Hi all, got a call from a customer. One piece of electronics keeps blowing it's AC line fuse. (120V 2A slo-blo.) I had them measure power supplies and such, nothing seemed wrong. So they sent the electronics back for me to look at. I got it today. Nothing seems to be wrong... As I turn on various subsystems, the current draw is almost exactly the same as another unit here.
Could it be something wrong with the AC power at their place? Something like a lot of neutral to ground current?
I'm not an AC power guy, but what about something else on the same outlet, but with a poor neutral.. all the current was going through ground, and then this piece of electronics gets added to the line and has a "better" neutral connection and all the current then flows through our unit... It's got fuses on both AC lines. I should ask if it was always one fuse that blew.
Your guesses are appropriate, but don't forget fuse fatigue. I learned that the hard way. Since it is a slow-blow, I assume there is inrush to be considered. How often is the AC applied? Is it a transformer input? Filter Capacitor input?
Neutral to ground current shouldn't affect your equipment -- you should treat neutral and power as "the white wire" and "the black wire" that happen to have a certain voltage between them, and an unknown voltage to ground. Ground should only be there as a safety.
Have them check the voltage of the AC power. It is very common for the 115 volt power to actually be 130 volt power. The power transformer could be marginally designed and not have a high enough ET. And be saturating during the end of each cycle.
They might also put a scope on the power. Might not be a nice sine wave because of some unusual load.
It also could be that the power is turned on and off a lot and occasionally it get a positive half cycle just before the power is shut off and then gets another positive half cycle when it is turned on.
They claimed to be blowing fuses, 50% of the time they turned it on*. The AC goes through a PEM (power entry module) and then to a ~100W linear supply (+/15 V /3A) there is also a 25W linear supply and fan.
*The first call I got was from a student asking for the fuse size, "What ever was in there", I told him. (not knowing the answer.) Finally looked up 2A slo-blo, but he may not have gotten the slo-blo part. (This all might be human error...though at some time the original fuse blew.)
Yeah I was thinking low voltage might cause higher current draw, the idle current of this box is 0.35A.
My current theory is that after the first blew.. (cause unknown) human error has not allowed the proper fuse to be installed, till it arrived in my shop. (I offered to send them a bunch of the right fuses in the mail.) I'm going to order some 2A fast blows and see if they goes ~1/2 the time.
Hah, probably a monthly task is remotely analysising a customers setup. Skype helps a lot, but I'm not bad over the phone or by email (w/pictures)
mostly iron tranny, (Today I took data on current draw as various bits were plugged in. (there is most likely similar data in a 15 yr old notebook, but quicker to take it again )
0.37 A at idle measured with a DMM and 1 ohm in series with AC neutral. On the DMM (fluke 87 89?) the first reading might have hit 1.x V once out of a few tries.. With everything maxed out and cold, the current started around 1.3A maxed at 1.45, and then dropped. I didn't look at the in rush,
Assuming a glass 3AG type fuse, what does the inside of a blown fuse look like? Is there vaporized aluminum spattered all over the glass? Is the break a clean break, or are there balls of molten metal on the ends of the fuse wires? That info will give me a clue as to how much of an overload caused the fuse to blow.
Are there one or more MOV (metal oxide varistor) over voltage protectors across the line, or from line to case ground? If these are too close to the peak line voltage, you'll see quite a bit of current through them, followed by a blown fuse. Check the value and the condition. Extra credit for removing them and testing the knee voltage with a high voltage power supply. Also check if they've been overheated.
I once had a run of base station power supplies (117VAC to 12VDC 10A) that had similar mysterious fuse blowing problems. The circuit was: >- Line -> Fuse -> Switch ->---+----> etc MOV >- Line ---------> Switch ->---+----> etc The switch was a DPDT. The problem was that the switch would sometimes short across the line, causing the fuse to blow. Checking the switch with an ohms-guesser, everything seemed normal. Fortunately, the Gods of Troubleshooting were with me that day. I put the ohms-guesser across the switch (between switch sections), a rather odd thing to do, and heard a tiny beep from the ohms-guesser when the switch shorted the line. Bingo. You might want to test the power switch.
Why does your device need a slo-blo fuse? Are you charging some big caps when you turn on the power? Tube filaments? Maybe an inrush current limiter (NTC thermistor) might be better? Or maybe something in the power supply regulator is momentarily acting like a crowbar which causes an inrush current surge which the slo-blo fuse might be hiding? Insert a 0.1 ohm resitor in series with the power line, put a scope probe from each channel to each end of the resistor, set the scope to A-B, and see if there are any spectacular increases in current when the power is applied.
Jeff Liebermann firstname.lastname@example.org
150 Felker St #D http://www.LearnByDestroying.com