if a power supply load is pulling 3A, does it means...

[jarthel]

3A is passing through the wires from the secondary to the diodes?

thank you.

 

[jan.didden]

3A is passing through the wires from the secondary to the diodes?

thank you.

Not at all!. Firstly, the diodes only conduct if the sec voltage is above the voltage on the cap after the diode. That interval depends on the circuit values, but can vary from 10% to 30%
of each mains cycle. So, if it is say 20%, in that 20% the sec has to sent enough current through the diodes into the cap to 'bridge' the following 80% time until the next charging interval. That current is therefore MUCH higher than 3A, possibly up to 20A pulses or more.

The larger the cap, the shorter and higher and sharper the charging pulses, and the more hf harmonics in the charging pulses. Alternatively, the smaller the cap, the lower and more 'round' the current pulses are, and the less high-order harmonics in the charging pulses. Makes sense?


Jan Didden

 

[jarthel]

Re: Re: if a power supply load is pulling 3A, does it means...

Not at all!. Firstly, the diodes only conduct if the sec voltage is above the voltage on the cap after the diode. That interval depends on the circuit values, but can vary from 10% to 30%
of each mains cycle. So, if it is say 20%, in that 20% the sec has to sent enough current through the diodes into the cap to 'bridge' the following 80% time until the next charging interval.

the way i understand this is: the cap is fully charged and the secondary stop supplying current. Once the cap has released the stored energy, the secondary starts supplying again.

is this right?

That current is therefore MUCH higher than 3A, possibly up to 20A pulses or more.

but people have used 1n4001 (which I believe is up to 1A) and diodes hasn't died yet. is it because the intervals is quite fast to fry the diode?

The larger the cap, the shorter and higher and sharper the charging pulses, and the more hf harmonics in the charging pulses. Alternatively, the smaller the cap, the lower and more 'round' the current pulses are, and the less high-order harmonics in the charging pulses. Makes sense?


Jan Didden

is there any pros/cons between large (I've seen replies in the pass forum section where people have mentioned 120mF) and small 1st cap?

thank you.

 

[jan.didden]

Re: Re: Re: if a power supply load is pulling 3A, does it means...

the way i understand this is: the cap is fully charged and the secondary stop supplying current. Once the cap has released the stored energy, the secondary starts supplying again.

is this right?



but people have used 1n4001 (which I believe is up to 1A) and diodes hasn't died yet. is it because the intervals is quite fast to fry the diode?



is there any pros/cons between large (I've seen replies in the pass forum section where people have mentioned 120mF) and small 1st cap?

thank you.

The sec is AC, half sine waves, and it can only supply from the moment that the AC wave peaks above the cap (+diode threshold of some .7V) until the AC dips below the cap+diode value. That is from just before the AC peak till after the AC peak.

So what you see is that the sec AC goes about its business swinging from 0 to peak value and back, 100 (50Hz) or 120 (60Hz) times a sec and when it gets close to the peaks the diodes start to conduct. The exact moment depends on how much the cap voltage has drooped due to load since last time that it was topped up.

If you download the supply designed from Duncan Amps website you can play with this and see the voltages and currents. Highly recommended little program.


The 1N400x 1A spec is average. Peak currents can be much higher. I don't have the data sheet handy but would expect repetitive peaks to 5 or more amps would be allowed. Non-repetitive (like at switch-on) spec is even higher.

Jan Didden

 

[jarthel]

thank you for the help

 

[AndrewT]

Hi,
the little 1n400x series can take surprisingly high single shot peak current. I recall 200A., but this would be for very short duration and only once in a while.
Definately not to be used at anywhere near this level for repeated long duration mains charging duty.
I would expect the 1n400x to cope with upto 10Apk on repetitive currents and an on duty of 10% or so, giving you an average of near 1A.

 

[gmphadte]

The peak current of 1N400x is about 30A for one half wave of mains.

 

[jan.didden]

The peak current of 1N400x is about 30A for one half wave of mains.

..... that's non-repititive, then?

Jan Didden

 

[AKN]

Hi,

Yes, Philips datasheet for 1N400x says 30A non repetive peak forward current and 10A repetive peak current.