> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf
> Of John Ferrell
> Sent: 13 February 2012 15:44
> To: Microcontroller discussion list - Public.
> Subject: Re: [EE] Need a cheap power supply
>=20
> How about a recommendation on the current limiting function?
>=20
> In the longer term I would add one of those digital voltmeter boards
> (can you still get them) that used a 40 pin LSI and 4 digit 7 segment
> display, so you have a permanent voltmeter on the output. For extra
> brownie points make a project out of a PIC that has a 12 bit ADC, and a
> keypad for setting the voltage, and maybe also current limit, and be
> able to display both current and voltage.

FWIW I have a PSU right here on my desk that has keypad entry of voltage an=
d current, and I really don't get on with it.  It's very easy to accidental=
ly press the wrong button, or not notice that the decimal point was missed =
and suddenly have 10x more voltage than you actually wanted.  To be fair yo=
u can set an over voltage protection level to ensure this doesn't happen, b=
ut you have to make sure you type this in correctly as well.

The PSU underneath in (Keithly 2400 Sourcemeter) is slightly better - you s=
elect each digit in the voltage and increment or decrement it, but it's sti=
ll not a pleasant interface.  If I was making my own digitally controlled i=
nterface, it would use a rotary encoder, and it would have pre-settable upp=
er and lower limits so you could crank the voltage up or down with impunity=
 whilst watching a scope display etc.

Whilst on the subject, an LM317T circuit is certainly better than nothing, =
but an adjustable current limit is a 'must have' for any remotely serious d=
evelopment work.  Applying 5v 1.5 Amps to your new circuit will probably sh=
ow you that you put an IC in backwards, but it's unlikely to work after you=
 correct it...  A bench PSU with adjustable current limit is also useful fo=
r battery charging duties.

Regards

Mike

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