Permanent magnet choke vs not permanent magnet ferrite choke?

[gary350]

I have some toroid permanent magnets from microwave magnetrons do they make better chokes than none permanent magnetic ferrite toroid chokes for current limiting?

I found several choke coils in old TVs and old computer monitors that have permanent magnets attached to the chokes???

 

[shortbus=]

do they make better chokes than none permanent magnetic ferrite toroid chokes for current limiting?

Of course, that's why they use them where needed.

 

[ronsimpson]

Chokes with magnets can be used to get more ability to handle current.
Take the coil in the first picture and knock off the magnet. It probably saturates at +10A and -10A. Add the magnet and it might work from 0 to +20A or -2 to +18. This does not affect the +/- range but it moves the center over.

The pictured coils are "linearity coils" from TV sets. I have used the same idea where there is a strong DC current that will never go negative.

 

[gary350]

Chokes with magnets can be used to get more ability to handle current.
Take the coil in the first picture and knock off the magnet. It probably saturates at +10A and -10A. Add the magnet and it might work from 0 to +20A or -2 to +18. This does not affect the +/- range but it moves the center over.
View attachment 136439View attachment 136440View attachment 136441
The pictured coils are "linearity coils" from TV sets. I have used the same idea where there is a strong DC current that will never go negative.

Coil saturates.................. this is probably the answer to the problem. My induction circuit works good to 33A, its a bit tricky at 34A, if current goes over 34A meter pegs out 50A lightning fast there is a loud AC hum and mosfets all explode. I changed to higher power mosfets 3 times circuit continues to explode. When meter pegs out it sounds like a short circuit pulling 300A BOOM 94A mosfets gone. Up to now I did not know what is going on or how to stop it. When I changed circuit wire from #12 to #10 solid copper current has sky rocketed. I tested, 2uh, 4uh ,6uh, 8uh chokes. PS voltage 15vdc = 14a, 16vdc = 25a, 17vdc = 300a BOOM

 

[ronsimpson]

Without a schematic cannot comment as to what might be happening.

 

[gary350]

Without a schematic cannot comment as to what might be happening.

This is the circuit. I have not worked on it for 6 months. I have the original 55A mosfets in the circuit they work best. Circuit idles with no load at 75KHz. Loaded KHz drops to 60KHz. Remove .47uf caps 1 by 1 KHz goes very high but that kills the power of the circuit it takes 1 minute to heat a small nail red hot. Try to heat a 3/8" solid steel rod red hot it pegs the amp meter and explodes the mosfets.

 

[ronsimpson]

What do the inductors look like? They could easy be saturating.

 

[gary350]

What do the inductors look like? They could easy be saturating.

Ferrite Toroids are 7/8" diameter by 5/16" wide. I tape these together in stacks then wind them with #14 solid copper wire. I use my meter to test them as I add turns when I reach the value I want I stop winding.

 

[ChrisP58]

As ronsimpson said, I think your inductor is saturating. What does the inductor current waveform look like when operating just before failure?

 

[gary350]

As ronsimpson said, I think your inductor is saturating. What does the inductor current waveform look like when operating just before failure?

I don't know, I did not know to test for that. I saw several videos yesterday how to test for that.

Yesterday I watched & read educational information about chokes they claim chokes store up magnetism. A Tube audio amp uses a choke to limit current to the power amp tubes. When I worked industry 40 years we used choke transformers as current limiting and also variable current control.

Online information shows flyback voltages 300v to 800v across a choke several people solder a diode parallel with the choke to block the high voltage pulse. Now that I know to do this I will try this.

If an industrial choke transformers is designed to limit current to 100 amps it will not go higher to 101 amps but toroid chokes appear to be different if the choke limits current to 30A when choke saturates the choke seems to act like it is no longer in the circuit so current can suddenly go up to 300 amps. Now I understand what I am working with I need to learn how to fix it.

I have no clue how to know if my chokes are large enough to limit current to higher amps without testing them in the circuit to see of mosfets explode. That will destroy a lot of mosfets.

Yesterday I put my largest choke in the circuit a 1/2" solid steel rod pulls 30A and heats up slow with PS volts about 11 volts, metal rod takes 2 whole minutes to get a dull red hot color not very hot.

 

[Nigel Goodwin]

Yesterday I watched & read educational information about chokes they claim chokes store up magnetism. A Tube audio amp uses a choke to limit current to the power amp tubes.

I suspect you're mistaken? - an old valve amplifier used a choke in the PSU to reduce hum, as long ago large electrolytics weren't available - adding the choke between two capacitors forms an LC 'PI' filter and greatly improves the ripple on supply. So something like two 8uF or two 16uF capacitors and a choke

In later times it was common not to have a choke, as larger electrolytics were freely available - so a couple of 47uF or similar.

A choke does limit current, but so does a resistor, which is a lot smaller and cheaper.

 

[rjenkinsgb]

I have no clue how to know if my chokes are large enough to limit current to higher amps without testing them in the circuit to see of mosfets explode. That will destroy a lot of mosfets.

The choke is for commutation, not exactly current limiting.

Those induction heater circuits are anything but "zero voltage switching" and both power FETs turn on momentarily each half cycle, causing a near short circuit of the power supply.

The choke opposes sudden changes in current, so if the short is brief enough, the supply current never has time to rise much.

If the oscillator stops or that choke saturates, the current will only be limited by the circuit resistance and things start to leak smoke!

(A similar "commutation inductor" concept is used with three phase thyristor drives, as under higher load conditions another phase thyristor will be switches on before the previously conducting one reaches a zero current point and turns off, causing a spike which is limited by a large three phase inductor).

 

[unclejed613]

I found several choke coils in old TVs and old computer monitors that have permanent magnets attached to the chokes???

those are for altering the current waveform in the sweep circuits. the magnet makes the core slightly nonlinear, and the nonlinearity is used to correct for nonlinearities in the sweep waveform. at the factory, if there is a nonlinear portion of the sweep waveform, the magnet is placed on top of the choke, and moved around until the nonlinearity in the picture is corrected, then the factory glues it in place permanently.

a similar effect was used (quite accidentally) in the "Cry Baby" wah-wah pedals in the 1960's and 70's. a wah pedal is a variable bandpass filter, and a choke was used as part of the filter circuit. from some step in the manufacturing process, the core of the choke would get magnetized to some random extent, and the coil would add some distortion to the sound. because the magnetization was random, no two of these mass produced pedals sounded exactly alike. that's why guitarists would try every one of them in a music store's inventory before picking one (Jimi Hendrix, for instance, who made the Cry Baby pedal famous). what happens is the core, already being magnetized is closer to saturation when current flows in one direction through the coil, vs current the opposite direction.

 

[gary350]

I need better chokes but how do I know what to buy and if its is the correct size not to saturate?

 

[gary350]

I used a transformer from a switching power supply as a choke I know these operate at about 200KHz I have tested them. My meter shows 44mh for this transformer/choke. I soldered a 1N4007 diode parallel with the transformer to block flyback voltage. I tested it and it works. I increased PS DC voltage & tested it again. Amp meter needle has never been this stable before & does not go over 15A. I heated several things red hot very quick the circuit is staying cooler nothing it getting over heated. DC goes through the choke but RF can not go through the choke. I kept thinking how chokes worked in older circuits and now I see how this choke works different in this circuit. Slow as a snail I'm learning new things. LOL.

 

[unclejed613]

a switching supply transformer has an air gap in the core (actually they use plastic spacers, but it has the same effect) to reduce the possibility of saturation. at least that's the simple version...

 

[shortbus=]

a switching supply transformer has an air gap in the core (actually they use plastic spacers, but it has the same effect) to reduce the possibility of saturation. at least that's the simple version...

Not all of them use the air gap though, only the flyback type to my knowledge, and some types of boost SMPS. But then I no way an expert on things.

A simple explanation of what I mean/understand - https://www.gowanda.com/application...formers/switch-mode-power-transformer-theory/

 

[rjenkinsgb]

I soldered a 1N4007 diode parallel with the transformer to block flyback voltage.

Those are not good at high frequencies, you need a fast recovery or schottky diode to be effective; though back EMF should not really be an issue with a series commutation choke anyway.

 

[gary350]

Those are not good at high frequencies, you need a fast recovery or schottky diode to be effective; though back EMF should not really be an issue with a series commutation choke anyway.

I have some schottky diodes I will change it. Are they rated 1000 volts?