Diy Dc Ups

[crenn]

I know that this is doable, but what I'm wondering is if I can get some help to design some custom circuitry so I can have a stable 12V output to drive a router and a modem (technically this should be a lot more efficient and last longer than the current UPS in place).

This is what I've found so far:
http://www.dansdata.com/diyups.htm
http://www.dansdata.com/upsupgrade.htm

That's given me a basis to work with, now I need to work out how to make it:
-so the voltage to the battery will increase (without affecting the output to the devices) every 3 months for 12 hours (I might be able to use a micro here depending).
-have a smooth transition from AC (I'll use a normal AC-DC converter for this) to battery power (maybe a large capacitor that can handle 15v?)
-turn off when the battery is too low to run the components safely (ADC with the micro?)
-a warning system for when the system is triggered

Any suggestions or links is welcome.

btw, this is the battery I'm looking at using for this build/idea:

http://www.jaycar.com.au/productView.asp?ID=SB2490&CATID=18&form=CAT&SUBCATID=250

[DaveW]

Try PM'ing CrazyBillyBob about this. I'm no good with AC circuits, but he managed to do R2. It runs of a car battery I believe, and he plugs it into the wall to charge; he then uses the same battery to power the PC and the motors. If my memory serves.

I'm sure he'll be able to help you. Make sure to let everyone else know what you come up with.

-Dave

[Spawn-Inc]

the only thing i can think of in all that is for the large cap a stiffening cap that is used in car audio can handle a 20v surge.

[xmastree]

Right.
The router and modem, how much current to they require? how much backup time do you need? Use those figures to determine the battery capacity. If you like I can dig out the aging factors to include in there.

Then, use a trickle charger which can deliver the required current plus a bit more to keep the battery topped up.

Why do you want the battery voltage to increase? I don't understand that part.

You're right about efficiency. A normal UPS will generate AC, which the router and modem's PSU's will drop back down to 12V again. Removing these elements from the chain will be much better. Just be sure that the two units are happy sharing a common supply. They should be, but make sure first.

The only tricky part is the low battery cutoff. Our Emergency Lighting systems use something similar, a contactor on the output which drops out when the circuitry detects too low a voltage. You can use a simple relay as you're dealing with lower currents.

Remember to monitor it after the relay, so that once it's off there's no load whatsoever on the battery, not even the monitoring circuitry.

Edit: low battery cutoff
http://www.discovercircuits.com/DJ-Circuits/9vbatmon1.htm
Just play with the values of R1/R2 to get the required cutoff voltage, then use the output to control the relay.

[crenn]

Router needs 2.5A peak, modem needs 1A peak. Both require 12V. I'd like them to run for 12 hours+ (in the event of a blackout). However, even they won't be running at peak. I need to get it so I can monitor power draw. But for now, I'll say each device uses half.
So a 18Ah battery should last 10h17m08s, which should be long enough for me. Average blackout doesn't go longer than 12 hours. Aging factors would be great.

I was also looking at using a couple of super capacitors so that the system has time to switch to the battery (it depends on how I design it). But so far I can only find 5.5V 22F capacitors, know where I can get some higher rated ones, or if I can work out a way to use these ones?

The low battery detection circuit is something I'll look into, I'll need to analyse the circuit first. The upping of the voltage is apparently a maintenance thing for Lead Acid batteries, I read about it in the first article.

This has given me a lot to think about now, thanks guys.

[xmastree]

I was also looking at using a couple of super capacitors so that the system has time to switch to the battery (it depends on how I design it).I wouldn't do it that way, as you're not driving an inverter.
Think about running the devices from the battery, and connecting a 4A charger to it as well. This will float the battery at maximum charge, and power the equipment. Power fails, the batteries supply the power instead. No switching involved.


The upping of the voltage is apparently a maintenance thing for Lead Acid batteries, I read about it in the first article.
Never heard of that one. We just use a power supply to float the batteries, which take over if there's a power cut. Nothing too fancy in there.

[xmastree]

Have a look at this:
http://www.elecfree.com/electronic/wp-content/uploads/2008/03/uninterruptible-alarm-power-supply-by-7805.png (http://www.elecfree.com/electronic/uninterruptible-alarm-power-supply-by-7805/)You'll see that the load is connected to the battery all the time. That's what I'm thinking about. Obviously this one's not powerful enough for your application, but the principle is the same.

You could easily make a redneck one with a car battery and a simple charger. Just don't keep it inside... :eek:

[crenn]

That helps more and simplify things, I may get a couple super capacitors (for the hell of it) anyway but not use them for this.

I'm considering putting up a switch mode power supply (because of what I read on wikipedia) so that I can keep the voltage as 12V constantly in the event the voltage level goes lower than 12v (which it will). Still working on a couple of other components.

[billygoat333]

when I first read the title I read "Diy D Cups" and I got all excited. but then I realized that this isn't about homebrew plastic surgery, and I feel like an idiot. lol

[xmastree]

I'm considering putting up a switch mode power supply (because of what I read on wikipedia) so that I can keep the voltage as 12V constantly in the event the voltage level goes lower than 12v (which it will). Still working on a couple of other components.
You'll probably find the modem and router will work happily on a wider voltage range, maybe 10 - 14V so I wouldn't worry too much about regulation.
The battery will be at 13.8 fully charged, you could feed that into a 12V reg if you like (as in the above circuit) but once the battery is running, that will drop and you won't have enough overhead to get 12V at the output even if the battery is healthy.

[crenn]

You'll probably find the modem and router will work happily on a wider voltage range, maybe 10 - 14V so I wouldn't worry too much about regulation.
The battery will be at 13.8 fully charged, you could feed that into a 12V reg if you like (as in the above circuit) but once the battery is running, that will drop and you won't have enough overhead to get 12V at the output even if the battery is healthy.
That's why I'm looking at a switch mode power supply. A little 60W one for a Mini ITX board is all I should need providing it has the features I want.

[crazybillybob]

Crenn,
For what your doing you don't need the "inverter" as listed in your linkage.
You really could do it all from a 12Vdc perspective. In your case the inverter is just a waste of energy, by skipping it you'll make a smaller, lighter, cheaper, more efficient design. Your looking for a DC to DC 2 source converter with auto switching.
I've got a circuit that does most of what you want except check that the batt isn't dead (under volt). It doesn't use any micros. I'm sure that you can find a chip that will do everything you need, I think maxim has a couple of IC's that will do it.

Good luck!
CrazyBillyBob

[crazybillybob]

I think with a little work you could use a microproc to measure the battery voltage, if it's low; trip a load relay off to disconnect the load from the battery.

[xmastree]

I think with a little work you could use a microproc to measure the battery voltage
What's wrong with a comparator?

[crazybillybob]

as long as you build a little hysteresis into the limits it should work just fine. I just don't have a comparator circuit within easy reach. :)

[crenn]

A comparator is more simple to implement than a HC12. I don't have any spare HC12 chips either.