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  Circuit Diagnosis

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 15 Minute Timer Does Not Time Out

January 26, 2012  

I received a call from an old client.  They said that some infrared LEDs in a circuit I had designed many years ago were fading. The light they emitted was no longer was enough to do the job. I asked them to send me a working unit and all the documentation.

The system was designed to produce an intense flash of 880nm infrared light using an array of 27 LEDs.  The LEDs were driven hard but for only 100ms at a time.  I had my doubts that even after 1000s of flashes the LEDs would not be producing the same amount of light.

After I had received the documentation package, I discovered that extensive changes had been made by someone else long after I was no longer involved with the project.  To my shock, I saw a 555 timer circuit being used as a 15 minute timer.

I questioned my old client.  Yes, they had made changes.  One of the last additions I was asked to make to the original circuit was to provide a means to control the LEDs through a computer link.  The link was to be used only during initial calibration so the LEDs could be aligned properly so they all pointed at the center of a large projection screen. But, it seemed that under certain conditions the LEDs could indeed be left turned on all night.  This only occurred when the power to the LED box was left on while the computer system it was connected to was turned off.  This cooked the LEDs. The 15 minute timer was added to prevent the LEDs from being turned on for more than 15 minutes at a time.  I knew immediately what the problem was.

A 555 timer is a great device and has been used for thousands of short timing applications.  But, it is not good for timing situations when the time stretches out to many minutes.  As in many simple timers, the 555 timer relies on a simple resistor and capacitor time constant to generate the time delay.  As an example, a 1M resistor and a 10uF capacitor will work as a 10 second timer.  A 10M resistor and a good 10uF capacitor will produce a 100 second time.  I myself have used similar circuits with the 555 timer for simple timing functions out to 3 minutes or so.  Beyond 3 minutes or so there is a very real possibility that the timer will not time out.  The reason?  Leakage! As the capacitor values grow larger to produce a longer and longer RC time constant, the capacitor leakage current can often exceed the charging current through the resistor.  As shown in the drawing below, my client’s circuit used a 1M resistor and a 1000uF capacitor to form a 15 minute timer.  This might have worked when the capacitor was new and if the capacitor was at room temperature.  But, as the device aged and if it were used inside a warm enclosure, the leakage could dramatically increase.  As soon as the leakage current exceeded the charging current, the capacitor would never charge up to the 2/3 supply voltage needed to reset the 555 timer.  As the capacitor aged, the times would gradually increase until they became infinite.

15 Minute Timer Circuit

If you were look at almost any aluminum electrolytic capacitor data sheet you will find a leakage figure such as “I = 0.01CV”, were C is the capacitance and V is the applied voltage.  I is the leakage current in Amps.  A 1000uF cap charged up to 5v could, by the data sheet, have a leakage current of 50 microamps.  Holy Cats!  When used with a 1M resistor and a 5v supply the maximum charging current would be only 5 microamps.  So, an older capacitor would never charge up and the 555 timer circuit would wait forever.  No wonder the poor LEDs were cooking.  There really isn’t a simple way to correct this circuit.  Even the best of electrolytic capacitors will still have unacceptable leakage currents.  I you were to increase the resistor value to say 22M with a 47uF electrolytic cap, the lower leakage would still be offset by a much lower charging current.  The best solution would be to replace the 555 timer circuit with a digital timer, which uses an oscillator and a multi-stage counter to form at long time delay.  The cost would be minimal and since no large capacitor would be needed, the circuit would not take up much more board space.  The lesson here is that a 555 timer should never be used for times beyond about 3 minutes or so.

LED Array 1000uF Capacitor

Second Quarter,  2012    

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