I finished my dual-mono LM3886 amp based upon the boards from chipamp.com. Everything was working and sounding great for the last 2 weeks.
Last night I went to turn it on and no sound. I noticed the LED's were not on so I checked the fuse and it was blown. So I disconnected everything, replaced the fuse and everything came on.
So, I reconnected the pre-amp only and the LED's lit up when I turned it on. I turned it off, connected the speakers, turned it on and the fuse blew.
So, I disconnected the speakers, replaced the fuse and it blew the fuse again.
Then I disconnected everything, replaced the fuse and it blew again.
How do I start troubleshooting this?
Last night I went to turn it on and no sound. I noticed the LED's were not on so I checked the fuse and it was blown. So I disconnected everything, replaced the fuse and everything came on.
So, I reconnected the pre-amp only and the LED's lit up when I turned it on. I turned it off, connected the speakers, turned it on and the fuse blew.
So, I disconnected the speakers, replaced the fuse and it blew the fuse again.
Then I disconnected everything, replaced the fuse and it blew again.
How do I start troubleshooting this?
Sometimes its better to be slow....
I agree with Nuuk.... If the fuse is on the primary side, it has to be slow blow
"Some notes on fuses: (a) It is best to use a "slow-blow" type in the power-line circuit, because there is invariably a large current transient at turn-on (caused mostly by rapid charging of the power-supply filter capacitors)." - The Art of Electronics ; Horowitz & Hill
I agree with Nuuk.... If the fuse is on the primary side, it has to be slow blow
"Some notes on fuses: (a) It is best to use a "slow-blow" type in the power-line circuit, because there is invariably a large current transient at turn-on (caused mostly by rapid charging of the power-supply filter capacitors)." - The Art of Electronics ; Horowitz & Hill
Re: Sometimes its better to be slow....
The purpose of the soft start resistor and/or thermister in the primary circuit is to reduce the start up current that establishes the flux in the transformer. Prior to this flux being established the transformer might try to operate as an air cored transformer and that produces very little if any output at the secondary
Horowitz and Hill are referring to the charging current and that can only flow after that start up flux has been established.
The best place for slowing down the smoothing capacitor charging is in the secondary circuit and requires a much longer time delay than the soft start.
Try an experiment:
use a >300VA transformer and find the largest fuse that blows when direct on line starting is used.
Now fit a timed delayed bypass across a soft start resistor. Set the relay time delay to 100ms. Does the charging of the downstream smoothing caps blow the soft started fuse? Can the fuse be made even lower and yet still not blow on start up? Remember to discharge the smoothing caps before doing these before and after experiments.
I think Horowitz and Hill are wrong here.stpeter said:
The purpose of the soft start resistor and/or thermister in the primary circuit is to reduce the start up current that establishes the flux in the transformer. Prior to this flux being established the transformer might try to operate as an air cored transformer and that produces very little if any output at the secondary
Horowitz and Hill are referring to the charging current and that can only flow after that start up flux has been established.
The best place for slowing down the smoothing capacitor charging is in the secondary circuit and requires a much longer time delay than the soft start.
Try an experiment:
use a >300VA transformer and find the largest fuse that blows when direct on line starting is used.
Now fit a timed delayed bypass across a soft start resistor. Set the relay time delay to 100ms. Does the charging of the downstream smoothing caps blow the soft started fuse? Can the fuse be made even lower and yet still not blow on start up? Remember to discharge the smoothing caps before doing these before and after experiments.
Hi,
motors and transformers should be fitted with a time delayed fuse of approximately
amps~ 3 * Watts / Mains voltage.
This information is repeated in many sources not just this Forum.
But, it means the cables after the fuse must be very big.
The fuse must be smaller than the rated current capacity of all unprotected cables downstream.
To avoid fitting a fuse that is three times over-rated use a soft start on your transformers (of any size, not just >300VA).
I like to close rate the fuse on all my equipment. This can often be less than the nominal maximum current consumption.
eg.
a 160VA transformer running on 110Vac has a maximum consumption of ~1.5A.
A soft start fuse could be anywhere between T1A and T1.6A.
Direct on line starting could be T4A or T5A or if you're unlucky T6.3A
ps,
I got taught this 3times rule in school science classes, probably as a 14year old or maybe a bit younger.
motors and transformers should be fitted with a time delayed fuse of approximately
amps~ 3 * Watts / Mains voltage.
This information is repeated in many sources not just this Forum.
But, it means the cables after the fuse must be very big.
The fuse must be smaller than the rated current capacity of all unprotected cables downstream.
To avoid fitting a fuse that is three times over-rated use a soft start on your transformers (of any size, not just >300VA).
I like to close rate the fuse on all my equipment. This can often be less than the nominal maximum current consumption.
eg.
a 160VA transformer running on 110Vac has a maximum consumption of ~1.5A.
A soft start fuse could be anywhere between T1A and T1.6A.
Direct on line starting could be T4A or T5A or if you're unlucky T6.3A
ps,
I got taught this 3times rule in school science classes, probably as a 14year old or maybe a bit younger.
I wonder what is the formula for this practice with soft start circuit and without soft start circuit....???
is it like this :-
VA / VAC (primaries) = max A rating = fuse size (with soft start)
VA / VAC (primaries) x 2 - 10% = max A rating = fuse size (without soft start)
Please someone correct this.....
is it like this :-
VA / VAC (primaries) = max A rating = fuse size (with soft start)
VA / VAC (primaries) x 2 - 10% = max A rating = fuse size (without soft start)
Please someone correct this.....
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