Battery currents. - Page 1
Forum section: electronics / beginners

author: Mint.
author_rank: Frequent Contributor
timestamp: August 12, 2012, 09:01:16 am
content: I have built a circuit that utilises a 7812 and two 9V batteries to power a larger circuit with two relays and four 555 timers. The highest current consumption that the circuit needs is about 200mA, but it is only used in very short bursts of 15 seconds with high and low states, otherwise it is in standby mode which consumes 50mA. I also will probably keep the circuit fully off by a DPST switch when I am not using it. I have two 9V batteries in series which are old and probably are on their last leg, but I used them anyways as test batteries for the circuit and they seem to be giving off too little current to switch on my relays. How do I chose a battery that will be able to switch on the relays?Here are some of the results:Standby:43.6mAOne relay activated:67.1mATwo relays activated:90mARelay activated 555 multivibratorHigh: 120mA Low: 88.8mABoth relays activated and with two 555 multivibrators runningHigh: 194.2mA Low: 162.3mAFrequency is around 1Hz

author: Mint.
author_rank: Frequent Contributor
timestamp: August 12, 2012, 09:04:53 am
content: Here is the circuit. The 7812 is not shown in this schematic. There are to identical copies of the circuit. One simply is connected to the other and they share the same voltage rails.

author: Astroplio
author_rank: Regular Contributor
timestamp: August 12, 2012, 09:40:42 am
content: When you have the highest current draw, at what voltage level is the Vcc dropping?I believe, since your batteries are old, it drops below or goes very near the required voltage for the regulator to work under spec.From my experience, 9V batteries don't mix well with voltage regulators, unless you use them with LDO 3.3V perhaps, but that's an assumption.I had used two 9V in parallel and a 6V regulator to get sufficient power output when I had built this DIY line follower robot:http://myrobots.gr/robots.html("babycloud") Otherwise it wouldn't have worked as expected.Of course now I know better, and I would have used a AA battery pack, which is also my recommendation for your project, since it will make it more reliable in the long run.George :-)

author: Mint.
author_rank: Frequent Contributor
timestamp: August 12, 2012, 09:50:34 am
content: Quote from: Astroplio on August 12, 2012, 09:40:42 amRight! Thanks for suggesting to check the voltage. Its dropping down real bad, to 10v at first and then 8v in operation D:What do I do?!

author: Monkeh
author_rank: Super Contributor
timestamp: August 12, 2012, 10:42:21 am
content: For the love of..Here, have some compression.

author: grumpydoc
author_rank: Super Contributor
timestamp: August 12, 2012, 11:06:48 am
content: QuoteBuy fresh batteries??

author: michaelym
author_rank: Regular Contributor
timestamp: August 12, 2012, 12:31:49 pm
content: The battery's series resistance increases with discharge. So even if the battery's measure 7v each and you have 14v in series, when you draw current the voltage drops. If the resistance is R and the current is I the remaining voltage is 14V-R*I

author: TerminalJack505
author_rank: Super Contributor
timestamp: August 12, 2012, 05:27:13 pm
content: Here's an idea. Buy one of those (for example) 8 AA battery holders that has a 9V battery snap connector.You can probably ditch the 7812 and just run your 12V rail directly from the battery pack. You'll get a lot more juice from the AAs too.

author: michaelym
author_rank: Regular Contributor
timestamp: August 12, 2012, 07:01:24 pm
content: You can simply use an old charger. I have a whole box of them.