Battery-PSU swapover - Page 1
Forum section: electronics / projects

author: Chet T16
author_rank: Frequent Contributor
timestamp: July 20, 2012, 11:09:28 am
content: Suppose you have a battery powering some devices at 12v and you want to switch to a PSU when the battery voltage drops. The attached is what i'm using and it works but i'm curious to see alternatives to how you would connect the power sources.

author: madires
author_rank: Super Contributor
timestamp: July 20, 2012, 11:23:12 am
content: KISS :-) Switching power inputs via MOSFETs or power management devices are more complex and expensive.

author: digsys
author_rank: Unknown
timestamp: July 20, 2012, 11:56:40 am
content: Usually it's done the other way. A Power Supply trickle charges a battery which supplies a load.You get a flatter voltage curve that way as well ie not waiting for the battery to go flat first,plus only use 1 diode. I'm guessing you have a reason to do it that way?

author: madires
author_rank: Super Contributor
timestamp: July 20, 2012, 12:52:00 pm
content: Quote from: digsys on July 20, 2012, 11:56:40 amA problem arises if you sell a device which charges the non-rechargeable battery and that battery leaks for whatever reason. You'll take all the blame because the battery's label clearly states "Do not recharge! May leak or explode.". It's simply CYA :-(

author: Chet T16
author_rank: Frequent Contributor
timestamp: July 20, 2012, 01:16:45 pm
content: Quote from: digsys on July 20, 2012, 11:56:40 amThe battery is connected to a charger/conditioner which is connected to a solar panel, the power supply is connected to the mains via a relay. When (if!) the sun is shining and the battery is charged the power supply is off. If the battery voltage goes too low the PSU is switched on until the battery is charged again

author: vk6zgo
author_rank: Super Contributor
timestamp: July 20, 2012, 02:51:05 pm
content: Quote from: Chet T16 on July 20, 2012, 11:09:28 amThis is the system used for years in the TV/Radio Broadcasting organisation I used to work for.The requirement was the opposite to yours,in that a -24V DC line needed to switch automatically to the Station 24V Diesel start batteries when the mains power was lost.Of course,the diodes were the opposite way round because of the negative polarity,but still the same idea.

author: digsys
author_rank: Unknown
timestamp: July 20, 2012, 03:00:49 pm
content: QuoteAh ok. Well, If you already switch a relay when the battery voltage is too low, why not simply also switch the power source on a 2ndset of contacts. Saves the loss in the diodes. All you'll need is a electro to store enough charge for the ~20mS-50mS changeover.

author: Chet T16
author_rank: Frequent Contributor
timestamp: July 20, 2012, 11:36:44 pm
content: You mean add two more relays in, one on each supply? Surely I'd need diodes as a precaution anyway?

author: digsys
author_rank: Unknown
timestamp: July 21, 2012, 01:20:23 am
content: Quote from: Chet T16 on July 20, 2012, 11:36:44 pmI was thinking something like this -You can either leave the PS on standby for a fast changeover, or drive a 2nd relay to connect it tothe mains first, if you want to save power.

author: Psi
author_rank: Super Contributor
timestamp: July 21, 2012, 07:58:40 am
content: If you design it to handle both connected at once for a little while you can use relays and make the connection before breaking the other. (Easy if you have a mcu in the system)You could also have that two diode arrangement and use one relay contact set to selectively short out one of the diodes.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 21, 2012, 10:49:13 am
content: You certainly add a lot of complexity to get around a 0.7V drop!Of course one thing with the diode method is that the normal source always has to be a bit higher in voltage,so the diode is reverse biased,This was not a problem with the TV sites,as the nominal "24V" from the mains operated supply was usually around 27-28V.Anything voltage critical normally had a 24V to 12 V regulator in its power circuit,& other stuff like relays weren't fussy!.

author: digsys
author_rank: Unknown
timestamp: July 21, 2012, 10:57:00 am
content: Quote from: vk6zgo on July 21, 2012, 10:49:13 amYes I did !!, and it is a valid common option, unless of course, saving power is a critical requirement. BUT -Check the OPs original comment ... and you'll see the light :-)Quote

author: EEMarc
author_rank: Regular Contributor
timestamp: July 21, 2012, 05:51:52 pm
content: KISS... Isn't that Keep It Shockingly Sophisticated? I suppose it makes for better job security.I saw a flashlight with a pair of rechargeable AA batteries and a solar cell. It had nothing but a diode to protect the solar cell. It was left out in the sun to charge. The batteries soon leaked everywhere.Sometimes keeping it too simple can be just as bad as making it too complicated. Case in point, always use a proper battery charger circuit. Meaning one that keeps within the charge specifications of the battery.

author: Chet T16
author_rank: Frequent Contributor
timestamp: July 21, 2012, 09:36:00 pm
content: As pointed out i have this up and running. It is/was for my brothers setup in the attic at home and i took it as an excuse to design a board and get it made up because aside from the little adapter boards i got done (see one of the mailbag vids for that) i've never made a board, hence the experimentation with uncovered wide tracks and animal silkscreens

author: lk
author_rank: Regular Contributor
timestamp: August 02, 2012, 08:07:27 am
content: Im no expert, but looking at the OP's first post, does that system not constantly draw power from the PSU, or is there some trickery that you can do with the diodes that i dont understand?-lk

author: Chet T16
author_rank: Frequent Contributor
timestamp: August 02, 2012, 08:15:07 am
content: Well assuming both voltages are the same then you would be correct but the diagram in the first post is just part of the circuit. In the actual circuit the PSU is off once the input voltage is high enough

author: vk6zgo
author_rank: Super Contributor
timestamp: August 02, 2012, 12:00:08 pm
content: The diode circuit like that relies on the main power source being normally slightly higher in voltage so the diode is reverse biased & the standby supply cannot supply current to the load.When the main supply fails,the standby can supply current to the load.At least,that's how it was done with the Station 24 volt supplies.