Can a loose connection lead to a blown mains fuse?

800 watt toroidal transformer operating at say 400 watts. Secondaries lead to conventional bridge rectifier and smoothing caps for +/- rails. A break appears in the DC rails central 0V return to the transformer, could that lead to a blown mains fuse, via inductive action?
Reply to
N_Cook
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Intermittent contacts within inductive circuits can be disastrous. Hail to the fuse for being fast enough.

Cheers!

Reply to
c4urs11

N_Cook-

It depends on the circuit. If the center tap opens, then voltage between each side and ground will be determined by the ratio of each side's resistance to ground. If one side's voltage gets so high that it causes components to break down, resulting high current might blow the mains fuse.

An open center tap might upset circuit balance so that everything is turned on hard, resulting in too much current. On the other hand, balanced resistances between each side and ground might not cause any problem other than the circuit not working correctly.

Fred

Reply to
Fred McKenzie

lead

break

The power amps always survive . In the process of exploring suspect caps etc and turning over the pcb, the spade-connector for this 0V connection came off the pcb spade. Looking under x30 on the spade and what I can see of the line spade-socket, I can see no sputtering or smoke trails or anything suggestive of a problem there. I'm aware that a valve output matching transformer working hard and there is a break in the speaker wire/connection /voice coil then enough voltage can be induced in the primary side to knock out the output valves and weld turns on the primary. But I do not know if similar in a mains transformer would give enough current , from the energy stored in the inductance , to throw back enough current , at over the mains voltage , to knock out the mains fuse.

Reply to
N_Cook

N_Cook-

For tubes, gain = Gm * R(load). A transformer coupled amplifier output stage has a very high voltage gain if the speaker is disconnected. With no load, tubes alternate between being saturated and cut off. When saturation current flowing in the output transformer primary is interrupted, inductance reacts to generate a voltage that attempts to keep the current flowing. Voltage = L * di/dt, where di/dt (change in current with respect to time) is almost infinite as current goes to zero. This is comparable to the points opening in an older auto ignition system, and voltage increases until insulation breaks down or a spark occurs.

The reason this effect would not happen to your power transformer, is that the two sets of filter capacitors are in series across the full output voltage of the bridge rectifier. Current interrupted when the center tap is disconnected, will be supplied to or from the capacitors.

Fred

Reply to
Fred McKenzie

It's like this. Every time you turn on the switch you are in effect "tighte ning" a connection. When it's a dirty lug or a shitty solder joint, the inr ush of current happens many more times per hour or whatever. Understand if your mains run at 50 or 60 Hz it matters not, with a shitty connection it c ould be effecively running alot lower. The equipment can even work if there 's enough filtering and regulation, but yes, because of having to recharge the main filter too many times a ten amp fuse could blow in a device that o nly pulls two amps.

In the past there were devices that cheated on power consumption by doing t hat. No, they weren't on the shelves at Cosco or whatever. I designed a cou ple. With the mechanical electric meters, inertia is your friend :-) I also found that back wayback if you need a lot of current you could half wave r ecify it and fool the electric meter. My cousin researched that alot more b ecause he had an electric furnace.

Anyway, the bottom line is the answer is yes. Think about it with basic mat h. You apply either 110 or 220 AC to it, the main filter cap has an ESR of less than an ohm, you might have a 2.2 ohm inrush limiter, figure out the c urrent assuming a 0.6 volt drop across the recifiers, actually 1.2 volts. I t's a hell of alot more than xis amps which might be the fuse rating. You k now a 120V US market Sony TV or 32" pulls like 70 amps upon being turned on ? Now THAT is not just the rectifiers, it's the degaussers, but the same t hing applies. Your basic seven amp fuse will pass 100, but only for millise conds.

In fact this inrush current has spawned two design approacxhes in power sup plies. One is using an extraa relay to jump the current limiting resistor s o it no longer wastes power. The other is to use a zero crossing trigger to make the relay contace right where the voltag5e is at or near zero so the peak current is not so high.

Enough. Scuse my typos because my eyesight sucks and I REALLY cannot sit he re and proof this.

Reply to
jurb6006

I've had amplifiers pop the breaker when turned on. (This is extremely rare.) And just the other day, the rear amps tripped their breaker when they were shut off.

Can you say "statistical variation"? Sure you can.

Reply to
William Sommerwerck

rare.)

You have a point there. This amp has an inrush limiter, the heatshrink over it looks fine. I don't know if a VDR variant or what but if it is the usual NTC thermistor then its cold resistance of 0.6 ohms looks suspiciously low. I'll have to strip off the heatshrink and try and find some numbers and data for it

Reply to
N_Cook

Inrush limiter marked with logo something like 8 in a half circle VEI or UEI , 20SP, 0R7 Having difficulty finding data on this but cold 0R7 agrees. In 240V land I usually find inrush limiters of cold value about 3 ohms. I'm wondering if as its a USA made amp then these limiters are used for all models and its not really high enough cold value for reliable 240V use. I'm thinking of odd magnetic domains building up in the core of torroids and occassionally reinforcing the inrush combined with an initially only marginal rating for 240V use.

Reply to
N_Cook

looks as though its N20SP007 on

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looks too low a value of cold resistance for all eventualities 240V inrush current use

Reply to
N_Cook

Its a bit much to see that UEI logo of an egg cup with a variant of the ON symbol inside instead of an egg, as the poor legibility 8 in a half circle here, on a roughish surface, but I'll accept it as so.

6 amp rating for the international model, internal wiring set for USA is the correct component, for UK 3 amp is fine but I suspect the cold 0R7 is just too low. I'll replace with a higher initial ohmage NTC.
Reply to
N_Cook

Yes, kinda. The trick is, an intermittent contact can accidentally rectify, and the primary (AC) current shoots through the saturated transformer. It'll more likely pop the small device fuse than the main breaker panel.

It'd have to be a LOT of DC on the secondary to cause saturation.

A bad connection could also generate heat at/near a fuse, and cause it to fail by heating from outside, but that's less likely.

Reply to
whit3rd

lead

break

A good introduction to this (I did only once) testing a 12V relay. 2 wires to the screw terminals of a power supply. Hold the relay at the pins , one finger holding one wire to one pin and make and break the other contact with another finger touching it. But that is high voltage , rather than high current.

Reply to
N_Cook

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