>>>>"JMini" <j.minih...@minihane.tzo.com> wrote in message >>>>news:VYvbk.450$4a3.312@trnddc04... >>>>> I'm building a PWM regulator for an incandescent bulb. Some of thos was
>>>>Cut lots of bad ideas!
>>> Cut another bad idea.
>>> He needs to regulate the power in a resistive load. The required PWM duty >>> cycle is inversely proportional to the square of the supply voltage. You >>> don't square anything with an RC network.
>>> You could get a square law by making the PWM on time inversely >>> proportional >>> to the supply voltage *and* the PWM period proportional to the supply >>> voltage but there are simpler solutions. >>> -- >> He needs to regulate luminous intensity, controlling power may be one >> way. A better way may be controlling current thru the lamp. VFF is a >> simple current control. We need a plot of lamp current and power vises >> luminous intensity. I'm betting on current being the more linear and best >> to control intensity. Where is Don Klipstein when you need him? Sorry if >> this is a rehash.
>> Cheers, >> Harry > 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?
>>> You could get a square law by making the PWM on time inversely >>> proportional to the supply voltage *and* the PWM period proportional to >>> the supply voltage but there are simpler solutions.
>>I'm all ears. I'm really looking for all input here. >>If someone has simple solution to keep the RMS voltage constant into a >>resistive load using PWM while the input voltage drops, I'd love to hear >>them. I showed you mine, now you show me yours. TIA, guys.
>For less that $1 you can get a PIC in SO8 (and smaller) which has built in >5v regulator, built in voltage reference, built in oscillator, 10 bit ADC, >10 bit PWM generator, and enough processing power to measure the supply >voltage, calculate and apply the required PWM duty at several hundred Hz >(and still have 4 pins left over).
Which SO-8 one(s) have an internal voltage reference (ie. not Vdd) for the ADC?
>If you can't do software the lamp filament *is* a thermistor providing >direct feedback of what you are actually trying to control (the filament >temperature). You can connect the lamp as one leg of a wheatstone bridge >and make a bistable circuit controlling the MOSFET from a comparator which >flips off when the filament resistance exceeds a value set by the rest of >the bridge resistors. A second comparator can make a timer to flip the >bistable on again after a fixed dead time or an oscillator which flips the >bistable on at constant frequency.
>Both are one chip (+ maybe something to drive the MOSFET hard) solutions.
Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" sp...@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
>>>>>"JMini" <j.minih...@minihane.tzo.com> wrote in message >>>>>news:VYvbk.450$4a3.312@trnddc04... >>>>>> I'm building a PWM regulator for an incandescent bulb. Some of thos >>>>>> was
>>>>>Cut lots of bad ideas!
>>>> Cut another bad idea.
>>>> He needs to regulate the power in a resistive load. The required PWM >>>> duty >>>> cycle is inversely proportional to the square of the supply voltage. >>>> You >>>> don't square anything with an RC network.
>>>> You could get a square law by making the PWM on time inversely >>>> proportional >>>> to the supply voltage *and* the PWM period proportional to the supply >>>> voltage but there are simpler solutions. >>>> -- >>> He needs to regulate luminous intensity, controlling power may be one >>> way. A better way may be controlling current thru the lamp. VFF is a >>> simple current control. We need a plot of lamp current and power vises >>> luminous intensity. I'm betting on current being the more linear and >>> best >>> to control intensity. Where is Don Klipstein when you need him? Sorry if >>> this is a rehash.
>>> Cheers, >>> Harry >> 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
>>>> You could get a square law by making the PWM on time inversely >>>> proportional to the supply voltage *and* the PWM period proportional to >>>> the supply voltage but there are simpler solutions.
>>>I'm all ears. I'm really looking for all input here. >>>If someone has simple solution to keep the RMS voltage constant into a >>>resistive load using PWM while the input voltage drops, I'd love to hear >>>them. I showed you mine, now you show me yours. TIA, guys.
>> For less that $1 you can get a PIC in SO8 (and smaller) which has built in >> 5v regulator, built in voltage reference, built in oscillator, 10 bit ADC, >> 10 bit PWM generator, and enough processing power to measure the supply >> voltage, calculate and apply the required PWM duty at several hundred Hz >> (and still have 4 pins left over).
>> If you can't do software the lamp filament *is* a thermistor providing >> direct feedback of what you are actually trying to control (the filament >> temperature). You can connect the lamp as one leg of a wheatstone bridge >> and make a bistable circuit controlling the MOSFET from a comparator which >> flips off when the filament resistance exceeds a value set by the rest of >> the bridge resistors. A second comparator can make a timer to flip the >> bistable on again after a fixed dead time or an oscillator which flips the >> bistable on at constant frequency.
>> Both are one chip (+ maybe something to drive the MOSFET hard) solutions.
>I know the uC path is the lowest component count path. I just have ZERO >knowledge with regard to uCs. This would be a simple thing if I had the first >idea what I was doing with those things. I've been pointed in the direction >of the ATTiny85 AVR as well as the AVRFreaks forum, but I need a lot more >learning. I don't even know what to buy to connect my PC to the board to >program the chip.
>Also, the circuit really needs to control RMS current or RMS voltage. The >bulb mary vary by application, so it's resistance would be different from >bulb to bulb.
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 -- "it's the network..." "The Journey is the reward" sp...@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Spehro Pefhany <speffS...@interlogDOTyou.knowwhat> wrote: >>For less that $1 you can get a PIC in SO8 (and smaller) which has built in >>5v regulator, built in voltage reference, built in oscillator, 10 bit ADC, >>10 bit PWM generator, and enough processing power to measure the supply >>voltage, calculate and apply the required PWM duty at several hundred Hz >>(and still have 4 pins left over).
>Which SO-8 one(s) have an internal voltage reference (ie. not Vdd) for >the ADC?
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. --
<hmm... repeated since my laptop clock seems to have been off by a couple of days>
>>Also, the circuit really needs to control RMS current or RMS voltage. The >>bulb mary vary by application, so it's resistance would be different from >>bulb to bulb.
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 -- "it's the network..." "The Journey is the reward" sp...@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
>>>For less that $1 you can get a PIC in SO8 (and smaller) which has built in >>>5v regulator, built in voltage reference, built in oscillator, 10 bit ADC, >>>10 bit PWM generator, and enough processing power to measure the supply >>>voltage, calculate and apply the required PWM duty at several hundred Hz >>>(and still have 4 pins left over).
>>Which SO-8 one(s) have an internal voltage reference (ie. not Vdd) for >>the ADC?
>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.
Too bad, I was hoping I might have missed something. A built-in 2.50V reference would be nice. Best regards, Spehro Pefhany -- "it's the network..." "The Journey is the reward" sp...@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
>>>>>I'm building a PWM regulator for an incandescent bulb. Some of thos was
>>>>Cut lots of bad ideas!
>>>Cut another bad idea.
>>>He needs to regulate the power in a resistive load. The required PWM duty >>>cycle is inversely proportional to the square of the supply voltage. You >>>don't square anything with an RC network.
>>>You could get a square law by making the PWM on time inversely >>>proportional >>>to the supply voltage *and* the PWM period proportional to the supply >>>voltage but there are simpler solutions. >>>--
>>He needs to regulate luminous intensity, controlling power may be one way. >>A better way may be controlling current thru the lamp. VFF is a simple >>current control. We need a plot of lamp current and power vises luminous >>intensity. I'm betting on current being the more linear and best to >>control intensity. Where is Don Klipstein when you need him? Sorry if this >>is a rehash.
>>Cheers, >>Harry
> Luminous intensity is fine if the flashlight will only be operating in a > constant ambient luminosity level, however it almost definitely won't.
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..
>> The output of the rms-dc converter has a very slow response time - >> measured in the 100s of milliseconds. In order to get the NCP102 to >> work with this inside the feedback loop, you're going to have to slow >> it's regulator down considerably. There's a model available if you >> want to see what a pspice-type simulator shows.
>> Although the converter has differential inputs, which will simplify >> interface to the low-side driver actually being employed, it's linear >> output is in the 0-400mV range. How are you matching this to the >> regulator's 800mV internal reference?
>> Fast-rising and falling current transients will radiate, even in an >> 800hZ pwm cicuit. The resonant frequency is determined by your >> battery, lamp and switch wiring loops reacting with the fet's output >> capacitance. Check it out with a scope. At this low frequency, you can >> probably be generous with snubbers.
>> RL
>In the design I had working with a 40kHz PWM controller, I level shifted the >RMS output using a resistor divider by +750mV because the Feedback voltage of >the TL5001 is 1V. I can do the same with this design. I can just level shift >the output by 600 mV. That puts the RMS converter output (in regulation) at >200mV. Right in the middle of that linear range. >Good catch on the linear range. I'll make a note.
>Please give a little more detail regarding the use of snubbers. Snubbers >around the FET? In a previous design of a simple PWM soft-start, scope trace >of the output under a 12 Amp load was pretty clean. It runs at 175 Hz.
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.
