In this post we discuss how to build a simple UPS circuit for backing up computers or PCs during sudden power failures or brownouts.
Introduction
Normally when we talk about uninterruptible power supplies (UPS) we imagine large inverter units with complex features, where it imperatively needs to be a pure sine wave type. Such inverters occupy enormous spaces, require bigger batteries and are immensely expensive.
A little innovative thinking shows that the above cumbersome design can be replaced by just batteries and a small circuit for implementing all the necessary actions of an efficient compact transformerless UPS circuit.
However the design also a few downsides. It is specifically intended for CPU type computers only and cannot be used for other applications.
The installations procedures are complicated and time consuming and requires expertise in the field of electronics as well as computers.
Having said these, once installed the unit will provide some very useful services for a very long perid of time. Moreover the efficiency of the system will be far better than the conventional UPS systems.
Looking at the circuit we see that its all about switching the motherboard of the CPU with a set of matched outputs from a battery source which exactly corresponds to the voltages that's obtained from the power supply of the CPU.
Using the Versatile LM338 ICs
The circuit is made up of two ICs LM338, which are set for producing exact 3.3V and 5V outputs which are appropriately bifurcated into many outputs via diodes.
The 12V outputs are taken directly from the battery, while a minus 12V output is derived by employing an extra battery.
One battery feds the LM338 circuit while the other battery generates the required -12V output for the CPU.
The switching action is implemented by a relay when power fails.
The relay simply selects the appropriate grounds while doing the reverting actions.
As long as power is available from the mains, the relay keeps the backup ground disconnected from the CPU ground, and keeps the power supply ground connected to the CPU ground via the N/C contacts.
The relay is powered by an external AC mains power supply source, which is also used for charging the batteries. Actually it can be an automatic battery charger unit, attached to the system for the required actions.
The moment AC fails, the relay disconnects the power supply ground from the CPU and connects the back up circuit ground with the CPU ground, so that the CPU now gets the required back up from the relevant outputs of the transformerless inverter circuit.
The reverting actions is done within a few ms, providing an interruptible power during power failures or brownouts.
All the outputs shown in the circuit should be carefully soldered to the relevant wires of the power supply by slightly stripping the wire insulation and then taping them. The voltages must be thoroughly confirmed before integrating the two systems together.
Part List
IC1, IC2 = LM338
R1, R2 = 240 Ohms,
P1, P2 = 4K7 presets
All diodes are 6 amp rated
Relay = 24V, SPDT
Battery as shown
Απόστολος Παπαστεργίου says
The above circuit Transformerless UPS Circuit for Computers (CPU)
Swagatam says
yes it can be used after doing the recommended wiring correctly
Απόστολος Παπαστεργίου says
can you tell me how to connect the purple +5VSB and gray POWER OK .
Praveen says
+5VSB and gray POWER OK is standby voltage used to trigger and switch on for SMPS ATX
Swag says
sorry, I am not sure about it at the moment, I don’t remember, because the circuit was designed by me 3 years ago
Swagatam says
sorry I do not have much idea regarding computer internal wiring details…
apostolos says
helo mr.Swagatan majumbar i have try to make the circouit Transformerless UPS Circuit for Computers (CPU)
and i have some problems can you help me please
when the supply returns and disconnect the battery the PC stops.
Swagatam says
hello apostolos, which circuit diagram are you referring to?
Paddie says
Dear Sir I built This Circuit and it is working. but i have following problems.
(1)What will happen the Battery voltage doped to 11V or 10V, It is Harm to the mother board or HDD or Flash Drive. So Can you give a low voltage cut-off circuit.
(2)The -12v side battery discharge slower than +12v side one,
Swagatam says
Dear Paddie,
You may battery the second circuit shown in the following link, the relay contacts should be wired in series with the common negative from the battery
https://homemade-circuits.com/2011/12/how-to-make-simple-low-battery-voltage.html
Auston says
And wat about -5V???
Swagatam says
I don't think there's a -5V out in ATX power supplies
Auston says
can i use mosfet to get high current output from LM338???….
Swagatam says
no a mosfet won't work, you will have to use an overboard high current transistor across the IC for upgrading current.
Auston says
OK……can i use 7912??….for -12V output?? if yea….what would be its Ground and input??…
Swagatam says
no it cannot be used here….
Auston says
i meant to say….How much time will my battery last?(if my Battery is 3.5Ah and my smps was rated at 500 Watts)…..why cant i give ground directly instead of -12V???….
Swagatam says
The circuit has nothing to do with smps rating, it's connected with the CPU, so the back up will depend on CPU consumption.
The CPU board requires a -12V tap also therefore this supply needs to be provided to it.
Auston says
i am gonna do this…..wish me a luck….but i cant understand why it needs -12V….instead can wee use ground or 7912???
Swagatam says
wish you good luck!
Auston says
haan…ok…..so how much time will it work???……my smps is 500Watt one….so how much A would it take for an hour?…..can i use A=W/V….Ex. V=220 and W=500 and A=500/220=2.7777…..?
Swagatam says
I did not understand your question…..
Auston says
And….Can i use one 12V battery???
Swagatam says
Ground is the common point for all voltages…
a single 12 volt battery will not work, a couple are required as shown in the diagram.
Auston says
hey……what should i connect for -3.3V n -5V……12V ground directly???
Swagatam Dash says
this is very interesting . can you give any idea to make a circuit to connect a 12v 7.2AH UPS battery to laptop.
Anonymous says
ok tnx for the update i will try and i will feed you back
EmbSysDev says
Good to see a fellow Mumbaikar,passionate about power electronics.
But I don't see how I could get in touch with you from here,email id or something ??
Anonymous says
good day what about the monitor where did it get they power source? its only for the cpu
Anonymous says
ok then, ill wait for your update tnx for the reply
siree says
haha okay sir… i will not try it now..
after becoming an experienced den i will do it…
siree says
helo sir..
its really very nice sir…
but sir, its very complticated to connect on pcb and soldering also must be done ery care fully isnt it sir.. is it recommended to do on our own sir??