The circuit below is the final one, after all the advice from Mishyoshi and especially Andy aka.
It includes Zener diode protection against over-voltage between gate and source on the Mos-fets and also a back-emf diode to deal with below-zero pulses caused by cable inductance to the lamp.
It seems to work well with the 20Khz PWM dimmer I needed to use, at least on the workbench. In the future I would like to add a circuit to cut the supply if it falls below 10 volts to safe guard the Mosfets. Hopefully this will help anyone faced with the same problem as I couldn't find a practical circuit when I looked.
Hi everybody! I am a newbie and although I have read a lot of stuff on this forum this is my first post.
I am an electrician working on a feature film and have been given the task of making several American trucks headlights dimmable. Nowadays every light the camera sees needs to be dimmable by remote control from the lighting desk. The industry standard wireless DMX dimmers (in the UK at least) are Lumenradio LV4's. These run on 12-24volts dc and give a Pulse width modulated output. They have four channels, at 4 amps each.
Unfortunately as well as being under-rated current-wise for the lamps,they are also designed for LEDs so they are common positive. The headlamps are hard-wired as common negative (the lamp fittings have one terminal going to the chassis of the truck which is connected to the negative of the battery).
This means I had to make a high-side p-channel mosfet switcher to drive the lamps. The lamps are 190 watt halogens (there are two filaments for high and low beam, one at 100w, the other 90w).
I did a lot of internet searching trying to find a circuit that would do the job but found it quite exasperating. There was a lot of 'be careful you don't make this value too high or this bad thing might happen, but if it's too low this bad thing might happen' but precious little in the way of a circuit I could just copy. I will admit I am no good at designing circuits to component level. For most electronics needs there are standard circuits which can easily be adapted to specifics, but on this subject It seems like there is an amount of witchcraft involved.
Anyway, I built the above circuit and it works but not with the preferred LV4 dimmers, but it does work with these cheap chinese versions;
I presume this is because they work at only 400HZ, whereas the LV4's work at 20KHZ. We settled for this as a solution but now they are going to be using high speed cameras, around 2000 frames per second so they are worried about flicker. I have seen high speed film of incandescent lamps and they do pulsate.
So now I have to make a circuit which will work at 20 KHZ. I do not understand what it is about the circuit which makes it frequency dependent. I wonder if it is because I used strip-board or if it is a more fundamental design/component choice problem.
I would be grateful for any help you could give me.
P.S. I fitted these filters;
to the truck battery side to deal with transients from the 'dirty' automotive side as I read that MOSFETS are prone to voltage spikes.
P.S. I used an opto-isolator because I didn't know what practically the power set up would be like and it seemed prudent as it keeps the dimmer safe from dodgy auto-motive power.
When I tried to power the dimmer and the power mosfet circuit from the same supply just to see if it was an option it didn't work, the lamp was on full constantly. Why? I am sure it obvious to most of you but I don't see what the problem is.
This is the output at the drain of the p-channel mos fet . Dimming level is about 40%. The PWM frequency is 20Khz.