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Power


Alan King on using caps in low power applications:

...[I found some] black filter caps with gold lettering said "gold cap" on them. Sure enough, those numbers really said 2.3v 10F, big F, no pesky little u in there! Charged it up to 1.93v and put a superbright red led and 1K resistor on it, and it's still glowing well four hours later. I can't wait to see how long I can get them to run a PIC doing some low power data sampling. Definitely an alternative to batteries for some low power apps, about 1/2" shorter and 1/8" less diameter than an AA cell. A bit pricey, $7.50ea from DigiKey in singles, but not as bad as I was expecting, and drops to around $3 each at 1K, probably less from some other distributors. And the current version is rated 2.5v, so you only need two to start out with 10F at 5v. I'm only 30, but I remember how they used to say it'd take a railroad car size to have a 1F cap not so very long ago, much less 10F. Neat to have a small train's worth of cap powering your PIC..

Andy Kunz on multiple power connections to SMT chips

You should keep the distance between the Vdd and Vss pins on the same chip as short as possible. If there are two Vdd pins, you need to put bypass caps on BOTH pins.

There should be one connection from a chip to the power supply. If ther are two leads on the chip, the best method is to make a T with equal left/right sides if possible. Don't kill yourself for it, though. If you need to make a main feeder which splits into two short (unequal but not greatly disparate) lengths, so be it.

Chris Eddy says:

I am going to go out on a limb and say that ground planes are overrated. They are good for three things; overcoming electrostatic field interference, making high density layouts possible, and compensating for an inability to route a decent ground net.

Just a hypothetical: let's say you had four circuit pieces on a single board. Further say that three are analog, and one is high current switching. Further say that none of them can withstand interference from the other.

The PSU is a split supply coming in at one corner of the board. Route each sub circuit in a clean manner, but return ground and power for each section individually to the power entry point. Do this for all four sections, and add generous capacitor decoupling at the sub circuits. Add both mono and electrolytic caps at the power source.

Take this example one step further and split the grounds from each component if necessary, maybe in a tree fashion.

wiring@ Cooling@ Safety@ Relays@ PWM@ Filters@ Gotchas@ PCBs@ "avoiding noises"@

See also:

Don't limit using external pass transistors to just linear regulators, you can also use them in switching regs if you are carefull about speed.

See:

Questions:

Archive:


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