AboutDavid Freitag Expertise I can answer questions about all kinds of computers, operating systems, software and hardware in clear and easily understandable language. I have dealt with sorts of software and hardware issues in PCs from before windows to the present. I can explain basic concepts and deal with complex detailed issues in the guts of operating systems, including drivers, bios, hardware issues and software problems.
Experience I have over 3 decades of experience ranging from mainframes and minis and microcomputers to all flavors of personal computers. I have programmed in assembler language, Fortran, C, VB, C# and I have worked with the major relational databases. I have been a CIO of an energy company and I have provided hands on technical management for Windows software development. In the course of my career, I have had to deal with all sorts of configurations and nitty gritty system problems which exposed me to all aspects of personal computer systems.
Publications I am an expert author in Ezinearticles and have had articles published in Buzzle, IdeaMarketer, ArticleCube and other article sites.
Education/Credentials I have a BS in Physics from CCNY. I have received credits from courses dealing with OLE automation and computer systems management from the AMA
Awards and Honors Phi Beta Kappa
Achievement awards for software development efforts in a leading Voip provider company.
Expert: David Freitag Date: 5/20/2008 Subject: AC adapter for HP Compaq nx7300
Question QUESTION: Hi,
A short question:
AC adaptors for HP Compaq nx 7300 have two output voltages:
The first voltage is about 18.8V/3.5 A (between the outer and inner cylinders of the output (DC) connector; and
The second voltage, a voltage between the central pin of the connector and the outer cylinder of the output connector, which was about 18.4 V (about 0.4 V less than the first one).
Is this central pin voltage (the second one) a dc voltage, or some thing else? Why is nothing written about this voltage on the AC adapter’s label?
Thanks in advance,
Bashkim
ANSWER: Hi Bashkim,
I need to look into this. I will get back to you shortly.
Best,
David
Does the cord coming from the A/C power brick have 2 wires or 4 in it? If it only has two, then the adapter is only outputting one voltage, and the adapter on the notebook can take either of two different power adapters (possibly one for the U.S. and one for Europe?)
If it has 4 wires, then the A/C brick is supplying two voltage lines because some other component inside the notebook needs a different voltage rail (although I can't imagine what).
Another expert that I contacted suggested the following;
I'm not familiar with that specific adapter, but here's my <guess>, based on other charging systems:
The first voltage you measured is the main output used for charging the battery and powering the unit.
The second voltage is part of a monitoring/regulation circuit that keeps the battery from being overcharged. The voltage there is coming 'backwards' through a signal conditioning network that, when the connector is unplugged, is disconnected from its 'other half.' The network could be as simple as a low resistance element that is used to measure the current draw according to ohm's law (V = I * R). The difference in voltage could be due to the presence of a diode in the circuit to make sure the system isn't back fed when the battery is fully charged and the power supply is unplugged.
To answer your second question, I <assume> the voltage is DC - if your meter measures both AC and DV voltages, switch it to AC mode and check again - it should read a low value in AC mode if the 'real' voltage is DC.
Best,
David
---------- FOLLOW-UP ----------
QUESTION: Hi David,
Thanks a lot for your quick reply.
Let me answer/specify the point/assumptions you rightly outlined.
1-The cable that comes from the AC “power brick” has three wires: one to the outside cylinder of the output connector (o.c.), the other to the inside cylinder of o.c., and an other to the central pin of the o.c.
2-Both voltages, the first one to the outside cylinder of the o.c., and the second one to the central pin of the o.c., both with respect to the “common” electrode (the outside cylinder) are present also without connecting the AC adapter to the laptop (idle voltages). I measured the voltages with a dc voltmeter, not with an oscilloscope. It seems that the input resistance of the laptop for the first voltage, is relatively low (one can measure some current there), while the input resistance of the laptop (central pin jack) seems to be pretty high (could not measure any current).
3-I tried to substitute the original HP AC adapter, with a general purpose AC adapter, applying a resistor divider between input cylinder- central pin-output cylinder, in order to get the second voltage. But the laptop did not function normally: it was very slow, and it did not charge the battery, although it signaled “recharging the battery”.
I tried to contact many people who claimed to be experts on ac adapters, and their answer was: “We do not know. This thing with “two voltages” is only to these HP Compaq nx7300 laptops”
I am just curious what is it.
Thank you again for all you time.
Bashkim
Answer Hi Bashkim,
One of the experts that I know said;
When you say:
'It seems that the input resistance of the laptop for the first voltage, is relatively low (one can measure some current there), while the input resistance of the laptop (central pin jack) seems to be pretty high (could not measure any current).'
Do you mean that you measured these with an ohmmeter or with an ammeter? An ammeter circuit would have required breaking into the circuit to insert the leads in series - it sounds that way from the 'could not measure any current.'
From the low current draw of the central pin I would think it is a monitoring circuit looking at the voltage coming back from somewhere inside the computer - probably the actual battery terminals. If you think of it as a 'voltmeter' this makes sense, as voltmeters have high input impedance.
The slow function of the system with the alternative power source may be due to the system's picking up a low voltage on the 'monitoring' pin. This would indicate a low battery or weak charger and the system responded by cutting back on CPU/mainboard frequency to conserve power.
If you set up a circuit to monitor the current and voltage on both legs while the computer is running with the original and alternative power supplies you could deduce the function of the connections - I would guess you would see high current on the main connection and very low current on the secondary, while the voltage on the secondary should be lower than the voltage on the primary when the battery is low, rising to be closer to the primary as the battery charges.
