Re: Power supply tutorial? (kinda long)

On 2005-10-16, Mark Haase <mehaase@xxxxxxxxx> wrote:
> Hey all--
>
> I'm looking to build a power supply for a mobile robot. I want to build
> a battery-based supply that can recharge the battery when plugged into
> the wall while supplying power for the robot at the same time. The long
> range goal is for the robot to find a base station when its batteries
> are low and recharge itself, then continue doing whatever it was doing.
>
> Problem is, most of my electronics knowledge is in logic. I'm pretty
> clueless about other areas, particularly power electronics. Are there
> any good tutorials (online or book form) for learning about this stuff?

the easiest way is probably to start with regular charger....

> I understand the basics of rectifying AC. It looks like I need to use a
> step-down transformer, a bridge rectifier, and a large electrolytic to
> smooth? My understanding is that the peak AC voltage into the rectifier
> is roughly the value of smoothed DC voltage I will get out of the
> capacitor. So if I need 7.5V to recharge the battery and supply my
> circuits, I want a 7.5V peak to peak AC?

7.5V ground to peak if you're using a bridge rectifier.
you won't need the capacitor the battery will fulfill that purpose.

> Another question: I need 5V regulated for my logic circuits. I was
> thinking of using a star power distribution scheme. A single 5V LDO
> (rated at .5-1A) should be adequate: I'm only powering a few AVRs,
> photointerrupters, etc. Total current consumption I expect to be 100-200
> ma, but I want headroom to grow. Also, a switched mode regulator is
> probably overkill, right?

yeah, I'd say.

> And ripple current is okay for motors, right? If not, how do I regulate
> enough current to power a motor with a 1A stall?

yeah, but there won't be much ripple even when charging.

> I'm also interested in learning about safety guidelines when working
> with AC. I'm familiar with safety rules regarding home repairs, e.g.
> installing a switch or outlet, but not for building AC-powered devices.
> I would probably build the power supply in two parts: the "base station"
> would have a grounded, metal chassis, and were perform the step-down,
> rectification and smoothing. Then it would provide unregulated 6V DC to
> the robot, which would have a regulator and star distribution. As an AC
> appliance, does my base station need a fuse? I'm not looking for a good
> housekeeping seal of approval here, just something that's very safe in a
> controlled environment.

A fuse in the transformer primary circuit is a good idea. The transformer
may have an internal fuse, the fuse stops the whole thing bursting into
flames should a fault or short-cuircuit ddevelop.

> Finally, if the power supply needs to power the robot and recharge the
> batteries at the same time, do I need to do anything special? I mean, I
> don't need to switch off the batteries connection to the circuit? This
> is probably a really stupid question, but in my head I feel like it
> might not be as simple as I think it is.

The battery voltage will be slightly higher during charging, but the
regulator will take care of that. so you won't need to switch the battery
out of the circuit.

> Anybody with the patience to respond to any or all of this: much thanks
> in advance!

:)

Bye.
Jasen
.

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