Electronic power supplies

Thread Starter


Joined Jan 17, 2008
I had an emergency led lamp consisting of 120 5mm white leds. The unit was powered by a rechargeable 12v 2Ah lead battery. When the battery finally packed in I removed it and via a plug-in connection attached a mains power supply giving 12v at 2A. This worked fine. Then to make the system more compact I replaced the power supply with a lightweight switching power supply as used to power a 12v external hard disk. The leds lit up alright at the beginning and then the lights started flashing. My query is why does this happen? Surely if there is enough continuous power from one of these units to drive the motor in an external hard disk then why do the leds flash?

Thread Starter


Joined Jan 17, 2008
I never checked the actual led lamp current drawn from the battery. But why would that be important if I'm using a power supply with a similar output? I am assuming that the flashing effect that I get is a result of using a switched mode power supply instead of the usual transformer supply, but why does this happen?


Joined Sep 17, 2013
It's possible that the SMPS has been optimistically rated (Chinese 'Amps'?) and can't manage the LED load continuously, so is going into self-protect mode when it overheats.


Joined Jan 29, 2010
hi b,
The 2AHr rating is the Ampere-hour capacity.
If the battery was specified as 2Ahr at 1Amp, this would mean that the battery could supply 1Amp for 2 hours or say 2Amps for 1 hour.
If the draw current was say 5Amps, the battery would be 'exhausted' after approx 0.4 hours.

Note: the actual relationship between duration of the Ahr rating and current drawn is not linear, the higher the current the shorter the duration.


Check out this AAC link.


Joined Aug 1, 2013
Hard drives are not nearly as hungry as they were 5 or 10 years ago, and it is entirely possible that the switcher is seeing an overload/overcurrent condition, shutting down, then auto-restarting, seeing an overcurrent, shutting down... What is the rating on the lightweight switching power supply label?