dave@onfcanim.UUCP (Dave Martindale) (09/16/86)
A components question for you electrical engineers out there:
I have a stepping motor driver sitting on my desk. The motor supply
voltage is 14V nominal, which is produced with a 28VCT transformer,
two diodes, and a capacitor.
Under full load, the power supply's output may indeed be 14V. But with
all motors freed (no load on the supply) the voltage goes to 20.5V.
The capacitor, however, is rated at only 15VDC.
I wouldn't use such a low voltage rating on the capacitor myself, but
given that someone else has, is it safe? Will this capacitor take a
35% overvoltage continuously due to its specifications being conservative?
Or will it explode one day? Should I go out and replace it?
Finally, what (book) is a good source of such information? I prefer
to look things up in my own references rather than request information
on USENET if I know where to find it.
Thanks for any info (send it via mail, I'll summarize),
Dave Martindale
{watmath,musocs,micomvax}!onfcanim!davewtm@neoucom.UUCP (Bill Mayhew) (09/17/86)
Hi, I tried to dig up my reference from Mallory, but it seems to have wandered off. A call to Hamilton-Avnet or a similar distributor of components should be able to yield a capacitor spec. catalog. Most electrolytic capacitors are designed to withstand overvoltages of 20 to 30 per cent for brief periods. Sorry, I can't cite numbers on the duty factor for overvoltage. A general rule of thumb that I was taught was to design in electrolytics that had woking voltages about 1.33 times the maximum expected input, irrespective of the cap.'s so-called surge rating. The general symptom of an over stressed electrolytic cap. is getting hot and then, perhaps, exploding. The voltage rating of the cap. is proportional to the thickness of the dielectric paste. Too much voltage will decompose the paste and cause resistive heating. If the cap. is not provided with an emergency vent plug, it will explode. I've seen serveral nasty capacitor explosions. The dielectric that gets sprayed inside the affected equipment can also be corrosive and quite smelly! The worst offenders are small electrolytics that have been completely dipped in plastic and thus have no vent. If a capacitor sits around in a parts bin for a long time, the paste will tend to depolarize somewhat and become partially conductive. Its usually a good idea to reform the dielectric in a capacitor that has been sitting unused for a long time. This can be done by connecting the cap in series with a 10K ohm or so resistor to a power supply. Charging the cap up slowly gives a chance for the polarization to reform. You can get an idea of how well the capacitor is doing by monitoring the voltage drop across the series resistor. --Bill Bill Mayhew Division of Basic Medical Sciences N. E. Ohio Universities' College of Medicine Rootstown, OH 44272 USA (216) 325-2511 (wtm@neoucom.UUCP)
dave@onfcanim.UUCP (Dave Martindale) (09/19/86)
Well, I've received a number of replies by now (over half of them from Tektronix!). The concensus is that the stress of working at a higher-than-rated voltage will greatly shorten the life of the capacitor. No one predicted it would explode (unless the voltage applied was increased to twice its working voltage). Someone phrased it as "when you select a capacitor's voltage rating, you are really selecting its wear-our rate". It seems that general practice is to use a capacitor which is overrated by about 50% - so a 30V rather than 15V capacitor is appropriate to this application. I'll replace the capacitor before I actually put the piece of equipment into service. (I've only been testing it so far). Thanks to all those who replied. Dave Martindale