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High Voltage Magnetics: Chokes

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LAN_403

monokel

Sun Nov 28 2010, 08:16AM

Registered Member #2981 Joined: Thu Jul 08 2010, 01:47PM
Location: Germany
Posts: 35

If the schematic is mains frequency transformer - rectifier - L - C then there will be a voltage of about 20 kV at the inductor at the moment of switching on. Thus there will be problems if the layers of the windings are not well insolated.
But if the schematic is mains frequency transformer - rectifier - "C L C" pi filter there will be less voltage stress at the inductor.

Proud Mary

Sun Nov 28 2010, 09:20AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

It's all a bit too technical for me - this inference that no energy is stored in the magnetic field of this choke on the basis that no voltage will kick back when the field collapses, if and when the circuit closes or arcs over etc... so I will beg your leave, and go back to doing whatever it is I do best elsewhere....

Sulaiman

Sun Nov 28 2010, 10:08AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

With 2uF a 250 Hz lpf would be formed with 318 ohms resistance
which at 160 mA would drop 50 V and dissipate less than 10W.
Of course an LC filter would roll-off twice as quickly.
I can't see the purpose of such a filter.
emc/emi/rfi filtering can be done at the low voltage side of the psu.
What are you expecting above 250 Hz that needs filtering?

P.S If this LC filter is the rectifier filter then
the inductor will see ENORMOUS voltage spikes as rectifier diodes comutate,
enough to over-volt your rectifiers !.
You should also put some (a few nF) capacitance directly across the rectifier output
- before your LC filter.
Schematically it looks like a PI filter
but it's a 'spike catcher' with an L filter.

monokel

Sun Nov 28 2010, 11:36AM

Registered Member #2981 Joined: Thu Jul 08 2010, 01:47PM
Location: Germany
Posts: 35

Sulaiman wrote ...

P.S If this LC filter is the rectifier filter then
the inductor will see ENORMOUS voltage spikes as rectifier diodes comutate,
enough to over-volt your rectifiers !.
You should also put some (a few nF) capacitance directly across the rectifier output
- before your LC filter.
Schematically it looks like a PI filter
but it's a 'spike catcher' with an L filter.

Maybe such a LC filter doesn'f make much sense. But I don't agree that there should be enormous voltage
spikes at the commutation of the rectifier diodes. The current through each of the diodes is continuous
(except at power turn off); there are no abrupt changes in the current and thus there are no "enormous"
voltage spikes. I have just simulated such a circuit (without the capacitor parallel to the output of the rectifier) with LTSpice. I used a resistor of 10K (and later 100 K) for load.
The voltage across each diode is never above 21 kV and - except at the moment of power switch on - the voltage across the inductor is never above 4 kV.
A cap parallel to the output of the rectifier is useful to prevent voltage spikes at power turn on and off - not at the moments of the commutation of the diodes.

sngecko

Sun Nov 28 2010, 01:14PM

Registered Member #3447 Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97

First, thank you all for the enlightening discussion.

The PSU is fairly simple: A variac drives a 120:1 Potential transformer (PTG5). The transformer then goes directly to a full-wave bridge rectifier, grounded on the negative DC side. The positive HV side is in series with the inductor first then the load (current regulated, 0-160mA), with a 2uF filtering capacitor to ground in between.

The purpose of the inductor is only to smooth the current and voltage spikes hitting the capacitor and reverse-biased diodes.

From the earlier discussions, I'm planning on taking a page from Proud Mary above: filling a ~1cm diameter glass tube with numerous lengths of varnished 28ga annealed wire (rather than ferrite), then seeing what kind of inductance I get as I wind the thing with a single-layer.

Proud Mary

Sun Nov 28 2010, 02:07PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

sngecko wrote ...

First, thank you all for the enlightening discussion.

The PSU is fairly simple: A variac drives a 120:1 Potential transformer (PTG5). The transformer then goes directly to a full-wave bridge rectifier, grounded on the negative DC side. The positive HV side is in series with the inductor first then the load (current regulated, 0-160mA), with a 2uF filtering capacitor to ground in between.

The purpose of the inductor is only to smooth the current and voltage spikes hitting the capacitor and reverse-biased diodes.

From the earlier discussions, I'm planning on taking a page from Proud Mary above: filling a ~1cm diameter glass tube with numerous lengths of varnished 28ga annealed wire (rather than ferrite), then seeing what kind of inductance I get as I wind the thing with a single-layer.

You might put some gaps in your annealed wires too - that is where the energy is stored after all, and it will also reduce a tendency to saturation. Annealed soft iron wires were used as the cores of induction coils in Victorian times, and after, so it will be interesting to see how
this old technology stands up today. If it doesn't work, you will have learned something, and so in that sense your efforts could not be called a failure.

Sulaiman

Sun Nov 28 2010, 03:53PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

You may like this simulator - simple & intuitive & helpful

I found it gives a good 'feel' for such psu circuits.

radiotech

Mon Nov 29 2010, 01:42AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

I'm planning on taking a page from Proud Mary above: filling a ~1cm diameter glass tube with numerous lengths of varnished 28ga annealed wire (rather than ferrite)

Wonderful, Proud Mary has given the thread a coherer
to detect the signal. All we need is bell and a tapper.

Proud Mary

Mon Nov 29 2010, 10:19AM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

radiotech wrote ...

I'm planning on taking a page from Proud Mary above: filling a ~1cm diameter glass tube with numerous lengths of varnished 28ga annealed wire (rather than ferrite)

Wonderful, Proud Mary has given the thread a coherer
to detect the signal. All we need is bell and a tapper.

Didn't Branly's Coherer use iron filings?

Nikola Tesla used soft annealed iron wire as a transformer core here:

TESLA PATENT 433,702 ELECTRICAL TRANSFORMER OR INDUCTION DEVICE.
SPECIFICATION forming part of Letters Patent No. 433,702, dated August 5, 1890.
Application filed May 26, 1890. Serial No. 345,390.

,702-electrical-transformer

sngecko

Mon Nov 29 2010, 02:53PM

Registered Member #3447 Joined: Fri Nov 26 2010, 11:10PM
Location: North Jersey
Posts: 97

After an analysis of coherer technology, I'm convinced that this inductor has nothing to do with it.
Also, owing to the large air path typical of these cylindrical, single-layer inductors, linearity should not be a problem. (It's like a big air gap from one end of the "rod" to the other)
I'll certainly post a couple of construction pictures and test parameters in the next couple of months.

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