Batteries and Voltage Regulators PLEASE HELP! - Page 1
Forum section: electronics / beginners

author: Mint.
author_rank: Frequent Contributor
timestamp: August 05, 2012, 08:41:55 am
content: I have a few questions on batteries and voltage regulators, so I would appreciate any kind of help.1. Is it a good idea to hook up two 9v batteries to a 7812 to drop the 18 volts? My circuit will be using 170mA maximum for a short period of 10 seconds. Otherwise in standby it will use 20mA and most of the time it will be turned off by a switch.2. How long will my batteries last and should I get somekind of special 9v? I will have access to the battery compartment to change them, but I'd prefer they would last as long as possible.3. How do I connect my switch, should I use a DPST to cut off both terminals, or just use a normal SPST?4. The 12v powers a relay, will the batteries current be enough to turn on the relay?5. Any other tips would be much appreciatedCheers!

author: T4P
author_rank: Super Contributor
timestamp: August 05, 2012, 08:45:12 am
content: 170mA? ... Good luck being able to draw that much ... or if possible even last 2 hours ...Use a DPST switch yes, the batteries? Again it will dissapear in a short while

author: Mint.
author_rank: Frequent Contributor
timestamp: August 05, 2012, 10:21:11 am
content: Quote from: T4P on August 05, 2012, 08:45:12 amThats why I asked!Well its only going to be used a max of 5 or 6 times a day.Just did some calculations and here is what I got:40 mA relay with 555 timer, circuit from relay is an astable multivibrator, 35mA in low mode, 75 mA in high mode.2 circuits with 40mA + 2 circuits drawing 75 mA during high = 230mA maxSuggestions would be much appreciated.

author: Mint.
author_rank: Frequent Contributor
timestamp: August 05, 2012, 11:52:00 am
content: Also forgot to ask, I'll be running the circuit power from the relay through 20 meters of wire, will the voltage drop be significant to impact the operation of my circuit at the other end?

author: M. András
author_rank: Super Contributor
timestamp: August 05, 2012, 12:58:29 pm
content: why waste that amount of power to drop 6 volts of voltage?. use aa or d cells to get few volts above to use that voltage regulator, and you have much longer battery life, 9 volts one are good volt nothing

author: mariush
author_rank: Super Contributor
timestamp: August 05, 2012, 08:55:39 pm
content: Why don't you generate the 12v locally from a boost converter? Shouldn't be that hard to generate 12v @ 200mA when needed.Then you could use 4 x 1.5v Ni-Mh batteries giving you 5.5-6.5v at high mAh, and you can use a low voltage drop to get stable 5v for your components... something like this springs to mind :http://www.micrel.com/_PDF/mic2940.pdf(up to 0.35v @ 1A)For the 12v you can probably use something like this :http://www.ti.com/lit/ds/symlink/mc33063a.pdf... just not sure if it can do 200mA @ 12v from 5v. Search for other ICs if that's the case, I'm sure there are out there.

author: Mint.
author_rank: Frequent Contributor
timestamp: August 05, 2012, 09:09:33 pm
content: Quote from: M. András on August 05, 2012, 12:58:29 pmCircuit will be running for 20secs max at a time, I do not think that's enough to make the thing hot. Also the 555 has a max rating of 16 volts, therefore I cannot use the 18v from batteries.

author: Mint.
author_rank: Frequent Contributor
timestamp: August 06, 2012, 07:08:15 am
content: Bumping, really need some advice on this.

author: T4P
author_rank: Super Contributor
timestamp: August 06, 2012, 08:32:08 am
content: Quote from: Mint. on August 05, 2012, 09:09:33 pmNo. energy is a precious commodity with batteries, heat we can deal with but extra draw is pointless

author: Things
author_rank: Regular Contributor
timestamp: August 06, 2012, 09:01:31 am
content: Is the only reason you're aiming for 12V is because of the relay coil?If so, they will often latch at a significantly lower voltage. I've had 12V relays even latch down to 6V.Keep in mind that running them under voltage means they will pull less current and thus may have issues latching, however this will more than certainly be more "efficient" than using a 7812 and 2 9Vs in series.A 555 will run fine from 9V, and a 12V relay probably will too, so you can parallel the 2 9V's and ditch the 12Vreg.

author: David_AVD
author_rank: Super Contributor
timestamp: August 06, 2012, 09:44:10 am
content: Quote from: Things on August 06, 2012, 09:01:31 amWhat the??? A relay coil has a fixed resistance value. If you run it under voltage it will drawlesscurrent.

author: Things
author_rank: Regular Contributor
timestamp: August 06, 2012, 09:50:39 am
content: Oops, yes, that's what happens to your brain after no sleep for a few days

author: Mint.
author_rank: Frequent Contributor
timestamp: August 06, 2012, 10:39:11 am
content: Ok here is how it is set up currently on the perfboard.We got 12v input rails, the rails also determine the relay voltage output.The voltage rails power the 555 timer.I am really lost now, what should I do?

author: Things
author_rank: Regular Contributor
timestamp: August 06, 2012, 10:41:13 am
content: What do you mean the rails determine the relay voltage output? Relays are simple on-off switches, they have no voltage output.Maybe if you post a schematic of your circuit people could give more relevant replies.Dan

author: Mint.
author_rank: Frequent Contributor
timestamp: August 06, 2012, 11:18:19 am
content: Quote from: Things on August 06, 2012, 10:41:13 amAhh right, sorry. I've attached the schematic now.

author: qno
author_rank: Frequent Contributor
timestamp: August 06, 2012, 11:41:37 am
content: You can use a mosfet or a transistor to switch the second 555 on or off.

author: Mint.
author_rank: Frequent Contributor
timestamp: August 07, 2012, 06:04:18 am
content: Ahh screw it, I'm just going to do this. If this is a mistake, at least I'll learn from it