Match Hard Disk Startup Current with Power Adapter Output Current

When I was looking for a new hard disk for my Network-attached storage (aka NAS), I found the startup current of Seagate ST2000DM001 (aka Barracuda or Desktop HDD) in my Buffalo HD-LX2TU3 is 2.5A and output current of the adapter is 1.5A. It requires 67% higher than the adapter could offer. Is it safe use of electricity usage and okay to my hard disk?

I didn’t know and decide to research.

What is AC Adapter?

Back to basic, let’s begin from knowing Ac adapter.

Originally, most AC/DC adapters were linear power supplies, containing a transformer to convert the mains electricity voltage to a lower voltage, a rectifier to convert it to pulsating DC, and a filter to smooth the pulsating waveform to DC, with residual ripple variations small enough to leave the powered device unaffected… Wiki: AC Adapter

An AC adapter has fix amount of capacitor, the main purpose of it is to provide fix and steady voltage. If the capacitor is dead, the power adapter won’t be able to provide steady voltage.

Also according to watt = amp * voltage, it may provide lower than the calculate current with fix voltage.

What is Starting Current?

Starting current is different from inrush current which has been discuss in IHS GlobalSpec CR4: Inrush Current and StartUp CurrentStarting current is during starting motor while inrush current might happen all the time.

I think the best explain for starting current are following two:

the current drawn from the mains by an electric motor upon starting. Since the starting current of an electric motor can be several times greater than the motor’s rated current, it becomes necessary to limit the current by means of a starting resistance. It is sometimes also necessary to limit the starting current in order to reduce the starting torque to a value dictated by the mechanical strength of the shafts and other parts of the driven mechanism or to achieve the smooth start required by certain production processes. The goal is usually to obtain the required torque with the minimum starting current… definition from The Great Soviet Encyclopedia 1979)

if you were to use a scope on a motor you would see why we say inrush and startup are not the same. Stick a scope on your motor…pull in the contactor…wham …very high current with very short duration. Afterall…a motor winding is just a short circuit until current causes a magnetic field to build. But the motor isn’t even spinning yet, is it? Now comes startup current and time that is different from inrush and must be dealt with differently. Take a 1hp motor at 480 without anything attached to the shaft. Measure it with a scope. You have inrush ..then the motor spins up to speed and during that period decreasing curve current flows until at rated speed. Now put a load on. Inrush current/time stays exactly the same…but your overall starting current/time curve is totally different and is load based… Switchman from IHS GlobalSpec CR4: Inrush Current and StartUp Current

Seagate SV-35 also describe its starting current as “2A current-limited startup to support the use of embedded system power supplies”. Without inrush current limiter, the inrush current can easily exceed 50 A at the peak of the ac cycle and severely stress the converter’s fuse and input rectifiers�significantly reducing the reliability and life expectancy of the modules.

According to all above, I realize starting current is not a marketing term and an important spec to take care.

Inrush Current is Normal

According to following information found on web, inrush current is common to see on many electronics.

A cold filament offers far less resistance than does a hot one. Energizing an incandescent lamp actually results in an inrush current as much as 10 times greater than normal operating current. You can minimize the detrimental effect of the resulting thermal shock by providing a preliminary warming current for lamps you expect to cycle ON and OFF frequently… Allen-Bradley: Illumination Selection for Indicator Lights

When an UPS is switched from Line to battery (and yes it can take several millisectons) the Inrush current is not a factor. The reason is that durning this switch over time the computer Power is not switched off. The Capacitors inside the PSU will supply the power for this duration. You might see some perabations on the output lines but they would be small. Inrush currents are normally due to the capacitance in the load charging being at 0 Volts and appears as a short circuit decreasing to an open in 5 time constants.

Even with a PFC circuit, the inruch current amplitute and duration is depentent on the load characteristics. Setting the MAX rating to 3 to 4 times rated is not abnormal…  RetiredChief in Tom’s Hardware: Forum: How Many Amps for This Power Supply?

What Happen when there is Inrush Current

When components needs more current (aka inrush current), because the watt is fixed at design time, the voltage needs to be decrease. The temperature also increase immediately due to the inrush current according to EXTENDING THE LIFE OF POWER FACTOR CAPACITORS by John Houdek.

Capacitor temperature rise is a function of physical characteristics, thermal (true rms) current and the capacitor internal equivalent series resistance. Thermal current squared times internal equivalent series resistance is equal to watts (loss) which is dissipated as heat. Therefore, a 10% increase in capacitor current is reflected as a 21% increase in capacitor watts loss (I2R = W, so 1.12 x R = 1.21 x R). Since watts loss has a direct effect on capacitor temperature rise, that same 10% increase in current can result in a 21% increase in temperature rise.

When the voltage is lower than the minim requirement by components, they stop working.

What happen when the adapter cannot provide enough current?

A good Q&A from Stack Exchange: Electrical Engineering: What happens when a device draws more current than the power supply can provide? explains everything:

There are many different things that can happen when you pull too much power and this will be dependent on the technology being used. The typical things you will see is the voltage dropping below the spec’d output or cutting out completely. Some systems might have a fuse that trips when you pull too much power. Or the worst case is you exceed the safe rating of some components that result in something over heating and potentially causes lots of problems (fire, etc).

The circumstances that this can happen pretty much all fall on to the board designer. Many times it is cheaper to build a power supply that can provide less current, so the designer will minimize the cost as much as possible while still being safe. If there is some case that the board designer hasn’t considered, then they might pull too much power. These cases might be things like running an RF module while doing some DSP work, while the designer originally intended for them to only operate at separate times.

The other time that you might pull too much power is under an actual fault condition. This is when something truly goes wrong, like an IC fails into a short condition, or someone accidentally shorts two connections together, or something along these lines.

As to what happens to the device, this also all depends on the device itself and the PSU being used. If the PSU drops its voltage, potentially the device might fail if the system can’t handle the voltage… Kellenjb in Stack Exchange: Electrical Engineering: What happens when a device draws more current than the power supply can provide?

Output Current is different from Inrush Current

I also get a private message from jacson1019 on mobile01. He mentioned that the output current on the label of adapter means guarantee for continuously. But the adapter output peak current (aka inrush current) is different, it is not available on adapter nor manual.

Therefore, vendors will need to to stress test on adapter with DC load. For a hard drive with 2.5 to 2.6A at 12V, an adapter labeled with 3A at 12V need to handle the current for 30 seconds at least.

When consider it take 10 seconds from start up to normal operating, the lost on converting 12V to 5V, and the power consumption by bridge, it should be okay for a 1-bay adapter with 18W.

To summarize, he think it is okay to use a 1.5A / 12v adapter on Buffalo HD-LX2TU3 even Seagate ST2000DM001 needs 2.5A for start up.

Capacitor Life Reduce for High Temperature

Now, I know an adapter may stand for inrush current for hard drive to start up. But every inrush current generate a suddenly temperature increasing, according to EXTENDING THE LIFE OF POWER FACTOR CAPACITORS by John Houdek, the capacitor life will reduce.

There are formula to measure factors affect capacitor life. Due to the increase in capacitor current, it increase the capacitor watts loss and rise capacitor temperature. The more inrush current, the higher capacitor temperature, and the shorter life.

It is generally recognized that the life of a capacitor can be affected by its operating temperature terminal voltage or current. To maximize capacitor life, we should apply capacitors with consideration for actual terminal voltage, total (true) rms current and minimal operating temperature… Capacitors are sensitive to transient over voltages and may experience damage to the dielectric system. This can be experienced either due to transient over voltages on the power system as well as inrush current caused by either single or back to back switching transients. Under these conditions the capacitor terminals are exposed to elevated voltages as well as fast rising current.

Also, based on the formula capacitor inrush current = 1.44 √(short circuit current * normal capacitor Rated Current), you can get the inrush current the adapter may handle. Even an adapter has a limit on inrush current.

S.J. Kulas explains in Capacitor Switching Techniques very clearly:

Capacitance switching applications involve not only interrupting capacitive currents, but also the energizing of capacitor banks, cables and overhead lines. The interruption of a capacitive current can cause dielectric problems for the switching device. The high inrush currents can cause damage to the capacitors of the capacitors bank and to weld the contacts of the switch together. The problems of the capacitive inrush currents and ways to reduce the magnitude of the inrush current, have been chosen for analysis in this paper.

How I match Hard Drive with Adapter based on Startup Current and Power Adapter Output Current

List of my HDD Box and NAS adapter and Current.
List of my HDD Box and NAS adapter and Current.

I decide to install Seagate ST2000DM001 which requires 2.5A in QNAP TS-112; Seagate ST2000VX000 which requires 2A in LevelOne NAS GNS-1001; and two Seagate ST500DM002 which requires 2A in Promise SmartStor Zero as RAID 1.

I am not comfortable with Buffalo HD-LX2TU3 and shall invest a 2A /12V power adapter for it.

Poor Adapter Damaged Hard Drive Issue

I found one issue in the forum complaining about the adapter damaged the hard drive.

In ICY DOCK 的不誠實經營手法, MB561US-4S 電源虛標 + 偷工減料, the customer checked the power adapter in 4-bay ICY DOCK MB561US-4S external storage case. There is only 4A / 12V which is different from 6A /12V in spec.

If you use 4 Seagate ST2000DM001, the inrush current should be 10A, and needs 32W during operation. Theoretically, the power adapter may provides guarantee output 48W which seems enough for this case, but the inrush current is much more than it may provide.

I won’t choose it to keep my data.


Special thanks to Kenny_Din who help me to think from the basic of power adapter in 從挑選電源供應器開始保護您的硬碟 which lead me to the key of my answer.


  1. Wiki: Network-attached storage
  2. Seagate Desktop HDD
  3. The Free Dictionary by Farlex: Starting Current (from The Great Soviet Encyclopedia 1979)
  4. Buffalo HD-LX2TU3
  5. Wiki: AC Adapter
  6. Wiki: Capacitor
  7. Wiki: Voltage
  8. Wiki: Inrush current
  9. EXTENDING THE LIFE OF POWER FACTOR CAPACITORS by John Houdek, President, Allied Industrial Marketing, Inc., and Cesar Chavez, Engineering Manager, ARTECHE / Inelap
  10. IHS GlobalSpec CR4: Inrush Current and StartUp Current
  11. Wiki: Inrush current limiter
  12. Seagate Surveillance HDD
  13. Hearts Electronic Products: Understanding power supplies and inrush current
  14. Stack Exchange: Electrical Engineering: What happens when a device draws more current than the power supply can provide?
  15. jacson1019 on mobile01
  16. mobile01
  17. Wiki: Load line (electronics): DC and AC load lines
  18. Capacitor Switching Techniques by S.J. Kulas
  19. QNAP TS-112
  20. LevelOne NAS GNS-1001
  21. Promise SmartStor Zero
  22. Western Digital
  23. Toshiba Storage
  24. HGST
  25. How to convert watts to amps
  26. Jameco Electronics: Wall Adapters & Table-Top Power Supply
  27. Allen-Bradley: Illumination Selection for Indicator Lights
  28. Tom’s Hardware: Forum: How Many Amps for This Power Supply?
  29. Mobile01: ICY DOCK 的不誠實經營手法, MB561US-4S 電源虛標 + 偷工減料
  30. ICY DOCK MB561US-4S
  31. mobile01: 從挑選電源供應器開始保護您的硬碟
  32. Kenny_Din on mobile01

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