
                        Last edited by Umai Kakudo; 12-19-2009, 04:45 AM.
                        The Nerd shall inherit the podium for knowledge is power.


                        • #13
                          Another example made by Paul Bird for his AIX Mustang project:


                          I have been spending a lot of time on my wiring over the last few months. I have been working with an electronics maker on adapting a DC switching box to be used as a motorports PDU (power distribution unit). These boxes will replace all of my fuses, relays and breakers. I am putting one in the front of the car for - ignition coils, Motec & dash, headlights, running lights, fan, IMRC, and starter. The other goes in the back for - break lights, fuel pumps (3), diff cooler, transmission cooler, cool suit, and helmet fan.

                          The great thing about these is that they REALLY simplify the wiring of the car. All of my switches are connected directly to the PDU's with tiny (24 gauge) wire and are simply switched to ground. The PDU's also output to a LED for status for each curcuit. In the interest of full disclosure I may be selling/distributing these at some point in the future. Here is a picture of them off the car.:

                          From the description and picture it looks like this may be a compact arrangement of solid state relays without any micro controller or other software/firmware programming like the high end PDU's have.

                          That would make it much less expensive and simple to use for club racer level at the expense of flexibility and monitoring features the commercial PDUs offer.

                          Another option is the ISIS system.


                          It uses fuses but works like a PDU with programmable channels and a multiplexed network buss system with prices starting at $800 for 10 circuits an $1000 for 20 circuits.

                          I'm planning on doing a stripped down OEM harness to get the routing and circuits figured out for what I need and then decide what I want to do for a custom system.
                          The Nerd shall inherit the podium for knowledge is power.


                          • #14

                            if you want a turn key solution
                            Racepak power control module
                            GT3 Nissan 240SX sigpic