Why do some inductors have magnets?

[gary350]

I tested an inductor meter shows it is 40 mh. After adding 2 magnets inductor value drops to .1mh ?

 

[Nigel Goodwin]

Generally magnets on 'inductors' were to make them adjustable, and often used in old CRT televisions for adjusting such things as line linearity - the adjustment holes in the magnets were usually square, and of pretty low power.

Your (presumably?) ex-magnetron magnets are vastly more powerful, and are really messing the inductor up.

 

[Diver300]

The magnets are saturating the core, so that the magnetic field isn't changing. Without any change in magnetic field, there is no induced voltage, so no inductance.

 

[gary350]

Ok 1 more question. I have a 1N4007 diode in parallel with the inductor the diode keeps exploding. PS voltage across the inductor is 19VDC. Meter shows RF frequency of 68KHz and RF voltage 72 volts. How do I keep diode from exploding all that is left is 2 wires.

 

[rjenkinsgb]

1N4000 series diodes are very "slow", and once conducting will stay conducting at 68KHz.

They are not much use at anything far beyond 50/60 Hz.

Use a fast recovery or schottky diode for higher frequencies.

 

[ronsimpson]

Core saturation; A air core inductor will have a low inductance. 0.1mH for example. Slide a core into it and the inductance will go up to 40mH, in your case. This is the inductance at zero amps. As current is applied the inductance will drop a little. At some point the magnetic field will get too strong the core stops working, causing the inductance to drop back. A magnet makes a magnetic field that also effects the core.

In CRT monitors there is a horizontal linearity coil. This one has a rotatable magnet on the side. In a CRT monitor current ramps from negative to positive as the picture is drawn from left to right. The problem is that the left and right side of the picture will be slightly different in size. This inductor is set (by the magnet) to saturate to near zero for current in one direction and not saturate and have 20uH for current in the other direction.

This is also a picture of a lin-coil. There is a magnet on top. I have used these in power supplies. Lets say, that without a magnet, it saturates above +5A and -5A. I have a buck PWM that needs an inductor that needs to work from 0 to 7A. By adding a magnet, I can move the saturation points to -1A and +9A.

I tested an inductor meter shows it is 40 mh. After adding 2 magnets inductor value drops to .1mh ?

In your case the magnets are so strong that you have saturated the core completely. If you removed the core, you will get an inductance near ".1mH".

RonSimpson

 

[Nigel Goodwin]

View attachment 139308
This is also a picture of a lin-coil.

I must say, I've never seen a lin-coil like that one, must be an American type? - pretty cool having a knob you can twiddle

 

[ronsimpson]

I've never seen a lin-coil like that one

I did not want to go to the lab and get more kinds to show. I tried to find pictures on the internet. Found a paper I wrote a lifetime ago showing how to design them. LOL

 

[Dick Cappels]

I Remember Stan Horton at Pram Magnetiics who used to design those adjustable linearity coils. Later on he came up with a linearity coil that could be adjusted by a microcontroller-driven current source. That was in the late 1970's.

 

[gary350]

Still smoking diodes.

 

[Nigel Goodwin]

Still smoking diodes.

View attachment 139310

Presumably you're doing something completely stupid with them? - you've made no suggestion what that might be, but from your previous threads it's going to something crazy and not at all surprising diodes get destroyed.

 

[Diver300]

Voltage across the inductor is 19VDC.

That implies something is wrong, although that could be something wrong with the measurement.

The voltage across an inductor is equal to the inductance multiplied by the rate of change of current. So a non-zero average voltage implies that the current is permanently increasing.

There will be some voltage from the average current and the series resistance of the inductor, but that will be very small compared to 19 V.

Can you show the circuit?

 

[gary350]

Here is the circuit. Diode is in parallel with the 40mh inductor. When circuit is on with no load amp meter reads 8 amps. With load meter reads 30 amps. I was told diode gets rid of high voltage spikes. I never had a diode in the inductor until now.

 

[ronsimpson]

I Remember Stan Horton at Pram Magnetiics

Thanks Dick, I do remember Stan. The lin coil in the top right corner is from Stan. I had different companies making versions of that. The bottom right is from Coilcraft. You can see the round magnet. Bottom came from Japan. The top left coil has a fixed magnet on the top and an adjustable at the bottom.

On the left side is a board I am working on today. The two gray capacitors are 1500 volt. You can just see a bank of 2kV ceramic capacitors. This board can handle peak current of 200A. U400 is an isolation amplifier that gets analog signals across 2000 volt isolation.
RonSimpson

 

[ronsimpson]

Diode is in parallel with the 40mh inductor.

I do not understand why this diode is there. It seems wrong to me.
There are a million circuits like this on the internet. Some of which actually work. Plese look at some other ones and see if they have a diode there.

 

[rjenkinsgb]

I was told diode gets rid of high voltage spikes. I never had a diode in the inductor until now.

If anything, the diode should be the other way around.
It also needs to be rated for higher than the maximum current through the inductor!

 

[gary350]

I do not understand why this diode is there. It seems wrong to me.
There are a million circuits like this on the internet. Some of which actually work. Plese look at some other ones and see if they have a diode there.

This is the original circuit. The circuit got better & better for a while then after I rebuilt the whole circuit with larger wire #10 wire instead of #12 something changed. I always had trouble with the inductor until now. No load current use to be 4 amps now its 8 amps. No load current needs to be very low 1 or 2 amps. I need to learn how to know when mosfets are ON they are all the way NO and when they are off they are all the way off. The circuit works but needs to work better.

 

[ChrisP58]

Here is the circuit. Diode is in parallel with the 40mh inductor. When circuit is on with no load amp meter reads 8 amps. With load meter reads 30 amps. I was told diode gets rid of high voltage spikes. I never had a diode in the inductor until now.

Yes, diodes can be used to help suppress voltage spikes. But they need to be the right kind of diodes, and they need to be in the right place.

What spikes are you seeing?
What is their waveform?
Where in the circuit are they?

 

[gary350]

Yes, diodes can be used to help suppress voltage spikes. But they need to be the right kind of diodes, and they need to be in the right place.

What spikes are you seeing?
What is their waveform?
Where in the circuit are they?

I only put the flyback diode across L2 because several circuits & several videos show it, so I decided my circuit needs that. Several people use 1N4007 for the flyback diode on L2 in this circuit. Diode is a safety to remove high voltage spikes like a car ignition coil from L2. I'm not having good luck with the flyback diode like other people claim they do, my circuit works without it so maybe I leave it off.

 

[ronsimpson]

A 1N4007 is a slow 1 amp diode built for 60hz. There is a uF4007 which is also 1A but made for switching power supplies. (fast)