On 9/3/05, Philip Pemberton <philpem@dsl.pipex.com> wrote:
> (For some odd reason my messages don't seem to be reaching the PICLIST - I
> sent this at 9AM and it hasn't appeared yet, so here's a resend...)
> 
> Hi,
>   I'm trying to design a bench power supply, seeing as all the ones I've
> seen
> on the second-hand market are either too expensive or a load of junk. I
> wanted something with a voltage output of 1-30V with a maximum load current
> of around 5A. It's going to use a combination of linear and switchmode
> technology - linear pre-regulator, switchmode output regulator.

Hi Philip,

Nice ideea, however what is instead of a linear preregulator you'll
use a controlled bridge followed by a power LDO ? I've guess the noise
will be much- much smaller at 5A load.

> 
>   The problem I'm having is with the switchmode regulator - a MAX724 (the
> 726
> is the same chip with a 2A current limit instead of 5A). It uses a resistor
> divider to set the output voltage, which is great for normal analogue
> control
> with a potentiometer. Problem is, I want to control it digitally.
>   All the digital pots I've seen either have a maximum voltage of 5V
> between
> both ends of the wiper. That rules out using a digipot because at 30V out
> there's going to be (30-2.2)=27.8V between the end of the track and the
> wiper.

I don't know well the MAX724/726 but every feedback could be realised
in two ways: feddback from the output voltage, resitive network
divider from the reference voltage to the amplifier error input (and
here a digital potmeter could work ok), both of the previous methodes.

> 
>   Can anyone suggest a way to control this thing? So far the only idea I've
> come up with is a stepper motor and a potentiometer...

no way, this is a methode used since my grandmother was a young girl...
:)

> 
>   Ideally I'd like 0.05V resolution for the voltage. I'm going to do
> current
> limiting with a current shunt, a bunch of opamps 

why a bunch ?


and an 8-bit DAC, which
> gives a theoretical resolution of 0.01A. I was thinking of something
> similar
> for the voltage control (but with a 12- or 16-bit DAC), but couldn't think
> of
> a way to convert the voltage output from the opamps into a resistance.

Because you're thinking wrong. If you have a variable reference
voltage why you should convert it into variable resistance? Forget
about using the MAX724.

cheers,
Vasile

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist