Ok full disclaimer, I am just learning about PWM and all its ins and outs.
I own a 4x4 modification shop and have now started to dabble in the PWM since its in pretty much all new vehicles and controls a variety things.
Here is what I am trying to achieve. The electronic locking differential in newer Ford trucks uses PWM to activate the Electromagnet inside the rear differential which engages the locking differential. I am trying to duplicate this signal so that the axle can be installed into a different vehicle that does not contain the more sophisticated computer control of the truck it was originally installed in.
I do not know the frequency that Ford uses in the original PWM signal. How big a deal is this? How could I find this out if needed?
I do know that the signal starts out at 98% duty cycle and after 30 seconds automatically drops to 40%.
This is all being done with an input of approx 13.5 volts DC and the load is an estimated 20 amp's.
What is the best hardware to get this accomplished? Anything else I should be aware of heading into this? is there such a thing as a water tight or resin that this could be contained in?
Much thanks in advance
David
You should use an oscilloscope to determine the PWM frequency. A small handheld unit would be fine. It is important to get the frequency approximately correct, but some leeway is allowed. Any Arduino should be able to generate the signal.
To control a 20 Amp electromagnet at 13-14 V, you could use a high power brushed DC motor driver, like this one.
All automotive electronics require good protection against severe voltage transients -- voltage reversals and spikes of hundreds of volts are common, and instantly destroy Arduinos. Most people use a TVS diode suppressor and other filtration components, plus shielded wiring. Some info here. You may be able to buy a protection module to build into DIY equipment.
https://www.amazon.com/gp/product/B071F1H3PG/ref=ask_ql_qh_dp_hza
Do you believe something like this will have the accuracy i need given I dont know what frequency to expect?
That might be OK. Some of the reviews are pretty bad, though.
You want the scope sample frequency to be at least 10x the maximum expected PWM frequency, which I would guess to be less than 50 kHz (maybe way less).
Do you have another one you could recommend for a hobbyist such as myself. Forgive my ignorance on this but what really is the big advantage to using Pwm in this situation? I have also been kicking around the idea of just using a timed relay and step down unit to drop voltage from 12 to 5 after 30 seconds. I mean this is just an electromagnet after all and its only function is to close an air gap and then hold closed until its released. not like it needs to be precisely controlled such as a stepper motor.
Some of the other guys online are reporting putting 12v to this magnet and just that no step down and nothing else. anyway to verify if this magnet is capable of a sustained 12v without burning up?
That the manufacturer went to the trouble to use PWM suggests that there is a good reason, with avoiding overheating being an extremely likely one.
What are the possible consequences of magnet failure? Anecdotal evidence that one magnet did not burn up when provided with 12V is not the basis for making decisions where people's safety or lives may be at stake.
Many multimeters measure frequency, that is probably all you need.
I should clarify. There is no safety hazard if said magnet fails. It would only disable the rear locking differential in the vehicle. Which would be costly and frustrating but hardly unsafe. And it appears as though there are hundreds of guys running direct 12v to these units with either unreported consequences or non. I put little value on that though.
The same manufacturer of this device (in Ford trucks) also produces a similar device for both jeep and Nissan however only the Ford unit uses pwm. The others are just a straight 12v.
Is a computer able to monitor the current draw of a pwm circuit more precisely then a standard circuit. The reason I ask is that the Ford unit is the only unit that does not have a separate sensor from the magnet coil to tell the computer full engagement is achieved. The factory Ford computer must be monitoring the circuit draw so that when the air gap closes the current draw is less and the computer knows it's engaged. Vs the others have a separate system entirely to tell the computer it's engaged.
I was wondering if this was perhaps the sole purpose they choose pwm.
Is a computer able to monitor the current draw of a pwm circuit more precisely then a standard circuit.
That seems possible and if so, not being able to exactly duplicate the original circuit with an Arduino might defeat the purpose.
If you think there is no safety drawback, then my feeling would be to not worry about PWM. But it is really quite simple to implement as outlined in reply #1. And as mentioned, many cheap multimeters are able to measure the PWM frequency.
Good luck and let us know how it goes.