Can I switch between analogue and digital ammeter?

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Can I switch between analogue and digital ammeter?

Postby Ischcuch » Sun Jul 07, 2013 2:14 pm

I have read that a digital ammeter does not react fast enough to show the max current when engraving and would like to fit both a digital and an analogue ammeter.

Is there any reason why i cannot fit a SPDT switch in the HV return line before the ammeters to select the signal for the meter i want to use?

Id also like to know what the expected voltage is for a 40w tube between the negative pole of the tube and the power supply. I need this to calculate the shunt resistor value for the digital ammeter using a 200mv voltmeter.

Any help will be appreciated.
Ischcuch
 
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Re: Can I switch between analogue and digital ammeter?

Postby cvoinescu » Sun Jul 07, 2013 4:26 pm

Yes, there is. If you interrupt the path from the negative terminal of the laser to ground, you'll have the high voltage present there. I'm sure your SPDT is not rated for tens of thousands of volts. For the same reason, it's dangerous -- the connection to ground should be permanent.

However, there's nothing that prevents you from having two ammeters in series. Digital ones usually want one of the terminals to be ground; analog ones don't care, so you can connect the analog one nearer the laser.
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Re: Can I switch between analogue and digital ammeter?

Postby Ischcuch » Fri Jul 12, 2013 9:08 pm

Hi cvoinescu,

Thanks for the advice, duly noted!

Can you please help with the second question?

What size resistor must i fit in parallel with a 200mV digitial voltmeter to show a scale of 0 to 20mA. I have a 40W power supply and tube.
Ischcuch
 
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Re: Can I switch between analogue and digital ammeter?

Postby cvoinescu » Fri Jul 12, 2013 10:37 pm

You want a current of 20 mA to cause a voltage drop of 200 mV on your resistor. Ohm's Law says R = 200 mV / 20 mA = 10 ohm. At full scale, the power on the resistors will be P = 20 mA * 200 mV = 4000 uW = 4 mW, so even a small resistor should be fine.
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