RE: D.C. Filament transformer and choke size.

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In Reply to: RE: D.C. Filament transformer and choke size. posted by TomWh on October 17, 2014 at 13:28:15:

Yeah, the choke ratings that are in use don't tell you what you really need to know.

What you really need to know is the peak current capability of the choke before core saturation reduces the impedance too much. Various manufacturers will give a rated current which is smaller than that peak but none of them will specify how much the margin is, so we just assume there is no margin unless the choke is specifically described as suitable for a choke-input filter. This is conservative but safe.

When a choke is just exactly the minimum inductance, the current in the choke goes from zero to twice the average current - that's the definition of the minimum value, no reverse current. (This ignores the higher harmonics of the current but they are attenuated by the inductance so it's close.) So the choke needs to be rated for twice the average (DC) current. Minimum inductance is Rload/940 for 50Hz power, and Rload/1130 for 60Hz power - roughly 1mH per ohm.

If the choke is larger, the AC current is smaller - for example, if the choke inductance is 4 times the minimum then the AC peak current will be a quarter of the DC current, so the choke rating must be at least 1.25 times the DC current - 3.125 amps DC for a 2A3 with a 4mH choke.

Since the physics is all about the peak current, RMS is not an accurate unit to use. It is an approximation to the real situation by allowing more DC current when the AC current is small, and when used with a suitable "secret margin" as above it can be safe.

a link to an article that I found with google (Open in New Window)

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Follow Ups Full Thread

Topic - D.C. Filament transformer and choke size. - TomWh 11:32:34 10/17/14 (5)

RE: D.C. Filament transformer and choke size. - Maxamillion 18:25:16 10/17/14 (0)

RE: D.C. Filament transformer and choke size. - Paul Joppa 12:11:00 10/17/14 (3)

RE: D.C. Filament transformer and choke size. - TomWh 13:28:15 10/17/14 (2)

RE: D.C. Filament transformer and choke size. - Paul Joppa 17:51:50 10/17/14 (1)

Thanks Paul - TomWh 19:22:33 10/17/14 (0)

Follow Ups

RE: D.C. Filament transformer and choke size. - Paul Joppa 17:51:50 10/17/14 (1)

Thanks Paul - TomWh 19:22:33 10/17/14 (0)

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