# AC behavior?

Discussion in 'The Projects Forum' started by Lightfire, Jan 28, 2011.

1. ### Lightfire Thread Starter Well-Known Member

Oct 5, 2010
690
21
Hello,

I know DC can increase and decrease its current and this is my problem.

I want the current of DC will just stay in one current. For example, I had 12 volts battery, if the voltage of battery is less than 12 volts then all the devices operated with it will go off.

Is there any such thing can done it?

P.S. Halogen lamp cannot be use in a series connection?

Anyway, why when I pull out one device on a parallel connection, the rest of devices in the parallel connection will become more stronger... (I mean if it's light, it becomes more brigther...) However, in AC (electric powered by electric company). no matter you add or remove, the power is still the same...

2. ### Wendy Moderator

Mar 24, 2008
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2,533
Each device pulls the current matching the resistance it has. This is a consequence of Ohm's Law. Ohm's Law is probably the first equation any electronics student learns. The standard format taught in text books is E=I/R, where E is the voltage, I is the current, and R is the resistance. Two devices in parallel each pull their own current, so it is additive from the battery (or wall outlet).

Series resistors are a bit more complicated, and the equation used is a bit more complicated. You look at the total resistance the circuit has (which is each resistor added together). This give you the current using Ohm's Law, then figure out how much voltage each one is dropping. This is referred to as a voltage divider.

Volume 1 - Chapter 5: SERIES AND PARALLEL CIRCUITS

3. ### Lightfire Thread Starter Well-Known Member

Oct 5, 2010
690
21
Thank you.

It says that if my both devices are the same (volt, resistance, watt) the current flowing on the connection are the same but the current is already half (12/2=6v). However if my first device consumes more energy, the last device is looks like no more current flowing on it. And, if the last device consumes more energy, the current flowing on both devices is almost the same, 12 volts...

Isn't it...

Anyway,

Any help???

4. ### Kermit2 AAC Fanatic!

Feb 5, 2010
3,760
924
Do the MATH. It will give you the answers and it will be correct.

Voltage = current x resistance.
Current =voltage/resistance
Resistance = voltage/current.

Electric current is like water current. Series circuit is ONE pipe, so all water(current) goes through everything on that ONE pipe. Parallel circuit is more than one pipe, so water(current) goes in many different directions.

Voltage is like water PRESSURE.

5. ### SgtWookie Expert

Jul 17, 2007
22,182
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You don't say what kind of battery you are using. If it's a deep-cycle battery, then discharging to roughly 12v (depending on temperature) then that's about as low as you should go. See my post on the 4th page in the "Tips & Tricks" sticky thread in the "General Electronics Discussion" forum, particularly the spreadsheet.

It sounds like you want a circuit to disconnect your load - and stay disconnected - when the battery voltage (under load) reaches about 12v. Is that correct?

If you had two 6v halogen bulbs with the same wattage rating, you could operate them from 12v. If they are rated for 12v, then you would have to operate them in parallel.

Batteries have internal resistance which is pretty low when they are fully charged, but the resistance increases as the battery becomes discharged. Wiring from the battery to the load will also cause power dissipation and significant loss of voltage to the load if the wiring is of inadequate size (the wire gauge is too small in cross-section). Loose and/or corroded connections will also cause a loss of available power.

6. ### wayneh Expert

Sep 9, 2010
12,081
3,019
This different behavior is ONLY because the power from the electric company has almost zero impedance or "internal resistance". It's an almost infinite supply. It will maintain its voltage while supplying increasing amounts of current and power, until you blow a fuse. All devices connected to it are more-or-less unaffected by the other devices that might be connected.

As you've heard, a battery cannot do that. Voltage at a battery's poles drops as it works harder, even when fully charged. Removing a connected device allows it do do less work overall and provide a slightly higher voltage to any remaining devices. This effect increases as the battery loses charge.

Designing something to be powered by a battery usually requires the designer to either accept the voltage swing between full charge and discharged, or to regulate the voltage down, say from 12v to 5v, so that the circuit continues to operate just the same no matter what is going on with the battery's level of charge.

Dec 26, 2010
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One way to reduce the effect of a varying supply voltage is to use a voltage regulator. Linear regulators are inefficient, and cannot give more output voltage than their input voltage. Switched-mode regulators can be more efficient, and can give an increased output voltage. A regulator should not of course be used to flog the last drop out of the battery, particularly if it uses a technology which does not tolerate deep discharge well.

In addition, it must be recognised that adding a regulator of any kind inevitably wastes some power. It may be better to try to get hold of appliances which are more tolerant of voltage variations. For instance, a halogen lamp varies in brightness quite markedly in response to voltage variations. A competently designed LED based lamp might give more consistent light output for less power input. It probably also contains a kind of switching current regulator...

Oh dear, that last point somewhat spoils my argument, but I might as well point it out before someone else does!

Last edited: Jan 28, 2011
8. ### Lightfire Thread Starter Well-Known Member

Oct 5, 2010
690
21
Let's say that I want a battery that had infinite 12 volts... Is there any battery like that??? P.S. I mean in infinite, it will stay just 12 volts and it looks like in electric company that no matter how many devices is operated in that battery. The volts across in every devices is still 12 volts. Is there any thing like that????

P.S. Battery can make me in trouble. I mean if I had 240 volts battery if I touch the terminal of battery, can I get electric hazzard???????

9. ### Wendy Moderator

Mar 24, 2008
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240V would leave a crispy kid where you used to be, a large chunk of charcoal. Keep it safe, use low voltages.

Car batterys come very close to what you describe, though if it you use it for more than a few minutes it will drain. It will make a large metal wrench glow red just before the explosion, so be safe there too.

Basically yes, you can use a deep current source like a car battery for many devices, but it will discharge. If you discharge it too deeply, it will be damaged.

10. ### Lightfire Thread Starter Well-Known Member

Oct 5, 2010
690
21
I don't understand...

Um, car batteries can make infinite 12 volts even I got a lot of devices operated by it (12 v batter)))???

Sorry for my childish mind...

11. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
Car batteries are designed to output a LOT of current for perhaps 30 seconds to a minute, and then they need to be recharged immediately. If a car battery is used to power devices for a long period of time, they will become damaged internally and will fail.

12. ### spinnaker AAC Fanatic!

Oct 29, 2009
4,866
988

I am 51 years old and I still have a "childish" mind so maybe I can help. .

I think maybe the OP was simply asking if there was a battery that always supplied 12V while it was powering a device.

As far as I know, the answer is no. The closet might be something like a car battery which gets recharged when it is used.

You can also have a power supply like and adapter that plugs into the wall that will supply a constant 12V.

Also looking at one of the OPs threads. I think the OP may have been asking about keeping the current the same or current limiting.

Did I phrase that for you right Lightfire? If so maybe repost your question saying you are using a 12V wall adapter.

I will let a more knowledgeable forum member tackle current limiting and possibly voltage regulation.

13. ### Lightfire Thread Starter Well-Known Member

Oct 5, 2010
690
21
Huh??? No. no. no. I'm not using wal adapter. Just battery. I mean battery which is 12 v but not the battery used in car...

14. ### spinnaker AAC Fanatic!

Oct 29, 2009
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I know but your question was if there was a battery that had "infinite 12 volts" even after it was being used correct? The wall adapter was just used as an example as an "infinite" 12V source. But even it is not really "infinite" if the power company no longer supplies power.

When you connect something to the battery the voltage will eventually drop unless the battery is recharged.

Sorry it sounded like you had another question that had to do with your "infinite" 12V battery. Like what would happen to the current.

Again it does not exist as far as I know. A battery mounted in a car would only supply "infinite" 12V as long as the car was running. This would be the closest to an "infinite" 12 V battery. Or maybe one connected to a solar panel.

15. ### Lightfire Thread Starter Well-Known Member

Oct 5, 2010
690
21
No, I don't mean 'after it was used'. What I mean in infinite, let's say we have a 12 v battery. Even though three devices are being operated in battery, every devices had the same voltage and it is 12 v. Because sometimes, when you add more devices in a battery that is already supplying volt in other devices, the volts in battery is becoming low that's why the voltage accrosing in all devices is not 12 v anymore... I mean ALWAYS, not sometimes... I GUESS....

Dec 26, 2010
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It is not possible to make a battery with a perfectly constant output voltage. The changing output voltage is generally a disadvantage, but may be of use to indicate how much charge is left in the battery. For some rechargeable batteries, a particular voltage is used as a warning threshold to avoid discharging too far.

To obtain a fairly constant voltage, a battery should have a relatively large capacity in relation to its maximum load current. Excessive load current relative to the Ampere-Hour capacity results in more voltage drop in the internal resistance, and the effective capacity will be reduced. The bottom line is that a battery is never a perfect voltage source. Correctly specifying the type of battery for its application may help, but if a really stable voltage is necessary some form of regulator will be required.

When learning about electronics, one of the things we need to accept is that "perfect" devices are generally not available. In engineering, much work has to be put into obtaining acceptable results despite the non-ideal performance of the components.

Last edited: Jan 29, 2011
17. ### Kermit2 AAC Fanatic!

Feb 5, 2010
3,760
924

The four on the left each weigh 124 lbs. total is 500 lbs!!

Quarter Ton of Deep Cycle Lead Acid Battery. As close as most home hobbiest will ever get to 'infinite' 12V DC.

Infinite 12V DC costs close to infinite \$\$\$. You have been warned and informed of why most people settle for something less than 'infinite'.

18. ### Lightfire Thread Starter Well-Known Member

Oct 5, 2010
690
21
Did I say something about perfect? Did my 'infinite' word says that it will never end? Did I say that if there is no power anymore, the power is still there and it's 'INFINITE'?

What I mean in infinite, INFINITE 12 VOLTS. THAT'S NOT MEANT, IT WILL NEVER STOP!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!1

P.S. Not only in electronics, in every aspect of our livings...
I've not been warned or informed... Also, infinite 12 v dc costs infinite bucks? How I hope I'm the only one son of the electric company CEO AND PRESIDENT AND OWNER there in my country so I could have infinite 12 v dc.

P.S. The thread is going to be flamed thread, I think it should be closed then. I suppose...

ENOUGH ENOUGH ENOUGH

19. ### wayneh Expert

Sep 9, 2010
12,081
3,019
A battery drops voltage as it discharges, and as the amount of current drawn from it increases. A big battery drops less. A huge battery drops still less. An infinite battery doesn't exist and cannot be afforded.

What more needs to be said?

20. ### kubeek AAC Fanatic!

Sep 20, 2005
4,669
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I am not sure if internal resistance of the battery has been properly mentioned, so just to be sure.
Every nonideal voltage source has internal resistance, which makes the difference between the "infinite mains power" and OP´s battery, apart from discharging.