In the Hardware thread, "is my shaevaplug dead",
http://plugcomputer.org/plugforum/index.php?topic=1318.0, there are two or maybe three users who appear to have a broken sheevaplug internal mains to 5V power supply unit.
The common theme in these faults appears to be connecting and running an external USB 2.5in disk drive that is drawing it's 5V supply via the sheevaplug's USB connecter.
Looking at the pictures of one of the blow ups, it appears that C1, a 6.8ufd 400v electrolytic capacitor dumped its electrolyte into the supply. This capacitor is connected across the o/p of the full wave bridge rectifier (4 diodes) that rectifies the incoming ac mains voltage.
Now a digression on electrolytic capacitors. (Wikipedia has a full explanation for enquiring minds)
In the theoretical world, a capacitor consists of two plates and an 100% insulating dialectric. In the real world, the dialectric has a dc leakage resistance and an ac leakage impedance. This impedance is composed of a reactive and resistive component. The in phase resistive component is called the "Equivalent Series Resistance" (ESR).
The ac component in the current flowing in the capacitor, is caused by the ripple on the dc current. If the capacitor is drawing a CONSTANT current, there is only the ripple current is the charging part of the cycle when the mains bridge conducts and charges the capacitor, and the constant load discharges it to the mean value. However if the output of the capacitor is connected to a rapidly changing load, then the ripple can assume a large percentage of the total supplied current.
The ripple current is converted to heat (Irip^2 x ESR = watts) and this causes the electrolyte to heat. When the electrolyte heats the ESR increases and the capacitor can go into thermal runaway. To stop the capacitor exploding violently the top of the capacitor is scribed in a cross, as a deliberate weak point. Capacitors that are on the way to self distruction sometimes bulge the flat end out into a dome instead of a flat before they vent.
To prevent this, capacitors that are going to be used in a power supply or high ripple environment ( audio amps etc), are normally carefully chosen for a low ESR and a high nominal operating temperature.
There are also stories that many manufacturers used a "magic" formula from another manufacturer and got it wrong, and made the ESR problem worse. PC motherboards have suffered from electrolytic cap failure for several years, and if you Google for motherboard cpacitors you will find dozens of sites talking about it and the replacement caps procedure.
Returning to the Sheevaplug power supply.
I was worried about these failures so I stripped the power supply out of my sheeva plug and had a look at the manufacturer and type of capacitor used. I found the two main electrolytics were Chinese in origin, Yalecon RX 400V, 105 deg C devices. I searched vainly for a proper specification for these capacitors. All I found was a one line buying spec from one of the Chinese sales factors. The capacitors are manufactured by Gagaku Yiyang Electronics Co,and have the following characteristics (Example RX only):-
Brand Domestic Model RX
Dielectric Aluminum Applications Filter
Profile Cylindrical Power characteristics Power
Frequency HF Adjusting Fix
Lead Type In the same direction pinout Tolerance ± 20 (%)
Withstand voltage 500 (V) Nominal Capacity 68UF
Loss 5.0
The USB V 2.0 spec defines that the host shall supply up to 0.5A at 5V per port.
A typical 2.5in 500GB hard disk drive spec, my Samsung HM500L1 states the power consumption is:-
Power Requirements:
- Voltage: +5V+/-5%
- Spin-up Current (Max.): 1000 mA
- Seek (typical): 2.6 W
- Read/Write (typical): 2.3 W
- Low Power Idle(typical): 0.6 W
- Standby(typical): 0.25 W
- Sleep(typical): 0.2 W
So on idle its using 0.6W = 0.12A, but when seeking it uses 2.6W or 0.52 A
Depending on HOW MUCH FILTERING is in the external enclosure, the drive is going to create some or a lot of ac ripple in the Sheevaplug 5V supply.
This is ignoring the ripple of the sheevaplug SOC's own current consumption variations
The other problem is that the capacitors are "domestic" rated at 105 deg C max. There are higher quality devices on the market rated at 130 deg C.
I have NO knowledge what the internal operating temperature of the Sheevaplug power supply is when it is drawing a heavy load.
Also I have NO idea what the design output current for the Sheevaplug power supply is.
Therefore I would recommend that you power your USB hard drive enclosures from a separate +5v wall wart or brick
WARNING.... MAINS SWITCHING POWER SUPPLIES, ESPECIALLY THE FILTER CAPACITORS CAN HAVE SEVERAL HUNDRED VOLTS ACROSS THEM EVEN AFTER THE SHEEVAPLUG IS DISCONNECTED FROM THE MAINS. THEY CAN KILL OR GIVE YOU A NASTY FLASH BURN. OBSERVE ALL NORMAL HIGH VOLTAGE SAFETY PRECAUTIONS WHEN WORKING ON MAINS SWITCHERS.
If you don't know the normal HV safety rules then you should not be into a mains switcher!
cheers
Patrick
