# Unregulated voltage from PicoUPS... switching?

Discussion in 'Power Electronics' started by Doktor Jones, Oct 18, 2016.

1. ### Doktor Jones Thread Starter Active Member

Oct 5, 2011
66
2
I'm looking at using a PicoUPS with my network equipment to protect them with a battery (SLA); right now I have a 12V/16A supply that is adjustable but very tightly regulated. The hitch with the PicoUPS is that its input voltage needs to be at least battery float voltage (~13.2V) to charge the battery properly, and its output is unregulated... I can adjust the 12V supply to put out 13.5V, but then my output is anywhere from ~11.5V (low battery) to 13.5V (regulated supply input).

My devices should be fine with 11.5-12.5, but 13.5 is >10% over the specified input voltage on them, and I'd really rather not go over 13V (ideally 5%, or 12.6V). My first thought was to throw 2x 10A10 diodes in series on the output for a 1.4V drop @ 10A capacity, but the diodes just have a constant voltage drop, so when the circuit switches to battery, 13.2V at the battery is instantly 11.8V, and as the battery voltage drops to/below 12V, the output ends up south of 10.6V.

What would be the best way to work around this? Is there some sort of regulator that can output up to full input voltage (so it's dropping ~1.5V on PSU, but scales down on battery and maintains a constant output voltage as the battery voltage drops)? Is there some way I could switch the output path through either a regulator or direct output based on whether there is input voltage from the PSU?

P.S.: The 16A supply is already in use powering all the equipment. I suppose I could use an off-the-shelf UPS to power that, but this strikes me as inefficient (UPS downconverts to 12VDC -- or maybe 24VDC -- to charge its battery, then on battery power it upconverts to 120VAC only to immediately get downconverted back to 12VDC in my supply). Plus, if I build it myself, I have only myself to blame if something goes wrong, and if something breaks I'm in a better place to fix it

2. ### tcmtech Distinguished Member

Nov 4, 2013
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2,444
Does your network gear actually need that tight of input voltage regulation?

Also for LA batteries the proper charging voltage is around 14.2 - 14.7 volts with a 13.4 - 13.8 volt float level, not 13.2.

Without information on the specific bands and models of your gear we have no real idea what you actual working range specs for each device or component could really be.

I for one have many old network devices that have 12 volt nominal inputs yet the factory 12 volt power supply adapters for some put out over 16 volts at light loads.

3. ### Doktor Jones Thread Starter Active Member

Oct 5, 2011
66
2
The PicoUPS is just a constant voltage charger; it's meant as a backup system, so the battery sees long periods of "no use" where its voltage is just being maintained, with short bursts of usage when there's a power outage.

I checked the specs on the batteries I'm evaluating for this project, and you're correct; most list standby voltage as 13.5-13.8V. At the moment I have the power supply tuned to 13.5V, but I believe it can be turned up a little higher.

When I say network equipment, I mean as in computer networking -- cable modem (Motorola SB6121), network switch (D-Link DGS-1016A), and router (a homebrew mini-ITX computer). All of these currently use regulated power supplies that output 12V ±0.2V regardless of load (within specification at least, heh). They were designed with such power supplies in mind, and are consumer/"prosumer" grade equipment; I'm not sure how tolerant they are to higher voltages and would really rather not find out the hard way that they aren't.

4. ### tcmtech Distinguished Member

Nov 4, 2013
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2,444
Pretty much all of that stuff is very tolerant of input voltage being they all use dedicated internal SMPS to drop the nominal 12 volt input voltage down to the required 5 and 3.3 volt power the digital systems need.

I dont have the exact same models of gear as you but I used to have a D-link unit that had the old transformer based power pack that although rated for 12 volt output was closer to 16 - 17 volts.
Same with my current Netgear and related network devices in my house. I've ran them on way higher than 12 volt inputs for some time because I don't have the proper power adapters for them.

My brother is a senior service tech for a local computer service center so I get all sorts of old devices for parts for free and it's rare that any ever come with any let alone the correct power supplies and I have yet to ever kill a single one form that mismatching either. In fact my understanding is in many applications they are ran directly off of battery backup systems just like you want to do so they obviously have to be able to work with the battery charging voltages that may be present.

So to me if the input is anywhere under 15 volts I seriously doubt that it would ever do anything to them. Quality name brand stuff just isn't built that cheaply or poorly.

5. ### Doktor Jones Thread Starter Active Member

Oct 5, 2011
66
2
Well I suppose I can give it a try... I can use my IR thermometer to get a baseline temperature on them using their existing power adapters, and then monitor temperatures on the new supply @ 13.8V to make sure they aren't running the regulators too close to spec (as an extreme example, I saw someone using a 7805 to drop 7.5V to 5V @ ~1A for an LED project, with a small heatsink -- something happened to the 7.5V supply, so he replaced it with a 15V supply... you can imagine what happened to the 7805 when it's thermal dissipation jumped from ~2.5W to ~10W).

The piece of equipment I'm most concerned about is the router, which having built it myself, is also the easiest to open and monitor internal component temperatures (it's also probably the most robust of the bunch, because I tend to over-spec my builds). If I fry the modem, well Comcast has been yelling at us that we need to upgrade anyways to "take advantage of the fastest speeds" (really I know it's a marketing gimmick to get us to rent from them, but FTN, I'll buy my own). If I smoke the switch, that's \$70 I'd rather not lose, but it's not the end of the world.

6. ### tcmtech Distinguished Member

Nov 4, 2013
2,682
2,444
From the dozens of network modem units routers, wifi units, access points and other devices I never did figure out what they did I have never taken one apart that did not use a SMPS type DC - DC converter to generate the stable lower voltages. I would feel pretty safe running them at up to 15 volts being you have good name brand equipment but yea if they get hot then obviously that's not good either.

7. ### tcmtech Distinguished Member

Nov 4, 2013
2,682
2,444
Just for personal curiosity I dug out several old devices I had and put them on 24 volt power supplies to see how they would take it.

No issues with any. All that could work with my system powered up and worked as designed. The others just powered up and blinked their LEDs suggesting they were likely working as well.