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-   -   inductor/choke/air core/ferrite core. . . same thing? (http://www.diyaudio.com/forums/pass-labs/62592-inductor-choke-air-core-ferrite-core-same-thing.html)

poolorpond 15th August 2005 06:41 PM

inductor/choke/air core/ferrite core. . . same thing?
 
2 Attachment(s)
I wanted to build a power supply for my Aleph 30 (great pcb, thanks Brian) that included a 2.2 mh inductor. Upon researching, I found "inductor chokes", plain inductors, radial, axial, air core (like in crossovers), ferrite core, very low dcr and dcr upwards of .5 ohms.

Question 1 : Does it stand to reason that a choke and inductor are electrically the same thing?

Question 2: Does the air core inductor (parts express 255-640, $13) typically used in a crossover behave the same way as a ferrite core inductor, like one found in a computer power supply?

Question 3: Although threads have recommended using the 2.2 mh value, what is the math that determines the correct value?

Also, is it correct that in this schematic that the bridges (-) and (+)combine to form the return? And, would you use this for stereo (with more capacitance) or go with a dual mono PS?

Thanks

eLarson 16th August 2005 03:06 PM

Q1: A 'choke' is another word for 'inductor'.
Q2: An air-core inductor is different from a ferrite-core inductor in that the latter has... a ferrite core. :D
Q3: I'm not sure there is any math for that. You can try this, though:

desired output voltage (V) / total current draw (mA) => inductor size (H)

so if you have a rail voltage of 22 V divided by 10,000 mA, there would be 2.2 mH. (Warning: values plucked from the ether for the sake of illustration.)

That little rule of thumb came from the ARRL Handbook, ca. 1970.

wuffwaff 16th August 2005 04:26 PM

HI,

Q2: Iīm not really a specialist but a ferite core inductor can be build smaller with less windings (and a lower resistance) than a air-core-inductor. The disadvantage is that the core will be saturated above a certain current making the inductor useless.
For smaller amps (up to 3A of bias) itīs no problem to buy air core inductors with around 2mH with reasonable resistance and size (like those from intertechnik with 1,4mm wire). For an Aleph-X with 7-10A of bias an air core would get very big or have big losses so here you can use other inductors like the torobars with a high saturation current.
Q3: There are no maths , try Duncanīs PSU Designer to get the ripple you want. Normally 1.8-2.2mH is enough. I think if you would make a choke input power supply you would need some calculations.
Q4: Yes, I would use a double mono setup.

William

PRR 16th August 2005 07:49 PM

> Does the air core inductor ...behave the same way as a ferrite core inductor,

Air-core has a constant inductance at any current (until it melts, which is a huge current).

Iron/ferrite/powder cores allow a smaller (and sometimes cheaper) part, but the inductance declines at high current. You MUST be sure it is rated for more current than you have, and you must accept that the actual inductance could vary 2:1 from low current to rated current. (Cored chokes sold for speaker use "should" have a reasonably constant inductance up to rated current.)

In this case: iron/etc-core may be fine. The exact inductance is not critical, and the inductor current is known.

> what is the math

http://headfonz.rutgers.edu/PS-choke.gif
Pick C1 as a reasonable compromise between ripple and cost/size. Generally you aim for 5% ripple, though you may have to re-consider this.

If that is not clean enough, add a R-C or L-C filter. For best ripple reduction, the impedance of the C should be much less than the load R, the impedance of the R or L should be more than the load R.

Speaker amp power supply impedance is around 50 ohms.

Take 100Hz as the ripple frequency (double the line frequency). In 60Hz lands it will be 120Hz ripple, but we don't need to be precise and I'm lazy.

32uFd is 50 ohms at 100Hz. We will want much-much more than that.

High-current chokes are hard to find. 1mH is readily available (you can use a pound of fat magnet wire on a plastic spool). 1mH at 100Hz is 0.6 ohms. This is "less" than our 50 ohm load. However, as air-core, we will probably find ~1 ohm DC resistance. In fact air-core is not a lot of good for reducing 100Hz ripple, unless you use a LOT of copper (big, costly). Iron/etc-core will give higher AC impedance at lower DC resistance. (However, even if the air-core choke acts like a resistor at 100Hz, it can have high impedance at 1,000Hz, which will reduce the most annoying part of the ripple.)

Using the above values, 32mFd is 50 ohms at 100Hz and 1mH is 0.6 ohms at 100Hz. The ripple reduction is about 0.6/(0.6||50), or about 1:0.99, not much at all. We need more!

> desired output voltage (V) / total current draw (mA) => inductor size (H)

An OK starting point. However this calls for 50V/2,000mA= 25mH. It may be hard to find 25mH at 2 Amps.

Note that C1 "has" to be >1,000uFd for tolerable ripple in a speaker-amp. A good first-guess is that C2 should also be >1,000uFd. Both act as reservoirs, the choke prevents the amp from getting full use of C1, so C2 should be large too. And in the real world, a bag of same-size caps is cheaper than an assortment of different values. And another 1,000uFd costs little.

We would like the choke's AC (100Hz) impedance to be very high (with low DC resistance), but that may not be easy.

We would like the capacitor's AC impedance to be very low. Since the choke can't be as good as we like, we pick a cap with very-very-very low impedance.

This works really well when caps are cheap, as they have been since 1940. On today's market, caps of any size are a commodity product, chokes are all "specials".

Taking 1,000uFd and sticking with 1mH, 1000uFd at 100Hz is 1.6 ohms, choke is 0.6 ohms, ripple is reduced about as 1.6/(0.6+1.6) or 1:0.7, still not much.

So the answer seems to be: find the biggest practical choke. At these currents, 2mH may be as good as it gets without custom-winding a heavy hunk of iron. Then super-size the capacitors, spending about equally on C1 and C2.

In this case the designer got to 2x22,000uFd before he either met his ripple spec or ran out of space/money.

44,000uFd at 100Hz is 0.036 ohms. 2mH at 100Hz is 1.2 ohms. C1 ripple is about 0.5%. Ripple is then reduced about 0.036/(0.036+1.2)= 1:34 by L1 C2. Output ripple is around 0.015%. If the amplifier has zero ripple reduction, output buzz is very roughly -75dB from full output.

poolorpond 16th August 2005 09:26 PM

Thanks for the response guys. Yes, power handling was one of my concerns. The coils sold at parts express are rated for speakers of upwards of 500 watts. However, I thought that $13 (x4 for dual mono) was a little pricey. So, I scanned the digikey cat. and found some high current inductors, about 12 A , but very low wattage ratings- calculating to low voltage apps. I will most likely end up buying the air core from parts express and might try a higher inductance depending how the numbers work out.

It might have helped if I was more specific about the power supply caps. I was planning on using about 40-50k uf before the inductor and 50-70k uf after. I was thinking of using a higher voltage transformer and shunting the extra voltage to ground (because the 18v tranny that I have will come up under the desired 25 volts at the power supply with the drain from the bridge and inductor . I thought that this might "bias" the inductor a little more than the idle current required by the amp. I am not sure if this sounds right to you guys.

PRR 17th August 2005 05:46 AM

> The coils sold at parts express are rated for speakers of upwards of 500 watts.

If air-core, the limit is melt-down. A series-choke on a woofer could waste 10% of power, and indeed 500*10%= 50 watts would be awful hot, but no woofer will take 500 watts average (many kilowatts amplifier) for long enough to melt a big choke. So they just picked "a big number".

If iron-core: 500 watts in 8 ohms is 63V and 8 Amps RMS, say 11A peak. Assuming that it really will stand 500 Watts without "cracking" (what happens when a woofer choke saturates and stops choking on a transient), and they are not just assuming none of their customers actually use 500 watts (or could hear flaws in all that racket).

It seems ample. $13 is certainly not excessive: a very-optimum commercial design might put almost as much cash in the choke as the caps, and you sure plan on more than $13 of caps per channel. With modern cap/choke prices, you get more for the buck with caps, so it may be best to spend more on caps, still $13 is fair.

> using a higher voltage transformer and shunting the extra voltage to ground

??? Going to take a HOT shunt to pull-down a 300VA transformer, and you will know it by your electric bill.

If this is going to a true Class-A amplifier, a few-ohm series resistor will both knock-down voltage and help reduce ripple.

> the 18v tranny that I have

Plan posted shows 2*28VAC. I'm confused.

poolorpond 17th August 2005 01:27 PM

yah, just being sloppy- 18v X 2.

My next question was going to be whether to use a series resistor rather than a shunt. But you answered that.

Is there a proper place to put the resistor in the string if components- ccLccRc- assuming 10k uf per cap?

Thanks for the input. I have little experience with this.

poolorpond 17th August 2005 03:38 PM

Sorry again. I have an 18v *2, 250VA tranny but was thinking of getting a 25v *2 300VA tranny and reducing the output voltage a bit. The 18v *2 tranny will not quite give me 25 volts after the bridge and inductor. These are both compromises since I have not been able to find a 21v *2 , 300VA tranny.

harryeng 17th August 2005 04:58 PM

Hi PPR,

Thanks for this explanation. Even a digital guy like me can follow these calculations.

Now I at last I understand what I am doing.

:xeye:

Greetings,

Harry

Variac 17th August 2005 05:39 PM

Although I really don't have much knowledge, this is what I've distilled from building my own power supply:

First, you already have a resistor inline if you use an inductor. An air core inductor has a relative lot of resistance when it is inline. A lot of current flowing through the thing and you are up to 50 watts like in the example above... Do the Ohm's law calc- Then put your hand on a 50 watt lightbulb or whatever is equal to your calc....:hot:

The speaker inductors are rated for AC for speaker components- you are using them as DC in a Power supply - that requires a higher rating.

It seems that any air core around or above 2 mH has to be pretty thick wire, that 500 watt rating is bogus as mentioned above since you want continuous DC vs varying AC in a speaker plus they are just optimistic! :whazzat: Since you need the wire thicker, the coil gets bigger, the lengths longer and the resistance end up being more than you hoped...


Steel cored inductors have much shorter wire, so the resistance is WAAAAAy less. If they can handle the load they have real advantages. Remember a class A amp has a pretty continuous load, so if it doesn't saturate at first, it prob won't ever...

I believe that Nelson mentions the 14 ga Erse steel core inductors from Zylatron(sp?) are good for about 7 amps.

Regarding voltage of the Transformer:

I was told that with a continuous draw on the supply as with a class A amp, you can put the inductor as the first thing after the diode bridge (an "L" filter) and instead of multiplying the transformer output voltage by 1.4 or 1.3, you multiply it by 0.9 -it lowers!!!

I am using this trick to lower the voltage of my power supply.
The concern is that if you under spec the inductor, then it saturates, and the voltage rises up towards the 1.3 output. Also if the amp stops drawing power. For this reason you have to spec the voltage of your capacitors using 1.4 rather than 0.9, to avoid possible exploding caps.

This trick along with sometimes running transformers using the 220v input wiring config with 120 volts input to half the output, have allowed me to use transformers that are surplus but not in the outputs I need.


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