CurtPalme.com Home Theater Forum

[wallace123456]

Hey, I have a bunch of various power supplies for laptop computers.

Problem is, I don't know if they work or not.

Any suggestions or ideas on how to test them?

Thanks.

wallace

[garyfritz]

The power supply box should have the output specs written on it -- e.g. 18V 400mA or something like that. I wouldn't worry about the current rating. If the voltage is OK, the PS is probably OK. Just stick a voltmeter across the terminals and see if it's delivering the voltage it's supposed to.

If you want a slightly more comprehensive test, you could put about a 220 obm resistor across the terminals for a brief test. E = IR so I = E / R. For an 18V supply a 220ohm resistor should produce a current flow of 18 / 220 = 81mA. You could just measure the voltage across the resistor and make sure there's 18V there, so you know it can deliver current to a small load. If you want to get fancy you could put a current meter in line with the resistor to measure the current. But I wouldn't bother. If the resistor is 220ohms and it has 18V across it, it's got 81mA flowing through it.

[Curt Palme]

No you can't run a power supply without a load, it almost always will measure fine. It's under load where the ripple kicks in, and the voltage drops as a result. The same crap capacitors that are used in computer monitor internal power supplies are also used in the laptop external supplies, and it's usually worse because they are completely encased, and have you ever felt how hot they run?

. Shame on you!

Definitely put a load on it. Your ohms law advice is bang on though.

Before Christmas I had a beat up MP3 boombox in. It was completely dead, and a POS to boot. Turns out the screws that held it together were under the speaker grille mesh, so I destroyed the box trying to open it. I thought it was a snap together thing.

Anyways, I broke it open, and couldn't see anything wrong with it. I measured the incoming 12 volts under load of the boom box, and it measured 6 volts. Eying the external power supply that had measured 12 volts without a load, I swapped it out with a new one. Bingo, the now destroyed boom box worked fine. Dammit!

[Curt Palme]

No you can't run a power supply without a load, it almost always will measure fine. It's under load where the ripple kicks in, and the voltage drops as a result. The same crap capacitors that are used in computer monitor internal power supplies are also used in the laptop external supplies, and it's usually worse because they are completely encased, and have you ever felt how hot they run?

. Shame on you!

Definitely put a load on it. Your ohms law advice is bang on though.

Before Christmas I had a beat up MP3 boombox in. It was completely dead, and a POS to boot. Turns out the screws that held it together were under the speaker grille mesh, so I destroyed the box trying to open it. I thought it was a snap together thing.

Anyways, I broke it open, and couldn't see anything wrong with it. I measured the incoming 12 volts under load of the boom box, and it measured 6 volts. Eying the external power supply that had measured 12 volts without a load, I swapped it out with a new one. Bingo, the now destroyed boom box worked fine. Dammit!

[wallace123456]

Ohms Law. My head is hurting thinking about learning this 36 years ago when I went to Central Office Equipment Maintenance and Installation school.

Thanks. I'll need to see what I can do.

wallace