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Registered Member #3832
Joined: Thu Apr 14 2011, 11:57PM
Location: Downtown Chicago, IL
Posts: 37
Thanks for all your help so far. I just noticed something- (keep in mind in my OP I said I know very little about power supplies) is the capacitor value on the sheet (the mfd one we just went over) is that for a CLC filter or an LC? It seems that usually LC caps are high mfd, so I have no idea what is going on now.
And should the LC cap be polarized?
FYI I'm looking to do an LC filter here, which is why I express concern over the value.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
steve516 wrote ...
Thanks for all your help so far. I just noticed something- (keep in mind in my OP I said I know very little about power supplies) is the capacitor value on the sheet (the mfd one we just went over) is that for a CLC filter or an LC? It seems that usually LC caps are high mfd, so I have no idea what is going on now.
And should the LC cap be polarized?
FYI I'm looking to do an LC filter here, which is why I express concern over the value.
Steve
The minimum inductance - also called the critical inductance - of an LC filter is given by: L = Vout/Iout
Where L is in henries, Vout is the supply output voltage voltage, and Iout is the output current in milliamps.
So long as the inductance exceeds this minimum value, then Vout will be limited to the average value of the rectified voltage appearing at the input to the choke.
Moreover, so long as the critical inductance is exceeded, then the rectifier peak anode current will not exceed roughly double the DC current drawn from the supply.
You'll see from the critical inductance formula above that if no current is drawn from the supply, then the critical inductance would be infinite - a very rum state of affairs.
To avoid this calamity, we must impose a minimum load so that current is drawn under all circumstances. The minimum load current is given by Ima = Vout/L
Example: If the filter choke has an inductance of 30 H, and Vout is 3kV, then your minimum current will be 100 mA.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Wee, it's time for history class. Choke input filters are a hangover from the pre-war days. (that's WW2 for you youngsters, not Desert Storm ) Electrolytic caps were not available so all filter caps had to be paper in oil. These were very bulky and expensive per CV, so the choke input filter was used, as it could give better smoothing and voltage regulation using a small capacitor. When good high voltage electrolytics became available, the choke input filter got dumped.
Another reason for its use was that tube rectifiers hate high peak currents. The choke input filter is very easy on the rectifier, as the peak current is well defined and never more than twice the DC load current. Mercury vapour rectifiers are less bothered, and with silicon diodes it's not an issue at all.
So, if you bought a modern linear amp, it would have a bridge rectifier made of silicon diodes, a series stack of electrolytics, and no choke at all. Some of the older Henry Radio ones, famed for magnificent build quality (and being the size of a small file cabinet) had a resonant choke filter: google it for design examples.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
steve516 wrote ...
Thanks so much for all that! Very useful. But, if I'm not gonna use an LC, what would you recommend for filtering?
I'm using a 1160vct transformer with max 165ma if that helps at all? Steve
Unless you happen to have some giant chokes that have escaped from museums and recycling plants, I'd suggest not using a choke at all.
As Steve implies (and noting the Freudian slip about 'hangovers' following his birthday!) you'll do best with a series string of electrolytics.
The Royal Wedding has completely done my head in, so you should find someone else to argue about the value of equalising resistors across the electrolytics, for today, at any rate.
Registered Member #3832
Joined: Thu Apr 14 2011, 11:57PM
Location: Downtown Chicago, IL
Posts: 37
I appreciate that guys. I have downstairs on my bench a 4H 3kv 200ma choke, which by the calculations should work fine! It is acutally above the minimum on the sheet so I don't see why it wouldn't work. I also happen to have a 2mfd 1.5kv oil cap. So, I would think I'm ok.
Anyway, it's 6:30am in Cleveland, OH, and I swear to God the Royal Wedding is on EVERY channel. Wow.
Registered Member #3832
Joined: Thu Apr 14 2011, 11:57PM
Location: Downtown Chicago, IL
Posts: 37
Hello again- it looks like I won't be able to use my choke filter idea. I had a 4H choke rated at 175ma and 3kv insulation, and an oil cap 2MFD at 1.5kv. Unfortunately with these specs I will need minimum current of around 180ma, which exceeds my transformers maximum of 165ma. I'm looking for other ideas- you had mentioned a string of 'lytics. Sounds expensive...
I'm bummed. I got the choke and cap for about $12 total, so not like it burnt a hole but I was hoping I had this figured out.... ;)
Do you think it would be useful to post this in the general forum instead?
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Transmitter DC Filtering, 866a full wave
