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In Reply to: Re: Choke only power supply? posted by Steve Eddy on November 10, 2005 at 13:27:00:
It is conventional practice to use a series CCS or similar but an all choke input would work.Mark Kelly
Follow Ups:
Mark, that is a great idea to use a shunt reg after a choke input supply to give the choke a "standing" current draw to keep it from running "dry". Better that using a bleeder resistor as it would regulate, and a very small, quality film cap on the output would be the only cap in the audio path. Have you tried this?
My preamp uses a choke input on the rectifier and uses a form of shunt reg (see schematic for details of glow ultrapath reg). I have a CCS and a cap between them beacuse I need the voltage drop.If you didn't have a CCS you would need a choke with high DCR or a series resistor. If you are going to use a series resistor you might as well have the CCS. If you are going to use the CCS you might as well have a capacitor on it...
Mark Kelly
"My preamp uses a choke input on the rectifier and uses a form of shunt reg (see schematic for details of glow ultrapath reg). I have a CCS and a cap between them beacuse I need the voltage drop."Thats a common mode filter.
In a\case it's not obvious the cap is before the CCS not after.
Mark Kelly
uses a current source feeding a shunt regulator. A choke-input filter on the supply is recommended.You can read about it in his Tube Preamp Cookbook, or buy a kit of parts to make one.
I still don't see how it would work to any degree whether using a resistor or a shunt regulator.I mean, the fundamental behavior of a choke is to oppose any change in the current flowing through it. Yet the circuit it will be powering will be demanding current on a continually changing basis.
How exactly do you reconcile those two diametrically opposed elements?
se
The idea is that the current through the shunt element varies so that the sum of this current and the load current equals the supply current.That's the fundamental action of a shunt reg. on a CCS, it shouldn't be any different for a choke.
Mark Kelly
The idea is that the current through the shunt element varies so that the sum of this current and the load current equals the supply current.
Yeah, that's how it works with a CCS and a shunt regulator. But in that case, as far as I'm aware, the CCS is being supplied a relatively steady DC voltage from a low impedance source.
If we're talking about just a series choke placed right after the rectifier, it's just getting slapped with half sine pulses. I just don't see how you'd get any sort steady DC voltage out of that.
se
As you noted in your first post, the action of the choke is to resist any change in the current through it. To the extent that it can achieve this it acts just like a CCS. The variation in the DC voltage at the junction of the choke and the shunt reg will be affected by both the series impedance of the choke and the input impedance of the shunt reg.Let's take a practical example- we have say 100V AC at 50 Hz into an ideal full wave rectifier going into a 10H choke followed by a shunt regulator with an input impedance of 1ohm and set to a current of 100mA (so that we maintain a current well above the critical current for the choke).
The largest and lowest frequency component of the rectified voltage will be at 2f and will be at 4/3pi times Vpeak so it's about 60 volts. The choke and the shunt reg form a voltage divider with the choke impedance of 2pif.L for around 6280 ohms and the shunt reg of 1 ohm, so the remaining AC voltage will be around 10mV. Maybe not ideal but not too bad.The problem with this scheme to me comes with trying to achieve a certain DC voltage. A choke by itself is unlikely to offer enough DCR to give enough voltage drop to keep the DC voltage steady in the face of line variation. As I noted above you would need a high DCR choke or a series resistor or a CCS....
Mark Kelly
As you noted in your first post, the action of the choke is to resist any change in the current through it. To the extent that it can achieve this it acts just like a CCS. The variation in the DC voltage at the junction of the choke and the shunt reg will be affected by both the series impedance of the choke and the input impedance of the shunt reg.Let's take a practical example- we have say 100V AC at 50 Hz into an ideal full wave rectifier going into a 10H choke followed by a shunt regulator with an input impedance of 1ohm and set to a current of 100mA (so that we maintain a current well above the critical current for the choke).
The largest and lowest frequency component of the rectified voltage will be at 2f and will be at 4/3pi times Vpeak so it's about 60 volts. The choke and the shunt reg form a voltage divider with the choke impedance of 2pif.L for around 6280 ohms and the shunt reg of 1 ohm, so the remaining AC voltage will be around 10mV. Maybe not ideal but not too bad.
Hmmmm. Sorry, but without a nice size cap in there somewhere, I can't even see a "not too bad" result. Perhaps it's just me. I'll see if I can put something together over the weekend and see what results.
se
It would beinteresting to see whether practice matches the theory."In theory, theory and practice are the same. In practice, they're not" anon.
Mark Kelly
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