[Amps] high voltage fuses

[Fuqua, Bill L]

   I'd like to point out that you should not use "FAST" diodes. 
Use power diodes. If you use fast diodes you risk rectifying RF signal and
getting odd readings on your meters. I have actually seen this in a friend's homebrew 
amplifier.  Power diodes have slower reverse recovery time and will not rectify the RF
voltages induced by the strong fields inside the amplifier. Also, they are self sacrificial
devices like fuses except internally they fuse to a short when if a large impulse of current
passes thru them and voltage drop goes to zero.
  One other thing. If you try to arrange it so that the voltage across the diode is around
.7 volts just beyond the meter peg, the meter will not be accurate. Don't forget that 
the diode draws current at lower voltages. The .7 volts is just an approximation used 
mostly for power supply design and testing purposes. 

73
Bill wa4lav



________________________________________
From: Amps [amps-bounces at contesting.com] on behalf of Bill Turner [dezrat1242 at yahoo.com]
Sent: Thursday, October 10, 2013 5:13 PM
To: Amps
Subject: Re: [Amps] high voltage fuses

ORIGINAL MESSAGE:          (may be snipped)

On Thu, 10 Oct 2013 09:13:53 -0700, you wrote:

>We used to call this the 'voltmeter method' of measuring current. You put a resistor in
>series with a milliammeter to convert it into a voltmeter -- and then use it to measure
>the voltage drop across another resistor (the shunt) in series with the load.

REPLY:

No, that's not the same method. Yours works fine too, but it's different.

My method uses a current meter, not a voltmeter. The meter would be the same
range you would use without any protection, i.e. 1 amp for plate, 100 mA for
grid, etc. NOT a voltmeter.

The current meter and a low value resistor are in series. The purpose of the
resistor is to increase the total voltage drop across the meter plus
resistor to about .6 VDC, at which point the protective diode will conduct.
The value of resistor  plus meter resistance should be so that the .6 VDC
figure is reached at about 110% of full scale.

73, Bill W6WRT


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