12V 5 amps vs 19v 3 amps? - Page 1
Forum section: electronics / beginners

author: DW1961
author_rank: Frequent Contributor
timestamp: July 08, 2020, 12:28:40 am
content: My laptop converter says output is 19V at 3 amps.I have a light strip converter that says 12v 5 amps.That's roughly the same watts, but my question is why 19V instead of 12v for the laptop converter?

author: wraper
author_rank: Unknown
timestamp: July 08, 2020, 12:39:37 am
content: Why you did not ask about why not 120V 0.5A? The question is about as meaningful. As of why you would not use 12V for a laptop, 3A current requires thinner wires and with 12V input you cannot use buck converter to charge 3S lithium battery (12.3V) they usually use.

author: DW1961
author_rank: Frequent Contributor
timestamp: July 08, 2020, 12:57:53 am
content: Well, I'm sorry it had no meaning. I thought this was a beginners forum. Sorry for the inconvenience.

author: MarkF
author_rank: Super Contributor
timestamp: July 08, 2020, 01:40:25 am
content: I would assume for a laptop power supply that it needs the extra voltage to charge the batteries and to overcome the regulator drop-out voltage.In order to charge a battery, the charge voltage needs to be higher than the battery voltage to have current flow into the battery.For a regulator, you need a higher voltage to allow for voltage loss through the active components.

author: DW1961
author_rank: Frequent Contributor
timestamp: July 08, 2020, 03:01:07 am
content: Quote from: MarkF on July 08, 2020, 01:40:25 amVery interesting. Thank you. Can you give me an example of regulators and active components that require 12V and how voltage loss interacts?Thanks so much.

author: Rick Law
author_rank: Super Contributor
timestamp: July 08, 2020, 03:08:21 am
content: Quote from: DW1961 on July 08, 2020, 12:28:40 amThis in no doubt an over-simplification. But using the simple case makes the answer to your question is easier to understand. So, here goes:Typical laptops with removable Lithium Ion batteries would use 3, 6, or 9 batteries (typically 18650's) arranged in 1, 2, or 3 column(s) of 3 batteries back to back. The "3 batteries back to back" is the same as saying 3 batteries in series. The 1p3S (1 column, 3 in a series/row to make a column) is the typical "light" pack, 2p3S is the typical standard pack, and 3p3S is the typical extended battery pack arrangements.In that "3S" arrangement,three 4.2 volt batteries in series would appears to the world as a single battery with 4.2V*3=12.6V. A 12 volt source will not be high enough to charge a 12.6 volt battery. Charger's electronics themselves would have somevoltage dropand those electronics sit in between the power-converter and the batteries. So, thetotal voltage requirements is 12.6V plus the voltage required by the charging electronics. Now you can see 12V is a non-starter -- even without charging electronics, you need 12.6V for the batteries alone.Say you go up to a 16V power converter, you would have 16V-12.6V=3.4V, that is 3.4V head room for the charging electronics. 3.3V components are common, but that leaves a mere 0.1V margin. For example, you may have a 16V power converter running just a little under like a very small 0.2V under. 0.2V is small, but you are out of luck just the same -- you are now below requirement giving just 3.2V for your 3.3V components. So, you need lower voltage components (lower than 3.3V) for your charging electronics - which limits your choice of components and probably increase cost. My early (1998-2001) Sony and Fujitsu laptops use 16V power bricks. A 19V design gives more head room. At 19V, the charging electronic have a 6.4V margin to work with.Some high-capacity Lithium Ion batteries has nominal voltage at 4.35V instead of 4.2V. So 3S the 4.35V cells would need 13.05V for charging (instead of 12.6V for charging the 4.2V cells). A 16V laptop brick would have only 2.95V head room for the charging electronics. The extra 3V more in headroom a 19V power-converter give would probably be welcomed in those cases.Now stuff that make it more complicated:Typically, laptop charging circuitry will have additional electronics to "balance" the cells so that all the cells are charged to equal degree. Some would have the intelligence to "lock out" bad cells while keeping good cells in service.The arrangements is not always 3 series than parallel these columns. They may also be 3 connected in parallel first which make a composite single battery with 3x the capacity (instead of 3x the voltage as in series), than serialize those 3 "composite battery" in a rows of 3. So, different designs are used.Typically, the first thing after the power converter is a voltage regulator first - to ensure that the power source is not giving out something silly. The rest of the charging electronics sits after the voltage regulator. So, there is yet more voltage drop...I hope this explanation make sense to you and helps you.EDIT: I know this reply has already been seen and quoted, but this stupid single character typo really bugs me and I have to correct it.Now stuff that make is more complicated:corrected the "is" to "it" so it now readsNow stuff that make it more complicated:

author: DW1961
author_rank: Frequent Contributor
timestamp: July 08, 2020, 03:30:27 am
content: Quote from: Rick Law on July 08, 2020, 03:08:21 amHell yeah! It makes a lot of sense. I have some rudimentary understanding of how voltage drop and over head works, and how stacking batteries in a series increases voltage (such as using 3 AA batteries in a series creates a 4.5V battery, so your explanations are extremely helpful.Aside from the battery needing more than 12V, could using more voltage also be part of having to run the screen, CPU, Ram, etc., also?

author: Brumby
author_rank: Unknown
timestamp: July 08, 2020, 03:51:30 am
content: Quote from: DW1961 on July 08, 2020, 03:30:27 amNot really. The rest of the laptop will be designed to run off the battery - and CPU, RAM run on much lower voltages.The screen, however, could be a different story, depending on what is used for the backlighting (for your typical LCD panel).

author: james_s
author_rank: Super Contributor
timestamp: July 08, 2020, 07:03:27 am
content: Using a higher voltage is advantageous as the current will be lower for the same amount of power. Current is what causes heating in wires and semiconductors and heating is wasted energy. A related effect of this is that a buck regulator to drop a higher voltage down is more efficient than a boost regulator that raises a lower voltage up because the current in the switching device and inductor are lower in the former than the latter. Similarly the power grid that supplies your house uses very high voltage to transmit the power over long distances, often as high as 120,000V and then substations and distribution transformers step down the voltage to lower levels, eventually down to the 240V that comes into the house. The high voltage is used because it can send much more power over the same size wire with the same losses from current heating up the wires. Similarly a small shaft spinning at high speed can transfer the same amount of mechanical energy that would require a much larger shaft turning at a slow speed because torque is what determines the mechanical strength of shaft required and power is a product of torque and speed.In the laptop it's more efficient to drop 19v down to whatever the battery voltage is sitting at rather than boosting a lower voltage up to a level sufficient to charge the battery. The rest of the circuitry in the laptop will run on 3.3V and various lower voltages.

author: DW1961
author_rank: Frequent Contributor
timestamp: July 08, 2020, 05:30:37 pm
content: Quote from: james_s on July 08, 2020, 07:03:27 amAwesome explanation and analogy. Thanks.

author: Nerull
author_rank: Frequent Contributor
timestamp: July 08, 2020, 08:17:20 pm
content: Most modern USB charging standards negotiate a higher voltage for similar reasons - you can deliver more power over the same wire with higher voltage. USB-PD can deliver up to 100W by operating at 20V/5A. Delivering the same power at 5V would require 20A. I don't think you want to plug your laptop in with 12 gauge wires.OnePlus' Dash/Warp charging is an exception, pushing high currents at 5V, and requires special cables built to handle the higher current.

author: Nominal Animal
author_rank: Super Contributor
timestamp: July 08, 2020, 09:37:58 pm
content: Internally, laptops have pretty high-current 5V (used for USB and storage media) and 3.3V (lot of internal logic) buses, plus lower voltages for the processor and associated circuitry. These are all derived from the battery voltage using buck converters.If we assume the battery pack charging circuit, including its internal voltage drop, requires say 14.7 V, why not use a 15 V charger, then?The nominal voltage of the charger output is not exact. At minimum, 5% variation is allowed (even with USB 5V), and 10% is more typical. So, if you have a 19V charger, its output could be anything between 17.1V and 20.9V, and still be within spec.Around 19V seems to be a pretty good compromise between power delivery and ease of conversion to lower voltages using very small circuitry. If you used a higher voltage to charge, you'd need better isolation between tracks; and there really isn't any room to spare in a laptop. (Louis Rossmann has mentioned in his Apple repair videos how some Apple laptops have a +20V backlight line next to a logic level signal in the display connector, and they tends to blow whenever a little moisture or gunk gets in, bridging the contacts.)Desktop PCs have additionally a +12V line for peripherals, a -12V line for serial port, and a +5V standby. In datacenters, an entire rack can have a common +48VDC 300A voltage source, which is individually stepped down to +12V, +5V, and +3.3V using small DC-DC converters inside each rack-mount chassis.There are also small DC-DC power supplies like PicoPSU, that act like desktop PC power supplies, but at a lower power level (150W typical maximum).As an example, M3-ATX can be powered from 6 to 24 V, and it'll provide up to 6A on the 5V and 3.3V lines, and up to 4A on the 12V line. These are often used for automotive PCs.

author: Rick Law
author_rank: Super Contributor
timestamp: July 09, 2020, 04:27:29 am
content: Quote from: Brumby on July 08, 2020, 03:51:30 amDW1961,Your question in the quote of the quoted reply is masterfully answered byBrumbyalready. But seeing your question, I think you may be missing one trick up your sleeve. If I may be so bold, allow me to bring that trick to your attention.You seem to think that to power something, we need to drop the voltage -- not necessarily so.-----------------------------------Case 1:-----------------------------------When you connect things in series, each connected thing will cause a voltage drop. In this case, you can think of powering those things with voltage drop. Say you have two incandescent bulbs, bulb-1 and bulb-2, connected in series to the power source.Source+ -- Bulb1 -- Bulb2 -- Source-Voltage will drop after bulb-1 and then drop again after bulb-2, so the voltage is split between them. The Current flowing through bulb1 must also flow through bulb2 before it returns to the "negative/ground" terminal of the power source, so the current through both is the same current.Bulb1 and Bulb2 splitting the voltage meansV1+V2=Vsource;and V1 can be different than V2 if the bulbs are different in power consumption.They have the same current meansI1=I2=Isource, or simply, I1=I2=ISo bulb1 and bulb2 split the voltage between them, but has the same current.Power = Voltage * Current; or more commonly stated as: P=V*I, or W=V*I. Some use W because Watt starts with W. So:W1=V1*IW2=V2*I-----------------------------------Case 2:-----------------------------------You can connect them in parallel. In parallel is both bulbs connects directly to the power source.Source +/ \Bulb1 Bulb2\ /Source -In this case, both get the full voltage but they each have their own current. What Current flows through bulb1 has nothing to do with what current flows through bulb2. The current provided by the source is split between them.They both are directly connected to the source so:V1=VsourceV2=Vsourceso, V1=V2=VsourceThey split the total current:Isource = I1 + I2and I1 and I2 may be different if the bulbs are different in power consumption.So bulb1 and bulb2 split the current between them, but has the same voltage.W1=V*I1W2=V*I2-----------------------------------Summary:-----------------------------------In case 1 (series) youadd more voltage to power more thingsconnected in series but keeping current the same.In case 2 (parallel) youadd more current to power more thingsin parallel but keeping voltage the same.EDIT: Minor wording change to make it easier to read

author: dcbrown73
author_rank: Regular Contributor
timestamp: July 09, 2020, 01:52:25 pm
content: Great explanation guys!