Finding And Replacing Faulty Capacitors – Switch-Mode Power Supply Repair Hints
Faulty And Blown Capacitors (Example On A PC Motherboard)
The above PCB has obvious faults…
- The capacitors have leaked electrolyte all over the PCB and have gone high in ESR (effective series resistance)
- They have “dried out”, and the ESR has gone high, rendering them useless in the circuit, causing other problems
- Some of the capacitors have filled with gas and exploded, breaking down, or totally blowing forcibly apart physically
- The capacitors have been exposed to excessive temperature and have leaked and exploded dramatically due to this
- The capacitors have been supplied with excessive voltage over their recommended working voltage
- They are an inferior brand of capacitor, or have been made faulty (rare unless they are cheap unbranded ones), and they simply have “worn out”
You switch on a power supply, or some appliance with a power supply in it, and nothing happens! what has happened 9 times out of 10?
No lights come on, not a sound; it is totally dead. You say, “last night/yesterday I used it and it was fine! Or you used it for 10 years leaving it switched on; and switched it off for a few hours for some reason, and it stopped working!
What is going on there?
Simple generally… The capacitor(s) have failed, or gone high in (ESR) effective series resistance)
See my other page… http://mark-lawton.com/capacitor-esr-meter-project/ This will explain what ESR is, and how to measure it, and possibly make this project or something similar.
Apple iMac G5 Switch Mode Power Supply Example
Most modern appliances either have a power supply built in, or they have a trailing lead “brick” power supply; you know, the “black box” that makes it all happen!
Well before you start jumping up and down, or you go to your repair shop, or even “bin it”, as they say; calm down!
(I don’t take responsibility for any omission or injury by the way; the information is a rough guide only) Also please note…
The main filter capacitor (rated 400 or 450V DC) will often store around 339 V DC long after the power cord (plug) has been removed from the wall. Not all power supplies contain a small “bleeder” resistor (typically 33K to 100K Ohms) to slowly discharge this capacitor. Any contact with this capacitor may result in a severe (DC)electrical shock.
Always disconnect before testing, and check that the voltages have gone down to more or less nothing before handling/working on/testing etc.
Most power supplies first take in the mains or “line voltage”, this is then rectified and smoothed with a bridge rectifier and a 400V capacitor (maybe less rated for 110V line voltage only; but generally we are talking 85 to 265 V AC input working tolerance on a switch mode PSU, and the 230V AC European and UK mains supply voltage).
The DC voltage from this power supply has to be reconverted back into a form of AC, and then isolated (for your safety, and the correct operation of the appliance), and reduced in voltage to the desired voltage level for the requirement.
Now here’s the clever part…
The reason that we now use a “switching PSU” (switch mode), instead of a heavy mains transformer “brick”, is the following…
Some switch-mode advantages over a conventional (“old fashioned” and heavy) transformer “linear” PSU are…
- A switch mode power supply is a lot more efficient, and uses far less power 70/80%, and sometimes 95%, compared to only 40% plus for the 50Hz transformer “linear” type
- The switch-mode power supply will only supply the current needed at the time, and therefore use less power
- They are lighter in weight
- They don’t use vast amounts of laminated steel as in a transformer, and may run cooler generally
- They are smaller in size
- You can get more current for the size compared to a linear power supply
Downsides are…
- They can be difficult to repair
- They are more complex
- High frequency noise can be superimposed on the mains and other equipment causing interference
- When they go wrong they have more components fail, and more chance of damaging other parts of the appliance/equipment to which it is supplying
OK, back to the PSU operation (cut down for clarity)…
The high voltage goes to a transistor which is connected in series with a very high frequency ferrite core transformer, designed to operate not at 50 HZ, but at maybe 50kHz (more efficient).
Nothing will happen until the this transistor begins to switch the transformer on and off quickly.
At the same time a chain of resistors and some more reguation and smoothing taken from the high voltage side is supplied to an IC, maybe a UC 2844.
Once supplied with the 12/13V DC the IC (UC3844 example) will switch the transistor in series with the transformer very quickly, and thus a current is induced in the secondary, which is then rectified with high frequency diodes, smoothed, and then regulated etc.
Where they are still cleverer, is that when a load is applied, the output voltage will tend to drop, and so this will be fed back to the IC (UC3844), and it will change the way that it switches the transformer so that it will supply more current.
It alters the waveform going into the transformer, and so it therefore only draws the current from the mains that is needed.
This is where they score on efficiency!
OK onto a “switcher” repair…
This PSU isn’t working because the capacitors have failed, so we remove the old capacitors and clean up the PCB; notice the leakage from the old ones…
Switch Mode Power Suppy Capacitor Replacement
The old ones below, notice the chemicals coming out of them (click to enlarge them), and that they are drying out…
The Old Faulty Worn Leaky Capacitors With High ESR
Prepare the new ones kinking the legs to lift them up off the PCB, and protect them from the heat a bit more, allowing more airflow around them…
Preform The Capacitor Leads Ready For Insertion Into The PCB
Line them up ready for insertion for soldering…
The Capacitors With Their Leads Formed Ready For Insertion And Soldering
Replace them, (the correct way around plus to plus, follow PCB art (if shown), and/or take pictures to remember it)…
Power Supply Capacitors All Replaced OK
Also replace the bus caps (high voltage ones on the mains voltage input side), and maybe the bleed resistors if they look a bit “cooked” or overheated…
Replace The Higher Voltage Bus Capacitors And Bleed Resistors
Showing a capaitor soldered into place, and proud of the PCB for a better airflow, and a longer life. Choose low ESR (low Z, or low impedance quality 105 degrees Celcius capacitors for even longer life)…
Switch Mode Power Suppy Capacitor Replacement Showing The Capacitor Soldered In, Raised From The PCB
A new fan would be an idea, as the old one was seized up, causing overheating, which also led to premature failure of the PSU
Power Supply Fan Replacement
The finished job, cleaned, and tested…
Power Supply Tested And Working Correctly With The New Capacitors
Obviously you do need to check the semiconductors for problems, and look for other burnt out components like on this LCD monitor example taken from the web to illustrate…
PSU Parts And Features (LCD Monitor Example) Things To Look Out For
OK I’ll wish you some happy fault finding on your equipment; but be safe! Maybe that ESR meter link will help!
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- Switch Mode Power Suppy Capacitor Replacement
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- Power Supply Capacitors All Replaced OK
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- Power Supply Fan Replacement
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- Power Supply Tested And Working Correctly With The New Capacitors
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- Preform The Capacitor Leads Ready For Insertion Into The PCB
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- Replace The Higher Voltage Bus Capacitors And Bleed Resistors
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- The Capacitors With Their Leads Formed Ready For Insertion And Soldering
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- The Old Faulty Worn Leaky Capacitors With High ESR
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- Switch Mode Power Suppy Capacitor Replacement Showing The Capacitor Soldered In, Raised From The PCB
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- Faulty And Blown Capacitors (Example On A PC Motherboard)
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- Apple iMac G5 Switch Mode Power Supply Example
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- UC2844 Common Application Circuit
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- Mark-Space Ratio variation in a Switch Mode PSU Dependant Upon Current Demands
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- PSU Parts And Features (LCD Monitor Example) Things To Lokk Out For
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Page 1
I need a power supply that can be powered by 230V -240V input and 110V DC output @ 10A
I have a mains isolation transforner at the moment which is rated at 240V input 110V output with a standard brige rectifyer rated at 10.8A which when measured gives me an output of 122V dc which is too high and not very smooth obviously, I could conect a cap to the output to smooth out but the output gets even higher, I have no way of varying the input voltage. What if any can you sugest please
Hi there Steve,
That’s a good question.
Easy ways that may work…
Use a cheap dimmer switch before or after the transformer.
My favorite cheap way of controlling the phase angle of the mains and thus reducing the voltage is to buy a simple thick film controller chip from RS components.
RS Stock No. 307-957 (www.rswww.com) or any good supplier.
Build the circuit as they show it, simple!
I have built one into a metal 3 pin 240V mains socket for speed reduction, and lamp dimming.
Earth the body which is an isolated tab, and the metal socket acts as a heat-sink.
Fit a potentiometer to it as shown, and you can vary the output a lot.
Adjust it to get what you want. It works on AC motors, lamps and all sorts of stuff. You still get AC coming out, so you are OK with inductive loads.
If it is a permanent installation, you can fit a fixed resistor, or a trimmer, and mount the whole thing on Vero board, and fit a heat- sink to the device.
Another way, you could go direct in with mains, and rectify the output and smooth it.
Obviously you’ll need a 1:1 isolation transformer before the unit as otherwise you have no mains isolation, so it can be done.
Use a very fast acting semiconductor fuse to protect the controller if you so wish, and is recommended.
Other ways involve using switching regulators like in a TV set, with many components, and much design needed, but the way above is simple, cheap and straightforward, but the switcher isnt so straightforward.
Don’t forget your DC will go up when smoothed ((square root of 2) x RMS voltage output), and use a high value bleed resistor to discharge the capacitor slowly once it is switched off, and to avoid a nasty shock if it is still charged up, (say 100K 3W).
Mark Lawton
I have a new blog that may be appropriate…
http://mark-lawton.com/speed-control-phase-angle-dimmer/
Enjoy… and be careful with line or mains projects!
Mark Lawton