Filter Chokes

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For a power supply I want to build, I need a pair of 300 millihenry chokes, at 7 amps, and with a DCR of less than .2 ohms.

After using several search engines, I have come to the conclusion that I should wind my own. The cores I tried, EI 96 (center column 32mm x 80mm) is too small, as the required 700 turns of wire will not fit in the winding window. I know that a larger core will require fewer turns, but the DCR will not decrease any if at all, due to the larger turns.
I want to use the smallest core possible. Also, the wire must be fairly large (three #12 AWG, trifiliar), to keep the DCR low. This requires a larger core, simply because of the physical window dimensions required for the copper wire.

I'm baffled. I only want to buy the core laminations once.

Failing that, where can I pick up a pair of 300mH, 7A coils??? :confused:
 
Sch3mat1c said:
#12 is good for 10A, why the heck would you need trifilar? Making a 2V 100A supply or something?

TIm


I want to keep the DC resistance low. This will be a choke-input +/- 60 Volt linear supply at 7 amps. The power supply will have a minimum load of a few hundred milliamps, and I want fairly good regulation and efficiency. I could add a shunt regulator, I suppose, but then there goes the good efficiency of a choke-input supply. I'd rather just keep the DC drop in the chokes comparable to (preferably less than) the diode drops. I calc'd out the resistance of the wire, and it is surprisingly high in smaller gauges, with the required length.
 
Hi.

Are you sure you need 300mA at 7 A ?

My initial calculations suggest you will need about 50kg (110lb) of core material for each (using M6 laminations) I've not calculated the size or stack, since they would need to be custom lams.

For 1 ohm resistance you would need about 22kg (44lb) of core.

Chris
 
Chris Jennings said:
Hi.

Are you sure you need 300mA at 7 A ?

My initial calculations suggest you will need about 50kg (110lb) of core material for each (using M6 laminations) I've not calculated the size or stack, since they would need to be custom lams.

For 1 ohm resistance you would need about 22kg (44lb) of core.

Chris


Ouch. I was afraid of that.

But then again, 7A is the max, and it can swing. The critical inductance is about 150 mH at a couple of hundred ma... I could eliminate the air gap and let it swing...

But capacitor input is starting to look more and more feasible :(
 
Hi,
a choke regulated supply must be designed for a narrow range of current. There is a formula somewhere.
If you fall outside the necessary current limits (both hi & lo) then the choke stops working for you.
You are talking about 0.3A to 7A over 20:1.
The numbers supplied by our contributors show why hi current chokes are VERY expensive and also confirms why choke regulated supplies are generally found in lo current applications e.g. valve amps.
 
AndrewT said:
Hi,
a choke regulated supply must be designed for a narrow range of current. There is a formula somewhere.

Actually it's only a minimum (look for critical inductance), usually supplied by a bleeder in case the load has a low minimum current draw. Maximum is determined by the ratings of the components.

and also confirms why choke regulated supplies are generally found in lo current applications e.g. valve amps.

NO!!!

Power is power. Things get bigger as power goes up. Period. The ONLY difference between SS and tube supplies is the ratio of voltage to current. The result is resistance and inductance are proportionally lower, while capacitance is higher. NOTHING more.

The REAL reason chokes are used in tube supplies is one, the designers are more practical, and two, more importantly, nostalgia.

Tim
 
Hi sch3mat1c,
I think we are talking LC PSU and not CLC here.
I am not sure about the method of calculation but I still believe there is an upper AND lower limit to the current draw from a choke regulated PSU. Further I recall that the range of 20:1 cannot be accomdated once the design has been fixed to suit the maximum draw. Yes your solution to increase the minimum draw is workable but equally our friend could try increasing his amp to hi bias and may gain more benefit that way. What I was trying to achieve was for him to carry out some further reseach having drawn his attention to a possible problem.
You took my quote out of context. The two preceeding words were "very expensive".
I do not agree "nostalgia", in lo current applications the choke regulated PSU performs very well and offers a performance benefit that is difficult to achieve otherwise. This applies particularly to ClassA stages where the range of current demand is lower than in ClassAB.
Finally there are choke reg SS amps on the market but they are expensive and heavy. But presumably the manufacturer took that on board & decided there was a market that he could satisfy.
 
Hi Leslie,
just spotted your comment on that other thread, that you consider you do not need a gapped iron choke. I think your requirement is for a DC type that must have a gapped iron circuit otherwise the iron saturates at a low DC current and the inductance does not achieve the spec required.
 
Sch3mat1c said:


The REAL reason chokes are used in tube supplies is one, the designers are more practical, and two, more importantly, nostalgia.

Tim

You could not be further from the truth. The real reason is the shear size and expense of the coils needed in a solid state amplifier. When the power supply in a solid state amp is called on to supply many amperes of current, the size of the wire and size of the iron needed to make a choke, becomes an expensive and very impractical undertaking.
 
Hi Sch3mat1c,
I don't think you are comparing like for like, even after taking account of the 30, 50 ratio.
You have not supplied details but your voltage and current specs for the 2 supplies are probably very different.
Finally, when you say filtering, are you talking LC or CL they are completely different.
 
frugal-phile™
Joined 2001
Paid Member
Originally posted by Sch3mat1c
I've never bothered with LC, CLC is the way to go with caps cheap as they are.


It only takes one look at this chart to see the huge advantage of a choke input supply in terms of a good solid power supply. CLC is a compromise having to do solely with $$$... of course as pointed out, an LC supply really needs a Class A amp (or a CCS loaded shunt reg :)) to work.

dave
 

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