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Puzzled by Laptop power supply signal circuit

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bigal_scorpio

Active Member
Hi to all,

I have a HP laptop that has a faulty PSU, it is a strange one with a 5 pin USB-like plug that is reversible as only 3 pins are used and all 5 pins in the laptop are mirrored.

The center pin is a signal to the laptop of some kind and without it the laptop does not see the PSU as present.

I just bought a new PSU from the net and though it is outputting the correct voltage I think the signal may be wrong.

The circuit below shows how the signal wire is connected through a small circuit from the output of the PSU but I can't figure out what its supposed to be doing, so I can't tell if the signal is OK!

Can anyone explain to me what the circuit does and what it should be outputting so I can check it?

Thanks Al
 

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MikeMl

Well-Known Member
Most Helpful Member
The extra pins are likely to be used for charging the internal battery, or monitoring the battery temperature and/or terminal voltage, while the main supply runs the laptop.
 

bigal_scorpio

Active Member
Hi Mike,

Its not the pins thats the problem. What does the small circuit do?

Al
 

ericgibbs

Well-Known Member
Most Helpful Member
Hi Mike,

Its not the pins thats the problem. What does the small circuit do?

Al

hi Al,

I guess Mike is tied up.?

The 5K and 7.5V zener would clamp the voltage at their junction to 7.5V [when the 19V is applied]

The small cap is a smoothing/filter across the 7.5V

The 1N4148 diode connects the 7.5V to the SIGNAL line [ Vdrop across the diodes about 0.6V]

As Mike says, its a sense signal for the laptop to detect the presence of the 19V and perhaps control an internal charger.

OK.

OT:
All outside painting done, starting decorating all of the inside later this month.
 
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Reloadron

Well-Known Member
Most Helpful Member
I would venture a guess the signal should be around 6.5 to 7.5 Volts. You have 19 Volts coming in from the laptop mains adapter applied across the circuit. The Zener voltage id Max 7.87 and Mn 7.13 with the switching diode in series with the signal out. The switching diode (1N4148) has a forward voltage drop of about .62 to .72 volts.

I am guessing the signal tells the laptop that the mains adapter is connected to it.

<EDIT> I need to type faster! :) as ericgibbs was right on it. </EDIT>

Just a guess...
Ron
 
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bigal_scorpio

Active Member
Hi again Eric,

Glad to hear the outside is done, bet its a relief in this weather. ;)

As to the circuit, what exactly would you say should be coming out of it? How would I best test the output?

I am certain that the output is indeed to tell the laptop that the correct (maybe branded) type of battery is present but sadly this new unbranded battery is ignored by the laptop altogether. If I had a working one to test and find out what the signal should actually be like then I may be able to alter it to suit?

But I need someone like yourself with more knowledge than I have to tell me the characteristics of the signal that the circuit SHOULD be outputting.

I actually measured the signal in respect to ground with about 7v dc but over 200 on the ac scale! Does this seem right?

Thanks again...........Al
 

ericgibbs

Well-Known Member
Most Helpful Member
hi Al,

You say the unbranded battery is not recognised by the laptop, is this battery supposed to a direct replacement for the original.?

OT: the outside was done during the summer.
 
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bigal_scorpio

Active Member
Hi Eric and Ron,

Yes the battery was listed as a direct replacement but it did come from a scource on china.

It cost £33 and the branded one was three times that and then some, more than my laptop is worth!

The large ac voltage measured puzzles me somewhat. Is that expected?

Al
 

ericgibbs

Well-Known Member
Most Helpful Member
Hi Eric and Ron,

Yes the battery was listed as a direct replacement but it did come from a scource on china.

It cost £33 and the branded one was three times that and then some, more than my laptop is worth!

The large ac voltage measured puzzles me somewhat. Is that expected?

Al

hi,
I have no immediate explanation of the measured ac.??

What do the battery terminals measure [voltage] when the battery is disconnected and connected.?

EDIT:

Look here: **broken link removed**
 
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bigal_scorpio

Active Member
hi,
I have no immediate explanation of the measured ac.??

What do the battery terminals measure [voltage] when the battery is disconnected and connected.?

Hi Eric,

I will try to measure the terminals but I will have to do it at the PSU end as the battery is a tight fit into the slot and once in I could not get to the terminals without a hacksaw! ;)

Al
 

Reloadron

Well-Known Member
Most Helpful Member
Hi Al

Like Eric I have no clue why you would see the 200 V reading.

Laptop batteries can be a strange animal since they have evolved into "smart batteries". These batteries send a wealth of information to the system via the SMBus (System Management Bus). That information includes but is not limited to cell voltage, internal temperatures (generally measured with a thermistor), battery manufacturer, number of charge and discharge cycles, and date and time group information. The battery itself contains the chip. The chip can also send signals to the charge circuit to control the rate of charge.

Sometimes replacement batteries that are generic don't show the system what it wants to see and the system in turn will not recognize the battery. That could be the problem, even though the battery is advertised as a direct replacement part.

This link contains some interesting information on the subject of smart batteries.

Ron
 
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