Replaced 5a smd fuse in laptop

[b0b69]

i had problem with my laptop not powering up i used a continuity checker and discovered the fuse behind the dc-jack was blow-en it said fs5a on top so ordered a replacement and replaced it but as soon as i plugged in the ac adapter it blow the fuse straight away. i dont think its the ac adapter as its putting out the right voltage was hoping someone could point me in the right direction as to why it would be blowing fuses

thanks for any help you can give

 

[AntiZig]

most likely there's a fault somewhere on the motherboard circuitry

 

[b0b69]

thanks for the quick reply, thought that myself dont think its a short maybe a voltage regulator or something similar, not to sure what that would look like though or how test it

 

[b0b69]

update put a bit of 5amp fuse wire across the fuse, now when i plug in the ac adapter it drops the voltage going into the jack from 18.9v to 3.9 volts so charger must be detecting bad power and dropping voltage to prevent damage

 

[mavroxur]

I'm going to bet that you've got a fault on the board somewhere. What's the make / model of the laptop?

 

[b0b69]

fujitsu v5535

 

[RetiredChief]

AC adaptor didn't drop voltage to "prevent damage"; you just exceeded its current capabilities. Had it not dropped you would have, in fact, performed what is referred to as a "Smoke" test.

Try doing a resistance check from the fuse to ground (Also reverse leads and recheck).
Two possibilities other than the voltage regulator would be a Zener diode to prevent over voltage input or a released biased diode to prevent reverse voltage from damaging the MB. If either of these components is used then they could have shorted out, indicated by identical forward/reversed resistance being the same.

Note: to varify the ac adaptor you need to measure the voltage under load. Open circuit measures can be OK, but when the normal load is applied the ac adaptor will fail a voltage test. Load resistor for load testing should be around 5 -> 6 ohms.

 

[b0b69]

thanks for the reply chief im a bit of a noob at this do i use the continuity setting on the multimeter if so there is no continuity between ground and the fuse. ive also tried my universal charger it does the same thing drops to 3.9v and the light on the charging block goes dim

 

[b0b69]

also not to sure what a zener diode looks like i goggled it there is it a small black square thing with 2 connectors on one side and 1 on the other it looks a bit like the ones ya get in the xbox 360 ive tested those before but had to remove them from the board to test is this correct

 

[frozenlead]

Load resistor for load testing should be around 5 -> 6 ohms.

AND the resistor must be capable of dissipating the expected amount of power. Don't try hooking up a measly 1/4 W resistor or something here.

If you used a different power supply and it does the same thing, though, it's highly likely it's the board anyway.

Did you check both sides of the fuse for continuity?

I wouldn't bother with the diode, really. I'd replace the board if you're confident both your supplies are good. You can do the resistor test on them, which should be pretty fool-proof.

 

[RetiredChief]

You use the ohms setting. Depending on the meter different results can be obtained.
The reason for this is really above many, but just incase. A meter uses voltage to (internal battery) when measuring resistance. In some cases the voltage is high enough to break down a "PN" junction (ie a Diode is a PN junction device).

The ohms measurement should be from the Positive side (Fuse) to the point that the negative (Return or ground) attaches from the adaptor. Remove the Laptop batter. There should be a noticeable differnce between measurement 1 and measurement 2 (revearsed meter leads).

frozenlead is correct - 1/4 Watt resistor would go POOF. Requires a rather large (understated LOL) Load resistor that is NOT commonly available

 

[b0b69]

[/URL][/img]

i tested thes three components for continuity and on 2 of them i get continuity one way only and on one i get resistance both ways do ya think that could be it?

thanks chief for help much appreciated

 

[b0b69]

ok used the ohms setting from fuse to dc jack ground i get 170 one way and 140 with leads swapped

 

[RetiredChief]

The resistance seems low to me and 170 vs 140 seams to close, I would have expected a higher difference. With out a schematice or physically looking at the board it is hard for me to say.

I tried to check my laptop. I cut the connector off of a AC adaptor and plugged it into my jack. measured the resistance. The readings I got were > 100k but inconclusive.
The readings were going up one way (like a capacitor charging) and when the leads were revearsed te Ohms reading was negative (No such thing as Neg resistance). This indicates that a capacitor is across the input. Read one way and the reading slowly increases, reverse leads and the capacitor discharges into the meter giving a false neg value.