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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 24th Nov 2004, 6:03 pm   #1
staresy
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Default Volt drop across a valve rectifier

What is the typical voltage drop across a valve rectifier?
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Old 24th Nov 2004, 7:11 pm   #2
GMB
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Default Re: Volt drop across a valve rectifier

They don't have a distinct offset like a silicon diode so it depends on the current which varies over the cycle in a mains rectification situation.

For example, the GZ34 data shows the anode voltage going from 0 at zero current in a gentle curve up to 50V at 700mA. Its average max current is about 250mA where it only drops about 16V.

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Old 24th Nov 2004, 7:35 pm   #3
ukcol
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Default Re: Volt drop across a valve rectifier

Unlike semiconductor rectifiers, valve rectifiers behave approximately like an ideal diode in series with a resistor. In a broadcast receiver a typical value for this resistor might be 100-ohms. So one important factor that affects the voltage drop will be the load current. The sort of figures you might expect are an AC input of 230 volts and a DC output of 265 volts. Since the peak AC voltage is ROOT2 times the RMS voltage you are looking at a voltage drop of the order of 60 volts from peak AC in this case. All other things being equal, a full wave rectifier will give less voltage drop than a half wave one.
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Old 24th Nov 2004, 8:25 pm   #4
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Default Re: Volt drop across a valve rectifier

A very useful databook which gives the average voltage drop of of mostly octal valves is the Raytheon 1937 databook of receiving tubes:

Some rectifiers are designed to be more efficient than others - viz the 5Z4, 6X5 which has an average voltage drop of 20 volts at 125mA (and 75mA) as compared to the 5Z3, 5Y3, 5x4 (to name a few) which drop aprox 60 volts at their respective current. But again this is dependent on the current being drawn by the load resistance and the AC voltage (RMS) per plate. What is really useful about the rAytheon databook is that it plots out this drop on a fraph so you don't need to do anycalculations if you know the AC plate voltage and current consumed. If there is any particular rectifier you are interested in I don't mind scanning the page for you.

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