Question Is there a limit to power draw from USB devices?

[Steelbom]

Hi guys,

I have quite a few USB devices attached -- sometimes a device (keyboard, mouse, webcam, other, etc.) will not power on.
Wondering if there could be a limit to the amount of power that can be drawn from the motherboard?

Many of the devices are using USB 3 extension cables (1-1.5m) as the cables do not reach the computer -- unsure if this could cause issues with power delivery.

Cheers,
SB

 

[Paperdoc]

Definitely YES, there IS a power limit. All such systems supply power at 5 VDC to connected devices to use. For the older USB2 system, the limit is 0.5 A total load per USB host port. For the newer USB3 systems (now all called USB 3.2 Genn) the limit is 0.9 A. So if you are using a small Hub to connect several devices to one host port, that's the limit. Many devices like mice, keyboard, and a few others use very little power. But others use nearly the max. A good example is portable "Laptop Hard Drives". ALL such drive units use MORE that the 0.5 A limit for USB2 ports. Older drives designed for USB 2 use came one of two ways. Some had their own "wall wart" power supply modules you have to plug in. Others came with an odd cable with TWO USB 2 connectors on one end, and you had to plug BOTH of those into separate host ports to get enough power to run the drive. NOW the new drives for USB 3.2 come only with one normal USB 3 Type A cable and CAN operate within the 0.9 A limit of the host port. But that uses ALL the port power limit and you generally can NOT plug an additional device into that port. NOTE that such USB3 Laptop Drives SAY they are "USB2 compatible", but that means ONLY that they CAN communicate with a USB2 port (at the slower speed) IF you somehow arrange to provide them with the extra power that you can NOT get from that USB2 port.

Here's a typical symptom of inadequate power when using portable hard drives on a USB2 port. The drive unit DOES appear in Windows Explorer because it CAN "talk back" when addressed. BUT when you actually ask for data to be obtained from the drive, it locks up and fails to deliver because the major electrical load is from the motor that drives the spinning disks. That load is more than the 0.5 A available from a USB2 port.

How to "get around" this limit? Use a HUB that has its own power supply module provided. Such a system provides power to each of its output ports from the module, but then can communicate with the host port normally (noting that the total data transfer rate of that port is shared among all the devices on the Hub).

BEWARE of Hubs described as "bus powered". This means they get ALL their power from the host port and have NO additional power supply module. This is the simplest Hub type that IS limited to the 0.9 A max load.

For a simple Hub used only for data, the power supply module included needs to provide enough power for all its ports. In older USB2 systems that's 0.5 A per port. But today you are likely to find only USB 3.2 Hubs, and you can plug any USB2 device into its ports anyway. So ideally with a USB 3.2 Hub your power module should able able to provide 0.9 A PER OUTPUT PORT. However, it is NOT common for the module included to meet that target. That is because the designers (correctly) assume that you will NOT be actually USING all the ports simultaneously, even if all those ports have devices connected. Thus it is likely OK to have a power supply module able to provide about 60% to 70% of the theoretical max - for example, a Hub with 7 ports might be OK with a power module able to provide 4.0 to 4.5 A total.

But here is where it gets a little complicated. Those power numbers are for 5 VDC at the specified Amps. A LOT of such Hubs come with power supply modules that provide power to the Hub at 12 VDC, and the Hub converts that to 5 VDC with a small power capacity loss. So IF you have that type of power supply module included, you have to calculate, based on the fact that power in WATTS is Voltage x Current. So you get module output at 12 Volts and n Amps (or in Watts already stated), and you have to calculate how many AMPS that comes to when it is converted to 5 VDC for the Hub's output ports. Do this as port amps (at 5 VDC) = Supply WATTS (= Supply Volts x Supply amps) /5. For example, a module that can supply 35 WATTS at 12 VDC can provide to the Hub 35 /5 = 7 AMPS. That is more than enough for 7 output ports at full use, maybe 10 ports allowing for the "sharing" factor of non-simultaneous use.

An alternative calculation method IF you have a power supply module at 12 VDC to the Hub, is to do it all in WATTS. Each data port should require up to 5V x 0.9 A = 4.5 WATTS. So for example a 10-data-port Hub might need 45 WATTS from its power module, maybe derated to 32 WATTS for non-simultaneous use. (That would be 3.75 A at 12 VDC for full load, or 2.67 A derated.)

MORE to consider. MANY powered Hubs now come with "Charging" ports. These ports can provide much more power than the basic 0.9 A per port of USB 3.2 systems for use with devices that can use more power for charging, OR are NOT even data devices and just need charging only. Normally such ports are labelled for their higher max current rating. So remove those from the calculations first, and base your initial calculations on the standard USB 3 data ports only. THEN go back and calculate how much additional AMPS will be needed from the power supply module to satisfy the extra power capacity of the charging ports. ADD that onto the data-only requirement to see how much the total max load is on the supplied power module. Is it still big enough?

An example calculation for a Hub with "dedicated charging ports" (i.e., ports that charge only and do NOT deal with data transfer) in addition to data ports. This unit

https://www.amazon.com/Splitter-Charging-Individual-Switches-Computer/dp/B0B8MQGFV9/ref=sr_1_22_sspa?crid=14TEK02HD0X9G&keywords=usb%2Bhub&qid=1686840409&sprefix=usb%2Bhub%2Caps%2C99&sr=8-22-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9tdGY&th=1

has 10 ports in total: 7 for data transfer use at USB 3.2 speeds, and 3 for charging use only. Among the data ports, three are rated as "USB 3.1", which now are called USB 3.2 Gen2 able to transfer data up to 10 Gb/s. Of these, two are Type C ports and one Type A. Four are type A ports labelled as "USB3", now called USB 3.2 Gen1 able to move data up to 5 Gb/s. Three type A ports are Charging only labelled as 5 VDC up to 2.4 A max load. It comes with a power supply module rated at 12 VDC output up to 5A.

Now the calculations. The seven data ports require max 0.9 A at 5 VDC each, max total 4.5 W x 7 = 31.5 Watts. The power module can supply up to 12 x 5 = 60 Watts. No problem supplying the data ports. But now add the needs of the dedicated Charging Ports. They each say they can output 5 x 2.4 = 12 Watts, and there are three of them - total load for these is 36 Watts max. That is a total max load of 66.5 W, more than the power supply module's capacity. HOWEVER, allowing for the NON-simultaneous use factor, that capacity certainly IS sufficient for this Hub to do all it needs.

I will note that I have seen similar Hubs with that kind of port count but with a power supply module of only 36 Watts, NOT sufficient for such a unit.

Last item. The length of the USB cable generally does not limit the power reaching the device. BUT it DOES impact the data transfer rate, and a very long cable may cause errors in data and thus a failure for that reason, NOT because of inadequate power.

 

[Steelbom]

Definitely YES, there IS a power limit. All such systems supply power at 5 VDC to connected devices to use. For the older USB2 system, the limit is 0.5 A total load per USB host port. For the newer USB3 systems (now all called USB 3.2 Genn) the limit is 0.9 A. So if you are using a small Hub to connect several devices to one host port, that's the limit. Many devices like mice, keyboard, and a few others use very little power. But others use nearly the max. A good example is portable "Laptop Hard Drives". ALL such drive units use MORE that the 0.5 A limit for USB2 ports. Older drives designed for USB 2 use came one of two ways. Some had their own "wall wart" power supply modules you have to plug in. Others came with an odd cable with TWO USB 2 connectors on one end, and you had to plug BOTH of those into separate host ports to get enough power to run the drive. NOW the new drives for USB 3.2 come only with one normal USB 3 Type A cable and CAN operate within the 0.9 A limit of the host port. But that uses ALL the port power limit and you generally can NOT plug an additional device into that port. NOTE that such USB3 Laptop Drives SAY they are "USB2 compatible", but that means ONLY that they CAN communicate with a USB2 port (at the slower speed) IF you somehow arrange to provide them with the extra power that you can NOT get from that USB2 port.

Here's a typical symptom of inadequate power when using portable hard drives on a USB2 port. The drive unit DOES appear in Windows Explorer because it CAN "talk back" when addressed. BUT when you actually ask for data to be obtained from the drive, it locks up and fails to deliver because the major electrical load is from the motor that drives the spinning disks. That load is more than the 0.5 A available from a USB2 port.

How to "get around" this limit? Use a HUB that has its own power supply module provided. Such a system provides power to each of its output ports from the module, but then can communicate with the host port normally (noting that the total data transfer rate of that port is shared among all the devices on the Hub).

BEWARE of Hubs described as "bus powered". This means they get ALL their power from the host port and have NO additional power supply module. This is the simplest Hub type that IS limited to the 0.9 A max load.

For a simple Hub used only for data, the power supply module included needs to provide enough power for all its ports. In older USB2 systems that's 0.5 A per port. But today you are likely to find only USB 3.2 Hubs, and you can plug any USB2 device into its ports anyway. So ideally with a USB 3.2 Hub your power module should able able to provide 0.9 A PER OUTPUT PORT. However, it is NOT common for the module included to meet that target. That is because the designers (correctly) assume that you will NOT be actually USING all the ports simultaneously, even if all those ports have devices connected. Thus it is likely OK to have a power supply module able to provide about 60% to 70% of the theoretical max - for example, a Hub with 7 ports might be OK with a power module able to provide 4.0 to 4.5 A total.

But here is where it gets a little complicated. Those power numbers are for 5 VDC at the specified Amps. A LOT of such Hubs come with power supply modules that provide power to the Hub at 12 VDC, and the Hub converts that to 5 VDC with a small power capacity loss. So IF you have that type of power supply module included, you have to calculate, based on the fact that power in WATTS is Voltage x Current. So you get module output at 12 Volts and n Amps (or in Watts already stated), and you have to calculate how many AMPS that comes to when it is converted to 5 VDC for the Hub's output ports. Do this as port amps (at 5 VDC) = Supply WATTS (= Supply Volts x Supply amps) /5. For example, a module that can supply 35 WATTS at 12 VDC can provide to the Hub 35 /5 = 7 AMPS. That is more than enough for 7 output ports at full use, maybe 10 ports allowing for the "sharing" factor of non-simultaneous use.

An alternative calculation method IF you have a power supply module at 12 VDC to the Hub, is to do it all in WATTS. Each data port should require up to 5V x 0.9 A = 4.5 WATTS. So for example a 10-data-port Hub might need 45 WATTS from its power module, maybe derated to 32 WATTS for non-simultaneous use. (That would be 3.75 A at 12 VDC for full load, or 2.67 A derated.)

MORE to consider. MANY powered Hubs now come with "Charging" ports. These ports can provide much more power than the basic 0.9 A per port of USB 3.2 systems for use with devices that can use more power for charging, OR are NOT even data devices and just need charging only. Normally such ports are labelled for their higher max current rating. So remove those from the calculations first, and base your initial calculations on the standard USB 3 data ports only. THEN go back and calculate how much additional AMPS will be needed from the power supply module to satisfy the extra power capacity of the charging ports. ADD that onto the data-only requirement to see how much the total max load is on the supplied power module. Is it still big enough?

An example calculation for a Hub with "dedicated charging ports" (i.e., ports that charge only and do NOT deal with data transfer) in addition to data ports. This unit

https://www.amazon.com/Splitter-Charging-Individual-Switches-Computer/dp/B0B8MQGFV9/ref=sr_1_22_sspa?crid=14TEK02HD0X9G&keywords=usb%2Bhub&qid=1686840409&sprefix=usb%2Bhub%2Caps%2C99&sr=8-22-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9tdGY&th=1

has 10 ports in total: 7 for data transfer use at USB 3.2 speeds, and 3 for charging use only. Among the data ports, three are rated as "USB 3.1", which now are called USB 3.2 Gen2 able to transfer data up to 10 Gb/s. Of these, two are Type C ports and one Type A. Four are type A ports labelled as "USB3", now called USB 3.2 Gen1 able to move data up to 5 Gb/s. Three type A ports are Charging only labelled as 5 VDC up to 2.4 A max load. It comes with a power supply module rated at 12 VDC output up to 5A.

Now the calculations. The seven data ports require max 0.9 A at 5 VDC each, max total 4.5 W x 7 = 31.5 Watts. The power module can supply up to 12 x 5 = 60 Watts. No problem supplying the data ports. But now add the needs of the dedicated Charging Ports. They each say they can output 5 x 2.4 = 12 Watts, and there are three of them - total load for these is 36 Watts max. That is a total max load of 66.5 W, more than the power supply module's capacity. HOWEVER, allowing for the NON-simultaneous use factor, that capacity certainly IS sufficient for this Hub to do all it needs.

I will note that I have seen similar Hubs with that kind of port count but with a power supply module of only 36 Watts, NOT sufficient for such a unit.

Last item. The length of the USB cable generally does not limit the power reaching the device. BUT it DOES impact the data transfer rate, and a very long cable may cause errors in data and thus a failure for that reason, NOT because of inadequate power.

Thank you for the detailed write up, it's much appreciated!

I'll go over this a few times to solidify it in my head!

Did I understand correctly that my motherboard is limited to 5 AMPs spread across all USB ports?
My motherboard (Aorus Xtreme X670E) has four USB 2, 6 USB C (type A) and 2 USB C (3.2 gen2). Each device is connected to an individual port right now -- no hubs involved.

Some symptions I've noticed are devices flicking on and off (especially at boot), LEDs turning off, several devices just shutting off after a new device is connected, etc.

 

[Paperdoc]

That's HALF an amp: 0.5 A, not 5 amps.

It applies to EACH separate mobo USB 2 port. Small technical detail about this. Almost all USB Controller chips used on mobos generate TWO separate USB ports on ONE mobo header. So, for HEADERS on mobos, the cable you plug into it that feeds out to an accessible port almost always leads to TWO external ports. But on the back panel of your mobo where there are ports, not mobo headers, that split-up has happened already, so each exposed PORT is just one port. Anyway, EACH PORT has its own power supply limit: 0.5 A max for USB2, 0.9 A for USB 3.2. There is no specified limit for the TOTAL from all ports; the spec from the USB creators is PER PORT only.

The exception to this is the very new USB 3.2 Gen2x2 port system. The controllers for that generate only ONE port per controller. IF your mobo has one of these, it VERY likely uses a quite different mobo header called a Type E, and the cable from that to a front panel will feed only ONE port socket of Type C.

However, when you get into a multi-port HUB with its own power supply module, that module imposes a limit for the total load for that HUB.

USB PORT sockets come now in two types. The older we are used to is Type A, a rectangular plug with a plastic insert supporting 4 contacts. Actually, that's the USB2 version. The USB 3.2 Type A socket and plug looks a lot like that, but it has five added contacts in the back of the socket. WATCH for this distinction is choosing which Type A cable to use - you must use the USB 3.2 version with the five extra contacts for USB 3.2 ports and devices.

The newer connector type is Type C, smaller with rounded ends and contacts on both surfaces of a centre-mounted support "tongue". The important difference here is that the Type A rectangular connectors can do very well for the slowest data transfer max of 5 Gb/s within the new USB 3.2 (Gen1) systems. For the Gen2 (10 Gb/s) it is strongly recommended you use Type C connectors although Type A may work but not at full rated speed. For Gen2x2 (20 Gb/s) it is REQUIRED that you use a Type C connection to get that max speed.

Your mobo's rear panel has several USB ports of a full range of features. Refer to the manual p. 17-18. It has 4 USB2 ports (key f) of the older style. These are the ones limited to 0.5 A current and 480 Mb/s (0.48 Gb/s), and they are ideal for slow devices like keyboards and mice. The rest all are within the USB 3.2 family (0.9 A per port) and you CAN plug a USB2 device into one and it will work, but that's a waste since that device cannot operate at USB 3.2 speeds.

There are six USB 3.2 Gen 2 ports all using Type A sockets: five of them (key g) are "normal", and one (key h) is "special" in the sense that it is the only one that can be used for "Q-Flash Plus" functions involved in BIOS updates. That last also can function as a "normal" USB 3.2 Gen2 port. Note that, although the USB people recommend Type C sockets for Gen2 ports, these are Type A for convenience for people with older devices and older plugs. The MAY work at the 10 Gb/s max rate. Also note that you do not have any of the basic Gen1 (5 Gb/s) ports. Devices of that type simply plug into one of these Type A Gen2 sockets and will work just fine at their rated max speed.

Your rear panel has two Type C sockets. ONE (key i) is another Gen2 port, but with the recommended Type C socket. The other (key j) is a Gen 2x2 port, the fastest version (max 20 Gb/s) of USB currently in wide use.

Devices flickering on and off briefly at start-up is normal. that is part of the POST process for each device - it is tested by the mobo to see if it responds to requests for info. But after the start-up is complete, that should not happen.

If devices shut off after a successful start-up there may well be a power problem. This is ESPECIALLY likely to occur for certain higher-power devices (like "portable hard drives") if you try to use them on the USB2 ports that cannot supply power over 0.5 A. If that happens, try plugging that device into one of the Gen2 (red) Type A ports.

 

[sdifox]

Just get a usb powered hub. The ones that come with its own power adapter.

 

[Steelbom]

That's HALF an amp: 0.5 A, not 5 amps.

It applies to EACH separate mobo USB 2 port. Small technical detail about this. Almost all USB Controller chips used on mobos generate TWO separate USB ports on ONE mobo header. So, for HEADERS on mobos, the cable you plug into it that feeds out to an accessible port almost always leads to TWO external ports. But on the back panel of your mobo where there are ports, not mobo headers, that split-up has happened already, so each exposed PORT is just one port. Anyway, EACH PORT has its own power supply limit: 0.5 A max for USB2, 0.9 A for USB 3.2. There is no specified limit for the TOTAL from all ports; the spec from the USB creators is PER PORT only.

The exception to this is the very new USB 3.2 Gen2x2 port system. The controllers for that generate only ONE port per controller. IF your mobo has one of these, it VERY likely uses a quite different mobo header called a Type E, and the cable from that to a front panel will feed only ONE port socket of Type C.

However, when you get into a multi-port HUB with its own power supply module, that module imposes a limit for the total load for that HUB.

USB PORT sockets come now in two types. The older we are used to is Type A, a rectangular plug with a plastic insert supporting 4 contacts. Actually, that's the USB2 version. The USB 3.2 Type A socket and plug looks a lot like that, but it has five added contacts in the back of the socket. WATCH for this distinction is choosing which Type A cable to use - you must use the USB 3.2 version with the five extra contacts for USB 3.2 ports and devices.

The newer connector type is Type C, smaller with rounded ends and contacts on both surfaces of a centre-mounted support "tongue". The important difference here is that the Type A rectangular connectors can do very well for the slowest data transfer max of 5 Gb/s within the new USB 3.2 (Gen1) systems. For the Gen2 (10 Gb/s) it is strongly recommended you use Type C connectors although Type A may work but not at full rated speed. For Gen2x2 (20 Gb/s) it is REQUIRED that you use a Type C connection to get that max speed.

Your mobo's rear panel has several USB ports of a full range of features. Refer to the manual p. 17-18. It has 4 USB2 ports (key f) of the older style. These are the ones limited to 0.5 A current and 480 Mb/s (0.48 Gb/s), and they are ideal for slow devices like keyboards and mice. The rest all are within the USB 3.2 family (0.9 A per port) and you CAN plug a USB2 device into one and it will work, but that's a waste since that device cannot operate at USB 3.2 speeds.

There are six USB 3.2 Gen 2 ports all using Type A sockets: five of them (key g) are "normal", and one (key h) is "special" in the sense that it is the only one that can be used for "Q-Flash Plus" functions involved in BIOS updates. That last also can function as a "normal" USB 3.2 Gen2 port. Note that, although the USB people recommend Type C sockets for Gen2 ports, these are Type A for convenience for people with older devices and older plugs. The MAY work at the 10 Gb/s max rate. Also note that you do not have any of the basic Gen1 (5 Gb/s) ports. Devices of that type simply plug into one of these Type A Gen2 sockets and will work just fine at their rated max speed.

Your rear panel has two Type C sockets. ONE (key i) is another Gen2 port, but with the recommended Type C socket. The other (key j) is a Gen 2x2 port, the fastest version (max 20 Gb/s) of USB currently in wide use.

Devices flickering on and off briefly at start-up is normal. that is part of the POST process for each device - it is tested by the mobo to see if it responds to requests for info. But after the start-up is complete, that should not happen.

If devices shut off after a successful start-up there may well be a power problem. This is ESPECIALLY likely to occur for certain higher-power devices (like "portable hard drives") if you try to use them on the USB2 ports that cannot supply power over 0.5 A. If that happens, try plugging that device into one of the Gen2 (red) Type A ports.

Thanks for the info!

I really look forward to the industry adopting the C style connector for everything... will make it so much simpler haha (but rip old USB type A devices).

Do you think RGB (e.g., in a keyboard or mouse) is enough to exceed the 0.5A limit of USB 2?

Just get a usb powered hub. The ones that come with its own power adapter.

Yeah, I've just ordered one. Provides lots of power so it hopefully will alleviate any issues.

 

[Paperdoc]

I really doubt a keyboard or mouse with LED lights inside will consume more than the 0.5 A current limit for a USB 2 port. I'd speculate there might be 10 to 15 LED's in a keyboard, certainly many FEWER in a mouse. I could not find a unit with actual specs on power consumption, although there are man y specified for use on USB 2 ports.The ROUGH "rule of thumb" is that one LED can consume at max 20 mA, or 0.20 A. So even 15 of those would add no more than 0.3 A to the small consumption of the non-light components. Only "problem" I might consider would be if you do use an UNPOWERED USB 2 Hub to connect lighted keyboard, mouse, and a few other items to a single USB 2 port. Doing that MIGHT get you close to the port limit, depending on what the other devices are.

If and when Type A sockets start to disappear in favour of many Type C, you can still get adapters to convert a Type A plug into a Type C. Howver, in that situation yuo;d have to pay attention to whether these items are for USB 3.2 (with the 5 extra contacts) or older USB 2 (plain old four original contacts only).