DC-DC UPS for Arduino

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Drunkard #1

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I need my Arduino to run during power failures, so I need a UPS. It's 9V input, so I'll just use a square 9v battery for now, rather than building a charger and so on. How do I wire this up? Do I need a chip to compare voltages and switch when voltage-1 drops below 9V?

I don't want to spend R 2 000 on a PSU that can do this. Current draw will be: average <20mA, max <200mA (LCD backlight)

Thanks.
 
I'd ditch the power supply you are currently using and run it off a 12V 7Ah battery directly as it sounds like it has an on board regulator to derive the 5V. Then add a cheap linear charger for the battery and you are good to go. Shouldn't cost more than about R150.

Also check the specs of the board to check the maximum input to the board to make sure it can tolerate the fully charged voltage (~13.8V). You could also knock up a 9V pre-regulator out of a LM317 if it can't, or if it makes you feel better :)
 
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I can help you get a cheap alternative but you need to post more specific details:

What kind of input voltage do you require (only regulated 9v?).
What kind of current requirements do you have (1A?)
 
Recommended voltage is 7-12V, so I'm happy with anything in that range, 9V is the "normal", but I think it's even got a 5V input, although I haven't tried that. Power is max 200mA.

kayvee, your idea seems about right, although I'd go with a smaller battery, maybe even a PCB mount. I assume somewhere like RS or Mantech would have the linear charger. Is that a self contained unit that keeps the battery at optimal voltage without overcharging? Will the LM317 need to dissipate a lot of heat? Thanks.
 
Yeah, it depends on your size constraints I guess, but the 7Ah version is commonly used (alarm panels, etc) so prices generally good.

You can get off-the-shelf 1A chargers modules for them reasonably cheap too from the likes of Mantech, yes, so you don't have to fuss about charging.

If your typical currents are around the 20mA mark the 9V LM317 pre-regulator will barely get warm, and at 200mA its still easily achievable, higher that that you will need a heatsink on it, but that's also easy enough.
 
What do you do when the voltage falls below the drop-out voltage of regulator? (if you just use the regulator).

It will probably damage the Arduino if the voltage is out of spec...
 
The LM317 will loose regulation then the input voltage falls to about 11V assuming the output is set at 9V, but the battery is technically flat at 10.8V for a lead acid anyway. It will not damage anything.
 
Thanks okes. I'm just deciding on components, then I'll order.

If you don't mind, I've just got 1 more question, because I'm not an electrical engineer. I need to count pulses. The "pulse" is an open circuit, and the normal condition is 100ohms. They occur slowly, like 1-2Hz max. I'm using the interrupt pins on the Arduino, which works perfectly, but I need to push 5V through the pulse "probe" almost continuously, and according to highschool electronics, that uses 50mA. There are 4 probes, so that's 200mA total.

I'm concerned that the resistor in the probe might die, which is ok as long as it doesn't happen too often, once every ten or twenty years is to be expected, considering the conditions they'll be living in, but if I need to replace probes monthly, I might as well read the meters manually. Also, I'd like to reduce the power consumption. Is there any point to pushing a lower voltage through the probe and amplifying the signal? Maybe invert the signal with some sort of "not" circuit , so that the pulse is 5V and the normal condition is 0V, but only put 1V through the probe and amplify the signal. I'm just thinking out loud, so I'll test this next week, but does it sound like a better idea or am I on a pointless mission?

Thanks.
 
I might be missing something here but any reason why you can't just add a series resistor of say 1000 ohms in series with the probe? Don't have enough info on your circuit to offer more. A link to the specs of your probe, perhaps?
 
kayvee said:
I might be missing something here but any reason why you can't just add a series resistor of say 1000 ohms in series with the probe? Don't have enough info on your circuit to offer more. A link to the specs of your probe, perhaps?
Click to expand...

Just tested, and it works perfectly. I don't know why I didn't think of that. Thanks.
 
Great! At least you found a solution. Should be more reliable in the long run too.
 
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