The most frequent visitors to the
workshop nowadays are printed circuit boards for a variety of
applications. Mostly lift controllers, but also chillers, central
heating, electric fences, and fuse panels for motorcars; all
sorts of odd things arrive each week.
The faults are often not dissimilar to those found in old CRT TV sets, being cracked solder joints and their consequences. One of the types of fault however that is not found in domestic appliances is a damaged varistor. With a thing like a lift controller there's always the chance the installation will get struck by lightning and, to combat consequential problems, the designers generally fit a varistor across each vulnerable input circuit.
Varistors come in various shapes and sizes dependent on the amount of surge protection likely to be needed.
When a varistor has done its job it may be still in good order, however if a significant amount of energy has to be dealt with it can burn up and sometimes catch alight. When this happens the thing will turn into a very low value resistor and blow any fuse in the circuit.
Most times the device does its job perfectly, protecting hundreds of integrated circuits, but will require changing before the circuit will work. Because of the dratted harmonization of European mains supplies varistors will go pop because the designers didn't realise what "harmonization" really meant... but that's another story..
Replacing a varistor without a circuit
diagram or a components list for the board is a bit of a black
art as usually little is left of the original markings and often
there's no firm indication of the circuit parameters. If there's
a row of the things, clearly protecting identical circuits, one
can see the rating of the device on the side of one that's still
nice and blue and shiny.
Unlike resistors and capacitors, varistors from different manufacturers, but nevertheless having the same rating, can be marked in different ways and some background knowledge is necessary when purchasing replacements.
Fixing things like lift controllers (pictured above) is not easy as sometimes the lift is a hundred miles away and it's not a good idea to have the circuit board winging its way backwards and forwards until the fault is fixed. Not just a pain for the engineer having to drive backwards and forwards, but certainly not nice for the lift users, who may be old ladies in a nursing home, having to walk up several flights of stairs while their lift is waiting to be fixed.
One lift I fixed was the one that went up to a main operating theatre in a hospital on the Isle of Wight. As the hospital is judged on the number of operations it can perform the chief executive was rather upset when his lift failed. Such was the panic that I was presented with, not one circuit board, but a huge box full of the things. I had to look at all of them even though it was most unlikely there were more than one or two faults in all. It's easy to diagnose a fault when there's a blackened and burnt diode but very tricky when faced with a faultless board full of microprocessors and logic devices. Finding that there's no fault present always takes a lot longer than changing a burnt diode.
Thankfully most faults are due to the failure of common components such as relays, diodes and varistors and very few due to failure of complex chips.
A lot of failures can be blamed on the original designer. These generally fall into the category of excess heating causing the solder to fail or the board material or the component to burn. Many faults are due to finger trouble such as connecting a wire wrongly or short-circuiting connections and some are due to lightning or surges on the mains supply. Rarely does a component fail when it is run within its ratings.
I recall that when I worked in Defence, many millions of pounds was spent calculating reliability figures so that spare parts could be made available to keep equipment running. Looking back I think that the figures that were turned out were absolute rubbish, as the most common reasons for failure were never considered.
I bet that there are MoD warehouses stacked full of components, costing many billions of pounds, that will never be used. No one will ever admit this, as goodness knows how many little empires are dependent on the calculations, supply and handling (and disposal) of the stuff.
The circuit boards and the IGBT module have been removed and you can see where a failure has occurred by the soot that's been ejected from the module.
In this instance the module was made by Semikron and contained all the major high power semiconductors necessary to control the 3-phase motor.
There's a 3-phase bridge rectifier, a set of IGBTs (insulated gate bipolar transistors) for driving the motor and operating the brake.
The circuit board carries the IGBT drive components, power supplies and interconnections for the module etc.
Unfortunately, when an IGBT module fails some 600 volts or so is placed on the drive circuits because the insulated gate often breaks down. This causes lots and lots of damage to the low voltage circuitry which is very difficult to repair because the parts are surface mounted and carry microscopic codes instead of part numbers.
Generally, one can reckon on a dozen replacement parts in addition to the expensive IGBT module.
This type of unit can provide typically from 7KWatts to 12 KWatts of power.
Above: a view inside a typical blown-up IGBT module used in a Kone drive unit. It's about100mm x 50mm or in sensible units: 4 inches x 2 inches.
The plastic lid has been removed so you can see the inside.
The square white areas are power transistors and diodes.
The black smudges are made by soot which is centred on parts which have failed. The soot is underneath a jelly-like substance which is used to encapsulate the assembly.
Amazingly, a module like this can supply 60 or 100 amps at 600 volts or more to drive a lift motor.
A failure can occur for several reasons. For example if a motor bearing is seizing, excessive current flows and this results in the module getting too hot. The hotter the module gets the less is it able to provide the high currents taken by the motor and it fails catastrophically often taking out a large fuse rated at 100 amps.
A pair of IGBTs for each of the 3 phases supplying the motor is usually connected in series (a "totem pole" push-pull circuit). If the top transistor collector carrying 600 volt HT breaks down to its insulated gate, a considerable amount of damage will be caused to the control circuit board.
Early drive units used SCRs (Silicon controlled rectifiers) but more modern types use a set of very high power IGBT transistors (IGBT=Insulated Gate Bipolar Transistor).
An IGBT can control huge currents at the expense of almost zero power input, much like a high power thermionic valve used in broadcast transmitters.
This is the top side of the board that connected to the module shown above.
The underside of the board is covered with tiny surface mounted parts and the IGBT module is soldered by over 30 tags which are located around the 4 blue capacitors left of centre.
The parts which are usually damaged when the module fails are located in 4 places. Below the 3 orange blocks (pulse transformers) top left; to the left of the pulse transformer at the centre, and underneath both these areas.
The damaged parts will be primarily the optical couplers (the white i/cs) the capacitors, diodes and zeners which feed the insulated gates in the module.
In addition the pair of contactors (bottom right) which are mounted on a second control board may have burnt or welded contacts.
These contactors govern the direction of rotation of the motor.
In this model of drive unit the contactors are not mechanically coupled together so, if both operate and connect the motor to simultaneously go forward and backwards (perhaps due to welded contacts) the IGBT module will fail catastrophically.
The six large black capacitors smooth the rectified 3-phase mains producing up to 600VDC or so; the HT supply to the IGBT module.
Not all jobs are straightforward and clinical. Take this circuit board for example where the designer forgot to consider localised heating from feed resistors.
Two areas were affected similarly, and both sides of the board needed fixing.
Why not chuck it away? Well sometimes a replacement circuit board is no longer available and there's no option but to make repairs, no matter how messy, as the only viable alternative is to replace major parts of the lift system costing tens of thousands of pounds.
If the burning is too serious a new section of board material has to be grafted in place once the burned area has been cut out.
In this case the board carries a pair of numeric indicators showing the floor of the building. Sitting at ground "G" most of the time resulted in the pull up resistors for the relevant LEDs in the matrix getting very hot. Eventually the solder melted and the joints began to go intermittent. One or more resistors then got even hotter and one eventually failed. The only warning something was amiss was when part of the "G" disappeared.
Here's an instance of what happens when a lift engineer connects a high voltage to a low voltage circuit. Because a connector was poorly numbered Pin 4 wasn't Pin 4 carrying a ground connection of zero volts it was Pin 21 carrying 24 Volts.
Unfortunately, in this case 24 Volts was connected to the local intercommunications bus network and damaged 3 boards.
On the left is a bus chip (an NXP PCA82C251T/N3) and on the right four zener diodes and a termination resistor. Clues to the mishap are the small holes in the top of the zener diodes and the burn mark on the 220 ohm resistor. Sometimes, one of the hardest jobs is to identify the damaged parts. Here the letter "J" lying on its side is a good clue as it's a date marker used in this particular orientation by one specific manufacturer. For example, the same marking "Y4" is used for completely different devices by 13 different makers on an SOT-32 or similar 3-pin package. Checking data sheets for each likely candidate reveals only one maker who marks the date of manufacture with a sideways "J". If this method fails to identify a likely candidate, or the markings are obliterated, the only option is to reverse engineer the circuit because circuit board schematics are not available. Some manufacturers even go to the length of filing off chip markings so board repairs are virtually impossible outside the manufacturers repair department.