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Old 2nd Jun 2013, 12:18 pm   #1
domm1kta
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Default 10-20H DC 300V choke

Hi all,

Advise needed... apologies if this is a really easy one.

I need to find a small 10-20H DC choke for a 300V line c20mA and having no fun finding one.

Could I just use the mains side winding of a small 15-0-15 mains transformer rated to 30VAC instead? Maplin code YN17 Physically it fits

I measured the mains side and I read 17.87H so seems to be exactly what I need.

Unless my math is really bad or have it wrong 30VAC @ 250mA on the secondary translates to 31mA on the mains side so should be OK.

The secondary I will either terminate or remove.

72

Dom
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Old 2nd Jun 2013, 1:32 pm   #2
G8HQP Dave
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Default Re: 10-20H DC 300V choke

When used as an AC transformer the secondary current largely cancels the magnetic field from the primary current so the core does not get saturated. When used as a DC choke this doesn't happen, so the core needs an airgap which then significantly reduces its inductance.

Chokes are available from several UK sources: Danbury, VVT, Sowter to name a few.
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Old 2nd Jun 2013, 2:56 pm   #3
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Default Re: 10-20H DC 300V choke

You could perhaps use a valve sound output transformer or valve TV frame output transformer which can take the DC current without saturation of the core.

Leave the secondary open circuit.
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Old 2nd Jun 2013, 3:09 pm   #4
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Default Re: 10-20H DC 300V choke

Or take a rummage around my shack.....
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Old 2nd Jun 2013, 3:10 pm   #5
domm1kta
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Default Re: 10-20H DC 300V choke

Quote:
Originally Posted by G8HQP Dave View Post
When used as an AC transformer the secondary current largely cancels the magnetic field from the primary current so the core does not get saturated. When used as a DC choke this doesn't happen, so the core needs an airgap which then significantly reduces its inductance.

Chokes are available from several UK sources: Danbury, VVT, Sowter to name a few.
Thanks
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Old 2nd Jun 2013, 3:11 pm   #6
domm1kta
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Default Re: 10-20H DC 300V choke

Quote:
Originally Posted by TrevorG3VLF View Post
You could perhaps use a valve sound output transformer or valve TV frame output transformer which can take the DC current without saturation of the core.

Leave the secondary open circuit.
Don't have either

I have the small transformers to hand so thought they might suit.
The air gap thing not sure how much of an issue it might be.

Dom
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Old 2nd Jun 2013, 4:08 pm   #7
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Default Re: 10-20H DC 300V choke

The air gap is the fundamental difference between (choke/SE OPT - carries DC) and (mains tr./PP OPT - no DC or balanced DC).
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Old 2nd Jun 2013, 4:13 pm   #8
domm1kta
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Default Re: 10-20H DC 300V choke

Quote:
Originally Posted by G8HQP Dave View Post
When used as an AC transformer the secondary current largely cancels the magnetic field from the primary current so the core does not get saturated. When used as a DC choke this doesn't happen, so the core needs an airgap which then significantly reduces its inductance.

Chokes are available from several UK sources: Danbury, VVT, Sowter to name a few.
Thanks for the suggestion I see Danbury do one at 100mA but nothing smaller (or cheaper!)
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Old 2nd Jun 2013, 5:25 pm   #9
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Default Re: 10-20H DC 300V choke

Try it and see but the lack of an air gap may cause core saturation and a massive reduction in inductance.
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Old 2nd Jun 2013, 11:07 pm   #10
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Default Re: 10-20H DC 300V choke

Out of interest, could the secondary be loaded to counteract the lack of an air gap?
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Old 2nd Jun 2013, 11:12 pm   #11
Leon Crampin
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Default Re: 10-20H DC 300V choke

If you load the secondary, you'll kill the inductance.

Leon.
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Old 2nd Jun 2013, 11:35 pm   #12
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Default Re: 10-20H DC 300V choke

Putting DC on the secondary to cancel the DC in the primary is interesting. You'd need a choke to allow the AC swing, and the choke would have to also handle an equivalent amount of DC with respect to the inductance it needs to do!

The stored energy comes out exactly the same as doing it with a proper choke in the first place, and so the choke needed to inject DC into the transformer is jus as big as the plain choke would have been in the first place.

No such thing as a free lunch ;-)

And if you do a current source to bias the transformer secondary, I think the energy cost will come out the same as replacing the choke with a linear regulator (There went the free breakfast as well)

Sorry,
David
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Old 3rd Jun 2013, 1:24 am   #13
AC/HL
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Default Re: 10-20H DC 300V choke

Foiled again!
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Old 3rd Jun 2013, 8:26 am   #14
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Default Re: 10-20H DC 300V choke

I've got a N.O.S. 10H 75mA Parmeko P474 choke if it's any use ? measures 6X6X8.5cm incl terminals (6X6X7 cm exc terminals).
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Old 3rd Jun 2013, 1:28 pm   #15
G8HQP Dave
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Default Re: 10-20H DC 300V choke

Typically, the smallest smoothing chokes you are likely to see are 60mA but 100-150mA is more common. This is because at low currents you can usually get enough smoothing using resistors.
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Old 4th Jun 2013, 11:48 pm   #16
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Default Re: 10-20H DC 300V choke

One thing not intuitive about choke-input power supplies is that they need to run with a minimum current, chosen to keep one phase of the rectifier in conduction or the other, meaning that the ripple current plus the minimum DC never subtract to zero (or less) net current., otherwise the peak rectification happens and the DC voltage rises coloser to the peak. This isn't the same problem with capacitor-choke-capacitor circuits.

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Old 5th Jun 2013, 9:52 am   #17
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Default Re: 10-20H DC 300V choke

Absolutely right. At zero output current - the limiting case - the smoothing capacitor charges to the peak voltage of the applied AC and the choke current is zero. The choke then has no function or effect in the circuit.

Generally, for low output current requirements a choke is unnecessary. A resistor (assuming sufficient voltage overhead is available) followed by a large value smoothing capacitor with a value chosen to give the required minimum ripple voltage at the load current drawn will generally suffice.

Leon.
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