Low side switch mode regulator (automotive rheostat replacement) - Page 1
Forum section: electronics / projects

author: BradC
author_rank: Super Contributor
timestamp: May 26, 2015, 11:12:50 am
content: G'day all,I'm having a complete brain fade, so I thought I'd ask here and hopefully get pointed in the right direction.The dashboard/instrument lighting in my car is comprised of a stack of little 12v bulbs in parallel. These are "anchored" to the 12V rail and dimmed by a transistor to GND wired to act as a variable resistor. The older cars used a rheostat, this is just a more modern version.Now, the problem is in the process of adding some new gauge hardware, I've increased the load on the dimmer by about 30%, which has exceeded it's dissipation to the point it now routinely shuts down or does random stuff to the brightness while I'm driving.I plan to replace this 26 year old heat generator with a modern switching equivalent, but I just can't seem to get my head around the simplest configuration to make it happen. My gut tells me I should be able to lash up a 555 to an N-channel MOSFET and some passives and get a satisfactory result but after a hard couple of weeks I can't seem to get my brain engaged to come up with something that might do the job and my google-fu is failing me terribly.Can someone point me in the right direction? Pretty please.

author: DanielS
author_rank: Frequent Contributor
timestamp: May 26, 2015, 11:46:15 am
content: All you want here is basically control the duty cycle of your FET. You can do that by varying the discharge resistor.

author: BradC
author_rank: Super Contributor
timestamp: May 26, 2015, 11:53:16 am
content: Quote from: DanielS on May 26, 2015, 11:46:15 amBasically, but I don't want to squirt a 12V square wave through the wiring harness. I had a think about it and I figure if I put a low ESR capacitor across the load (lamps) and an inductor between the lamps and the FET then I should be able to turn the variable duty cycle PWM into something a bit smoother.

author: Seekonk
author_rank: Super Contributor
timestamp: May 26, 2015, 05:40:25 pm
content: Your fears may be unjustified. There are many instances where people have tried putting a cap on it and they have either destroyed the FET or capacitor. The capacitor supplies too much current to the FET or the cap can't take the current surges. If you must, consider making it into a standard buck converter, A diode to 12V, inductor to the lamps and capacitor.Noise generally just concerns the transitions. Use a RC combination on the gate drive slowing the transitions a little. Causes a little extra FET heating but that should be minimal if PWM rate is low like 200hz.

author: DanielS
author_rank: Frequent Contributor
timestamp: May 28, 2015, 09:17:05 pm
content: The circuit for a low-side PWM would look something like this.R1 would be your lamps, C1 an adequately sized smoothing cap, L1 a reasonably sized inductor to limit/smooth current change, D1 a high-side free-wheeling Schottky diode with adequate current and voltage rating, same goes for the NMOS. V2 is whatever you end up using to generate your variable duty cycle drive waveform and the unlabeled resistor is just an arbitrary value I used to get LTSpice unstuck when I tried simulating the circuit for sanity-checking.

author: BradC
author_rank: Super Contributor
timestamp: May 29, 2015, 07:12:58 am
content: Much appreciated Daniel. That's roughly what I came up with after some more thinking time. I'll lash up some prototypes and see how I get on.One of these days I must learn how to use a simulator.