What happens if two DC power sources are plugged into a device in parallel? - Page 1
Forum section: electronics / beginners

author: monster_of_electronics
author_rank: Contributor
timestamp: April 16, 2015, 09:19:27 pm
content: I have a radio my friend wants to make battery powered. It normally uses a 6v/1A DC wall power supply via a barrel jack at the back of the unit. Because of the fairly low power consumption, there is enough room inside the unit to add an internal ni-mh battery pack w/regulator circuit. I planned to add a switch to turn on/off battery power when the wall power supply is to be plugged in.My only concern is this - what if someone plugs in the wall power supply while the battery pack is still connected (switch is in the "on" position)?

author: Anand
author_rank: Contributor
timestamp: April 16, 2015, 09:23:25 pm
content: Boom.

author: Red Squirrel
author_rank: Super Contributor
timestamp: April 16, 2015, 09:26:38 pm
content: Depends on the adapter's voltage. I'm not familiar with lithium ion charging but if you were to use lead acid it would charge the battery if you provide a float voltage. float voltage is the voltage at which the battery can be happy with continuously and it will simply absorb current it needs. Typically this is 2.25v per cell (1 cell is 2 volts). If you can manage to put a 6 volt lead acid battery in there and find a 6.75v DC adapter you'd be set. Or you use a 9v adapter and have a regulator circuit that keeps the battery in float then radio runs off that too.Not sure where you'd find a small enough lead acid battery though. I'm sure they make some the size of D cells, that would probably work nicely. Someone with experience with lithium ion charging proceedure may be able to help better though and you can stick with those.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: April 16, 2015, 09:27:52 pm
content: Electricity will flow from the higher voltage supply to the lower voltage supply, with a current equal to the voltage difference divided by the sum of the two sources' ESR. The higher voltage source, as long as the extra current stays within its limit, won't be affected. The lower voltage source will all of a sudden have a bunch of current dumped into it. What happens then depends on the design of the lower voltage source, and what it does with a bunch of incoming current. It might just shut off, or in the case of non-rechargeable batteries, the previous poster said it best:Quote from: Anand

author: Rick Law
author_rank: Super Contributor
timestamp: April 16, 2015, 10:20:45 pm
content: Use a 1p2t or 2p2t switch. 2p2t=2 pole 2 throwSo, the switch you plan to add, instead of ON-OFF, get a 2pt2. 2t is like an A/B switch. The circuit is connected to either A side or B side.1p with you switch the V+ only. Use 2p if you want to switch both +V and Ground.Rick

author: SL4P
author_rank: Super Contributor
timestamp: April 17, 2015, 12:45:04 am
content: A diode and current limiting resistor.Think of it as always running on the battery - and when external supply is connected - it is charging the battery.

author: DavidDLC
author_rank: Frequent Contributor
timestamp: April 17, 2015, 01:11:02 am
content: Or adapt a barrel jack with internal switch to automatically disconnect the battery when a plug is insertedDavid.

author: timb
author_rank: Super Contributor
timestamp: April 17, 2015, 07:25:32 pm
content: Quote from: DavidDLC on April 17, 2015, 01:11:02 amThis is the best answer.The alternative is to use a pair of diodes coming from the battery and the DC jack. This will create an OR switch and only allow the supply with the highest voltage through.Sent from my Smartphone

author: Jeroen3
author_rank: Super Contributor
timestamp: April 17, 2015, 08:07:19 pm
content: Look herehttp://www.linear.com/products/PowerPath_Controllers_*_Ideal_Diodes for more high-tech solutions.