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Old 21st January 2015, 06:52 PM   #1
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Default Field Coil Power Supply DIY

Has anyone out there built their own field coil power supply? There are a few commercial designs out there but in the spirit of DIY those aren't to my liking. I have a pair of German field coils and Lowther basket assemblies.

I have read diy posts on batteries, and using antique PSUs. I'd like to build my own based on the following theme:

120V-> isolation transformer->variac->HV Bridge rectifier->HV filter caps-> field coil. (Fusing for safety assumed)

My field coils will work 0-120VDC and 0-3 amps. Should be ridiculously simple but as I have no experience with field coils I'd appreciate any advise from those with experience. I'd prefer not to get into a long discussion of "sound of " power supply components. Posts or PM appreciated.
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Old 31st January 2015, 07:58 PM   #2
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Are you sure of your figures ? 120vdc and 3A will be 360w of heat that just
doesn't seem right. Could that be 12v at 3A or 120v at .3A ?
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Old 31st January 2015, 08:41 PM   #3
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Yes, you are correct. My Field coil has a 450 ohm field coil. At 150 V (max) it should require a current of .3A. However, it can hand a lower voltage at higher amperage(3A max). The manufacturer states "up to 150VDC with no current limitation".

I picked up a used paper chromatography power supply that will do 0-200V and will supply up to 1.5 amps. I am going to give this a try to start. I have found a number of 5U4GB based tube supplies in a 1930's Radio Handbook. This appears to be what a few commercial units are doing. I really appreciate your reply. Doesn't seem to be much interest.
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Old 31st January 2015, 11:20 PM   #4
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I've been working with a couple of Jensen field-coil speakers in older guitar amps lately, but those speakers are different than yours, as they were designed to use higher voltage.
(One of the amps used the common technique of putting the field coil into the PS where a choke would normally be; the other uses a voltage doubler from the 120V AC supply to power the field.).
The guideline I picked up somewhere for the Jensens was max of 12W dissipation. Yours are rated a lot above that.
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Old 1st February 2015, 01:13 AM   #5
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Quote:
Originally Posted by davelang View Post
Yes, you are correct. My Field coil has a 450 ohm field coil. At 150 V (max) it should require a current of .3A. However, it can hand a lower voltage at higher amperage(3A max).
Some kind of misunderstanding here. At a lower voltage the current will be less than 0.3A, e.g. 100V/450 ohms = 0.222A
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Old 1st February 2015, 08:51 AM   #6
AndrewT is offline AndrewT  Scotland
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Will a DC coil exhibit a saturation (in the VC gap) limit similar to an AC transformer that starts to pass excessive AC current as the flux maximum is reached?
How would that look for a DC coil if one were to plot DC current vs DC voltage?
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Old 1st February 2015, 11:32 AM   #7
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by AndrewT View Post
Will a DC coil exhibit a saturation (in the VC gap) limit similar to an AC transformer that starts to pass excessive AC current as the flux maximum is reached?
Yes, of course: at some point, the B/H curve will begin to flatten, but it isn't particularly detrimental: the B value will be clipped (softly) against further increases in current, that's all. It is certainly not going to decrease
Quote:
How would that look for a DC coil if one were to plot DC current vs DC voltage?
The DC current is affected solely by the copper resistance, magnetic parameters having no influence whatsoever (unless of course you look at very tiny higher order effects, totally irrelevant in this case)
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Old 1st February 2015, 02:47 PM   #8
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I think you are correct. Ohms law!
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Old 15th April 2015, 04:55 PM   #9
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I run vintage Graetz field coil speakers. I built my own PSU based upon the general principle that a Constant Current Supply delivered superior results. I didn't do anything fancy. My coils are rated at 100V 100mA or about 1Kohm.
My power supply is a 70V AC secondary (35-O-35 in series) power supply rectified with just a standard bridge rectifier. I applied about 1Kuf of filtering followed by a Microwave Oven transformer as the Choke (big and heavy but cheap). It delivers the required 100V DC with no ripple and the field coils are whisper quiet.

The principle on which a CCS or choke output power supply is based is that the field coil will be subjected to a substantial back EMF which will generate an inverse signal in the field coil which will distort the signal been fed by the main amp and possibly create ringing between the power supply and the speaker. The Choke resists this back EMF and offers a more stable current and voltage to the coil.

I am very happy and my power supply was cheap as chips to build with the parts I had lying around. The only thing I quite fancy trying is a simple MOSFET source follower voltage regulator before the choke to allow adjustment of the DC voltage. I have a spare small variac lying around which I could try - but it seems overkill considering the source follower will do just as well.

Shoog

Last edited by Shoog; 15th April 2015 at 04:58 PM.
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Old 15th April 2015, 05:46 PM   #10
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Thank you very much. I am contemplating the same or similar setup. Could you explain how you wired the microwave transformer. I understand the principle of the choke for back EMF but I am not clear on how you executed that with the transformer. I too am having a HV voltage regulator made for the same use.

thanks for taking the time to respond to my query.

David
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