Filter choke voltage rating

[stevef]

I've read that the filter choke in a choke input power supply should be rated for the full transformer output voltage.

I could not locate the topic in a search, but I think someone on the forum told me a few years ago that I could put the choke in the center tap leg and the voltage rating is not as important (or something to that effect).

Can anyone confirm this?

For instance, 
- a transformer puts out 650-0-650 vac no load.
- the filter choke is 10H at 65 mAdc and rated 400v
- load will draw no more than 40 mA from supply in service.

So, the voltage rating is exceeded in this application.  But what if the choke is on the CT leg of the transformer?

Tnx, Steve

 

[WQ9E]

Steve,

Bill Orr commented upon this in one of his handbooks but I don't have it handy at work.  But from memory, a couple of points he made:

1.  The transformer center tap will be above ground by the ripple voltage present and in some transformers this can prevent a problem since the center tap may not be sufficiently insulated.  This probably won't be a problem with the voltage present in your case.

2.  The effectiveness of the filter choke is reduced (I believe that Bill attributed this to shunt capacitance but hopefully someone else can clarify).  He was very clear that this is an important consideration where maximum filtering is necessary such as for a VFO or low level audio stage.  This might be an issue in your case.

Otherwise, your example should work fine.

[WA1HZK]

Your only going to make around 375 volts DC for the choke input. This will not be a problem at all.
If it flashes over and fills your cellar with smoke let me know and I'll send you another choke!
How's that for a warranty.
Keith
WA1HZK