choke current rating

[jlsem]

Is a choke like 10H/100mA rated at maximum current of 100mA or it has an inductance of 10H at 100mA ?

Most mil-spec power chokes I've seen are rated for a minimum inductance at 10-50 volts rms or so.

John

[metebalci]

> Chokes are usually rated for DC current, but just because you
> are passing a DC current within their rating doesn't mean that
> you can't overload them with an additional AC current. The
> current peaks must be less that the DC rating.

Ok. As in the book "Valve Amplifiers by Morgan Jones", choke must be rated for Idc + Iac (peak) ? But is this value rated DC current or saturating DC current or neither ?

For example, in hammond catalog, enclosed DC filters (chokes) have specifications below:

- Inductance
- DC Current
- Resistance
- Maximum Operating Volts DC

Lundahl LL1638 have specifications below:

- Inductance
- Recommended DC Current
- Saturating DC Current
- Max Ripple Voltage at recommended DC Current
- Resistance
- Isolation Between Windings/Between Windings and Core

What I understand here is:
- Inductance = Inductance value when specified (recommended) DC Current drawn.
- (recommended) DC Current = Recommended DC Current that must be drawn for specified Inductance. (less or higher value will change the inductance)
- Saturating DC Current = Maximum DC Current that can be drawn from this choke. Exceeding this value will prevent usual behaviour of choke.
- Max Ripple Voltage at recommended DC Current = Maximum AC voltage that is not harmful for this choke.
- Resistance = Resistance of choke.
- Maximum Operating Volts DC or Isolation Between Windings = Maximum DC Voltage that this choke is permitted to work. Exceeding this value will destroy something.

Here there is no reference to Iac(peak) of the choke. Only specified AC parameter is Max. Ripple Voltage at recommended DC current.

> An LC filter in a power supply is a resonant circuit. Whether it
> actually rings is another matter. My personal opinion is that
> things that ring are not good, and should be avoided - critical
> damping is a possibility (Q = 0.5).

Why do you think ringing is not good ? I need some technical explanation why or why not it is bad in a power supply designed for audio power amplifiers.

> You'll need to give a page reference for your RDH comment.

RDH: Chapter 30- Section 3 - Page 1182.

Thanks...

MB

[metebalci]

What is the disadvantages of putting a bleeder resistor in parallel with the load in order to decrease required choke size ?

[EC8010]

Saturating current, AC, etc: The issue has been somewhat confused by some iron manufacturers who have designed chokes for a specific choke input supply yet only give the DC load current rating, rather than the true current rating of the choke. In their defence, they frequently have customers who don't know how to calculate what's going on. It's also not helped by the fact that although iron manufacturers know about winding coils on iron, they very rarely know anything about electronics, so explaining how you are going to use the iron doesn't always help.

At the end of the day, an iron choke has a finite number of magnetic dipoles that can be aligned, once they have all been aligned, the iron choke is saturated, and it doesn't matter if this was done by pure DC or pulses of AC. The magnetising force that causes saturation is measured in Ampere turns, and since the number of turns on the choke is fixed, it follows that a given current causes saturation. Unfortunately, flipping those very last magnetic dipoles is difficult, so saturation doesn't tend to occur abruptly, it's more of a gentle knee, and that's why there are so many definitions of when saturation occurs. Play safe and keep below the maximum DC operating current, and you should be fine. As you enter saturation, inductance falls, and the effect on a choke input supply is catastrophic because it causes even greater AC current to flow, driving the choke harder into saturation. This can often be the cause of buzzing that a previous poster noted.

Why do I not like ringing? Would you like your amplifier to be supplied from a source that isn't constant DC but might bounce up and down by tens of volts?

The RDH equation refers to the "critical current" of a choke. Leaving the rectifier is a waveform composed of pure DC plus a series of harmonically related AC. The output of the choke is short-circuited to 0V by the following smoothing capacitor, so the choke draws AC current that is inversely proportional to its reactance at the AC frequencies of interest. Obviously, if there is AC current, at one instant it will add to the DC load current, and at another, it will subtract. The critical current is the DC current required to ensure that the AC current never causes the total current through the choke to fall to zero.

Adding a bleeder resistor helps with the size of the choke if the load current is low, but wastes power. If you have a variable load, then a swinging choke might be a good idea, otherwise, it's time to rethink the design.

Hope this helps.

[gerd]

hello

try to get more information from the manufacturer. Older types give the Henries with no dc and max dc for the rated power losses.
Or the values are explizit given in Henries at the spezified value of dc, like 10H@100mA.
With curves you can choose the right choke by Henries, dc and power losses, voltage drop at the used current...

look here

an example:

[jlsem]

What is the disadvantages of putting a bleeder resistor in parallel with the load in order to decrease required choke size ?

The advantages far outweigh the disadvantages. If you are using a choke input filter, bleeding 10% of the load current will help a lot. In your case , I'm guessing a 40K ohm 10-20 watt resistor would work. I personally like to bleed off as much as the power transformer can stand without getting too hot; even in class A this type of brute-force regulation is advantageous.

John

[fdegrove]

Hi,

I personally like to bleed off as much as the power transformer can stand without getting too hot; even in class A this type of brute-force regulation is advantageous.

While I entirely concur with the above, this is also where I often like to waste even more power by doing away with the resistor and use a shunt regulator instead for a money no object project.

"Slam" fans, take note...


Cheers,

[metebalci]

Thank you all for the answers...

MB