Lift Controllers

The most frequent visitors to the workshop nowadays are printed circuit boards for a variety of applications. Lift controllers, chillers, central heating, electric fences, and fuse panels for motorcars, and all sorts of odd things arrive each week.
The faults are often not dissimilar to those found in 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.

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 (see picture opposite) 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.

Return to customer repairs