Power supply for LM3915/LM3916 vu meter - diyAudio
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Old 30th May 2011, 05:48 AM   #1
Frosteh is offline Frosteh  United States
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Default Power supply for LM3915/LM3916 vu meter

Hi there. I'm building a vu meter based on the design located here. All is well, except that I'm a little troubled on how to power it. I'll be using this amp powered by this filter, supplied by a 40V 500VA toroidal transformer (here). My question is, how do I connect the VU meter to this system? Somehow I'll need to add resistors to drop the voltage from 40V to 12V, but where do I put the VU meter in the system?
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Old 30th May 2011, 06:32 AM   #2
jaycee is offline jaycee  United Kingdom
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dropping 56V to 12V is quite some dissipation... I'd recommend getting a small 12v transformer - 3-5VA will be more than enough. The VU meter would be connected to the signal-level input of the amplifier.
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Old 30th May 2011, 06:56 AM   #3
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you have to have a regulater that will bring the voltage down to at least 12v to 12.5v to make it stable. jer
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Old 30th May 2011, 06:59 AM   #4
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most National regulater can only handle 37v input voltage. jer
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Old 30th May 2011, 07:17 AM   #5
jaycee is offline jaycee  United Kingdom
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Much better would be a simple transistor-following-zener regulator, but the drop in voltage would still make the pass transistor get very hot
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Old 30th May 2011, 07:22 AM   #6
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A simple buck regulator would be cheap and efficient.
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Old 30th May 2011, 08:43 AM   #7
Frosteh is offline Frosteh  United States
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Quote:
Originally Posted by jaycee View Post
dropping 56V to 12V is quite some dissipation... I'd recommend getting a small 12v transformer - 3-5VA will be more than enough. The VU meter would be connected to the signal-level input of the amplifier.
Something like this? Though I'd have to get another filter because this would still be AC. Is there a transformer you could recommend that would be able to take the voltage from the DC filter?

And I'm also confused why it goes at the signal input and not output. The VU meter is measuring the power level of the amp, so wouldn't it go on the output? I'm a bit new to building my own amps so any help would be appreciated with explaining this to me.
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Old 30th May 2011, 08:43 AM   #8
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I second jaycee. Your VU circuit uses a bipolar 12V supply. Instead of trying to patch into the main amp supply, I think it would be easier to just get a small 12-0-12 center-tap transformer. 1/4 amp should be more than enough current. Use 7812/7912 or 317/337 voltage regulators.
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Old 30th May 2011, 08:53 AM   #9
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No, no. not that Avel. Waaaay overkill. Just a cheapee transformer. It ought to be small enough to tuck away from any sensitive signal circuitry. If you do need benefit of a toroid, look for the smallest made.
And yes, whichever the transformer, you'll need to rectify and filter the output. There is no transformer to recommend for DC.
The device connects to the input because that is a low level signal. The device is designed to operate with just a couple dozen volts on its power supply pins. If connected to the amp output, it would need a much higher supply voltage. And to what end? It can measure the power at the input, which is just a smaller version of the output. They're the same except in amplitude. IE it's just scaling.
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Old 30th May 2011, 09:08 AM   #10
Frosteh is offline Frosteh  United States
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Ok I understand now. Thanks.

So would these two combined do the trick?

CT Transformer

Regulator

This psu draws 1.5A. Would I be able to drive two of these vu meters with one power supply? I'll be using two so that I can have both channels.

Last edited by Frosteh; 30th May 2011 at 09:28 AM.
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