constant current/power load for automotive use - any design ideas? - Page 1
Forum section: electronics / projects

author: MachineToy
author_rank: Newbie
timestamp: December 05, 2017, 10:18:40 pm
content: Hello ladies and guys, I long time reads the forum and watch Dave's videos and now I have a question I hope I can get some knowledge about.I have a friend who makes custom show motorcycles (not used on street - they used in shows, exhibitions, movies, things like that) and asked me to develop him a light module/array which is programmable and can be use for brake, turn signal, and such.The problem is that his bikes start as BMW, and BMW has the engine computer that monitor the current draw of the accesorries. If the draw is too low, it gives the error on dashboard. It seems easy - just to add resistor in parallel to consume more current, but not so simple if I want to build an array of LED's which can be customized. So maybe it's 100 LED's and all them are brake.. but when use the turn signal, some of the LED change to yellow to be turn signal. So the current load on brake can change lower when activate turn signal. And current load on turn signal can vary if different turn signal option is selected.I wanted to make a programmatical option - so when he use this light system for a new design, he can make a program to design how the brake and signal will work, and also can choose what the current consumption will be for brake and signal. To achieve this, of course I will need four circuits - on incoming run light, brake light and left and right turn light power wires, which have variable resistance and so variable load.I like Dave's video about variable electronic load... in this way, power level can be set with potentiometer input into op-amp. I can easy use DAC in my microcontroller to allow programmatical choice and send into op-amp which will control voltage into gate of FET based on voltage over small value resistor on source of FET. However, I do some search on this forum and see people have some issue with that circuit... ringing and it need tweaking. And in automotive, input voltage can vary depending if vehicle running or not, and in general much more noisy environment. So I think it can be even more difficult to get stability from such circuit in automotive situation.So maybe someone has an idea? Maybe I complicate it too much, and should just make some kind of terminal screw connector and suggest to use through hole-resistor depending on what program is made (because I think engine computer will check only if current draw go below a level, not that it must be constant at exact level). Or maybe better use demux chip with outputs on fets switching some power resistors to give options of power draw?Or I hope is more simple way I did not see? I thank everyone for informations and advice!

author: IDEngineer
author_rank: Super Contributor
timestamp: December 05, 2017, 11:21:00 pm
content: I suspect it reports if the total current consumed gets below some minimum threshold. You don't necessarily have to maintain a constant current, just some minimum. A couple of ideas:* Your variable load idea could work, built in a variety of ways. Are there heated handgrips? Can you integrate that load into yours so the power isn't literally wasted?* Decorative LED's that are illuminated when nothing else is active. Say, a frame of LED's around your display array that is on when quiescent but turns off in favor of an active display. You could have a lot of fun with this... sequenced lights, company logo, etc. Could do different colors to highlight the effect or stay with (I presume?) red. The minimum threshold current could be quite low, requiring only a few active LED's at any one time.

author: Rerouter
author_rank: Super Contributor
timestamp: December 07, 2017, 07:37:25 pm
content: I would say go the keep it simple method. Have a load resistor per bulb and use the voltage to trigger say a relay or input signal to your controller. And work from there.

author: IDEngineer
author_rank: Super Contributor
timestamp: December 07, 2017, 07:45:38 pm
content: That would get him started, but hopefully it doesn't require a separate dummy load per device! A single dummy load would be dramatically simpler and cheaper, switched on only when the rest of the load was insufficient to keep the upstream circuitry happy.