Absolutely correct! I learned this the hard way when I accidentally shorted the screen to
ground in my 813 amplifier. Poof, no more screen meter, despite back-to-back diodes.
On 9/14/2013 8:12 PM, Bill Turner wrote:
ORIGINAL MESSAGE: (may be snipped)
On Sat, 14 Sep 2013 19:13:02 -0700, Jim wrote:
also use the 6A10 as a safety diode
across meters.
REPLY:
A diode across a meter provides almost zero protection. One must put an
appropriate value resistor in series with the meter and place the diode
across both the meter and resistor. Perhaps this is what Jim meant to say?
Diodes such as the 6A10 and its cousins don't conduct until the voltage
across them reaches about .6 VDC. By the time the voltage across the meter
reaches that, the meter is already destroyed.
Choose a resistor value such that the voltage across the meter + resistor
will reach .6 volts at about 110% of full scale current. At that point the
diode will conduct and shunt any current above that.
In nearly all circuits, the resistor value is so small it has essentially no
effect on the rest of the circuit. Cheap insurance.
73, Bill W6WRT
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Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco/
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