Delete Delete Delete - Page 1
Forum section: electronics / beginners

author: The_Almighty_Bacon_Lord
author_rank: Contributor
timestamp: March 23, 2016, 11:23:43 pm
content: Delete Delete Delete

author: ez24
author_rank: Super Contributor
timestamp: March 23, 2016, 11:25:49 pm
content: 1. Fuse

author: 8086
author_rank: Super Contributor
timestamp: March 23, 2016, 11:26:05 pm
content: Transistors, diodes...at least in my experience these are the most common failures.

author: Brumby
author_rank: Unknown
timestamp: March 24, 2016, 12:38:32 am
content: It's difficult to point to particular components unless you have access to the history of the particular model (or at least family) of this PSU as experienced by others. Fuses, semiconductors and bad joints are all good contenders, but where do you start?Answer: Start with the easy -- Check the fuses and continuity through the mains circuit (you do have a multimeter, don't you?).- If that's all good, then look for the obvious. Louis Rossmann makes a good point of just doing a visual inspection looking for things that stand out, such as a charred piece of PCB, vaporised tracks and other signs that something has gotten way too hot.- A close look at solder joints is useful - and if you're unsure about any, just re-solder them.If there is still no joy, then it's time to get a bit more involved with the circuitry. Having a schematic will be very helpful here.To test various components while in circuit is not straightforward. All the surrounding circuitry of the component under test will affect a measurement - but there are some basic rules that can help. For example, when checking a resistor, if the measured value is less than the marked value, then itmightbe OK - but if it is significantly more (as in outside the tolerance range), then you are likely to have a problem. You also have to be very careful about any residual voltages floating around in caps - and that your meter can introduce voltages into the circuit when doing tests like resistance and diode check.Component testing isn't always an efficient process, unless you have clear indications, such as historical reports that point to specific components or a nicely baked area.I've struck out the following because of this statement:Quote from: The_Almighty_Bacon_Lord on March 24, 2016, 12:08:20 amYou need to get a bit more grounded in what you're doing before I'd suggest powered testing.We then step up to powered testing. The first rule here isDon't kill yourself. Know where the dangerous areas are and keep away. Respect the mains - because it won't respect you.That said, start checking from the secondary of the power transformer and work your way towards the output. This will be much easier with a schematic - but even then, the layout of a schematic and a PCB isn't always similar.That's where I would start.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 24, 2016, 03:03:10 am
content: First, you mean most likely in general? Or first as in, first in the cascade of events?Because failures are most often cascades. Consider:1. Suppose an output diode fails shorted (due to overheating at high current and high temperature).2. Now the primary side transistors are switching into a short. Then those transistors die (possibly explosively, as they discharge the big electrolytic capacitor in the process).The diodes won't have failed visibly, but will be damaged.3. Surge current is drawn from the mains, eventually blowing the fuse. This current (perhaps 2000A) is drawn through the input FWB or PFC rectifier.4. That current possibly blows up one or more of those diodes (there are always at least two diodes).5. If the unit has an active PFC stage (looking at pictures of the unit in question, it looks like it does), then those transistors will be trying to switch against the PFC diodes, which are now carrying fault current. So those might die, too.Anything connected to a faulting transistor will probably also die:6. In a conventional flyback supply, one transistor does the switching, driven directly by a controller IC (e.g., UC3842) on the primary side. During the fault, the transistor's input (gate) terminal surges to some ~100V, damaging the chip's gate driver section. The series gate resistor if present) may be blown like a fuse.In a conventional forward converter, the transistor drive is transformer coupled, which is pretty safe. (An old ATX power supply is a typical design using bipolar transistors and transformer coupling.)A newer style design, including two-switch forward or flyback types, half bridge and push-pull forward converters, and resonant converters, might use an HV gate driver IC (IR2110 as an example). This IC would be toasted. The controller IC connected to it may also be damaged (when the driver IC fails, it lets through some surge voltage as well, through its input or supply terminals).Things that aren't likely to be destroyed:- Low voltage secondary stuff. Surges don't couple through transformers well, so the secondary side usually rides it out through fault conditions like this.- Most passives. Surges cause thermal stress in resistors and voltage stress in capacitors, but they're much more robust than semiconductors are. Needless to say, transformers and inductors are just balls of wire; a fuse is enough to protect them.- Junction diodes. They're amazingly robust; together with SCRs, they are the only semiconductor types that can actually be protected by the right kind of fuse (you need a special ultrafast "semiconductor" type).So, between these lists, you can see, if you have a damaged power supply, it's quite likely there are many components, buried deep within its bulk, that need to be tested and replaced. Many of them can't be tested in circuit (e.g., the diodes are wired across a 0-ohm transformer winding, good luck ohming that out!). So, you're better off just buying a replacement.Things that are likely causes:- Capacitor failure (electrolytics drying out; ESR rises; heat rises; eventual release of guts)- Thermal overheating, subsequent diode or transistor breakdown- Surges applied to the input or output (wiring faults in a system?, lightning induced mains surges)- Poor design, operation, environment, etc. (overheating, clogged with dust, marginal design limits, poor current-limiting design, etc.)As for more mundane things like open circuits, like cold solder joints, those are possible too, but don't usually result in catastrophic faults so much as errant behavior (output runs in pulses; voltage too high or too low; doesn't start up; etc.). You can trace the circuit (once you're familiar with the basic building blocks of a power supply, that is) to find where the miscommunication is taking place.Tim

author: station240
author_rank: Unknown
timestamp: March 25, 2016, 03:26:31 am
content: These things usually have a 5V standby supply. If that isn't working, then nothing else does.Common parts in this small supply are:1. blue disk capacitor2. optocoupler3. small electrolytic capacitor (eg 10uF, 16V).