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Learning about voltage, amps and watts.

Aeoelwulf

Sep 7, 2014
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I just got my first voltmeter and would like to learn how to use it by modifying a circuit to run a laptop.

what i want to learn is:

- About volts, amps and watts. i know watts are an amount so are volts the speed of a hypothetical train and amps the power behind it?

-if i have a PSU giving 90W 19.5V 4.7A these numbers are just max amounts right? do the volts have to match? laptops get 19.5ish volts to power the PC use and charge the battery. I want to understand that better and figure out if i would hurt anything just giving the laptop fewer volts without the battery.(my desktop PSU is a 550W (maxed used yet is 220 watts) single 12v rail platinum rated with a connectors that can supply me with (+3.3V@20A,+5V@20A,[email protected],[email protected],[email protected])

-experiment in using resistors, capacitors and transistors.--i think i might be able to use a resistor on a circuit from a molex connector on my PC power supply to drop from 12v to 10.8v that the battery normally would supply to my laptop motherboard. would the type of electricity be different from the battery as apposed to what i would get from my PSU?(AC/DC). if so my power brick normally sends out 90W 19.5V 4.7A.

-anything else that you can add that you feel like might help me catch on correctly.

Thanks friends and happy posting xD
 

Aeoelwulf

Sep 7, 2014
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I forgot to mention that this laptop can run without the battery. It has never had one actually I bought it from Best Buy as a display model
 

Gryd3

Jun 25, 2014
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Watts is a rating of total 'Power' which is a product of Voltage and Amperage.
Voltage is commonly referred to as a 'potential' and is said to be a 'speed' or velocity in many explanations.
Amperage is not necessarily the 'power' or 'force' behind something... as both of those descriptions should refer to Watts. Amperage is more of a measure of how many electrons are travelling.
There are two ways to get more power. Higher voltage or higher amperage.

Think about a car accident. A SMART car will need to travel faster to get the same power behind it in an accident than a fully loaded dump truck. In this case, the slow dump truck has a lower voltage and higher current. (This is not an ideal example, as the energy calculation is done differently... but the general idea is there)

Power = Voltage * Amperage.

Typically, you want to match the voltage as closely as possible to the required amount for a device. This value is the damaging one and is almost always constant. The amperage provided by a device is variable, the label is typically it's maximum rated value that it can supply. A device will only pull as many amps as it requires, which means the supply is still capable of delivering more amps to an additional device connected at the same time, or providing more amps is the device requires a momentary surge of energy.
Because the Watts are a product of Voltage and Current, the rating for a supply in 'Watts' is the highest it is capable of if the device is pulling the maximum amount of Amperage permitted by the supply. In the case of a power supply with multiple voltage rails, each voltage rail would need to supply the maximum amperage permitted to reach this rating.

Also. I had pointed out the Power formula above. Here is another for you:

Voltage = Amperage * Resistance

These two formulas are going to be used a LOT in your learning path. This second formula could be used to determine which resistor you must use to drop the voltage down from 12V to 10.8V
10.8 - 12 = 1.2V that we must have the resistor drop
There is a problem though... We have voltage, but we don't have the other two values.
We could estimate that the Amperage draw would be 4.5A which would mean that we would need a
Resistance = Voltage / Amperage
Resistance = 1.2 / 4.5 = 0.267Ω resistor

There is one more problem though... what if the laptop is not drawing 4.5Amps? Perhaps the CPU is idle, or there is a period of low activity.
Let's assume the current draw dips to 3.5Amps, this will cause the voltage across the resistor to change.
Voltage = Current * Resistance
Voltage = 3.5 * 0.267 = 0.933V
So we are now supplying 12V - 0.933 = 11 Volts instead of the desired 10.8V

This has been a poor example, because the voltage drop required is a very small percentage of the supplied voltage, but the point still remains that as the device changes and draws more or less current, the voltage being provided will inadvertently change. This change may not be in an acceptable range. It is always best to use a constant voltage source to power something with a variable current draw then to tamper with a resistor in-line with it.


...and I forgot to mention... remember that Power formula above? If you do use a resistor, it will waste power.
Power = Voltage * Amperage.
Power = 1.2V * 4.5A = 5.4Watts.
You want the resistor to drop 1.2V to bring 12V down to 10.8V, and if you are pulling 4.5A through it, the resistor will waste 5.4Watts of energy as heat. If your resistor is not rated for this power it may smoke or catch fire ;)
 

Aeoelwulf

Sep 7, 2014
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Im just wondering as i process all of your information. could i use a transistor to switch between one circuit for say 4.5A switchable to a 3.5A circuit? maybe even have a series of them to keep as honest as possible?

5 watts of loss might actually end up being nothing as a trade off. the OEM supply for the laptop got really hot.
 

Arouse1973

Adam
Dec 18, 2013
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You don't need to change the transistor switch if you are drawing less than the maximum rating. The current in a d.c circuit is proportional to the applied voltage and resistance. So your 4.5 Amp circuit will have no problems running a 3.5 Amp circuit. But you can't do it the other way round. A 3.5 Amp circuit trying to give 4.5 Amps will get very hot and probably fail quite quickly.
Adam
 

Gryd3

Jun 25, 2014
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I would still strongly suggest against using a resistor in-line with a power supply that is over-rated, even if you do setup switching transistors to pick the correct value resistor to use.. Adding the transistors would add more complexity than is worth, and if you were to go that far, a replacement circuit to go from 12V to 10.8 would most likely be better.
Perhaps Adam can confirm that a 10.8V power supply can be made from a 12V source? I can't remember the the minimum voltage difference required...
Alternatively, I have heard of ATX power supplies being modified to alter the reference voltage to produce a different output. The power supply contains some pretty high voltages though, so great care should be taken to avoid the capacitors and any other lines inside that may be carrying high voltage.
 

Aeoelwulf

Sep 7, 2014
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I think i should mention what im looking at. basically as far as the vaio goes i salvaged it from a wreck we got into. was going to do the mineral oil toy fish tank build but with using the pc psu i reckon i could mount this vaio board on the door of my case with a nice plexiglas enclosure stuffed with a riser board for a small r7 graphics card and this really cool conversation piece i can use to explain what im learning from you guys while having a 2 phase PC xD low and high power PC this FX 9370 costs 10 dollars a month alone to run :(

what do you mean by not suggesting using a resistor in line with a over rated power supply? what could happen?

hoping for adams confirmation. i have almost everything here and a really good electronics store down the street to get the supplies and help piecing the them together and started tomorrow evening.
 

Gryd3

Jun 25, 2014
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what do you mean by not suggesting using a resistor in line with a over rated power supply? what could happen?
Well. because we don't know the minimum current draw of the device, if we use a power supply that is supplying a higher voltage than required and use a resistor to drop the voltage down, the voltage actually being delivered will vary depending on the draw of the device. I am also unsure of the tolerance of the device, so providing a voltage higher than expected may cause damage to the device. Depending on the setup, the resistor could take drop too much or too little. As explained above, running a higher than required voltage could cause damage, running a voltage lower than required could cause at the very least, system instability.
 

Arouse1973

Adam
Dec 18, 2013
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I would still strongly suggest against using a resistor in-line with a power supply that is over-rated, even if you do setup switching transistors to pick the correct value resistor to use.. Adding the transistors would add more complexity than is worth, and if you were to go that far, a replacement circuit to go from 12V to 10.8 would most likely be better.
Perhaps Adam can confirm that a 10.8V power supply can be made from a 12V source? I can't remember the the minimum voltage difference required...
Alternatively, I have heard of ATX power supplies being modified to alter the reference voltage to produce a different output. The power supply contains some pretty high voltages though, so great care should be taken to avoid the capacitors and any other lines inside that may be carrying high voltage.

If you use a standard LDO regulator or quasi LDO you should be able to do this with no problems. Just check the maximum voltage drop at that current from the datasheet.
Adam
 
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