Why did the designer put a 350v cap when the max voltage of circuit is 20v?

[Ehsan]

Here is a simple current source being derived from an unregulated 20v DC coming from a bridge rectifier:


My question is that why does the designer have to place a cap that stands 350v while the DC voltage is only 20v (referring to C4)?

The DC voltage coming from a bridge rectifier being feed by a transformer.

 

[KrisBlueNZ]

I can't suggest a reason. But I notice that the capacitance and voltage markings for C4 are in a different font from all the other markings on the diagram. Maybe it wasn't originally designed with a 330V capacitor. Maybe someone has made a mistake.

 

[chopnhack]

My uneducated guess would be one of a few reasons: supply on hand, schematics were drawn up to include parts already owned or a simple typographical error - an extra zero or a missed decimal point.

How large a value does the secondary produce before feeding the bridge?

 

[Ehsan]

But I notice that the capacitance and voltage markings for C4 are in a different font from all the other markings on the diagram.

Nice observation, I didn't notice that!

 

[Ehsan]

How large a value does the secondary produce before feeding the bridge?

It has two transformers, I assumed both secondaries produce 20v based on the information related to the circuit.

This is not my design, It is a power supply circuit that I am studying over.

I think it is better I upload the full schematic:

 

[BR-549]

I would set supply for highest voltage and current, Attach scope probe to C4. And then short the output. But they wont let touch a screwdriver any more.

 

[Ehsan]

I would set supply for highest voltage and current, Attach scope probe to C4. And then short the output. But they wont let touch a screwdriver any more.

Who won't let touch a screwdriver ? I can't get your point.

 

[davenn]

wasn't an overly helpful post
I don't think English is in their top 3 languages

Ehsan

Are you able to confirm that there is only 20V on that top rail
ie. on the top side of D16 ( on the collector of Q8)

Dave

 

[Ehsan]

Are you able to confirm that there is only 20V on that top rail
ie. on the top side of D16 ( on the collector of Q8)

There is no information about transformer rating in the datasheet, so I can not confirm, but it is designed to have maximum 20v regulated DC output. Considering filter capacitor C5 (50v rated) the top rail could not exceed 50v. I think it should be only 2 or 3 volt more than 20v, to minimize voltage drop across the regulator.

 

[BR-549]

Pardon me, Ehsan, I wasn't able to respond right away. I assumed the the power supply was working properly and you were curious as to why the high voltage rating on C4. I believe the supply to be rated at 40 DC at 3 A. I was suggesting that you observe C4(with scope,looking for spike,high frequency) while shorting the output. If you don't see anything......then spark the output. Sparking will cause the most extreme output load. Now if you still don't see anything on C4.....attach 3 A, 40 VDC load on output. Then pull the mains quickly. Now you have checked the most extreme input mode. I believe there is a reason for that voltage rating......we just don't know what it is. I am sure it makes the unit reliable. In the USA, when techs get a little age....they take them out of the field and put them on the bench.......bench work. When they get to old for that.....it's called taking your screwdriver away from you. Sorry.

 

[Ehsan]

I assumed the the power supply was working properly and you were curious as to why the high voltage rating on C4. I believe the supply to be rated at 40 DC at 3 A.

I don't own the unit, I just studied its schematic. So I can't apply your suggested experiments, but spikes under extreme conditions is probably the reason for that high voltage rated cap, I agree.

 

[davenn]

There is no information about transformer rating in the datasheet, so I can not confirm, but it is designed to have maximum 20v regulated DC output. Considering filter capacitor C5 (50v rated) the top rail could not exceed 50v. I think it should be only 2 or 3 volt more than 20v, to minimize voltage drop across the regulator.

why I ask is because the voltage selector switch shows a 40V setting