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 Post subject: Speaker field coil power supply
PostPosted: Nov Fri 03, 2017 1:16 am 
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I would like to build a power supply with the soul purpose of powering a Philco field coil.
It is a 12 in speaker with an 1140 ohm coil and since I won't be using it as a choke for B+ I don't know how to design one.
Ed


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 Post subject: Re: Speaker field coil power supply
PostPosted: Nov Fri 03, 2017 2:57 am 
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Ed,

I think the first step is to determine how much voltage was dropped across the coil in the radio it was designed for.

That info could be gotten from the schematic if it can be reviewed.

From there we can determine the voltage and current needed.

Tim


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 Post subject: Re: Speaker field coil power supply
PostPosted: Nov Sat 04, 2017 2:06 pm 
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All you need after figuring the voltage dropped across it and the correct transformer is a bridge rectifier and a filter cap of at least 10,000uF to ensure a well filtered supply.


Last edited by Tube Radio on Nov Sat 04, 2017 3:11 pm, edited 1 time in total.

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 Post subject: Re: Speaker field coil power supply
PostPosted: Nov Sat 04, 2017 2:32 pm 
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Tube Radio wrote:
All you need after figuring the voltage dropped across it is a bridge rectifier and a filter cap of at least 10,000uF to ensure a well filtered supply.

How did you come up with that number?

Thanks,
Jason


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 Post subject: Re: Speaker field coil power supply
PostPosted: Nov Sat 04, 2017 2:50 pm 
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Need about 60 volts to get 50 ma for the field. That should give plenty magnetic force. Assume that is what the OEM radio that once owned this speaker pulled.

Do find the residual magnetism by using a compass. Pass current in the direction to yield the same compass direction..

A 48 volt or near transformer, full wave bridge. Try a 1 or 2 mf yellow cap across the field, the idea is to resonant the field without resorting to complex math. The old cut and try. At 1100 ohms the coil's resonance can be taken advantage of.

In the late 20's it was popular to provide an add-on cone speaker, line powered these typically had a field coil of less than 100 ohms powered by a copper oxide rectifier, 6-volt transformer. Many, did not have any filter cap.


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 Post subject: Re: Speaker field coil power supply
PostPosted: Nov Sat 04, 2017 3:14 pm 
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reeves03 wrote:
Tube Radio wrote:
All you need after figuring the voltage dropped across it is a bridge rectifier and a filter cap of at least 10,000uF to ensure a well filtered supply.

How did you come up with that number?

Thanks,
Jason


Just thought of it off the top of my head.

When I'm dealing with something of lower voltage that can draw a bit of current I'll often use large value electrolytic caps within reason to be sure I have a well filtered supply.

Probably don't need 10,000uF, but the capacitor isn't expensive so why not :P


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 Post subject: Re: Speaker field coil power supply
PostPosted: Nov Sat 04, 2017 3:44 pm 
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Tube Radio wrote:
reeves03 wrote:
Tube Radio wrote:
All you need after figuring the voltage dropped across it is a bridge rectifier and a filter cap of at least 10,000uF to ensure a well filtered supply.

How did you come up with that number?

Thanks,
Jason


Just thought of it off the top of my head.

When I'm dealing with something of lower voltage that can draw a bit of current I'll often use large value electrolytic caps within reason to be sure I have a well filtered supply.

Probably don't need 10,000uF, but the capacitor isn't expensive so why not :P

I can give you one good reason: surge current. With just a S/S diode and a capacitor, the startup surge will be significant.

Why wouldn't the typical C-R-C filter work here?

10 milliFarads at 100volts is not expensive? Hmmmmm

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Last edited by pixellany on Nov Sat 04, 2017 3:48 pm, edited 1 time in total.

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 Post subject: Re: Speaker field coil power supply
PostPosted: Nov Sat 04, 2017 3:46 pm 
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Chas wrote:

A 48 volt or near transformer, full wave bridge. Try a 1 or 2 mf yellow cap across the field, the idea is to resonant the field without resorting to complex math. The old cut and try. At 1100 ohms the coil's resonance can be taken advantage of.
.

Does that work when the coil is the load?

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 Post subject: Re: Speaker field coil power supply
PostPosted: Nov Sat 04, 2017 4:10 pm 
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Surge current may be, but that shouldn't be an issue or a small value resistor between the cap and rectifier will reduce the surge.


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 Post subject: Re: Speaker field coil power supply
PostPosted: Nov Sat 04, 2017 4:23 pm 
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I can't even FIND a 10,000 uF 100volt cap......based on some superficial searching, an equivalent parallel or series combo is going to be well over $10.

Let's guess that 1000 uF might be more in the ballpark. If the load is 60mA, then the 1/2 cycle droop is 60E-3 * 8.3E-3 / 1E-3 = 0.5 volts. That might be good enough even without another stage.

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 Post subject: Re: Speaker field coil power supply
PostPosted: Nov Sat 04, 2017 4:24 pm 
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Since the speaker field doesn't need a whole pile of current, why not make it part of the filter system, like they were in the radios, and have the load be a wire wound or sand type power resistor of high enough value to create a bit of current, but not get too hot?

In other words, the same idea that Mark is proposing, but the field is L instead of R?

cheers

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 Post subject: Re: Speaker field coil power supply
PostPosted: Nov Sat 04, 2017 6:17 pm 
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Yes, the coil has inductance, but--if it is run from a steady DC source, we never see it. But then, I guess it DOES affect the ripple voltage / current? Much as I would love to spend the rest of the day with Spice, I have work to do....:)

Meanwhile, if I were the OP, I'd be lashing up a breadboard with a basic CRC filter. Based on the earlier rumination, the first cap can probably be in the range of 100-1000uF (the latter gave ~ 1/2 volt of "droop", which is fine, but probably not optimum).
Make R about 200 - 500 ohms, depending on how much extra voltage is available from the transformer / rectifier.

finally, increase the value of the second cap until the ripple is acceptable.

Each cap should have a MINIMUM voltage rating 1.5X the steady-state working voltage.

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 Post subject: Re: Speaker field coil power supply
PostPosted: Nov Sat 04, 2017 6:50 pm 
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Yes a 10,000uF 100 volt cap would be hard to find, but I doubt the OP needs that much voltage.


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 Post subject: Re: Speaker field coil power supply
PostPosted: Nov Sat 04, 2017 7:01 pm 
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Yawn....

Look at Schematic for RCA 106 speaker. Line powered field, via copper oxide bridge. Only caps line bypasses at .1 :shock:

http://www.nostalgiaair.org/pagesbymode ... 040249.pdf

Line powered :D not going to matter if speaker enclosed...

Yep, copper oxide has lots of forward resistance. That can be madeup by series resistance, not much... Only needed to reduce voltage/current for Philco field.


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 Post subject: Re: Speaker field coil power supply
PostPosted: Nov Sat 04, 2017 9:21 pm 
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Tube Radio wrote:
Yes a 10,000uF 100 volt cap would be hard to find, but I doubt the OP needs that much voltage.


The main point---IMHO---is that we don't need that much capacitance.....

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 Post subject: Re: Speaker field coil power supply
PostPosted: Nov Sat 04, 2017 11:12 pm 
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Agreed but it wouldn't hurt.

It's just what I pulled off the top of my head.

Keeps it simple as only one cap is needed and no resistor required.


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 Post subject: Re: Speaker field coil power supply
PostPosted: Nov Sat 11, 2017 4:29 pm 
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The places that rebuild these old types of speakers must have some sort of variable power supply that would accommodate all different varieties of these field coils that they run into but there are just too many unknowns at this point so I will skip that idea for now.

BUT I have always wanted to power up my Magnavox R-3B horn speaker without having to bring a 6 volt battery into the house. This may be a stretch but since it was designed to be powered with a car battery, what about using a hefty 6 volt battery charger? This brings me back to the same type of conundrum of not being able to find the power requirements for a R-3B.
eD


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 Post subject: Re: Speaker field coil power supply
PostPosted: Nov Sat 11, 2017 4:33 pm 
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http://plymouthcolony.net/starcity/radios/pages/unispkr.html

Here it is.

Jason


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 Post subject: Re: Speaker field coil power supply
PostPosted: Nov Sat 11, 2017 4:58 pm 
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Ed Jacobs wrote:
The places that rebuild these old types of speakers must have some sort of variable power supply that would accommodate all different varieties of these field coils that they run into but there are just too many unknowns at this point so I will skip that idea for now.

BUT I have always wanted to power up my Magnavox R-3B horn speaker without having to bring a 6 volt battery into the house. This may be a stretch but since it was designed to be powered with a car battery, what about using a hefty 6 volt battery charger? This brings me back to the same type of conundrum of not being able to find the power requirements for a R-3B.
eD
The Magnavox pot coil is 1A/6v... A full wave charger will work Probably without any filter cap. A half wave charger a 1000mf at 10-20volts would be fine. Car radios from 30's to early 50's used a 6 volt FC speaker.


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 Post subject: Re: Speaker field coil power supply
PostPosted: Nov Mon 13, 2017 1:49 pm 
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Definitely wouldn't need much filtering with an antique horn speaker as many would not reproduce anything to 120Hz.

For that one of those EICO battery chargers/eliminators set for 6 volts would work just fine.


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