Help with 0 - 120 V linear DC power supply - diyAudio
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Old 24th June 2010, 05:00 AM   #1
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Default Help with 0 - 120 V linear DC power supply

I'm building a linear power supply to power some electromagnets. These electromagnets are field coil speakers and require very clean DC in order to prevent hum in the speakers. The power supply needs to be variable from under 50 volts to about 120 volts DC. The electromagnets vary from 12 to 20 watts with a DC resistance of 700 ohms max. I'll only be powering one speaker at a time so current requirements are about 200 mA max.

I had some power supply design courses over 25 years ago... Just about the time the wheel was invented. What we designed in class was a simple linear supply that I am basing my project on.

Transformer -> bridge rectifier -> ripple filter circuit -> output

Very simple. However, this was all textbook and I'm beginning the see that it's not all that simple. I am going to use a Staco 171 variable transformer to get 0 to 130 VAC. Feed that into a full wave bridge rectifier to get choppy DC. Run it through an LC filter to reduce the ripple (10 henry choke and 800 uF cap). This will reduce the ripple by about -72 db. Probably overkill. I know it's overkill.

I already have the Staco transformer and the Hammond 193Q 10 henry choke (all 21 pounds of it). Here are my questions:

1) Most of the mfgrs of the bridge rectifiers recommend use of a "snubber network." We never learned about these in class. Do I really need this to protect the diodes? If so I'll spend the next two weeks reading about it.

2) When I de-energize the system there will be an EMF spike from the electromagnet load and/or from the choke. Do I need to build in some type of protection for the power supply and/or the field coil speaker so the voltage spike does not cause any damage to the load or the PS?

I was thinking about adding a diode to the +V output to prevent the back EMF spike from going into othe PS. Then add a resistor and diode (in reverse) across the output to dissipate the spike straight to ground. Not sure if this is the right way to do this because it really isn't going back to ground. It's going back to the transformer. I've been doing a lot of reading this last week and not making much progress on finalizing my design.

I'm hoping someone can give me some advice. I looked at voltage regulator circuits and none of them gives me the range <50V to 120V that a brute force linear approach gives me.
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Old 24th June 2010, 06:36 AM   #2
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Do you want an active regulator? I don't think that makes sense. So you just need a variable voltage. Realize that your load, being inductive, offers some filtering as well.

You only need snubbers if you are going to switch at the load; switching at the primary is best. A parallel reverse diode across the load is all the protection you need.
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Old 24th June 2010, 01:19 PM   #3
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I don't think I need an active regulator. I'm going to attach a Simpson 150VDC panel meter across the output so I can keep a close eye on the voltage. I should be able to get a pretty good idea of how the ps is performing.

I wish I could have used a voltage regulator but I couldn't find anything that had a wide enough range. I even looked at buying a bench power supply but they ranged in price from $300 to thousands of dollars. So far I have about $90 in parts and probably another $50 more to buy. The most expensive part will be making a case for it.

Thanks for the advice on the diode protection!
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Old 24th June 2010, 05:00 PM   #4
AndrewT is online now AndrewT  Scotland
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how about powering the mag coils from a CCS. Vary the CCS current setting resistor to change the mag flux.
As the coils warm up the current stays substantially constant, showing just a slight increase in voltage when the resistance is higher.
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Old 24th June 2010, 05:03 PM   #5
star882 is offline star882  United States
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Rectify 120v AC to 170v DC, then use a buck converter designed to regulate current.
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"Fully on MOSFET = closed switch, Fully off MOSFET = open switch, Half on MOSFET = poor imitation of Tiffany Yep." - also applies to IGBTs!
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Old 24th June 2010, 05:07 PM   #6
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What is a CCS? I will google and read up on it.

Will look at all options.
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Old 24th June 2010, 06:52 PM   #7
star882 is offline star882  United States
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That's a constant current source.

Your application sounds very similar to a small HID lamp or large LED array supply, although your load is linear. You can build a buck converter using a generic controller for a mains power supply (UC3842 or similar), deriving the feedback using a sense resistor in series with the load.
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