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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment. |
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2nd Jun 2013, 12:18 pm | #1 |
Triode
Join Date: Mar 2009
Location: Cambridge, Cambs. UK.
Posts: 34
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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 M1KTA |
2nd Jun 2013, 1:32 pm | #2 |
Rest in Peace
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Location: Solihull, West Midlands, UK.
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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. |
2nd Jun 2013, 2:56 pm | #3 |
Rest in Peace
Join Date: Apr 2011
Location: Matlock, Derbyshire, UK.
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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. |
2nd Jun 2013, 3:09 pm | #4 |
Dekatron
Join Date: Feb 2005
Location: St.Ippolyts, Hitchin, Hertfordshire QRA IO91UW
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Re: 10-20H DC 300V choke
Or take a rummage around my shack.....
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2nd Jun 2013, 3:10 pm | #5 | |
Triode
Join Date: Mar 2009
Location: Cambridge, Cambs. UK.
Posts: 34
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Re: 10-20H DC 300V choke
Quote:
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2nd Jun 2013, 3:11 pm | #6 | |
Triode
Join Date: Mar 2009
Location: Cambridge, Cambs. UK.
Posts: 34
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Re: 10-20H DC 300V choke
Quote:
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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2nd Jun 2013, 4:08 pm | #7 |
Rest in Peace
Join Date: Sep 2008
Location: Solihull, West Midlands, UK.
Posts: 4,872
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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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2nd Jun 2013, 4:13 pm | #8 | |
Triode
Join Date: Mar 2009
Location: Cambridge, Cambs. UK.
Posts: 34
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Re: 10-20H DC 300V choke
Quote:
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2nd Jun 2013, 5:25 pm | #9 |
Retired Dormant Member
Join Date: Jan 2011
Location: Egham, Surrey, UK.
Posts: 219
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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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2nd Jun 2013, 11:07 pm | #10 |
Dekatron
Join Date: Nov 2003
Location: Heckmondwike, West Yorkshire, UK.
Posts: 9,637
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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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2nd Jun 2013, 11:12 pm | #11 |
Octode
Join Date: Nov 2005
Location: Surrey, UK.
Posts: 1,869
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Re: 10-20H DC 300V choke
If you load the secondary, you'll kill the inductance.
Leon. |
2nd Jun 2013, 11:35 pm | #12 |
Moderator
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Location: Fife, Scotland, UK.
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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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3rd Jun 2013, 1:24 am | #13 |
Dekatron
Join Date: Nov 2003
Location: Heckmondwike, West Yorkshire, UK.
Posts: 9,637
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Re: 10-20H DC 300V choke
Foiled again!
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3rd Jun 2013, 8:26 am | #14 |
Octode
Join Date: Jan 2013
Location: Barnsley, South Yorkshire, UK.
Posts: 1,517
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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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3rd Jun 2013, 1:28 pm | #15 |
Rest in Peace
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Location: Solihull, West Midlands, UK.
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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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4th Jun 2013, 11:48 pm | #16 |
Moderator
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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.
David
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5th Jun 2013, 9:52 am | #17 |
Octode
Join Date: Nov 2005
Location: Surrey, UK.
Posts: 1,869
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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. |