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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 15th Jul 2020, 3:40 pm   #1
TonyDuell
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Default Estimating the forward resistance of a metal rectifier

Firstly a bit of background.

I am currently working on a Bryans 1806 XY plotter (or 'X Y Coordinate Plotting Table' as it says on the panel). I guess this is marginally on-topic here, it has nothing to do with radio really but the technology is much the same (steel wires round drums and pulleys to move the pen, making a Philips tuning drive look simple), PL82s to drive the motor fields, etc.

Anyway, the motor armatures are powered straight off rectified mains. There's a pair of metal rectifier stacks each consiting of 2 'diodes' in series, connected as a bridge rectifier. The output of that goes to a 32uF 450V capacitor. Then a BIG 470 ohm resistor in series with each lead to the 2 armatures in series. Total load resistance about 1.3k

The metal rectifiers have broken down and were arcing and giving off evil-smelling magic smoke. By replacing metal washers with fibre ones I've turned them into fancy-looking 3 way tagstrips. My idea is of course to put a silicon diode and a resistor in series acoss each section.

But what resistor to use? Any idea how to estimate the value?
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Old 15th Jul 2020, 4:41 pm   #2
paulsherwin
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Default Re: Estimating the forward resistance of a metal rectifier

I don't know anything about this technology, but I would expect a high voltage high current silicon bridge to be OK without additional resistors. Any resistor would just be to give the smoothing cap an easier life. The 470R resistors will limit the current.
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Old 15th Jul 2020, 5:11 pm   #3
G6Tanuki
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Default Re: Estimating the forward resistance of a metal rectifier

Do you have any idea what voltage the motors are expecting? (it may be marked on them...) If so, add series-resistance sufficient to get the rectified supply-voltage within 10% or so of the motors' intended working voltage.

[of course this doesn't work if there's something 'intelligent' controlling the power to the armatures so you could have 0 1 or 2 of them powered depending on what the thing is doing]
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Old 16th Jul 2020, 4:13 am   #4
TonyDuell
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Default Re: Estimating the forward resistance of a metal rectifier

Alas the only things on the motors are the maker's name and part number and (amazingly) a plate telling you what brushes and ball races are used (I guess if you need to replace them, these being the parts that would wear out). No voltages. And I think the motors were custom made, they have 6 connections (2 'armature' to the brushes, 4 'field' to a pair of field windings which are interconnected to give a centre tapped winding driven by a pair of PL82 valves in push pull.

There is however nothing complex about the armature circuit. It is as I described. Mains to the input of the bridge rectifier, output smoothed by the 32uF, then resistor in each lead to the 2 armatures in series. The motor control is done via the field windings.

I think I need some resistance in the circuit if only to limit the inrush current at switch-on charging that 32uF. But maybe not that much resistance.
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Old 16th Jul 2020, 11:40 am   #5
paulsherwin
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Default Re: Estimating the forward resistance of a metal rectifier

100 ohms or so should be enough to protect the capacitor.
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