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Author Topic: PWM EMI capacitor - How to work out the value?  (Read 1420 times)
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Please somebody help me understand how to work out the value of capacitor I need to add to my PWM CCR circuit. I read that the Arduino's PWM frequency is at about 500Hz, and that I will have 255 dimming steps. But the recommendation for adding a capacitor in my CCR datasheet leaves me very confused: smiley-eek

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Reducing EMI
Designers creating circuits switching medium to high currents need to be concerned about Electromagnetic Interference (EMI). The LEDs and the CCR switch extremely fast, less than 100 nanoseconds. To help eliminate EMI, a capacitor can be added to the circuit across R2. (Figure 11) This will cause the slope on the rising and falling edge on the current through the circuit to be extended. The slope of the CCR on/off current can be controlled by the values of R1 and C1.

The selected delay / slope will impact the frequency that is selected to operate the dimming circuit. The longer the delay, the lower the frequency will be. The delay time should not be less than a 10:1 ratio of the minimum on time. The frequency is also impacted by the resolution and dimming steps that are required. With a delay of 1.5 microseconds on the rise and the fall edges, the minimum on time would be 30 microseconds. If the design called for a resolution of 100 dimming steps, then a total duty cycle time (Ts) of 3 milliseconds or a frequency of 333 Hz will be required
http://www.onsemi.com/pub_link/Collateral/NSI50350AS-D.PDF
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Please somebody help me understand how to work out the value of capacitor I need to add to my PWM CCR circuit. I read that the Arduino's PWM frequency is at about 500Hz, and that I will have 255 dimming steps. But the recommendation for adding a capacitor in my CCR datasheet leaves me very confused: smiley-eek

Do you really care about a little EMI?

The PWM circuit they show achieves a pretty good efficiency by switching fast (that is, the BJT is in the linear region for a very short time). Adding a C or RC circuit to "reduce EMI" will slow down the switching speed of the BJT and make it run much hotter.
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Do you really care about a little EMI?

I'm not really sure...
The two critical parts of my circuit will be the Capacitive sensor (the electrode trace will be within ~10mm of the RGBW LED), and the Serial comms. I figured that over 1A of switched current might be an issue? but maybe not?
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If EMI was an issue, what equation would I use to work out C?
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just try a few different cap values and see what happens....

15pf to 560pf

or until you start noticing adverse effects on the LED. The datasheet for the ATmega328p recommends 15pf caps on either side of the 16MHz crystal to give you some idea.

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If your worried about emi, place it is a grounded aluminum enclosure,  keeps wire leads short and parallel (or twisted), if using a pcb use a ground plane,
usually you design to minimize the emi interference,  not that you kill all emi radiated just to treat it properly and use shielding, etc to keep it away from other devices
btw 500hz isn't that much to produce alot of emi, unless you have decently long leads and switchin alot of current
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