#### csdave

##### Member level 5

hi all,

As you might remember, I am designing the power supply part for a small circuit drawing a max of 250mA and have a few more questions.

My initial design was the following:

**broken link removed**

I computed the 800uF by doing:

Vpeak=9Vac * sqrt(2) = 12.73

the rectifier is quoted as having a max voltage drop per element of 1V (what does per element mean? It's not per diode is it?)

VdropRect=1V (is this correct?

VrippleMax=12.73-7 - 1= 4.73

Then I considered 300mA to be on the safe side...

C= 300mA / (2 * 50Hz * 4.73) = 0.63mF

so I figured 800uF would be fine...

Then however, I realized that a 6V transformer could be enough if I used a larger filter capacitor and was a little bit less conservative so I redid the design by considering a 6Vac 3VA transformer.

here I considered

Vpeak=8.46V

Vdrop=1V

VrippleMax=0.46V

C=300mA / (2 * 50Hz * 0.46) = 6500uF

Would this work?

The power I am asking the transformer to provide is 8.46V*300mA = 2.54W so 3VA should

be enough right?

However, 8.46*400mA=3.4W so I need a smaller fuse for F3, right? (300mA)

A few more questions

1- Was I correct to assume 1V of voltage drop through the rectifier? (cfr http://www.datasheetcatalog.org/datasheet/mcc/W005M.pdf

2- What happens to the current when the filter cap charges and in particular when it charges for the first time? Can't it exceed the 300mA? Do I need a slow-acting fuse? Can this harm the transformer? My equations tells me about the average current in each half cycle of the input sine wave, but aren't there instants when the dissipated power is higher?

3- how big should F1 and F2 be? Back of the envelope tells me 6Vac*300mA = 220Vac*9mA. Should they really be as small as 9mA???

I was thinking of putting the same 400mA or 300mA fuses I have on the other side (they are rated 250Vac), would that be good?

4- if I had designed the circuit to provide 400mA, it would have required a cap of 8.7mF... however 400mA would have been too much for a 3VA transformer, wouldn't it?

Do you have any other suggestions?

Thanks

Davide

---------- Post added at 11:51 ---------- Previous post was at 10:32 ----------

this page Power Supplies seems to give different numbers (apart from the 1.4V on the rectifier) the way they compute the DC voltage is different... I would say mine is more precise, but who's right?

As you might remember, I am designing the power supply part for a small circuit drawing a max of 250mA and have a few more questions.

My initial design was the following:

**broken link removed**

I computed the 800uF by doing:

Vpeak=9Vac * sqrt(2) = 12.73

the rectifier is quoted as having a max voltage drop per element of 1V (what does per element mean? It's not per diode is it?)

VdropRect=1V (is this correct?

VrippleMax=12.73-7 - 1= 4.73

Then I considered 300mA to be on the safe side...

C= 300mA / (2 * 50Hz * 4.73) = 0.63mF

so I figured 800uF would be fine...

Then however, I realized that a 6V transformer could be enough if I used a larger filter capacitor and was a little bit less conservative so I redid the design by considering a 6Vac 3VA transformer.

here I considered

Vpeak=8.46V

Vdrop=1V

VrippleMax=0.46V

C=300mA / (2 * 50Hz * 0.46) = 6500uF

Would this work?

The power I am asking the transformer to provide is 8.46V*300mA = 2.54W so 3VA should

be enough right?

However, 8.46*400mA=3.4W so I need a smaller fuse for F3, right? (300mA)

A few more questions

1- Was I correct to assume 1V of voltage drop through the rectifier? (cfr http://www.datasheetcatalog.org/datasheet/mcc/W005M.pdf

2- What happens to the current when the filter cap charges and in particular when it charges for the first time? Can't it exceed the 300mA? Do I need a slow-acting fuse? Can this harm the transformer? My equations tells me about the average current in each half cycle of the input sine wave, but aren't there instants when the dissipated power is higher?

3- how big should F1 and F2 be? Back of the envelope tells me 6Vac*300mA = 220Vac*9mA. Should they really be as small as 9mA???

I was thinking of putting the same 400mA or 300mA fuses I have on the other side (they are rated 250Vac), would that be good?

4- if I had designed the circuit to provide 400mA, it would have required a cap of 8.7mF... however 400mA would have been too much for a 3VA transformer, wouldn't it?

Do you have any other suggestions?

Thanks

Davide

---------- Post added at 11:51 ---------- Previous post was at 10:32 ----------

this page Power Supplies seems to give different numbers (apart from the 1.4V on the rectifier) the way they compute the DC voltage is different... I would say mine is more precise, but who's right?

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