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Thyristors and AC DC Diode Converter help

Tony Sharman

Jun 12, 2015
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Hi from Australia,
I was looking for some help in sourcing a metal stud Thyristor out of a Wadkin Pattern Milling Machine (approx made in the 70s) see photo of machine attached.
Please find attached a photo of the Thyristor and also a sketch including the identification numbers on it.
Any of your help would be greatly appreciated.
Regards
Tony Sharman
Grate Access Top Specialists
613 57871117 Wadkin.jpg Thyristor.png Diode AC DC Converter.jpg
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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According to this cached Google page...

It is an SCR. 600V 21A

Here is a search on Digikey for a similar part. Unfortunately none are stocked :-(

I think the package is the problem. If you can use a TO-220 device there will be more options.
 

hevans1944

Hop - AC8NS
Jun 21, 2012
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You are going to have to do some real troubleshooting to replace this.

First, no matter if NO13RH06 is an obsolete or factory part number, it may mean the stud is Reverse connected to the cathode instead of the anode because of the R in the part number. Some power supplies did this so the heat sink didn't require insulators for the stud, although the heat sink itself might be at either line or neutral potential. If this is so, there should be another similar thyristor of normal construction, anode connected to the stud, mounted nearby on the same heat sink. The only way to tell for sure is to trace the circuit out and make a schematic, noting whether one, two, or none of the thyristors is insulated from the heat sink. If you do that and post the schematic here, we may be able to offer better advice.

In a typical arrangement, two thyristors are connected in inverse-parallel: anode of first one to cathode of the second one, and cathode of first one to anode of the second one. The gates will be driven from two separate secondary windings of a pulse transformer. Each of the two secondary windings is connected between the gate and cathode terminal of a single thyristor, so one thyristor conducts during one half-cycle of the AC line and the other thyristor conducts during the remaining half-cycle. Precisely when each thyristor is triggered within each half-cycle determines how much AC current the pair passes. Sometimes this inverse-parallel connection of two thyristors was placed in series with the primary of a power transformer and supplied directly from the power line. The actual conversion of AC to DC occurred on the secondary winding using rectifiers suitable for the full-load voltage and current expected in the secondary winding.

If this is the only thyristor in the power supply, the circuit is a phase-controlled half-wave rectifier, very unusual for industrial machinery. In any case, you need to find a suitable replacement that will hold off the peak voltage applied, as well as the peak current and average current provided to the load. Thyristors are still available in reverse polarity and standard polarity packages, and you can use a search engine such as the one found on Digi-Key to select one suitable to your needs. If the device you have is indeed one with reverse polarity (stud is the cathode), you can substitute a standard thyristor (stud is the anode) and electrically insulate the stud from the heat sink. Just make sure the trigger wires are connected between gate and cathode.

Clearly it would help to have a seasoned (old) electronics tech on site to do the troubleshooting.

Hop

Edit: corrected in red the convention for reverse parts versus standard parts. Note that power diodes are also available in reverse as well as standard packaging.
 
Last edited:

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Perhaps the choice of a TO-220 plastic or isolated tab SCR might be the best option?
 

Tony Sharman

Jun 12, 2015
3
Joined
Jun 12, 2015
Messages
3
Than
You are going to have to do some real troubleshooting to replace this.

First, no matter if NO13RH06 is an obsolete or factory part number, it may mean the stud is Reverse connected to the anode instead of the cathode because of the R in the part number. Some power supplies did this so the heat sink didn't require insulators for the stud, although the heat sink itself might be at either line or neutral potential. If this is so, there should be another similar thyristor of normal construction, cathode connected to the stud, mounted nearby on the same heat sink. The only way to tell for sure is to trace the circuit out and make a schematic, noting whether one, two, or none of the thyristors is insulated from the heat sink. If you do that and post the schematic here, we may be able to offer better advice.

In a typical arrangement, two thyristors are connected in inverse-parallel: anode of first one to cathode of the second one, and cathode of first one to anode of the second one. The gates will be driven from two separate secondary windings of a pulse transformer. Each of the two secondary windings is connected between the gate and cathode terminal of a single thyristor, so one thyristor conducts during one half-cycle of the AC line and the other thyristor conducts during the remaining half-cycle. Precisely when each thyristor is triggered within each half-cycle determines how much AC current the pair passes. Sometimes this inverse-parallel connection of two thyristors was placed in series with the primary of a power transformer and supplied directly from the power line. The actual conversion of AC to DC occurred on the secondary winding using rectifiers suitable for the full-load voltage and current expected in the secondary winding.

If this is the only thyristor in the power supply, the circuit is a phase-controlled half-wave rectifier, very unusual for industrial machinery. In any case, you need to find a suitable replacement that will hold off the peak voltage applied, as well as the peak current and average current provided to the load. Thyristors are still available in reverse polarity and standard polarity packages, and you can use a search engine such as the one found on Digi-Key to select one suitable to your needs. If the device you have is indeed one with reverse polarity (stud is the anode), you can substitute a standard thyristor (stud is the cathode) and electrically insulate the stud from the heat sink. Just make sure the trigger wires are connected between gate and cathode.

Clearly it would help to have a seasoned (old) electronics tech on site to do the troubleshooting.

Hop
Thanks so much for your help it is really appreciated
 
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