Phædrus2401

Is there a simple and/or inexpensive way to convert 12V direct current (from, for example, a car battery) to 120V alternating current (like you get from a wall outlet) with an output of at least 30W for three hours?

octaneman

Hi Phaedrus2401

Yes there is, pick up an DC-to-AC converter, they are pricey though depending on how many amps you need.

Phædrus2401

Well, I'm looking at this:
http://www.tigerdirect.com/applicati...IN#detailspecs

And then connect one of these:
http://www.radioshack.com/product/in...ductId=2111060

Which at 12V/7200mAh gives 86W, or enough to power my 15W amplifier for 5.75 hours. Factoring in inefficiency from the inverter, I'm looking at about 4-4.5 hours of play time.

Hm, is this battery rechargeable? If not I'll need to make at least $7.50/hr to break even busking.

Now tell me I made a mistake figuring out how long I can play on 7200mAh@12V.

octaneman

Interesting choice, are you planning to plug it into a car ?

Phædrus2401

I wouldn't so much call it a choice as "blundering in the general direction I want to go."

What I was thinking was I'd connect the car battery to the inverter by attaching the positive terminal to the 0V "bump" on the car plug, and the negative terminal to the center (+) pin on the car plug. I'd accomplish this with a spare set of jumper cables I have lying around and some electrical tape.

If you know an easier way to accomplish what I want I'd be very happy to hear it. I'm also not sure how I'd recharge the battery. Since it's a car battery it should be rechargeable, but I'm not sure how I'd go about charging it short of "jump starting" it from my car.



EDIT: Idea. Could you charge a car battery from the +12V output of a computer PSU?

Basementgeek

The ad for the inverter is confusing, to me. It says is 200W, but says it will shut down:

Then it states

At full power it can up 20 AMPS battery power. The battery you looked at is only rated at about 7 AMP Hour. A 20 AMP draw of the car battery would be about the same as leaving you head lights on.

Yes, the battery is rechargeable but don't use a PSU to do it, battery chargers are different. That battery should only be charged at about 1 AMP at most.

You wrote:

The center pin is always the positive (+) side.

Just plug into the cig lighter, run a decent size extension cord to the Amp.

I assume you will take breaks, during you 5+ hours of playing, start the car to charge the battery.

BG

octaneman

I'm suspecting that the direction you want to go is to build yourself a portable mobile amplifier, with it's own internal power source with the added bonus of being hooked up to a cigarette lighter. If that's the case, then your idea is absolutely brilliant! Jamming with your buddies from the comfort of your own vehicle would have you guys playing anytime! anywhere! To build such a setup, would require that your power inverter would have to be hooked up to a heat sink with a fuse large enough to handle the loads and cooling system. Another factor you must consider is that you will need to use light weight material to build its outer shell and what type of sound you want coming from it. There are schematics available online on how to hook up the inverter, but it's location will have to be planned very carefully.

Good luck!

EDIT: Idea. Could you charge a car battery from the +12V output of a computer PSU?[/quote]


No. It violates the second law of thermodynamics which states, that you cannot get more power than what you put in.

Phædrus2401

They do have little mini amplifiers that run off 9V batteries, and I had one, but it broke after I dropped it. I've been thinking about opening it up to see if there's an obvious problem, but haven't gotten around to it. The problem is its maximum volume isn't all that loud, and when you turn up the gain it sounds really muddy, and I don't mean NOLA blues muddy, I mean "could your please stop your chimp from flinging its faecal matter in my general direction," muddy.

I can't run it from my car, Basementgeek, because driving a car through the French Quarter is more or less impossible. I'd have to park at least four blocks away from any decent busking sites, and I'd be worried about the cable getting stepped on or knocked loose, not to mention the risk of someone breaking into the car. What I want is something that I can carry, either in my hand or in a suitcase, onto the ferryboat, across the river, and into the French Quarter that I can just turn it on, plug in my guitar, and play for three or four hours. For that I need a battery or similar power source.

I figure the amp, inverter, and a battery would fit into a small suitcase (which I have) just fine. The problem is finding a battery between 9.5V and 16V capable of providing ~20W of power for several hours. Only thing I can think of that comes close is a 12V car battery, and I'm seeing that there are a lot of problems with that.


So I'm open to ideas. I'd be willing to hack something together from parts if someone can figure out exactly what needs to be done. I've got a 15W amplifier, a broken mini amplifier, a 700w computer PSU I could scavenge for parts, jumper cables, a soldering iron, electrical and duct tape, and access to a RadioShack. Where do I start?

Basementgeek

Do you have access to lawn/garden tractor battery? They should fit in a 5 gallon bucket? That should have enough power, to power the adapter.

I would forget about anything using a PSU, for charging unless you are going to watch like a hawk. It would over charge it I think.

BG

Done_Fishin

late to the party and I saw alot of info being said here that could be taken as misleading..

12V/7.2AH is not what it seems. One would normally discharge the battery down to about 9Volts and then arrange for it to switch off. Problem is that the battery voltage immediately after charging might be anywhere from 13,8 Volts to 14,2 Volts and so the current you will be drawing will ( without inefficiencies ) proportional to battery Voltage.
120V ac means that you would need to create a voltage greater than 120V in order to get the volts you want.

assuming 12V to 120 Volts that means (using rule of thumb) if you want a constant 120 Watts output (One amp or Ampere) you'd need at least 10 Amps from the battery at 12V, about 8,5 Amps if fully Charged (14 Volts) and about 13.5 amps should the battery volts drop to 9 Volts ..

A single car headlamp bulb 60 Watts draws about 5 amps

I repair UPS systems and one of the units I have been repairing a lot of recently gives 230Vac output. It takes the incoming 230Vac and converts it to 800Vdc before recreating a stable 230Vac output. The 800V is actually plus / minus 400V with reference to neutral and / or Battery Ground

It uses 3 batteries of 12V/5AH to create a 36Volt power pack in the event of mains failure and is designed for a 1000VA load.
I usually test at 240 Watts using 4 * 60 Watt light bulbs and from fully charged (42Volts ) to the switch off point at about 25 Volts (approx 8 volts / battery) , I like to see a 20 minute "life". Obviously the more batteries an inverter uses the less current it uses and so the "life " on battery increases. From the same company there is also a 1500VA UPS which has 7 batteries of 12V/ 7,2AH creating almost 100Volts and lasts a lot longer. However it is far more dangerous to work on .. 100Volts DC can do some serious damage to body tissue. DC tends to freeze muscles making you "hold on" whilst AC creates a spasm "flinging you away".

The 1000VA unit weighs an impressive 16Kg's, the 1500VA weighs a lot more and the last 3 times I have fixed it, it came back broken after transport problems.


Most of the guys I see around Athens playing guitars and having Amplifiers seem to use a battery for power. most inverters are only about 80% efficient, similar to power supplies, and you might be better off using a dc supply rather than converting.

A battery can easily be charged and placed on a trolley for transport. Car batteries are a bit more bulkier but are heavier duty. Just take care you don't short the poles ..

when it comes to charging a battery, when empty it will take whatever charge current you are able to give but you should look for instructions about what current you should limit it to. as the battery Voltage increases so the charge current will fall. You should never let a battery charge and boil .. and you should always try to keep it cool. Likewise the Maximum Voltage should be kept withing certain limits .. if you do standby charging a 12 Volt battery should not be allowed to exceed approx 13,8 volts .. however if you want to boost the charge you might find the manufacturer will recommend Voltages of 14 to 15 Volts for short periods of time.

gcavan

Phaedrus - What you have there will work fine. I would suggest, though, you snip that lighter plug off and crimp on terminals to the cord. Also I'ld suggest you use a battery that has screw down terminals.
By the way that battery is rechargeable.

Have you looked at something like this? Most of the power tool companies all have something similar.
http://www.tigerdirect.com/applicati...2576&CatId=237