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Another headlight modulator

Rik Steenwinkel looked at my headlight modulator and offered this design and some ideas. His design looks good to me, and I'll add my comments at the end of this article (Rik, if you want to reply to my comments, just let me know)


Hi Jim,

I found your design for a headlight modulator on the 'web (via the BMWMC pages) and have the following comments, if I may.

First: if all the ground reference points in your schematic are connected together, but not to the actual frame ground, the unit can also be inserted in the positive lead of only one of the headlight filaments, giving you a modulated low beam but a steady high beam without additional wiring and switches. Hooking up the modulator across the existing low-beam switch will then make S1 superfluous (modulated low beam with the headlight switch off, continuous with the headlight switch on); to switch off the unit a switch can be inserted between D1/C1 and the rest of the circuit, or in series with R1. This switch can be small, carrying only a couple of tens of mA, so wiring and mounting is less restricted.

Second, and unfortunately I haven't been able to test this yet, replacing Q1/Q2 by a power MOSFET should reduce power losses in the unit (I should have a couple of them to build a test circuit, but so far they're nowhere to be found). In your circuit Q2 still has appx. 1.2V Vce when on; with a 60/55W headlight bulb this results in 5.4W dissipated (50% duty cycle: 2.7W). Not too bad, but still requires a little cooling. A suitable MOSFET will have an on-state resistance of 0.1 Ohm or less, resulting in 2W dissipated (50% duty cycle again: 1W). Also, the 'full' voltage to the bulb will be closer to the battery voltage, which might be beneficial for a halogen bulb (yes, I've read Tony Turner's comment. Whatever little harm a voltage drop might do, less drop will harm less, won't it?) Attached is a uuencoded TIFF, with a power MOSFET replacing the conventional transistor combo. The BUZ71 used is a Siemens SIPMOS, Vds=50V, Ids=12A, Rds(on)=0.1 Ohm, TO220 package. Motorola and International Rectifier should have suitable equivalents. With a 90/130W or dual 60/55W headlight, a BUZ10 (Ids=19A) or BUZ11A (Ids=25A, Rds(on)=0.06 Ohm) would be better suited. There are also power MOSFETs in a TO238AA package. These have 6.3mm Faston connectors (4.8mm Faston for the gate) and an isolated metal flange for mounting (appx. same size as TO3, but with the connectors on top). One such FET is the BUZ27 (Vds=100V, Ids=26A, Rds(on)=0.06 Ohm). Easy to mount, though probably expensive ...

Schematic here

Third, to reduce parts count (not really necessary, but once you're redesigning ...), you could

- use a 556 (dual 555)

- connect the Vref pins together, and use a single capacitor there.

- remove R3/R5, and connect the end of R2/R4 that goes to pin 7 to pin 3 instead.

Feel free to add this to your page if you want. Credit? Nah, it's still your design, essentially.


'86 R80ST
'91 R100GS/PD

--
// Rik Steenwinkel
// OS/2 Warp from home
// rsteenw (at) ibm.net or rik (at) apd.dec.com


Jim's Comments:

> high beam without additional wiring and switches. Hooking up
> the modulator across the existing low-beam switch will then
> make S1 superfluous (modulated low beam with the headlight
> switch off, continuous with the headlight switch on); to
> switch off the unit a switch can be inserted between D1/C1
> and the rest of the circuit, or in series with R1.  This
> switch can be small, carrying only a couple of tens of mA,
> so wiring and mounting is less restricted.

This should work, but I chose not to do it for two
reasons. No matter what mode the modulator failed in, I
wanted the "OFF" switch to bypass it and ensure a working
headlight. I also wanted to have only two leads coming from
the modulator, as the original intent of my design was to
prove to some naysayers that a two-lead headlight modulator
was practical.

> In your circuit Q2 still has appx. 1.2V Vce when on;
> with a 60/55W headlight bulb this 

I'd experimented with hooking the collector of Q1 (in my
design) to the cathode of D1, which would cut the voltage
drop across Q2 quite a bit, but it took too much base drive
current and required a larger C1, already the physically
largest part in the circuit. A MOSFET would solve that
problem.

> - use a 556 (dual 555)
> 
> - connect the Vref pins together, and use a single capacitor there.
> 
> - remove R3/R5, and connect the end of R2/R4 that goes to pin 7 to pin 3 instead.

The first two ideas are good.

I've never seen (well, maybe I've forgotten) an 555/556 used
the way the 3rd point describes. It looks good at first
glance, I'll have to try this.

Good looking design, Rik, I'll have to try it out.

Thanks for sending it!


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