Design for an Uninterruptible Power Supply

[hughgilhespie]

Inspired by Gina's thread on power supplies I have been rethinking the design of the main DC power supply for my Pulsar dome. Where we are in North Devon, power outages are fairly common, sually only lasting a few seconds.  So, an uninterruptable supply makes a lot of sense.

I already have a nice 240 VAC input 14 - 16 volt output PSU from fleabay. It is an American unit , old but very high spec with a lot of protection built in. Overvoltage, overcurrent, short circuit, reverse polarity, thermal, etc. It is a switched mode unit but is supposed to have very low noise output and it has a ludicrously high maximum output, maybe 50 amps or so.

I want to couple this unit with a Yuasa 12 volt 22 AH VRLA battery to give UPS capability.

I have come up with a design based on using  4 off 10 amp Schottky diodes. The ones I have selected have a forward voltage drop of only 0.35 volts.

I will put two diodes in series to drop the voltage from the mains unit to give 13.65 volts at the battery terminals. This is apparently the ideal float voltage for the battery. Then each supply line (mains PSU and battery will have a diode in it to provide isolation between the mains PSU and the battery. 

The finished PSU will have a few more bells and whistles. One of those delightful dinky fleabay voltmeter/ammeter units and 10amp fuses and 100 nF capacitors scattered about .

For now I only have one question. Will it work ?

[perfrej]

Yes, it will work. There is, however a lot of watts to cool off and the charge voltage for the battery is a bit low.

In my remote obsy in Provence I have used an even simpler approach. Use a 240V powered battery maintenance charger, or simply a power supply that delivers exactly 13.8V, then add the battery as it is. It will keep the battery properly charged, and it just works...

/per

[Tinker1947]

Would the UPS not require some sort of timer, say power outage of more than 30 seconds and then it starts a shut down of the Obby........

[Gina]

As seen in my thread, I just use a 13.8v PSU directly connected to the battery (as Per said too).  Can you not adjust your PSU to 13.8v and dispense with the Schottky diodes?  Works for me

Other very important items missing from your diagram are fuses.  These are essential to prevent the possibility of a fire caused by the very high energy vailable from a (charged) battery.  You need to protect both the inpput (charging) side and the output. 

Please don't skimp on fuses - not only your expensive equipment but your very life could depend on them!

[thanders]

Skip the diodes and adjust the power supply to 13,65V. If not possible, use an ordinary diode (0,7V) between the ps and battery. In both cases your equipment should be connected to the battery.

Keep it simple!

/T

[Gina]

The correct maintenance voltage for lead acid batteries is 13.8v according to all the sources I've found.  And as I said, I find this works well,

[hughgilhespie]

Thanks for the comments. I am very happy for Per's confirmation that my design will work. I should have made it clearer that my fleabay PSU can only go down to 14 volts so I do need to drop some voltage before connecting to the battery. I definitely don't want to risk gassing the battery as the idea of pumping hydrogen into a closed space is alarming to say the least. The other diodes are to prevent a catastrophic battery discharge through the PSU if it suddenly short circuited. Unlikely I guess but better safe than sorry and I do have power to spare and then some. I calculate that at my maximum current of about 7 amps, I will be dissapating around 10 watts, should keep the psu box nice and dry!

Gina, thanks for the comments about fuses. Again I should have made it clearer that I will be using 10 amp quick-blow fuses in both output lines. The schematic I posted was only to show the diode wiring, not the full circuit.

The same applies to Tinker's comments. The finished unit will also contain a circuit that will send a signal when a mains power outage occurs to the Arduino in the main Observatory Control box. This is all yet to be finally designed but the intention is to have a preset period, say 15 minutes, for the power to come back on and if it doesn't to trigger a shut down routine. The battery is sized to allow for at least 1 hour's running at maximum current consumption so 15 minutes plus a shutdown shouldn't cause it any problems.

Thanks to all, Hugh

[Gina]

Fair enough re. fuses

If your PSU goes down to 14v you could set it to 14.15v and use one Schottky diode in the PSU line.  That will also stop any reverse current from battery into PSU.  Your output will be 13.8v on mains and the full battery voltage during a power cut - no point in losing 0.35v in a diode and losing the time the kit will run off the battery as it discharges.  eg. some SW mounts need 12.5v minimum and won't work on 12.0v even if they say they will!

[hughgilhespie]

Nice One Gina,

One diode is better than 4. That said, the minimum pack size from RS is 5 diodes but always nice to have a stock!

The charging voltage of 13,65 volts I took from the spec for the Yuasa Yu-Power 4 amp charger type YPC4A12. This gives

Input Voltage 220-260 VAC

Input Voltage frequency 50 Hz

Output Voltage - Absorption 14.4 VDC

Output Voltage - Float 13.65 VDC

Output Current 4 A

I guess floating at 13.8 volts won't make much difference so it looks like a revised circuit is in order.

Thanks again, Hugh