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Power supply tutorial? (kinda long)

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?

I'm planning on using two 6V batteries: one for logic and one for actuators. The robot is small enough that I could probably get away with one, if that makes the power supply circuit any simpler, but I chose two to eliminate problems with motor-related brownouts.

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?

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?

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

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.

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.

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

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mehaase(at)gmail(dot)com
Reply to
Mark Haase
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I don't think you mentioned what type of cells? You can't just connect some types of cells to a constant voltage source. It's much more complex - particularly if you want fast charging.

The best approach might be to buy a suitable charger and just concentrate on the other parts of the system.

If you are using lead acid cells then consider using a charger with a "float" setting. That will only charge the battery to about 95% full but on the float setting it won't matter if it stays on charge forever. (eg it's fail safe). This setup will supply power to the robot at the same time no problem. For info - Lead acid cells need a constant voltage charger with current limiting.

If you are using NiCAD or NiMH cells then you could look for an automatic charger with an LED that indicates "charge complete". Use an opto sensor to detect the status of the LED. The main problem is you want to draw power from the battery at the same time. Thats quite tricky as any current surges might fool the charger into thinking that the cells were full. For info - NiCAD and NiMH cells need a coustant _current_ charger - usually with a "peak voltage detector" (NiCAD) or a "negative delta V" detector (NiMH) to detect when the cells are full - plus a thermal and/or timer safety cut off to prevent overcharging.

Reply to
CWatters

Reply to
w_tom

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

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

yeah, I'd say.

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

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.

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.

:)

Bye. Jasen

Reply to
Jasen Betts

Mark Haase wrote in news:mehaase- snipped-for-privacy@news1.east.earthlink.net:

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Reply to
roma

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