El-cheapo AC inverter

[erick (Eric)]

Wanted to run my lappie off 12V for the lowest cost possible - eeeek!

Bought a 150W 230V inverter from Jaycar (MI-5121) for $39.95. Had done some calcs based on my lappie powerbrick specs and figured it might just do.

It works!

Q1. Haven't put a scope on the output yet, but the powerbrick makes a fair buzzing sound when plugged into that inverter. I guess the waveform is anything but a sine wave? It ran for a few hours and the powerbrick didn't get particularly warm, so I'm hoping it will survive?

Q2. I got the lappie down to the usual condition it will be in, while running Stellarium in the field - screen brightness minimum, hard disk spun down, only two USB "devices", a trackball and a light for the keyboard. At that level the current drain on a 12V battery was around 3.5A. I thought it would be less. Anyway, how long can I draw 3.5A from my humble car battery before I'll be pushing the car home?

Thanks guys.

[acropolite (Phil)]

Eric, you are right in assuming it's not a sine wave. Modified sine wave inverters vary the duty cycle of a square wave to achieve voltage regulation, they really shouldn't even be called modified sine wave. Most plugpacks will run happily from modified sine wave type inverters.

Your average car battery will be around 40-60 amp hours, I wouldn't discharge any more than 30% which would be around 13-20 amp hours, roughly 4-6 hours should be OK if your battery is in good condition.

Basically car batteries aren't designed to be discharged significantly, they're really designed to supply high currents for short periods (hundreds of amps for seconds only) and not for lengthy discharge.

[h0ughy (David)]

get the MP3472 converter DC/DC for the laptop from Jaycar and lose the inverter. it connects to the laptop and you select the required voltage on the back of the unit. I have two of these - does up to 6 amps 17-24 volts from 12V

[erick (Eric)]

Thanks Phil. Good news (but park my manual car facing down hill, just in case!)

[erick (Eric)]

Originally Posted by h0ughy

get the MP3472 converter DC/DC for the laptop from Jaycar.....

$20 bucks more. Probably consume much the same current from the battery. I lose the protection that would be in my IBM powerbrick and probably not in this supply. I'll stick with the inverter and see how it goes. (But maybe in a few months, Dave, you can say "I told you so" )

ps. I think IBM are 16V 4.5A

[Starkler (Geoff)]

Originally Posted by erick

$20 bucks more. Probably consume much the same current from the battery.

I expect not. Using an inverter you are adding an extra conversion stage where energy efficiency is lost.

12v > 240v > laptop voltage.

Vs

12v > laptop voltage

FWIW Last year at Snake Valley I flattened my car battery over 2 nights just running my scope from the inverter via a 12v power supply, and about 3 hrs of laptop usage, to the point where I didnt have enough juice to crank over the diesel.

[erick (Eric)]

Originally Posted by Starkler

I expect not. Using an inverter you are adding an extra conversion stage where energy efficiency is lost.

12v > 240v > laptop voltage.

Vs

12v > laptop voltage

FWIW Last year at Snake Valley I flattened my car battery over 2 nights just running my scope from the inverter via a 12v power supply, and about 3 hrs of laptop usage, to the point where I didnt have enough juice to crank over the diesel.

I agree it will consumes more current from the battery, but I wouldn't expect it to be that much more - I reckon half an amp at most. We can test at camp if anyone has a DC/DC converter - I have a 12V gel battery with a 10A meter in series for monitoring my dew heater consumption.

This is it - I only have to run the lappie - at minimal use (just Stellarium ticking over with a dull screen). If I had to power anything else - eg. dew heating, mount - I'd think twice about using my car battery.

[acropolite (Phil)]

FWIW an inverter like that should run about 80% and upwards efficiency. That's without any losses the from conversion back to DC from the laptop plugpack.

[digby dart]

erick "...but park my manual car facing down hill, just in case!" ...