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Old 2nd December 2004, 09:57 PM   #1
jazz is offline jazz  Netherlands
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Default Swinging choke?

Hi there,

For a while now i've been wondering: there's so much talk about choke input psu's and yes i can see the advantage and wanna try. However as far as i've understood one need's a 'swinging choke' for this. What's the difference between a swinging choke and a regular coil. That's given something like 250 volts/100mA. I just don't understand what makes an lc input combination function as a swingin choke (rec bridge l/c) instead as a filter choke in a standard clc combo.

Any input?

Regards joris

(currently listening to katie melua's 'call of the search', which is a great album!)
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Old 2nd December 2004, 11:06 PM   #2
jlsem is offline jlsem  United States
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Quote:
For a while now i've been wondering: there's so much talk about choke input psu's and yes i can see the advantage and wanna try. However as far as i've understood one need's a 'swinging choke' for this. What's the difference between a swinging choke and a regular coil. That's given something like 250 volts/100mA. I just don't understand what makes an lc input combination function as a swingin choke (rec bridge l/c) instead as a filter choke in a standard clc combo.
A swinging choke is only necessary when there are are large current swing requirements as in big class B amplifiers and class C transmitters. The inductance of the choke swings with the current demand, decreasing when current demand goes up. You don't need a swinging choke on an audio amplifier, just a choke that is designed for choke input operation. Most military surplus potted chokes fit the bill.

John
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Old 2nd December 2004, 11:43 PM   #3
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Quote:
Originally posted by jlsem

A swinging choke is only necessary when there are are large current swing requirements as in big class B amplifiers and class C transmitters.
The requirements of large amplifiers are no different from that of a single 12AU7 in class B. It's just scale, you want to use a more weight- and cost-efficient swinging choke than a standard choke.

Quote:
The inductance of the choke swings with the current demand, decreasing when current demand goes up.
The point being you run it on the edge so as current goes up, inductance goes down along the regulation curve. So you get better voltage output, regulation and efficiency.

Tim
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Old 3rd December 2004, 01:44 PM   #4
jlsem is offline jlsem  United States
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Quote:
The requirements of large amplifiers are no different from that of a single 12AU7 in class B. It's just scale, you want to use a more weight- and cost-efficient swinging choke than a standard choke.
Swinging chokes are usually more expensive and harder to find than regular chokes. Where's the efficiency in that? Ordinary chokes swing a little bit anyway and for home audio, a bleeder resistor provides good enough regulation.

John
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Old 3rd December 2004, 04:09 PM   #5
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Really? Then why are they used..

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Old 3rd December 2004, 04:23 PM   #6
jazz is offline jazz  Netherlands
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Hi there,

so given the above comments i should be fine when using a standard choke in a lclc configuration. For example: 200v input 10H/200mA (EIcore 100r dc), 120uF, 10H/200mA (ei core 100r dc), 120uF. Which simulates to sometihing like a nice and steady 185 volts output, giving me just about the headroom for the regulater to put out something like 150V @ 50Ma?

Only when i'm trying to make a power beast in class c or b and want to save on parts a specific swinging choke would be needed. And even then, given availability, a standard ei core choke with adequate ratings should do the job? And given the fluctuating currents in this case, a very big and rather low value bleeder resistor should equalize things out. Which would then reduce effeciency and hence equalize the cost benefit.

So is it save to say that as long as the values for the choke are correct and give some margine. a standard choke will be just as adequate as a swinging choke. That is given the better availability of standard ei core chokes like mentioned above. Hence this is what is mostly used?

regards
Joris
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Old 3rd December 2004, 09:12 PM   #7
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I believe this is largely the case. A minimum DC load needs to be always imposed on a swinging choke input supply, or else the DC output voltage given a sine wave AC input will rise by up to 40% when the effective load resistance starts to approach or exceed that of the low current reactance of the swinging choke which could create all sorts of problems with circuits that use components close to their voltage limits as well as losing any sort of regulation at lighter loads.

That said, IMO, a properly applied swinging choke is a pretty neat way to get output stage power supply load regulation for a class AB power amp without adding any active components.
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Old 8th January 2009, 09:40 AM   #8
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Hi all,

Excuse me to have resurrected this old thread!

I am learning about chocke input PSU.
I have read the chocke is very stressed and will produce very fastidious noise-vibration thus it is necessary it is designed expressly to work in this special application.

Second, normally a parallel resistor ( bleeder?) is always required

third , a series RC in parallel to the chocke can save from high tension peak.

What is method to calculate the parallel resistor ohm value?

Can you suggest a good" brand" for those special chockes , possibily
in europe ?

Cheers,
Paolo
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Old 8th January 2009, 12:32 PM   #9
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hey-Hey!!!,
The swinger is going to have some sort of variation in its gap length to get the core to saturate gradually. They need more turns to lower the AC contribution to flux too. A wedge shaped gap should do it for standard E-I.
cheers,
Douglas
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Old 8th January 2009, 01:50 PM   #10
45 is offline 45  Italy
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A swinging choke is desirable in input-choke supplies where the current demand is highly variable.
The reason is simple: an input choke filter works only if the L value is greater than a critical value.
Such critical value for the inductance is given by the load divided by 6*pi*f (where f is 50 or 60 Hz depending on where you live!).

If the inductance is less than its critical value the LC filter will work as a simple RC filter where R is the DC resistance of the choke.

In a class B amplifier amplifier, for example, the idle current is much lower than the anode current at max output.
So if one uses a choke that works at max output only it could not work properly at lower power output.
Instead if one chooses the choke with high inductance value it could be very big and expensive (to work also with much higher currents).

One way to guarantee the minimum load for an effective L-input filter is using a bleeder resistor. However this is not always cost effective or possible.

Let's consider a 108W class B EL34 PP (penthode connection).
The anode voltage is 800V and the idle current is 2x20mA.
The current at max output, for 11K a-a load, will be approx. 2x100 mA.

For simplicity, let's assume other series resistances to be zero.

Consider 50 Hz AC line so that 6*pi*f = 942.5 1/s

Load = 800V/200mA = 4K

The critical value at max output will be: L = 4.32 H

It is a good rule to use a higher value than critcal, say twice.

Anyway if we use 8.64 H/ 200mA standard choke it will be below the critical value for lower output.

At 10W output, for example, the anode current will be 2x30 mA and the input choke critical value would be: 14.1 H!!

So, there will be no effective input choke!

Cheers,
45
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