Are these iron core RF Inductors suitable for power amp power supply? - diyAudio
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Old 21st November 2013, 09:37 AM   #1
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Default Are these iron core RF Inductors suitable for power amp power supply?

Hi,

I modelled that even 3.3uH inductance in a CLRC filter will dramatically improve RF filtering comparing to a CRC filter for a power amplifier. I also modelled that the inductor in that position have current pulses of 22A peak when the amplifier output draws the maximum current of 7A DC. The difficulty is to find inductors that can stand that sort of current without saturation in which case they can generate a lot of harmonics.

I have just found these JW Miller 5218-RC, 3.35uH, 20A, Iron core:

5218-RC - BOURNS JW MILLER - INDUCTOR, 3.35UH, 20A, AXIAL LEADED | element14 Australia

Datasheet: www.bourns.com/data/global/pdfs/5200_series.pdf

Do you think they will work?

Thanks a million.
Bill
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Old 21st November 2013, 10:57 AM   #2
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They should work, however they will radiate quite a lot of magnetic field because the magnetic circuit is open. A toroidal inductor like the Bourns 2301-V might be preferable from that point of view.
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Old 21st November 2013, 07:09 PM   #3
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I'm using salvage toroid filters from dead PCAT switcher power supplies as an RF choke coming into my disco mixer. Totally choked out the lamp dimmer hash and the CB radio guy that played Dixie all the time while driving by. I unwound everything off the transformer but 20 turns.
I think that fat 5 vdc at 22 amps winding on the main switcher transformer toroid might make a nice CLC pi filter on power amps. I don't know what inductance it measures, but the wire is fat enough for 22 amps and the price is free. I have a stack of dead $30 PCAT power supplies; they seem to blow up about once a year. The IEC connector, switch, MOS surge supressor, and UL listed x2 input cap are very useful, too. I installed the latter two on my 1966 design dynaco amp on the AC input. There is lots of lightning here.
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Last edited by indianajo; 21st November 2013 at 07:11 PM.
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Old 21st November 2013, 07:41 PM   #4
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Quote:
Originally Posted by scopeboy View Post
They should work, however they will radiate quite a lot of magnetic field because the magnetic circuit is open. A toroidal inductor like the Bourns 2301-V might be preferable from that point of view.
It looks like I should use toroidal inductors.

The issue I have with toroidal inductors is that I have never read one datasheet that provides the resonance frequency of the inductors, and most indicate that they test the inductors at 1kHz, 10kHz or something. That makes me think that they may not work at higher frequencies. On the contrary, the 5218-RC datasheet indicates 107MHz SRF and it was tested at 7.96MHz.

I am also worried that they give very little information about the saturation point. Take 2301-V for an example, L=10uH but at the rated current of 20A, L=6.7uH. Although 6.7uH is good enough for me, I am not sure when inductance is reduced by this much would the core saturation cause a lot of extra harmonics?

Last but not least, I am thinking that the CLRC circuit is really a low impedance circuit so am wondering if the EMI radiation from iron core inductors would have an effect on it. My sensitive circuit is moved far away from the CLRC circuit. It was for this reason I thought I might be able to use iron core inductors.

Please help me with these questions further and correct me if I am wrong.
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Old 22nd November 2013, 10:07 AM   #5
DF96 is offline DF96  England
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At RF frequencies you can reduce magnetic coupling by using a metallic screen/shield. Just put some metal between the choke and whatever you don't want picking up the RF.
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Old 25th November 2013, 07:17 PM   #6
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Metallic screen / shield would definitely work but there is too much work to build them and space is tight.

It is for the power amp power supply so RF is radiated at the surrounding filtering capacitors and rail tracks only but nothing else due to distance. But if the filtering capacitors and rail tracks pick up the RF again then there is no point having the inductors. I am just not sure if the filtering capacitors and rail tracks would pick the RF up because they are low impedance nodes.

Nobody has answered the question about toroid inductors' effectiveness at RF. I can't figure out a reason why they are not good at RF, but then I can't figure out the reason why manufacturers never test them at RF either.
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Old 25th November 2013, 09:34 PM   #7
DF96 is offline DF96  England
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Toroids can be fine at RF provided they are intended for RF and wound in a way which minimises stray capacitance. Toroidal coils are routinely used in RF equipment. It is not possible to give general advice, as it all depends on how the winding is done.
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Old 26th November 2013, 01:56 AM   #8
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CB radio transmissions are RF. Lamp dimmer hash is RF. These salvage toroids from a PCAT power supply stop both from coming into my high gain (50 x) disco mixer, through the power supply line. 20 turns, I used between the external wall transformer, and the internal 50 x gain op amps, with a steel box in between the wall transformer and the toroid. The toroid was right in the steel box with the 50x gain op amps.
Japanese weigher (Ishida) power supplies use one turn of the 220 VAC 3 phase main power, wound in a toroid, to keep hash out of the switcher DC supplies.
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