IR Beacon circuit optimizations (8-7.2v unreg, 5v reg)

[Kevin Watson]

Originally Posted by Dale(294engr]

Are you saying the logic level FET is not extra ESD sensitive...

For a few reasons this is a non-issue. I've seen more than a few semiconductor devices fried due to mishandling, but never have I seen a fried power MOSFET (well, due to ESD anyway ). I originally specified a different N-channel device for the low-side switch, but the folks at IFI suggested switching to this device because they're bulletproof (they have a lot of experience with this device).

Your scheme would make more sense if the beacon were in a sealed box that prevented folks from handling the circuitry. Any electrostatic discharge that happens will be to the unprotected beacon circuitry, on the gate side of the resistor, not on the completely insulated PWM cable side of the resistor. Moreover, if the MOSFET were susceptible to ESD damage, wouldn't you imagine it would be more likely damaged before it was wired into the circuit?

The mating/de-mating of the PWM cable doesn't induce any triboelectric charging because the impedances are far too low.

Originally Posted by Dale(294engr]

My suggested 10K ohm in series with the gate does NOT slow down the FET (check it out on a scope) Series resistance isolates the gate a bit & if used aids zener effectiveness. The 1M parallel keeps the FET gate drained, i.e. off. A very much higher series resistor than 10k would indeed slow turn on, but I do not suggest that.

It's an RC circuit, so it does slow it down. It's a common practice to maximize gate drive in MOSFET power switches, so it makes no sense to slow the switching down and increase complexity at the same time. The IFI controllers have ~2.2K series resistors to prevent damaging the PIC in the event the outputs get shorted. We spent a lot of time evaluating MOSFETs to ensure that this wouldn't be a problem.

The megaohm resistor doesn't help because the IFI controller has a built-in pull-up resistor that's a few orders of magnitude lower in resistance.

Originally Posted by Dale(294engr]

Our Beacon doesn't get turned off much so depleats 800 mAHr batteries fast.

How much time are you getting? Should be several hours. How 'bout using a ~7.2 volt wall-wart for power?

Originally Posted by Dale(294engr]

(is FRC using the LEDs distributed to Teams or the more powerful Sony?)

Where does it say a Sony device was used? As far as I know, all documentation says a Lumex OED-EL-1L2 device is used.

Originally Posted by Dale(294engr]

...I agree that it is of little consequence re: LEDs getting HOT when not pulsed...

...The DUTY cycle thus LED heating goes WAY up if the switching stops...

Again, the resistors are sized for continuous operation of the LEDs.

Originally Posted by Dale(294engr]

We use two series LED Strings, two resistors, and 5V regulated. Two series strings same Ipk = 1/2 current switched by FET.

This does make sense, not because it lowers current through the MOSFET, but because it lowers the power losses in the resistors. This was considered for the beacon, but I wasn't sure how consistant Vdrop across each LED would be (and didn't have any time for testing), so went with the more conservative approach.

I guess my point really is that with all of the infrared beacon & tracking documentation problems, the last thing we should be doing is to make it even harder for teams to experiment with this technology.

-Kevin