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deicide67 5th September 2013 02:11 PM

Choke ratings and resistor values in choke input supply? ... page2.html

I am looking at this PS site from Patrick Turner (the traditional choke input), and noticed that the 6K bleeders are 30W! Do these really have to be 30W?

The other question is that the second choke (L2 in the image) I have is only rated at 715 WVDC. My supply transformer is 1100VCT. Would a current limiter help the startup spikes? Is it OK to use the 715V choke in the second position?



TheGimp 5th September 2013 02:35 PM

He is using the resistors to load the transformer and thus lower the output voltage.

Another way to accomplish this might be to add a capacitor at the output of the rectifiers and input of the first inductor. This would be a cap input filter, although the first cap would be small (probably 1-10uF). The larger the cap the lower the voltage out.

deicide67 5th September 2013 02:44 PM

It looked kind of strange.

He is using a higher voltage transformer than I have. Mine tests at 1200vct unloaded, so I assume it will drop quite a bit under load.

I do not mind using his schematic. I found some 6K ceramics that are 25W on eBay. It just seemed like a hefty W rating and load on the transformer.

deicide67 5th September 2013 02:50 PM

What is the typical load drop from an unloaded transformer to a loaded transformer? As I mentioned, it is 1200VCT unloaded. Would a typical loaded V drop be around 10% or so? 1080VCT?

Is there a load I can strap across the winding to test the drop?


Frank Berry 5th September 2013 03:12 PM

You might want to put L2 on the negative leg of the supply to avoid exceeding the voltage limit of your inductor.

deicide67 5th September 2013 03:14 PM

From the center tap to ground? Won't that make it look like one big inductor?

DF96 5th September 2013 03:23 PM


Originally Posted by TheGimp
The larger the cap the lower the voltage out.

Other way round.

One way to limit the off-load voltage of a choke input supply is to use a string of high power zeners. If their combined voltage is a bit higher than the on-load voltage then they don't waste any power under normal conditions. You still need a small bleeder too.

Putting chokes in the negative side is a standard technique for avoiding high DC voltages on the choke. For the first choke in a choke input supply this gains you less, as it still has a high AC voltage to cope with.

deicide67 5th September 2013 03:29 PM

I read his post right. I keen the cap size vs. output voltage is the other way around.

I have a 10H, 1kv 500mA choke for L1, and a 5H, 715WVDC 500mA choke for L2.

So, I should use 10H as L1 from the rectifiers, and use the 5H from Center Tap to ground?



DF96 5th September 2013 03:45 PM

Any choke placed in the CT connection becomes part of L1 - unless you connect the negative end of C1 to the CT instead of ground.

homeskillet 5th September 2013 04:03 PM

Putting the choke on the center tap also puts a bias on your power transformer. I was taught to never put anything on the centertap and maybe this is why. I didn't know or consider that doing so lowers the dc on the choke there. Could be handy. I think if your chokes are rated for your draw at that voltage then keep them off the center tap. If you still have hash on your line after your supply is built then that small value high voltage cap after the rectifier is the only cure i've found. On lower voltage supplies i've never needed it.

There's also a range of current draw that gives the best regulation and the bleeder can be adjusted to arrive at this. I have to remember where I read thisthough. The math didn't look to bad if i remember correctly.

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