Raspberry Power DC Power Supply

Thanks Rew and shabaz. Good feedback.

One of the earlier suggestions about using a 5v PSU directly indicated that many people have them lying around, so I thought I'd play on that strength, which is where the readily accessible standard connector may be an advantage.

If over-voltage (which is very likely to present an issue) needs to be mitigated, how about placing a resettable fuse in series with the +5 line and then installing a zener diode (maybe 2 or 3 in parallel) that breaks down on a reverse voltage of over 5.3v for example. Would this trip the resettable fuse?

Resettable Fuse:

http://uk.farnell.com/bourns/rm008-240-2/fuse-resettable-ptc-240vac-80ma/dp/2366925

Example Diodes:

http://uk.farnell.com/nxp/plva653a-215/zener-diode-0-25w-5-3v-sot-23/dp/2439627?ost=zener&categoryId=700000004649

http://uk.farnell.com/rohm/kdztr5-1b/diode-zener-5-1v-1w-sod-123/dp/2143976

Hi Paul,

You could use such an approach based on zeners as you say.

I think just a text label could be "good enough" for RPI protection maybe (not everything needs an electronic solution), but a zener/fuse option may be cheap enough too (the zener

needs to handle a high amount of current for a length of time, otherwise it could be damaged too - there are other tricks that use zener + MOSFET as an example).

There are even dedicated ICs for this situation too - that's possibly a bit 'over the top' for protecting an RPI though.

Thing is: what are you prepared to handle... I have one of those 5.5/2.1 connectors lying on my desk here. It is connected to a 12V/30A ATX powersupply. That's some serious stuff to try to absorb that with a resettable fuse or a zener. And wasn't the idea to be able to provide more than say 100mA to the USB ports? So if you want "normal operation" at 4*500mA + RPI, your worst case becomes that the powersupply ends up delivering only 5.5V at 2.5A  (with the RPI+devices only using < 0.2A) (it's a lowsy 12V 2A or 2.5A powersupply), and you'll have to dissipate that heat in your zeners, at a current that the fuse should be able to tolerate....

The kind of circuit I was thinking of employing is as follows:

Would the Zener(s) have to dissipate any heat during normal operation in this configuration? There is a risk of zener damage when the reverse voltage is exceeded, though i'm wondering if the offset in cost of repairing/replacing this small board vs an RPi would make it an advantageous addition to the board.

The resettable fuse could be rated at 2-2.5A. Following my (somewhat limited) experience with Zener barriers I may parallel two or three diodes to increase the infallibility factor. Might this also reduce the current each zener has to withstand?

Another device I was considering was this: http://uk.farnell.com/on-semiconductor/ncp361snt1g/voltage-detector-20ua-20v-tsop/dp/2382339

Unfortunately it includes built-in overcurrent protection that limits the load to 750mA. This may not be sufficient for some use cases.

Hi Paul,

I'm not sure about that device, and I think finding high current zeners/making them parallel while it may work is not ideal but there is a useful voltage protection PDF here that explores a few options.

It is -3 degrees (why no super/sub on text editor?) Fahrenheit outside here in Chicago.  Fretting about 5 to 15 W of static dissipation (presuming this heat is properly spread) at the corner of my bench seems neurasthenic today.  People make the gadget you describe all of the time.  The problems a prior poster mentioned about the ubiquity of the connector do obtain.  Here are some solutions:

The plug could be polarized the wrong way:  If you want to have non-functionality in this context, use an input diode.  If loss in this diode upsets you, use a shottky.  If you want to have functionality in this context, use a bridge rectifier (absolute value circuit) on the input instead.  Throw in a cap and a protection diode to guard the regulator during discharge and you are even proof of AC adapters.

The supply features too great a potential:  Add an input fuse, really you should anyway.  Shunt the supply with a backwards power (clamp) Zener.  This can be fashioned out of a regular-old Zener and a bipolar power transistor.  Use NPN, in Si electrons are lighter than holes.  With this scheme you can get reversal protection by shunting the power Zener with a backwards (to normal operation) power shottky.diode.  No static dissipation, but tape a couple spare fuses under the lid of the chassis.  In a non-conductive box, of course.

The plug could be polarized the wrong way:

We had some field equipment that sought to eliminate 'oops wrong way round' on the battery.

They cleverly used a relay, that had a diode in series with the coil.

If the input volts was correct it would close the relay contact and apply the voltage to the rest of the circuitry.

You could also use this to ensure the voltage was 5v was within whatever tolerance you want, and prevent the relay from closing.

Mark

Thank you for that document link shabaz. I had never heard of a crowbar circuit before.

Looks very promising.

IRL I use these alot:

http://www.microcenter.com/product/317906/AC-DC_Power_Adapter_%282AMP_,12V-5V%29_w-_4-pin_Molex_Connector

Fig 20 seems to be a clever crowbar:

http://www.ti.com/lit/ds/symlink/lm431.pdf