In our last article we looked at a few of the possible sources of electronic equipment damage due to power transients. In this article Jim and Chuck will further investigate ways to troubleshoot and correct these transient conditions to protect electronic equipment.
Although there are four types of power anomalies, there are three ways that transient voltages can enter an electronic system (including scales):
•The power source
•Electrostatic Discharge (ESD)
The basic goal of any protection device is to divert the excess charge along a path to ground that does not include any of the sensitive electronic components that will suffer damage. This is done in a number of ways and the methods are tailored to the source of the excess voltage.
The power source
Most electronic scales need a source of AC power. Some battery-operated units can be exempt from this type of disruption, but during their charging cycle, they become susceptible, sometimes even if the unit is turned off. Most electronic devices use a power supply that converts the raw AC power into a lower DC voltage. A linear power supply can shield against passing many transients through to the circuitry, but more modern switching power supplies can block many common transients also. The power source can contain any combination of surge (overvoltage for one half cycle or longer), sag (under voltage for one half cycle or longer), or transient, over or under voltage that is very short in duration (less than one half cycle), but can contain very high voltage peaks.
Most modern electronic devices are internally grounded and protected from a static electricity discharge from any outside surface of the device. The problem becomes greatly magnified when either the grounding is subverted or the case is opened. The use of a three-to-two wire adapter on the AC cord is one of the most common ways this is accomplished. The ground pin on an AC cord and receptacle is provided to do just that—connect the device to a legitimate ground (see sidebar “Respect for the AC receptacle”).
If an extension cord is used, be sure that not only is the grounding pin intact on both ends, but that they are connected. Use an ohmmeter to confirm this before plugging in either end of the extension cord. A simple method to check the basic wiring and to ensure that the ground, neutral and hot wires are connected properly is to use a simple tester as shown below. These are available in hardware and electrical supply houses and are quite inexpensive. Any problems that show up using this test require immediate action. NO EQUIPMENT OF ANY SORT should be plugged into any receptacle showing a wiring problem. Remember this device will only tell you if the wires are connected properly, not if the quality of the connection is good or even adequate. Once the wiring has been tested for correctness, the problems of surges, sags and transients need to be addressed. These will require more rigorous testing. All three can be monitored using a device which plugs into the receptacle and monitors and collects information such as the voltage, frequency and any transients outside of specified norms. The data is stored and can be downloaded later into a computer for analysis. Click here to read the rest of part two.