Trying to understand Power Supplies - The first Cap in a CRCRC

[Brit01]

Just trying to understand the concepts behind putting the first cap in this configuration and in partucular why we can only use a small value with tube rectifiers and much larger values with SS recitifiers.

eg.

with my 6X5 I use only a 0.47uF

but in a SS we can use maybe a 47uF or more.

What effect does using a 0.47uf/1uF first cap in the SS set up have instead of 47uF?

[janneman]

Originally posted by Brit01
Just trying to understand the concepts behind putting the first cap in this configuration and in partucular why we can only use a small value with tube rectifiers and much larger values with SS recitifiers.

eg.

with my 6X5 I use only a 0.47uF

but in a SS we can use maybe a 47uF or more.

What effect does using a 0.47uf/1uF first cap in the SS set up have instead of 47uF?

Hi,

Are you talking of the first cap after the rectifier (tube or ss)? It seems your values are very low. Common values are 50-100uF for tubes and 10,000-20,000uF for ss.

Anyway, there are a few issues. First of all, the cap holds the energy to supply the amp during the interval that the secondary transformer voltage is lower than the cap voltage and the rectifier doesn't conduct. You may be surprised that that is some 80-90% of the time. So during that time, the cap is the only energy source. Since the energy is a function of voltage and capacity, it follows that lower voltage (ss) tends to require higher capacity, especially since ss amps are usually more powerfull (require more current) than tube.

So, if your cap is too small, it will discharge rapidly between top-ups and that increases the ripple.

OTOH, too large a cap can really stress the rectifier: when the diodes again start to conduct when the secondary transformer voltage rises above the cap voltage, a huge cap looks like a bottomless pit and takes a LOT of current to top up. In ss it is not uncommon to see current spikes into the cap of 10A and more. SS rectifiers can take some abuse, but for tubes the cap value should not be too large.

Does this help?

jd

[Brit01]

I'm referring to the first cap after the bridge before any resistors.

In my 6x5 psu I've got

0.47uf
1k
100uf
1k
200uf

But maybe in a SS I could use:

47uf
1k
470uf
1k
680uf

Now what would be the result of using 1uf or 0.47uf as the first cap in a SS bridge? No point at all?? I can use a high quality PIO or something as in my 6x5 psu.

[chrisb]

It never hurts to follow the original manufacturers' spec for cap input filter - some are very low.

A quick search at
http://tdsl.duncanamps.com/tubesearch.php



will reveal that the 6X5 calls for max of 4mF

http://www.pmillett.com/tubedata/HB-...5_6X5-GT_G.PDF

[Brit01]

Thks chrisb.

I'm happy with a 0.47uF for my 6X5GT. This works just fine.

I'm really interested in the SS bridge and if I could use such a low cap here but followed by large caps 470/680uf.

so intending:

1uF high quality WIMA or PIO
100 ohms
470uF
100 ohms
680uF

[ChrisA]

Originally posted by Brit01
...why we can only use a small value with tube rectifiers and much larger values with SS recitifiers.
...

One of the specs on a rectifier tube is the max size of the first cap in the filter. The reason for the limit on cap size is "in rush current". When the AC mains power is flipped on the un-charged cap looks to the rectifier just like a dead short and a very large current goes through the tube. The only limit is the resistance in the transformer. The tube rectifiers simply can not handle this kind of current for any length of time, or they melt. SS Diodes can handle a longer in-rush period but they have specs for this too. The in-rush current is limited by what the transformer can supply but the in-rush period is determined by the size of the capacitor. The rectifier is between the two. All three must be "in balance" or you get either smoke or you have paid to much for one of the parts.

If you want a larger input cap but are limited by the in-rush energy limit of the rectifier there is still a way. There are in-rush limiters. These look like 200 ohm resisters when powered off but. Put one in series with the reccitifier as the device heats up the resistance drops. The 200R limits current in the first few seconds. After that they act like 2R. Every switch mode computer power supply uses these and they cost about $2 each. The correct term to Google is Current Limiting Thermistor.

You see different designs over the years because 50 years ago capacitors where very expensive and chokes where cheaper. Today we have cheap caps and chokes cost more. Today with SS diodes we cn have larger input caps so we can lower the values of the others. It's all a cost trade.
A lot of what you see in amp designs is an engineer's attempt to be cost effective.

[Brit01]

Thanks ChrisA. Good info there. makes sense.

So there is no harm in using a small cap in the SS set up really just as long as I have large caps in the RCRC to reduce the ripple.

From another Chris! ( I should be ChrisC)

[Sheldon]

Originally posted by Brit01
Thks chrisb.

I'm happy with a 0.47uF for my 6X5GT. This works just fine.

I'm really interested in the SS bridge and if I could use such a low cap here but followed by large caps 470/680uf.

so intending:

1uF high quality WIMA or PIO
100 ohms
470uF
100 ohms
680uF

Yes, you can use low values with a solid state bridge. But the question is; what are you trying to accomplish? What is your transformer delivering? What voltage output do you want? What ripple level do you want? How much dampning do you want?

All these things you can predict if you model the supply in PSUD. It's free from the Duncan Amps site. It's easy. It's accurate.

Sheldon

[Brit01]

All these things you can predict if you model the supply in PSUD. It's free from the Duncan Amps site. It's easy. It's accurate. Sheldon

Thks Shedlon.

Yes I'm a regular user of this. It's great software.

I was trying to use a 1uF cap here just because of financial restraints. Got a load of WIMAS 1uF and a box of 470uf and 680 uf caps.

the 47uF caps are expensive here. But I'm getting 4 new ones in a few weeks from someone travelling.

But in the meantime I simulated using a 1uF as the first cap and everything looked fine. Very low ripple with the big caps after.

275V transformer secondary.
Trying to get the max possible out of this to run 150-200mA.

[Sheldon]

If it meets your requirements, then it's fine.

Sheldon