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Old 13th February 2010, 02:57 PM   #1
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Default Shunt Regulator Issues/ Questions

I bought a 500mA Class A (+/-)Shunt Reg Kit from eBay Hong Kong, for use with an RIAA circuit I built.

The RIAA requires 2x 27v DC and pulls about 20-30mA per channel.

I can't get the shunt reg to work as I'd hoped. I have a toroidal with 18v secondaries, as recommended for the kit, and when I wire it up I get 13.7v DC per side. When I use it bipolar, of course, I get 27.4vDC, very close to my desired voltage, but I also get some small voltage fluctuations, and I can't get more than 1 volt difference with the whole range of the pots, so I don't think that's ideal at all.

I'm not sure if it is regulating properly, as I had thought I would be able to tune the voltage with the pots, and I expected absolute voltage stability.

If anybody can advise me, I would be most grateful. Here's the schematic:

Click the image to open in full size.
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Last edited by LucasAdamson; 13th February 2010 at 03:25 PM. Reason: To hopefully make the image show up!
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Old 13th February 2010, 03:10 PM   #2
iko is offline iko  Canada
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If you have a 470R or 500R resistor, or two 1k in parallel, test your reg first with this passive load.

It's possible your reg is oscillating.

The schematic does not show.
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Old 13th February 2010, 03:16 PM   #3
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OK Thanks. Image fixed now I think! I will test with a passive load.
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Old 13th February 2010, 03:25 PM   #4
iko is offline iko  Canada
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I still can't see the image. You could attach an image to the post if you don't want to rely on the site where the image is hosted.

You should probably describe the psu before the reg, bridge, CRC filter, what value C, R, and C, etc.

Edit: probably something wrong with the site that the diyaudio attachments uses.

Last edited by iko; 13th February 2010 at 03:31 PM.
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Old 13th February 2010, 04:00 PM   #5
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Lucas
You need around 40-42V AC on the transformer secondaries , to get 27V DC with your shunt
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Old 13th February 2010, 04:18 PM   #6
iko is offline iko  Canada
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+1

Definitely you need higher voltage before the shunt.
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Old 13th February 2010, 04:42 PM   #7
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Looking at the notes on the schematic it says that 18V in gives you 13.5V out, so it seems you have it working perfectly.
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Old 13th February 2010, 05:58 PM   #8
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OK that makes sense. Thanks to you all.

I wasn't sure if I could get the full range of voltages out of an 18v transformer, as I've never built a shunt reg before.

I tried with the jumper leads in, and it seems I can go up to about 24v DC before the lights start flickering and then go out. Without the jumper leads I get 27v DC and no lights and it's impossible to tune the voltage by more than +/- 1volt.

I'll get a 40v traffo then. Many thanks, Lucas
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Old 13th February 2010, 06:08 PM   #9
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If I have dual 18v secondaries, that are actually reading 20v on the DMM, can I wire them in series to get 40v? I have little experience of AC multiplying, so I am a bit cautious to try it as an experiment, and I also don't want to waste the transformer.

Thanks
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Old 13th February 2010, 08:19 PM   #10
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No, you will have to get another identical transformer if you want more voltage and don't want to modify the power input board into a half wave voltage doubler.
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