Feedback for low frequency PWM regulator

You have three Flips which equals one Flop (see De Morgan)

Harry

The required current changes just as much as the filament resistance and controlling current would be a terrible thing to do anyway.

Yes the target filament resistance would have to be adjusted for each type of lamp and controlling filament resistance is a bit more sensitive to lamp to lamp variation than controlling voltage.

Luminous intensity is fine if the flashlight will only be operating in a constant ambient luminosity level, however it almost definitely won't.

I see a chart on Wikipedia (SP?) of efficacy (Lm/W) for 120W lamps from

5W to 300W and the efficacy changes from 5 to 20.7. Power dissipated in the lamp is not good intensity control, maybe current control is just as good or better? It sure is simpler! Cheers, Harry

Regulating RMS current would eliminate the need for a soft-start. The bulb pulls a lot of amps during startup. For example, a 100W bulb on 12V pulls a

50+ amp spike then settles in around 8.3A. If the circuit only allowed an RMS current of 8.3A max, the bulb would soft-start itself. Good thoughts. But how would controlling RMS current be easier than controlling RMS voltage?

After further review, VFF (Voltage Feed Forward) would keep the voltage across the lamp constant with changes of input voltage. The current would vary with filament resistance. Now, is controlled voltage adequate for your application? If yes, a simple VFF, one 8 pin IC, will do the trick. We still need the curves of lamp Voltage, Current and Power vs. Luminous intensity output to determine the best variable to control. Cheers, Harry

12F615 has a 0.6 and 1.2v bandgap reference but looking closer you can measure it (to calibrate) but not use it as an ADC reference. The HV615 shunt regulator is derived from the bandgap so maybe Vdd is good enough anyway.

If you measure the voltage v(t), then the required PWM output % is proportional to 1/v^2(t).

You'd need to calculate a voltage divider to give you the reference voltage at maximum input, and specify the output % at that input.

Eg. 10V maximum input with 25% output at 10V in (say it's a 25 ohm 1W bulb). Then at 6V in you'd have about 69.5% pwm %, for that same 1W output. It would hit the end stop at 50% of the maximum input voltage in this example, below which you could extinguish the light, flash it or whatever, or simply allow it to drop naturally below that level.

Best regards, Spehro Pefhany

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Too bad, I was hoping I might have missed something. A built-in 2.50V reference would be nice. Best regards, Spehro Pefhany

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I don't why you're making it so hard on your self.. Simply place a photo detector diode in the path of the light and use that as a feed back to the PWM circuit.. the circuit can be operating on a low voltage fixed regulator .. etc..

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If you've checked it out with a scope and seen no funny stuff at turn-on and turn-off of the switch, then there's no issue. Dedicated gate rivers tend to be pretty quick, though ~ it's their job.

RL

There are at least 20 different bulbs that could be used. That's why I'm hesitant to try controlling luminous intensity or something other than a basic electrical property. Voltage or current. But would it be possible to use VFF in a low frequency design and limit RMS current to the bulb?

Because the output will change with ambient light conditions. I don't want that.

By regulating the rms voltage across the lamp, you're not regulating luminous intensity or power any more effectively than by regulating the average voltage. For a resistor and pwm, these two voltage quantities are the same.

Neither has a predictable relationship to power consumption in a load with a positive temperature coefficient of resistance. PTC's with low mass are strongly self-regulated for power. In the E^2 / R relationship, as E increases, then so does R.

The use of the rms converter is simply complicating this circuit unnecessarily. The use of PWM to reduce power loss in a control element IS effective...you should be satisfied with this and concentrate on simplifying the interface to a low-side switch and to additionally reducing inrush current to values that will improve lamp and switch operating life.

RL

VFF will control average voltage across the lamp. Since the waveforms are almost constant, the RMS voltage across the lamp is just a constant factor. This is the first circuit you should try and then see what else is needed. Tell me your requirements, input voltage range, switching frequency, dimming control voltage, lamp current, lamp control voltage range and any other info necessary. If you have parts already selected, I will try to use them. A simple pdf design on "alt.binaries.schematic.electronic" will be posted for your viewing pleasure. Cheers, Harry

Brings to mind, why not just regulate the bulb current?

...Jim Thompson

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"Jim Thompson" wrote in message news: snipped-for-privacy@4ax.com...

That was my first thought. Slightly harder than VFF because of current sensing. IMHO the OP has not tried the simple things to determine what is adequate for his application. Sometimes you have to put the pencil down and grab the soldering iron. If the human eye is in the loop, it is very insensitive to light intensity changes and control methods must be tried. So the order of progression from simple to difficult control, voltage, current and then power. In most lighting applications I have seen, current is the best parameter to control. Cheers, Harry

Schematic posted on "alt.binaries.schematics.electronics" as "VFF 10A Lamp Driver". Just a starting point. Need lots of added features the OP must specify. Cheers, Harry

Over the supply range of 9 to 14v the lamp light output will vary by a factor of about 3 and the lamp life vary by a factor of about 300.

So it isn't much of a starting point.