June 6, 2013 by Kevin Keras
Power To The People!
One of the most challenging problems faced by field service techs and engineers is intermittent failures that appear to have no discernible root cause. Well…as least not an obvious one like a loose cable or belt. More often than not, the most common cause of such failures is overlooked…electrical power. Incoming AC (alternating current) power has been around so long that most of us take it for granted. So long as the lights come on, we assume all is well. That’s not always true.
In North America, most labs have either 110VAC (actually somewhere between 105-125VAC) or 240VAC (for larger instruments like freezers or floor mount centrifuges). The AC power that feeds most lab instruments is converted into DC power via the instruments internal power supply which also steps it down to power integrated circuits, dc motors, relays, solenoids..etc (generally in the 5-24VDC range). Power supplies are pretty robust devices that can provide constant, clean DC voltage, but like many things in life the quality of the output is a function of the quality of the input. Garbage in = Garbage Out.
Unlike DC voltage which if looked at with an oscilloscope would show a flat line, AC voltage is actually a sine wave and in most cases, the rated voltage of a circuit is represented by the average (RMS) of the voltages under the curve over time (usually 50 or 60 HZ or cycles per minute). Now, garbage might be a harsh term but what we are really talking about are several common problems;
Voltage Spikes – Sometimes called a surge, a result of incoming voltage exceeding the rated voltage by 10% or more. This typically happens when an inductive load (like a centrifuge) is turned off. The centrifuge pulls a lot of current and taking that load away (current) allows the voltage to quickly increase (ie, spike). If an instrument has a well designed regulated power supply then no problem, but transient spikes (think lightening) have been known to take out the best designed power supplies.
Voltage Dips – a temporary drop of more than 10% (ex: 120V * .9 = 108V). Probably not a killer, but what if your device is spec’d at 120V and the input line power is only 110VAC? Now with a voltage dip you are talking 99VAC … Some instrument power supplies have the ability to detect under voltages and report errors, many do not. And…guess what usually happens after a dip? You guessed it, a spike!
Noise – AC-powered devices can create a characteristic hum at multiples of the frequencies of the AC power that they use. Hums are commonly produced by spinning motor and transformer core laminations vibrating in time with the magnetic field. The noises can wreak havoc on under-voltage situations as they can temporarily cause an instrument on the hairy edge to work temporarily. Shut the noisy device off and the line dips down again causing the instrument to fail (or act really weird).
What to do if you suspect power issues? Well for starters, whenever an instrument starts to show ‘random’ failures;
- Have facilities verify incoming power. A digital voltmeter can be used for this, but make sure they are using the RMS (root, mean, square) setting to capture the average voltage. When in doubt, put a scope on it. Scopes can also show noise as well as nominal voltages.
- Isolate the instrument in question. Make sure there are no other devices on the same circuit.
- Put a digital or analog chart recorder on the circuit and monitor the line over several days. Sometimes called a strip recorder, the analog version looks like the lie detectors you see in crime shows. A needle draws on the paper producing peaks whenever it sees a spike or dip. Newer digital units do the same thing but are much less intimidating to less truthful members of society…
- Note the time that failures occur. Not surprisingly, spikes and dips tend to occur in larger facilities when people arrive at work, go to lunch, take breaks or go home around the same times. PC’s , HVAC, lights are turned on/ off – all in the name of conservation…the laws on unintended consequences.
- Plug the instrument into a line conditioner, then plug the conditioner into the circuit. Power conditioner are good for removing noise and higher-end models (don’t go to Home Depot for this…) offer some protection for spikes and under-voltages. Not to be confused with Un-interruptable Power Supplies (UPS) which offer a measure of time insurance in the event of a total power loss. Even basic UPS’ are available with spike and dip protection but even without they are still not a bad investment if you have dirty or unreliable incoming power and no easy way to fix it…
Net/net – don’t be so quick to blame an instrument for abhorrent behavior. Sometimes it is best to recall a bit of Shakespeare…”the fault dear Brutus lies not in our stars, but im ourselves (or our facilities).”