Choke inductance

[bill knox]

Hi

I will very soon to build a power supply that will give out approx. 200v, it has to be able to give out approx. 60Ma.

I am going to use a 10H 90Ma choke and my question is, if the choke is as I stated earlier, will the choke still have an inductance of 10H at 60Ma or will the inductance have either gone up or down at 60Ma drawn.

Regards

Bill

[Argus25]

The DC magnetizes the core and pushes the core up the B-H curve, which flattens off toward the top and magnetic saturation. So with too much magnetization the inductance will drop, but likely if the choke is rated for 90mA it probably will have its rated inductance, or close to that value, up to that current. Normally the choke is butt stacked with an air gap to help prevent core saturation and keep most of its rated inductance up to its rated current, which also relates to the resistance of the choke and the heat that will be generated in it.

[joebog1]

Chokes are sort of "close enough" in ACTUAL inductance. There are two BASIC types.
One is called the "swinging choke", the second is a "filter choke".
From your value of 10H @ 60 mA I strongly suggest it is a filter choke, and will operate very close, or at least close enough to its ratings to do the job you require.
A "swinging choke" is one that is normally found in a large transmitter or very biggish amplifier. Its inductance value will "swing" beteen two " values" that can be anywhere across the current that is required by the equipment it is fitted to.

A "normal" broadcast transmitter in Australia is generally in the 5 kilowatt to 10 kilowatt range, and will contain at least two chokes.
The first choke is the swinging choke and will supply current to the modulator and the RF output stage and may require in excess of 2 amps at some thousands of volts.
Yes !! we are talking POWER MANY, MANY watts.
Your choke, as stated will be a lil tiddler of perhaps 1/2" X 1/2" core,
or in modern speak 12 mm X 12v mm core, so the outside will be about an inch and a half by an inch ( including feet).
My previous example of a largish choke ( swinging choke in "average" transmitter) will be the size of a small to large steamer trunk, so read that as about two feet square. ( 600 mm X 600 mm core ) or an overall box of almost half a cubic metre.

Those examples are extreme but come down to the fact that your very small choke was probably designed to operate a small radio or record player, that runs almost pure class A across its normal job.

The BIG choke may be required to supply almost no current ( EXCEPT that needed by the modulator and transmitter at idle) to some several amps, say when an announcer coughs into a microphone, or an over zealous kettle drum player becomes excited, and can push the instantaneous current well into the satuaration region of a class A choke.

A big audio amplifier ( Williamson springs to mind) has two chokes. A swinging choke and a filter choke, even though Williamson amps are class A.


SO !!! Exact value is typically unimportant.
Current rating IS important, as TOO much current drawn will burn a design intended for class A operation.
A swinging choke can suffer quite larg abuse, as its not " normally" required to run into overload.

Swinging chokes make smoother DC than no choke at all, using capacitive only regulation/filtering, BUT much noisier than a smoothing choke.

A smoothing choke makes quite nice DC if run within its ratings.

In any case, chokes make the job of the rectifier easier, reduce ripple current, reduce overall hum levels and generally make a much more predictable or controllable power supply.

Hope my ramblings help a little

Joe

[bikerhifinut]

Or in a nutshell................ Your choke is fine and dandy for the job.
I'd use that with confidence.

What sort of choke is it. its just that I have a 90mA 10H choke here which atruck me as an odd current rating, I'd have expected it to be 100 as a round number. It's a Repanco "Admiralty" model choke in a can, I suspect it could be oil filled inside? Anyway I used it for yonks in a low current preamp PSU with no problems.

A.

[bill knox]

Hi all

Thank you for your response's to my question.

For bikerhifinut

The choke that I am looking to purchase is on Ebay and the value of the choke is written on top of it, I can only imagine that the 90Ma chosen value is calculated by the SWG of the wire used in it's construction together with the laminations used, the choke that you have, is it for sale and what is it's size, is there any chance of a photo?

Bill

[bikerhifinut]

I@ll get a shot of it Bill, yours for a nominal sum.

A.

[bikerhifinut]

Is it this one?

https://www.ebay.co.uk/itm/High-Grad...frcectupt=true

The one I have looks like this one i figure mines about 2" square by 4" high.

Looks like this.

https://www.ebay.co.uk/itm/2-4H-150M...JSl:rk:14:pf:0

But I don't want £76 for it, that seems rather steep!

PM me if interested.

Andy.

[Radio Wrangler]

Originally Posted by bill knox

Hi all

Thank you for your response's to my question.

For bikerhifinut

The choke that I am looking to purchase is on Ebay and the value of the choke is written on top of it, I can only imagine that the 90Ma chosen value is calculated by the SWG of the wire used in it's construction together with the laminations used, the choke that you have, is it for sale and what is it's size, is there any chance of a photo?

Bill

A choke has two current ratings. One is the current at which saturation in the core reduces the inductance out of its specified range. The second one is the current which due to to resistance in the winding causes overheating.

The latter one is directly affected by the wire gauge.

The former is independent of wire gauge except indirectly... if you wind a choke with heavier wire, you may need a bigger bobbin and that needs a bigger core which for the same inductance has a bigger gap and a higher saturation limit.

Usually saturation is more limiting than heating.

For a straight forward power supply, your choke current rating is probably the saturation point and so long as you stay below it, all should be well.

David

[trobbins]

It's a pity the seller doesn't have any better specifications for the choke.

Bill, you should also add some more detail about your power supply application, especially if you intended it to be used for a 'choke input filter', or for a CLC filter application, or something else? The main difference is the voltage across the choke during operation.

The ebay choke identifies itself as a smoothing choke, which is a common reference to use in a CLC filter, where the voltage across the choke may be something like 10Vrms at twice the mains frequency.

Some vintage choke manufacturers (eg. Rola) would often state the inductance value at the rated DC current and at 10Vrms voltage. That type of choke would have a slight rise in inductance as DC current reduced, and would likely reach a temperature rise limit at not too much higher DC current (perhaps 20-30% higher).

If you're keen you can even measure the inductance at your application's DC current to give yourself more confidence.

Is your application just general purpose, or are you aiming for the sky with performance of some kind?

[Herald1360]

Fixed vs swinging input chokes in choke input filters-

If the load is constant, a fixed choke will be fine, its inductance chosen to maintain continuous current through the rectifiers. Further smoothing if required can use another fixed choke and capacitor.

If the load is highly variable like a class B amplifier or an SSB RF power amplifier, a swinging choke is used because it will be cheaper and smaller than a fixed choke of large enough inductance to maintain continuous choke current at low currents whilst still not saturating at full current. It uses careful design to give a choke which provides high enough inductance at low current whilst allowing the inductance to fall in a defined way as dc load current increases such that it is always still high enough to maintain the continuous choke current criterion.

(If current becomes discontinuous, the filter tends towards capacitor input and the output voltage will rise, negating the desirable good voltage regulation capability of a well designed choke input filter).

Getting back to the original question, if the design is for a C-L-C filter, the choke will be fine as long as its dc volt drop is acceptable on load but care needs to be taken to check that the choke resistance provides adequate damping at the resonant frequency of the L and Cs. If it's for an L-C(-L-C) filter, 10H will need to be enough to maintain continuous choke currrent at low load unless low load voltage regulation doesn't matter. You still need to keep an eye on damping as well, especially if the load varies rapidly.

A poorly damped filter circuit can lead to some wild voltage overshoots at switch-on if solid state rectifiers are used and/or if there are sudden step changes in loading.

[kalee20]

Practically everything's been said!

Inductance at 60mA is likely to be similar to that at 90mA - may be a bit higher. But unless the choke has been designed to have a wedge-shaped tapered air gap as a swinging choke, it shouldn't vary much.

The OP's post - his 200V 60mA power supply - the requirements are somewhat different, depending if Bill is using it as smoothing choke or the input choke in a choke-input filter. In the former, as far as ripple is concerned you can make up for less inductance by having more capacitance following (though don't go silly and use a 1μH choke and make up for it with 10,000μF of capacitance or the rectifier valve will just 'see' it as a massive reservoir capacitor). In the latter, a critical minimum inductance must exist, which depends on load current, else the output voltage will rise and the thing becomes a rather soggy capacitor-input filter.

[bikerhifinut]

For what it's worth I now know what Bill wants a choke for.
He's building a PSU for an HRO receiver, I had a quick look online and the circuit I found showed a fairly standard CLC filter arrangement. The Parmeko Choke I'm donating will be fine in that application.

Andy.