Simple power switching circuit - Page 1
Forum section: electronics / beginners

author: Harvi007
author_rank: Newbie
timestamp: December 01, 2015, 10:04:37 am
content: Hello all,I'm new here and have very basic knowledge on electronics.I was wondering if anyone could help me with a simple power switching circuit I'm thinking about?I have a Apple TV that I've converted to take 12v from the car and drop it down to 3.4v 1.5a (found a circuit on ebay to do this which takes 40v to 5v input).Currently the device only powers up when the car is started. It takes a bit of time to boot up and make it's connections as it has to turn on from a power off and not from standby.I wanted to use a UPS but I thought this may be a bit excessive.So I went out and brought a backup battery pack, it was cheap due to the sales. It has it's own input for charging and 2 output at 5v (2a and 1a).I was thinking of using a relay SPDT to switch between the 2 power sources (car and battery pack)BUT... there would be a loss of power when the switch occurred.Right now I'm not sure how quick the switch will be and will there be enough power in the Apple TV circuit to compensate for no input while the switch takes place.I had a look around on the web and found that if I used a capacitor as a buffer, this may compensate for the second or 2 of power loss when the relay kicks in.I suppose my questions are, Is this a viable and working solution? and if so, how would I incorporate a capacitor to aid with the power buffer?Your help would be much appreciated.Thank you,

author: tron9000
author_rank: Frequent Contributor
timestamp: December 01, 2015, 10:36:28 am
content: Quote from: Harvi007 on December 01, 2015, 10:04:37 amCould you not change it to come on when the ignition is in the first or second positions?

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: December 01, 2015, 12:05:53 pm
content: I don't know the power draw an AppleTV pulls during startup, but even if it's low you tend to need a bit of capacitance to buffer the relay time (numbers can help us here).My first sollution is by just stopping the problem from occuring in the first place: Why not just power on the device manually once the car is running? Just a simple switch in series with the 12V line (or heck, even just unplug it and plug it in) would do the trick.

author: moya034
author_rank: Regular Contributor
timestamp: December 01, 2015, 12:11:15 pm
content: Your car already has a DC UPS built in side of it. It's the car battery and alternator. Just do like radio operators do, run a separate fuse and power wires directly to the battery posts in the engine bay. You can usually find a rubber grommet to slide the wires through without having to drill holes through the firewall.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: December 01, 2015, 12:23:25 pm
content: Quote from: moya034 on December 01, 2015, 12:11:15 pmThat does bring with it the disadvantage that his appleTV will always be draining batteries. I know a car battery has a huge capacity compared to the current drain, but still, forget to unplug it once over a long weekend and you might be in for an unpleasant suprise.

author: Harvi007
author_rank: Newbie
timestamp: December 01, 2015, 12:38:03 pm
content: Hi,Thank you for your replies, I was thinking about powering it directly from the car battery, but I fear that it may drain it?I'm not sure of the calculations, but I know the device runs on 3.4v with a max of 1.75amps. When in use, it pulls 2 watts and when in standby it uses .5 of a watt.I believe the car battery to be 12v and approx 40amps.I'm using my car 6 out of 7 days, and I'd say it's turned off for 156 hours a week which would mean that the device is sat in standby all that time.Will the car battery be able to handle this without draining out on me?Thank you,

author: moya034
author_rank: Regular Contributor
timestamp: December 01, 2015, 12:57:03 pm
content: I'd just put a good old fashioned toggle switch in there if it will be using that much current. Or if you want to get fancy, rig up a relay and 555 timer that wont switch it on until the car is fully started.

author: Harvi007
author_rank: Newbie
timestamp: December 01, 2015, 01:57:45 pm
content: Hi,Thank you for the reply, I don't think I made it clear enough on what I'm looking to achieve, sorry.I'd like to keep the apple tv on all the time, in use or in standby, but when the car is on, to draw power from it and when off, to draw power from the backup battery pack (this will charge up when the car is on).I was looking to use a relay to switch between the 2 power sources, but at the point of the switch, there will be a period when the connection is broken.I'm not sure how quick the relay will flip and if this will affect the operation of the apple tv, but I was thinking of using a capacitor in the circuit as a buffer so that when the relay flips, during the period of no contact, the apple tv will draw its power from the capacitor.I assume the flip will be in ms, but even if it takes 2sec's, I was hoping the Cap will provide the power required and thus being uninterrupted?I could just be making all this up as I'm no expert, but that's the theory.The end result being the apple tv constantly powered, but in standby when not in use, and when the engine is on to draw its power from the car, and when the engine is off, to draw from my backup battery pack.I hope this has cleared up any misunderstandings.Your help will be greatly appreciated.Thank you,

author: moya034
author_rank: Regular Contributor
timestamp: December 01, 2015, 02:10:00 pm
content: Oh OK, I get it now. Switching between two power sources will be a pain. So what I would do is run it off the separate battery all the time, and rig up a way to charge the 2nd battery battery when the engine is on. They do this in RV's... not sure what it's called, something like a "battery isolator" which lets you hook up a separate battery to your vehicle's electrical system, without worry of draining your main battery..

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: December 01, 2015, 02:12:37 pm
content: I believe something very similar has been posted in the projects page a few weeks ago, although they didn't use any mechanical relays but instead worked with a Pchannel mosfet. I myself have at one point done a similar setup (which worked - 24V at 10 amps IIRC) using that schematic they provided. Let me have a look if I can find it.

author: moya034
author_rank: Regular Contributor
timestamp: December 01, 2015, 02:14:20 pm
content: Example of an off the shelf device to do this:http://www.amazon.com/NOCO-IGD140HP-140-Battery-Isolator/dp/B001DKRF2M

author: Cliff Matthews
author_rank: Unknown
timestamp: December 01, 2015, 02:42:37 pm
content: The relay flip is just milliseconds, but with that small of a current draw you can save using a relay and extra battery. EG: Full shutdown could be done at ~3-days with a 555 timer feeding a CD4020 17-stage counter chip and a transistor to trip Dave's simple switch circuit to the off state. If my math is OK, 555 period of 7.91 secs/0.126hz (R1-220k, R2-10k and 47uf cap).

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: December 01, 2015, 02:52:53 pm
content: Couldn't find the thread, so here is the "core" of the schematic I used a year ago. We also used an optocoupler (but you can do without) to signal a control system when we went to battery-backup mode. This could be implemented in your situation as well to signal the appletv to go into standby?