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In Reply to: RE: D.C. Filament transformer and choke size. posted by TomWh on October 17, 2014 at 11:32:34
A lot depends on the actual circuit, specifically whether the filter is choke-input or capacitor-input, and whether the rectifier is full-wave center-tapped or a bridge.
A specific example: I am currently working on a design for 2A3s with fullwave bridge Schotty rectifiers and capacitor input. Looks like it needs at least 3.15 volts at 4.5 amps RMS input. Trying 3-amp rectifiers but may need to get larger ones to get the output up to 2.5 volts.
PSUD is a big help in scoping out a design, but actual voltages are not close enough to rely on, mostly (I think) due to inaccuracies in the diode model. Just my experience.
Follow Ups:
Hi Paul
It will be bridge to choke input.
My question is more about the current rating needed for the choke to handle the ripple after the bridge. I read that double is a safe bet. So for a 2a3 with 2.5 amp filament a 5 amp choke would be a good choice. (4mh should also more than enough inductance for 2a3 fil)
My other question is regarding how hammond rates in dc current and triad rates rms, not real sure what that means and how that would relate to the 5 amp choke above? Or to word it a different way if the double 5 amp is correct would that be 5 amp dc current or rms or are the same?
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.
I have a better feel for whats going on. There is a lot going on here, looks like make sure you got plenty of mh and double the current rating and you should be in pretty good shape. I down loaded the article you posted will have to go over it a few times to understand it better.
Its nice you are hanging out around here again, your knowledge is most welcome.
Thanks again
Tom
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