Serious power problem? (Interesting)

[Dominick22]

I just completed my 4 channel LM4780 amplifier, and fired it up.

The lights dimmed and the transformer got very warm very fast! I think I may have even heard a pop come from the wall outlet! I shut her down in seconds. The actual chips were cool. This is my second chip amp so I feel confident in the wiring and before the final build everthing ran great in testing!?

Any ideas where I should start trouble shooting??
Oddly enough-the fuses DID NOT pop.

6 months of work on halt 1 inch before the finish line! Help please!


Dominick

[anatech]

Hi Dominick,
You have a high current fault. Look in the primary circuit of your supply, also check for shorts directly across your caps. A shorted rectifier may do this too.

Do you have a variac or socket with a light bulb in series?

-Chris

[Dominick22]

Anatech, I knew you would reply-your alwasy a big help.

Your talking greek to me though!LOL

I dont know what a variac or socket is?

I can start checking for a short across the caps and rectifier now though.

Out of curiousity. If one of the chip pins came loose during the build...could that cause this problem? I dont think one is, but that would be the hardest to identify I think!

Thanks,
Dominick

[Stocker]

Variac is the Xerox of the variable autotransformer world.

It works kind of like a very big potentiometer, with a coil of wire across your mains and a wiper acting as a take-off point for the voltage, which changes depending on the position of the controls. The output AC voltage is variable.

Using a light bulb in series with your gear will provide a good indication of current draw... if you are drawing lots of current the bulb will light...

How is your amplifier fused? It sounds like the transformer has a short to ground (possibly to the core?) or else you have a drastic wiring error. IF the fusing is before the primaries of the transformer, replace with 1A fuses and try again. If it is fused after the transformer that's a problem in itself.

/edit: if one of the pins was shorting the AC input to the chassis, that could cause a high-current fault, but hopefully your test setup or house wiring is GFCI protected and would shut off the supply faster than you could jump back or else the pin might either a: vaporize or b: weld itself in place and conduct current or c: fly off into nowhere, preventing a recurrence of the problem.

What I would do: Inspect carefully for obvious problems. Plug the thing into a power strip at the end of a long cord, with a long inlet cord to your device, with a blast-shield of some sort just-in-case, and switch the powerstrip on, just for a moment, to check it again.

p.s. AFTER making sure the fusing is right!

[Dominick22]

I have it fused before the transformers with 5a slow blows. 1a seems like it would be too low even if the wiring was correct? I am running dual 22v 400va transformers with dual secondaries. Each primary has a dedicated 5a fuse.

Still wondering...And I checked for shorts anatech. (None)


Maybe I should try one transformer at a time? I guess if they both fault, then it HAS to be a wiring error---Right? Should I do that?

Dominick

[Stocker]

hey look who's online!

YOU!

Grab the multimeter, set to measure ohms or else the continuity "beep" check

You should have no continuity between the hot and neutral wires (read: couple of ohms of primary winding, not 0 ohms, nor to ground from either) that plug into the wall and good continuity between the ground lug and the chassis (you DID ground the chassis right? )

1A slow blow should not pop if you don't have huge capacitance installed, I think. 2A probably wouldn't if 1A did. If you can disconnect the secondaries from the rest of the circuit, do so and try again.

everything worked before final build... the final build is the difference. You may have done what we have all done (I just killed a computer motherboard with a stray bit of cut-off screw last week) and made a small, critical error. basic checks are the first ones to make; please see my /edit comments above.

[DJ Exprice]

How did you mount your Chips? Did you forget to use silpad? If your case is metal, and the 0v is connected to the case of the amplifier, you want to make sure that V- is NOT connected to the case. That could be a problem, or, in fact, you could just have a wacked up transformer.

[lgreen]

Hey, sounds like something that happened to me. In my case I had a "shorted turn" problem. The bolt from my toroid was touching the top of my chassis, which causes the toroid, chassis and bolt to get red hot, make lots of nosies, and a buzzing sound. Look into this, if you have a toroid, if not, sorry I could not be of more help.

[AndrewT]

Hi,
if your transformer got warm in just a few seconds then you were pulling enormous current from the mains.

Your fuses should have blown.

CHECK OUT YOUR FUSING.

Next wire up a mains light bulb in series with the hot from the mains. Fit a plug top to one end and a socket outlet to the other. The cold (neutral) and ground (earth) wires go straight through. Plug your amp into the socket outlet and the plug top into the mains. Insert a mains filament bulb (60W to 150W) into the bulb holder and then switch on.

If all is OK the bulb will flash briefly and go dim or even appear to go out.
If there is a fault drawing excess current the bulb will be either bright or glowing more than dim. This bulb trick absorbs most of the mains voltage and only allows a little power (voltage) into a faulty appliance. IT STOPS THINGS BLOWING UP. Keep it and use every time you test fire an alteration or new build.

The transformer now has some voltage on it and you can start to carefully measure voltages. Start with the internal mains side and identify the hot, cold and earth voltages. Then the transformer secondaries. You may need to step down to the mV range, but ALWAYS start with high range.

I suspect a rectifier fault or a shorted turn.

Good luck and work safely. On second thought do not rely on luck, be methodical.

[Dominick22]

Thanks everyone,

I will get right on those suggestions, but just to let you know, the case is wood so I dont have to worry about grounding the chassis.