DIY DC UPS for router - Page 1
Forum section: electronics / beginners

author: tlovie
author_rank: Contributor
timestamp: May 20, 2022, 07:19:06 pm
content: I've been trying to design an uninterruptable power supply for a router that is supplied by a 12V dc power brick. My design is like a 12V battery charger for a SLA battery that runs at a constant 13.5 volts, which has a low voltage detect circuit that cuts the power to the load if the battery voltage gets too low. The whole thing is powered by one of those 19.5V laptop power bricks. I had made an earlier prototype of this circuit that used an LM317 as a current limiter, but I wanted to try to eliminate the LM317 and implement the current limit function by lowering the regulated voltage point.Schematic attached.My main question is the current limit that is implemented via U1B and D1 that supplies current to the voltage divider R13/R14. The way that I think about this is that the current limit artificially moves the voltage sense of the voltage regulator U2A. I've copied this idea from some power supply schematics that I saw (0-30V 0-3A power supply). Would there be any upside/downside to do this instead at Pin 2 of Q1, which is the voltage control transistor?If anyone would also care to comment on the design, I'm interested in feedback from more experienced folks.Many Thanks,Tom.

author: David Hess
author_rank: Super Contributor
timestamp: May 21, 2022, 10:15:56 am
content: Quote from: tlovie on May 20, 2022, 07:19:06 pmThe disadvantage of that method is that the phase delay of the current sense amplifier and voltage sense amplifier add together complicating frequency compensation. It also makes the current sense response slower. Whether these disadvantages matter depends on the application and it should be fine in yours.

author: mariush
author_rank: Super Contributor
timestamp: May 21, 2022, 11:36:39 am
content: Your router has a switching dc-dc converter inside which converts the input 12v to lower voltages like 3.3v / 3.6v for the processor and 2.5v or 1.8v for the flash memory and sdram.They're using 12v as input to reduce the current going through the wires, and therefore voltage drops, and because 12v adapters are more mass produced, easier to make - the router probably consumes around 5w with maybe peaks to a few more watts, so using a 5v 2A adapter would have been a worse choice, as you'd lose a few .1v on the cable.Anyway, the point I'm trying to make is that the switching regulator chip inside the router should work with much less than 12v ... it would probably work just fine with 9v or even 7.5v, which would make it possible to power your router directly from a lithium-ion battery pack , 3 cells in series would give you 9v..12.6v and even 2 cells in series would give you 6v..8.4v when fully charged.You should try to measure the actual current power pulled by the router (place your multimeter in series with the adapter's DC positive wire and measure the current. or separate the positive and negative wires going to the barrel jack and use a clampmeter with DC measurement functionality to measure current.

author: tlovie
author_rank: Contributor
timestamp: May 21, 2022, 12:17:07 pm
content: Quote from: David Hess on May 21, 2022, 10:15:56 amI also felt that the current sense amplifier would be better if it drove the voltage control transistor directly, rather than driving the voltage sense amplifier. Thank-you for the insights.edit: showing revised schematic.

author: tlovie
author_rank: Contributor
timestamp: May 21, 2022, 12:32:52 pm
content: Quote from: mariush on May 21, 2022, 11:36:39 amThat is very likely the case. My router is actually one of those tiny passively cooled PCs that has SATA, USB, wired NICs, WIFI, HDMI outputs, etc. I made the 12V UPS not because it was the optimal choice, but because I had a surplus 18 Amp hour SLA battery. I'm sure its fine, but since the router runs OPNSENSE I didn't know if the filesystem would get corrupted with unclean shutdowns. The PC draws around 1.5 A peak during boot up, and around 400mA at idle, which I measured with a multimeter.I have not really played with lithium batteries too much, but building a balance charger might be beyond my pay grade. I will perhaps think of it in the future.

author: strawberry
author_rank: Super Contributor
timestamp: May 21, 2022, 12:44:53 pm
content: make some 12V OVP protection and run from SLA directly. When voltages are too tight use buck-boost converter. Less power loses and less complicated but making inverter could be fun. SLA wont last forever

author: tlovie
author_rank: Contributor
timestamp: May 21, 2022, 12:54:02 pm
content: Quote from: strawberry on May 21, 2022, 12:44:53 pmI think that is what I am doing - the SLA is connected to the load, with a NPN low side switch that has a low voltage shut off. Of course it would be more efficient with a switching power supply, but I just did a linear power supply to charge the SLA. Definitely not making an inverter... that's well beyond my capabilities.