Efficiencies can exceed 85 (compared to 50-60 for typical LPSs) with improvements being made continuously in this technology.
Disconnect the supply from the equipment it is powering if possible.White: -5, Blue: -12, Orange: Power_good (output).Also, smps may fail upon restoration of power after a blackout if there is any kind of power spike since turn-on is a very stressful period - some designs take this into account and limit turn on surge.It's handy for quick checks of semiconductor junctions - in general if it produces a slight glow it's leaky, but transistor B/E junctions have an inherent zener voltage, so there is usually some glow.However, what do you do if 5 V isn't exactly what you need for a project?That voltage must be more than.87 VDC.The following are suggested starting wattages: 40 W bulb for VCR or laptop computer switching power supplies.Multiple parts often fail and just replacing the transistor may cause it to fail as a result of something else still being bad.If the design uses a hard SCR crowbar, an overvoltage condition may trigger one of the symptoms!Nowadays, an IC PWM controller chip is almost always used.Most of the theory of operation and troubleshooting techniques apply to these as well.There is a chance that a blown transistor took out the controller chip as well.
These devices generate the PWM pulse control to the switchmode (chopper) transistor as well as various fault sensing and other control functions.
And, ICs at least, are usually readily available.
It should still be possible to run the unit somewhat reduced line voltage on the Variac.
A bad cap will lose all it's voltage in less then 2 seconds.
For the specific case of smpss for standard computers (PC, Macs, workstations, servers it often doesn't make sense to spend much time or money on repair.Providing open-loop line regulation and minimizing or in some cases, even eliminating the need for feedback control.Remove and check the suspected components.These then drive the 2 transistors via a transformer and the power supply is running.The current decreases to 3 mA, the voltage increase (coming from the 300V) the oscillator start again, ad-infinitum.In additional, you will find DC-DC converters which are smpss without the AC line connection, internally in an increasing number of consumer and industrial applications including things like portable CD players.The power transformer is itself, self oscillating.Under normal conditions, controllers like the UC3840 or UC3842 should current limit on a PWM cycle-by-cycle basis.The battery can be added with a button from a VCR front panel - but don't blame me if you kill yourself because you didn't insulate the added components properly!
This high frequency transformer provides the isolation barrier and the conversion to generate the multiple voltages often provided by a smps.
You better hope so in any case - replacements are usually only available from the original manufacturer at outrageous cost.