Usually, I never care about the theory behind all of my DIY project. Just plug it, hear it, like it, and finish it. But now, due to some reason (though somehow I also don’t know the exact reason why), I want to do it from scratch and want to be backed-up with some theory and calculation behind it.

My next project (again could be the last one) is a high end power supply for my Aikido pre-amplifier. I think I’ve mentioned about this project before.

This is the first part discussing about the theory behind my next DIY project (about the tube rectifier IFRM and capacitor/choke input). Next part will come soon.

I’m considering the usage of tube rectifier (I have Svetlana 5U3C or 5U4-GB on hand). So far, I only used to play with silicone rectifier where I don’t have to care about the size of filtering capacitor. Now, with tube rectifier, I have to learn the theory back again.

Tube rectifier, in this case my 5U3C or 5U4-GB, has only 1 Ampere of IFRM (Repetitive Peak Voltage Current) and 99 Ampere of IFSM (Peak Forward Surge Current). FYI, most silicone rectifier like IN4007 has both 99 Ampere of IFRM and IFSM. So any size of capacitor will be ok for this silicone rectifier (it won’t stress the rectifier, 99 Ampere is a huge number!).

The low capacity of IFRM of the tube rectifier makes the usage of big first capacitor is impossible. If you use big size of capacitor on the first stage after the rectifier, then this capacitor will stress the rectifier due to high load of current. Bigger capacitor will pull a lot of current. If this amount of current is bigger than the rectifier limitation (IFRM), then you probably could damage the rectifier. Please check the rectifier datasheet before you plug any big size capacitor (over 30uF) as your first filtering.

As you can see from the picture above (I use PSU Designer II from Duncan). With a too big first capacitor (errrr, 220uF is not that big, right?), you will put too much stress on the rectifier exceeding its IFRM. The big capacitor will need a big current to charge itself. If it exceeds your rectifier maximum IFRM value, then it may cause trouble. The result? I don’t think you want to know (or see) the effect 😉

But I need a big capacitor to give smoother filtering… Then continue reading 😉

Bigger Capacitance with No Heart Attack on Rectifier
Picking a right first capacitor is not difficult. You can refer to the rectifier datasheet. But from my experience, a small amount (4.7-10uF) should be a good start. Followed by resistor or choke, the bigger capacitor, and so on.

See my new schematic above. I use 3-stage filtering:

  • The 1st capacitor is 4.7uF.
  • The 2nd capacitor is 22 uF.
  • The 3rd capacitor is 560uF.
  • I use 10H choke (195 Ohm DCR) between those stages.

You may ask, why with multi stage filtering, we could use bigger capacitor? The answer is simple. The choke (or resistor) will slow down the charging process for the capacitor behind it. So only the first capacitor (4.7uF) will have direct contact with the rectifier and withdraw the current directly. The 4.7uF capacitor will not withdraw such big current (so your rectifier will not have a heart attack). The second capacitor (22uF) charging process will be slowed down by the choke (choke has resistance/DCR, remember that!). The third capacitor will be charged even slower due to two chokes in front of it. So everybody’s happy. No one has to be buried due to heart attack syndrome!

You can use combination of resistor and choke to slow down the capacitor charging process (because all chokes solution will rob your wallet). Just use appropriate value (resistance and wattage). But as far as I know, the resistor could give “stiffer” sound and less dynamic compared to choke (but choke is expensive! So there are some trade offs here). Make sure you will use good resistor. An above average resistor is still cheaper than a standard choke. So pick your correct team mate here…

Choke or Capacitor Input
Beside capacitor input, you can also use choke input for rectifier. Please be noted that the final voltage will be lower if you use choke input (the voltage drop is quite big). So instead of C-L-C-L-C filter as my schematic above, you can change it to L-C-L-C filter.

Theoretically, choke input design will provide better regulation with the cost of higher voltage drop. On the other side, capacitor input design will provide lower voltage drop at the cost of poorer regulation.

Please try yourself to pick whether you like choke input or capacitor input. Again, the rectifier datasheet is your best friend. It will explain the parameters with choke or capacitor.

I will continue with part two later. Time to sleep, dude! 🙂