UPS for 24VDC

[bursach]

Hi everyone!

I need to design an UPS for 24VDC, 5A. Input is power supply 220VAC/24VDC 5A. Battery pack 2x12V 12Ah. When PS is active, batteries are inactive.When PS is off, battery pack is in function.Again,when PS is on, it charges battery(automatic turn off charging when charged, not complete PS) and provide 24VDC 5A on output.

I have got usual PS without control electronic for smart charging, and lead acid 2x12V 12Ah batteries. What do you suggest me to do to make this work. Any schemes are welcome.

thanks in advance.

 

[Hero999]

https://www.ebay.com/

 

[Sceadwian]

Go with what Hero said. Just buy one off e-bay or from a store, it's going to cost you significantly less than if you design one yourself.

 

[bursach]

There is some problems with shippment in my country(Serbia) with ebay. I have to make this ups by myself. any new suggestions

 

[eblc1388]

This assume you want 24V 5A DC output and not the usual 120V or 240V ACvoltage output.

While most UPS selling on eBay output AC, I much doubted one can buy a 24V 5A DC output UPS from eBay.

Your problem is:

1. Charge batteries and stop charging when batteries full
2. Support a load of 24V, up to 5A all the time

What you'll need is those GEL battery, either single 24V or two 12V in series. These batteries use constant voltage charging and will not get overcharged if you follows the manufacturer's charging requirement exactly. So one problem solved.

Then there is a small problem about the total system voltage. It is now much higher than 24V, may be 27.2V. One would need a voltage regulator circuit to regulate this 27.2V down to 24V for your load, if they cannot take 27V directly. This can be done using a MOSFET to drop off 3.2V when the AC supply is healthy and the MOSFET is switched hard-on when the AC supply is lost.

 

[Dave377]

This block diagram (not really a schematic) may help as a starting point. I've used this with LM2940 5V backup schemes with a 9V rechargeable battery. There are myriad ways to make these linear regulators handle more current (pass transistors MOSFETS, etc.). So getting to 5A should be ok (refer to Mfgr's App Notes). We assume here you already have a DC supply that is sufficiently higher than the target regulated/backed-up +24 VDC you require (plus the steering diode drops and regulator drop-out).

Beware, the circuit isn't as simple as it seems. You need to take into account the drop-out of the linear regulator and you may want to search for a low drop-out (LDO) device instead of the 7824. There are nice newer solutions to solving your problem using switching regulators, but I'm going to assume you have limited access to parts in your part of the World (like me) and need to stick to venerable linear regulator parts as a basis. Good luck... Dave

 

[panic mode]

what is this 24V UPS for?

 

[bursach]

It is for carbon monoxide detection panel.

 

[Hero999]

Easy, build a 28V power supply using a 24V 7.5A transformer, an LM338 regulator, a bridge rectifier and a huge capacitor and connect 12V batteries connected in series across the output.

 

[bursach]

what is about controlled charging batteries? Protective against overcharging?

 

[Sceadwian]

Batteries last 2-10 times longer if they're inteligently charged, and even longer if the application is designed around the fact that it will be operated from a battery. Ideally you want to keep a battery between 20% and 80% charge, and only charge it when it's at it's lowest end using as gently a charging cycle as possible, while keeping heat to a minimum.

 

[Hero999]

Lead acid batteries don't need any special charging requirements, providing you don't drain them too much a constant voltage source is all you need.

 

[JTECH]

I agree with HERO. Just to be on the safe side, charge the 24V battery with a constant voltage not exceeding 28,4V dc. (Charging regulator). Then regulate the voltage supplied to the load with a LM338 adjusted for 24Vdc. (Load regulator).

 

[Sceadwian]

It's usually good to do constant current when the battery is low and then near the end of the charge switch it over to constant voltage. It charges faster that way, and it limits the current going through the cell when it's very low which should prolong cell life.

 

[bursach]

Ok, I figured out. But can somebody post a schematic or link of smart battery charging or current limiter that I can complete the project?
thanks in advance

 

[Hero999]

There's no need to build a separate constant current load circuit, the LM338 has current limiting built in.

bursach,
Look on the datasheet for the LM338.