Dummy load for testing laptop chargers

[JonathanAnon]

Hi guys, I'm sure I'm missing something obvious here, but I've gone through quite a few videos on testing laptop chargers But none seem to suggest an accessible dummy load to test them... like the way that they normally suggest you use car bulbs to test 12V rails.

They all seem happy enough just to connect the red to the inner pin and the black to the outer pin, and take that as the result, without testing under load. Anything about the house I could use as a dummy load for ~19V 4.24A charger?

[b700029]

Re: Dummy load for testing laptop chargers

2 12V lamps in series, of the appropriate wattage.

[mariushm]

Re: Dummy load for testing laptop chargers

You could try to build yourself a few of these :

http://www.sleepyrobot.com/?p=136

Schematics and everything under the image. Each should do about 1-1.2 A of load as far as I can tell from the forum posts on eevblog.

It's based on this video : http://www.youtube.com/watch?v=8xX2SVcItOA

another dummy load here: http://forums.adafruit.com/viewtopic.php?f=44&t=28487

[budm]

Re: Dummy load for testing laptop chargers

You need 5 Ohms 80 Watts resistor to about full load. So if you do not have any resistor like that in your house, you can use two 100 watts home light bulbs in parallel, resistance will go up a bit when the filaments heat up, you just have to monitor the voltage and current draw to see if your power supply can handle it, just add more lamps as needed. I use this setup for dummy load. The lamp resistance can rise as much as 10~15 times the cold resistance when lamp is at full voltage.

[JonathanAnon]

Re: Dummy load for testing laptop chargers

Yeah, I was thinking about the two 12v in series, just thought there might be something easier.. I soldered the two bulbs together in series and it did the job alright. I'll use it in future. Appreciate the other suggestions guys..

[JonathanAnon]

Re: Dummy load for testing laptop chargers

Was down at the electrical wholesalers today for RJ45 ends, and I asked the guy did he have any bulbs that were 25V... He gave me a 25V / 60W bulb for a couple of euros, so I have my 25V source which will pull 2.4A ..... I'm gonna buy one of these to house it (instead of having to solder wires to the bulb itself..



Then I just need to work out some way of attaching the different jack sizes, as the different laptop adapters have different size DC coax ends.

[JonathanAnon]

Re: Dummy load for testing laptop chargers

Okay guys here's what I've got so far:



I've drilled two 4mm holes in the 5A connector so that you can safely check the voltage from the adapter.

The idea is that you grip the outer connector of the dc coax connection (usually the ground), and fix the brown wire to the centre pin (usually hot). Another alternative would be to buy the different coax cords and connect the appropriate one for your particular laptop to the 5A connector box.

[budm]

Re: Dummy load for testing laptop chargers

Did you check the cold resistance of that 25V (odd voltage rating) 60WATTS bulb?

[JonathanAnon]

Re: Dummy load for testing laptop chargers

Did you check the cold resistance of that 25V (odd voltage rating) 60WATTS bulb?

Why would that be important Bud?... It's 25V and 60 Watt so I would assume
that it draws 2.4amps and the resistance is ~10 Ohms..

It's a special bulb.. I asked them in the electrical wholesaler if they had a 20V bulb and that was the closest they had..

[budm]

Re: Dummy load for testing laptop chargers

Just want to make sure it is not someting like 1 Ohms when cold which can cause the power supply to go into over current shutdown. Its resistance when fully hot 25v at 60 Watts is about 10 Ohms, but when cold it can be 10 time LESS. Just check it with your Ohm meter, light bulb has positive resistance characteristic.

[JonathanAnon]

Re: Dummy load for testing laptop chargers

hmm, The resistance IS 1 Ohms when cold... But I havent had a charger that did not work with it... Is it possible that the thermistor in the SMPS is counteracting the possibility of overcurrent?

[budm]

Re: Dummy load for testing laptop chargers

That is what I thought, the cold to hot can be 10 times.
It can be that some power supply is better design with lots of head room, or bad designed with slow response to over current without going into current limited mode.