blown fuses in dual fuse iec inlet

JAY X

Member
2010-08-29 12:27 pm
Hi All!,

Just finished building my psu with pcb transformer. (Talema Nuvotem 10Va).

I calculated the fuse: (10VA/230V) X 1.414 = 61,47 ma; roughly 63maT FUSE.

That's what i put in the filtered IEC AC inlet (SHAFFNER FN9260-10-06) rated at 10 amps.... big filter... It features DUAL FUSE holder, so two fuses to protect LIVE and NEUTRAL. Ok so far.

Before this psu i already built other one with the same trafo, and 63maT fuse (1 unit in the LIVE wire), and had no problems...

After blowing 4 fuses.... i re-checked the trafo specs sheet, and they recommend 80maT fuse in the primary. I suppose to hold the initial inrush current. But, i already have only 100maT and 125maT fuses right now...i don't know if they will be ok or a bit too big...

Thanks for your help!
JAY X
 
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The fuse rating does not change the start up current surge.
It's the resistance in the primary circuit that affects the start up current while the transformer core is partly saturated.

Very low VA transformer use extremely long thin wire for the primary winding. These inherently have very high primary resistance. This resistance fairly effectively limits the start up current and usually there is no advantage to adding any more current limiting resistance.

This is confirmed by the manufacturer's recommended fuse rating of T100mA for a maximum power current rating of 61.47mAac. The maximum primary current is actually the output current rating divided by the transformer efficiency while delivering full power. Maybe ~90%.

If you don't have a T100mA fuse, then maybe an F200mA, or F160mA, fuse will give sufficient pass capability during start up to prevent nuisance blowing.
You should fit a fuse that prevents any cable (or equipment) downstream catching fire during a fault incident. A 1A fuse usually does that.

My soft start transformer of 230Vac, 6VA rating uses an F500mA fuse.

Fit a very high fuse rating into the Neutral side of the dual fuse IEC. You don't want the neutral fuse to blow leaving the Live fuse still powering all the live bits to 220Vac after you think the unit is apparently OFF !
 
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JAY X

Member
2010-08-29 12:27 pm
Hi All!

Four eyes see better than two....Friday afternoon i went to an audio shop which also repairs, and after blowing a pair of 500ma fuses, we checked the psu and found that the primary wires were bad wired. After cutting the wrong traces, and placing two 63maT fuses...¡voilà!.. psu alive!.

It is all about checking, checking and checking...

JAY X
 

JAY X

Member
2010-08-29 12:27 pm
Hi All!

Right now i'm waiting for a custom case for my audio project. Meanwhile i would like to know your opinions on "how to" and "where to" hook the secondary psu gnd to the chassis gnd

Some say to take the gnd wire to chassis from the secondary at the last rectifier capacitor before regulator.

others say at the gnd regulator pin

others at the gnd output (0v)

the regulators are lm317/337. CRC filter: 4400uf-2r2-2200uf


As i don't have a case, i have a big amount of ripple at the psu output. ¿is that normal without a box? I suppose the box acts as a faraday cage diverting ripple and noise to chassis ¿right?.

thanks for your advise!

JAY X
 
The audio side does not need a metal chassis to operate.

Adding a metal chassis improves attenuation of interference.
Connecting the PSU Zero Volts to the Chassis does not improve the audio.

If you use connections and circuits that have good attenuation of interference, then your circuit will operate without a metal chassis.
Much of your electronic equipment that you buy comes without a metal chassis and it operates well. eg. TV, Radio, DVD, CDP, Xbox, telephone, etc.......
 

JAY X

Member
2010-08-29 12:27 pm
Hi Andrew!

Right, Ok, this makes a lot of sense!!

The fact is that when i increase the volume of the audio board i hear the psu ripple!

The psu board is made with a gnd plane. two layer board. And the transformer is pcb type.

Maybe with a scope i can check some nodes...this afternoon i will post some measurements.

jay x
 

JAY X

Member
2010-08-29 12:27 pm
Well, i don't know if it is hum, more hiss and ripple bo-bo-bo-bo-bo like pulses. in any case, annoying...:( at zero volume, zero noise. I will post an audio sample asap.

I have a quite old soundcard. ESI wt 192m.(2003), with unbalanced outputs. when i connect the ouputs directly to my speakers (KRK V6), the outputs are not so noisy, maybe because the krk's balanced input rejects the noise...

jay x
 
JAY X said:
Meanwhile i would like to know your opinions on "how to" and "where to" hook the secondary psu gnd to the chassis gnd

Some say to take the gnd wire to chassis from the secondary at the last rectifier capacitor before regulator.

others say at the gnd regulator pin

others at the gnd output (0v)

the regulators are lm317/337. CRC filter: 4400uf-2r2-2200uf
Two different issues getting mixed up here.

The 'ground' connection from the PSU to the audio circuitry must come from the point in the PSU ground bus which is furthest from the rectifier/reservoir - this is the cleanest part of the PSU ground.

If the audio ground is to be connected to the chassis/safety ground (which is usually a good thing from a safety point of view) then where this is done depends on a whole lot of issues. If thr audio grounding is correct then you should not get much difference in hum/buzz in any case.

As i don't have a case, i have a big amount of ripple at the psu output. ¿is that normal without a box? I suppose the box acts as a faraday cage diverting ripple and noise to chassis ¿right?.
No. Ripple comes from PSU capacitors. Anything else is not ripple. Ripple is completely unaffected by boxes, screening etc. PSU buzz (not ripple) may be affected by PSU grounding details - people often inject charging pulses into their audio ground and then wonder why they get buzz.
 

JAY X

Member
2010-08-29 12:27 pm
Two different issues getting mixed up here.

The 'ground' connection from the PSU to the audio circuitry must come from the point in the PSU ground bus which is furthest from the rectifier/reservoir - this is the cleanest part of the PSU ground.

ok, this is what i did. ok!


If the audio ground is to be connected to the chassis/safety ground (which is usually a good thing from a safety point of view) then where this is done depends on a whole lot of issues. If thr audio grounding is correct then you should not get much difference in hum/buzz in any case..

ok.:)



people often inject charging pulses into their audio ground and then wonder why they get buzz.

¡This is interesting!..how does it happen? (injecting charging pulses). where could be the mistakes that lead to this?.

JAY X
 
Two common mistakes:

1. use a 'star ground' and connect everything to it, including the transformer secondary CT and reservoir cap -ve (or common, when dual polarity supply)
2. connect CT to chassis, then connect reservoir cap -ve to chassis, then use chassis as signal ground too - very common in old radios, guitar amps etc. which is why people expect old stuff to hum