# Duty Cycle to Frequency

Discussion in 'General Electronics Chat' started by bwilliams60, Nov 21, 2014.

1. ### bwilliams60 Thread Starter Active Member

Nov 18, 2012
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I first want to state that this is not an automotive modification question. It is just using it as an example.
On a vehicle using daytime running lights, the module would reduce voltage supplied to the headlights by 50%, either on low beam or high beam depending on manufacturer. In my way of thinking, you could do the same using PWM at 50% duty cycle. Can there be a frequency associated with this and if so, how would one go about calculating this?

2. ### wayneh Expert

Sep 9, 2010
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I'm not sure about the premise, that DRLs are dimmed, but yes you could dim a bulb using PWM. The frequency of the PWM is somewhat arbitrary as long as it is fast enough to not flicker to the human eye. 100Hz and up would do, especially for filament bulbs that "coast" a bit between pulses. LEDs go dark and need a higher frequency to appear steady.

Brightness is proportional to the duty cycle of the PWM, not the frequency.

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3. ### bwilliams60 Thread Starter Active Member

Nov 18, 2012
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Thanks wayneh. I could have thought of something else but it was a question posed to me by a colleague for a project they are working on. There is no automobile involved but a mock setup using automotive bulbs, relays etc is being used. It is part of the learning curve our students are going through and some like to go deeper into it. I had never been asked the question before and thought perhaps someone here might have more experience. I understand PWM, frequency and duty cycle, but never had to calculate the relationships or even knew it existed.

4. ### jpanhalt AAC Fanatic!

Jan 18, 2008
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If you actually want duty cycle to frequency, it can be done by converting DC to voltage and then voltage to frequency. The LM2907's datasheet describes a dwell-time to voltage converter. Note the low-pass filter and direct coupling to the signal.

John

5. ### alfacliff Well-Known Member

Dec 13, 2013
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duty cycle is related to on off time, not frequency. each frequency has a different time, higher means shorter times, lower means longer time. you have to choose a frequency then the duty cycle.

Jul 18, 2013
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On GM's there are two methods out there, some switch the lamps in series (very simple) for DTRL the other method is a dropping resistor.
If any one modded one of these, the method presently used would have to be considered, IOW, it could not really be done universally.
The only down side to the series method, you lose both lights if one fails.
Max.

7. ### alfacliff Well-Known Member

Dec 13, 2013
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as an example of how cars wire their lights differently, my 74 Alfa spider has an individual fuse for each headlight, one for bright and one for dim on each side. 4 fuses means it will be a long time before you dont have any headlights at all.

8. ### bwilliams60 Thread Starter Active Member

Nov 18, 2012
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I'm an automotive technician by trade and understand how DRL's work. More to the story that i found out today. Student has a modue and wants to create a project simulating DRL. His choices from the module are %pwm or frequency. I suggested 50%duty cycle but he wants to use frequency setting. I have never seen anybody offer a frequwncy measurement for DRL. Any ideas or something to experiment with?

Jul 18, 2013
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I am not sure I see the point for DRL's, there appear more useful applications to use either of these in, designing for DRL seems a bit mundane, is this just for the academic purposes?
And how is varying the frequency going to vary the brightness?
Max.

10. ### bwilliams60 Thread Starter Active Member

Nov 18, 2012
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Hey Max. It is for academic purposes and not sure why the push for frequency. I have been pushing him toward PWM and it makes sense to go that way, but he insists on doing things "differently" Go figure.

11. ### MrChips Moderator

Oct 2, 2009
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Maybe your colleague is on the right track but misunderstands frequency vs duty cycle. A 50% duty cycle PWM would have the same effect as frequency if the waveform is symmetrical, i.e. 50% duty cycle also.

12. ### cmartinez AAC Fanatic!

Jan 17, 2007
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MMMhhhh.... I'm not so sure about that "brightness is proportional to the duty cycle of the PWM, not the frequency"
That assertion is right, of course. At least in theory. But the thing is that in real world applications there are a number of factors that would produce non-negligible hysteresis effects in some circuits, such as in controlling the brightness of an incandescent light bulb. While the average current (and illumination) would somehow be non-dependent on frequency, maybe flicker would affect the outcome, or even some particular resonant frequency in the bulb's filament itself would cause it to behave strangely... and if the load were an inductive one, then things would get far more interesting...
What I'm trying to say is that, although the effects of PWM's frequency in most applications would probably be minimal, maybe it would be worth considering the "what if's" of the behavior of your intended circuit while it's being run on certain parts of the frequency spectrum... at least academically speaking...

EDIT (before I hit the sac): Maybe "resonance" would be the keyword to consider here...

13. ### studiot AAC Fanatic!

Nov 9, 2007
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Trying to control light bulb brightness by controlling frequency instead of duty cycle is not a good example since they are resistive in nature and once you get past the flickering stage thermal inertia defeats such control.

However controlling frequency snesitive devices such as electric motors is a feasible example.

These can be controlled by pulse width modulation, pulse height modulation, frequency control , burst fire control and more.