excess power draw on PowerBoost
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excess power draw on PowerBoost

by harleyfrazee on Sat Aug 03, 2019 2:55 pm

I'm building two products which are all IR beam break timers. I use a 18650, Lithium Poly 3.7V battery and a Powerboost 500C. One product has three IR beams and the other has 7 IR beams. The 3 IR beam system works dependably, but the 7 IR beam system is flaky,

The transmitter uses (Vishay TSAL6200) which normally uses 100 mA each with a max draw of 200 mA.

This design uses PWM cycling at a 50% duty cycle and it pulses that signal for 50 uSec then off for 260 uSec; so it's about a 30% duty cycle.

Therefore, the peak draw could be (200 mA / 2 / 3) 33 mA each eye or 233 mA for the 7 beams.

I'm wondering if my math is correct and perhaps my issue is that the Powerburst 500C has a peak limit on the boost part so that when the IR LED is on, it's actually using 1500 mA.

I'm now testing with 4 LED's (900 mA peak load or 500 mA for normal load) and that appears to work dependably.

I should receive my Powerboost 1000C tonight so I'll replace the 500C with the 1000C to see if all 7 can function dependably. Please let me know if there are other considerations I need to take.

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Re: excess power draw on PowerBoost

by adafruit_support_mike on Sat Aug 03, 2019 4:07 pm

You can't compare average PWM power directly to the continuous output from a power supply. You have to compare the instantaneous currents, or scale the power supply's output down to the same PWM duty cycle.

If you have 7 LEDs that use 100mA turned on at the same time, the instantaneous current load is 700mA, which is more than the PowerBoost 500's rated 500mA output. If you want to take the average of 700mA at a 33% duty cycle (233mA), you also have to scale the PowerBoost's output down to a 33% duty cycle: 500mA x 0.33 = 165mA.

If you stagger the LEDs so two are turned on for t=(1-33), another two are turned on for t=(34-66), and the last three are turned on for t=(67-100), you'll still get the same average current consumption of 7 LEDs on a 33% duty cycle, but the instantaneous currents will be 200mA, 200mA, and 300mA. All three are lower than the PowerBoost 500's rated output, so the PowerBoost will be able to handle that load.

It's basically the same as lifting weights: if your maximum lift is 50kg, and you can lift 10kg 7 times, it doesn't mean you can lift 7 10kg weights at the same time.

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Please be positive and constructive with your questions and comments.