# VU Panel Meter

Discussion in 'General Electronics Chat' started by KCHARROIS, Aug 2, 2014.

1. ### KCHARROIS Thread Starter Member

Jun 29, 2012
292
1
Hello,

Below are the specs for a TR35 VU panel meter...

Professional, high Class
dB : -20~+5dB
0dB=1.288V+/-10%
DCR= 650Ω, If=450uA
Lamp : White LED x1pcs ,10~20mA (has no resistor inside)

I don't quite understand the specs in respect to what they mean. For example if I put exactly 1.288V going in the meter, the dial will point to 0 dB? DCR and If are confusing me but I do get the Lamp section.

Thanks

2. ### gagwd New Member

Aug 2, 2014
6
1
The characteristic DCR (DC resistance) is 650 Ohms when the meter is given 415 uA input. This is important since the meter is likely to be driven by an amplifier circuit and the circuit must be able to meet that requirement.

When 1.288 is driven across the meter, Yes, it should read 0dB.

3. ### KCHARROIS Thread Starter Member

Jun 29, 2012
292
1
Ok so what your saying is if I input 1.228 volts I need a minimum current of 415uA, there must be a maximum current for these things no?

4. ### GopherT AAC Fanatic!

Nov 23, 2012
5,979
3,686
I think you mean 1.228, not 1.288 volts.

Read this as a reference. Note that this meter is "professional audio", not consumer audio. It is for Line Level connections between components, not to speakers.

http://en.wikipedia.org/wiki/Line_level

Also, dB is a ratio measurement vs a standard reference measurement. In the case of professional audio, the reference is 1.228 volts (meaning you have zero decibels difference at that voltage.

5. ### KCHARROIS Thread Starter Member

Jun 29, 2012
292
1
I understand that it should not be connected to speakers. My question is, it gives you a minimum required current of 415uA correct? What's the maximum current so that I don't destroy it?

Thanks

6. ### THE_RB AAC Fanatic!

Feb 11, 2008
5,435
1,305
Normally with moving coil meters the stated uA is for full scale deflection. So it will read full scale (max meter movement) with 450uA through the meter.

In that case your 650 ohms is wrong, a 450uA moving coil meter has a MUCH higher coil resistance.

7. ### KCHARROIS Thread Starter Member

Jun 29, 2012
292
1
Now I'm completely confused... I'll just buy one and test it out. I'll put 1.228V limit the current and see what happens.

8. ### THE_RB AAC Fanatic!

Feb 11, 2008
5,435
1,305
Don't test it like that!

Get a 100k pot and put it in series with the vumeter, and put another uA meter in series too (like a multimeter set to 500uA range).

Start with the pot set to max resistance. Then slowly turn the pot until you get full scale deflection on the vumeter, at that point you can read the vumeter actual current needed for full scale deflection on the other uA meter.

Then you can measure the voltage on the vumeter pins. After that you will know its uA needed for full scale deflection and its resistance (using ohms law).

9. ### KCHARROIS Thread Starter Member

Jun 29, 2012
292
1
Thanks for the advice, after doing some more reading I found out that full scale deflection will happen when you apply a voltage equal to the DCR times If. There for in my case 650Ω * 415uA equals .26975V.

10. ### THE_RB AAC Fanatic!

Feb 11, 2008
5,435
1,305
The trouble with that, is that how does it display 0dB?

Normally 0dB is a point within the scale, so if full scale is 0.28v and 0dB is 1.288v something is not adding up...

11. ### KCHARROIS Thread Starter Member

Jun 29, 2012
292
1
Your absolutely right mind you this 1.22V is AC audio signal there for I don't believe DCR would apply to this. I will get a pair test them out an post my results, should be fun.

Thanks for the help