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Diode Test Proceedure

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willeng

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Hello Everyone,

I have an SCR controlled 240v to 12v DC 100A or better power supply that was built by a very good friend that I have lost contact with unforunately some years ago.

The last time I used this, when under full load the power supply would jump on & off & on etc very rapidly along with the circuit breaker in the mains board, it was quite an issue at the time.

I found the problem to be an unsoldered loose tap on the secondary side of the transformer, it was making intermittant contact.

I have just now years later connected a smaller transformer 240v to 12v 50A to see if all the rest of the circuitry was working still.

There seems to be a problem, into a 0.55 ohm load the secondary output is only 4.5v & about 8A instead of being 12V & approx 22A.

I get 240v to the primary side of the transformer through the SCR control, I get 12v on the secondary side but only 4.5v DC output after the rectifier.

The Power supply has four SKN 100/08 diodes to rectify the output.

Is it possible that these may have been damaged with the problems above mentioned before?.

What is the correct test proceedure for these diodes so I can check them?.

Could this cause the lower output if they were infact damaged?

Thank You.
 
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laurenrodriguez,

Ok, so I should only get a high resistance reading when connected one way & very low resistance when connected the other way, is that correct.
Does this apply to the SKN 100/08 power Diodes as well.

I guess a Diode is a Diode?
 
laurenrodriguez,

Ok, so I should only get a high resistance reading when connected one way & very low resistance when connected the other way, is that correct.
Does this apply to the SKN 100/08 power Diodes as well.

I guess a Diode is a Diode?

hi,
A silicon diode will measure approx 800R to 1200R in one direction and many meg ohms in the other.

A Germanium diode will measure approx 200R to 500R in one direction and 100K's in the other.
 
Most DMM's have a diode scale that measures the voltage drop. Usually it's associated with the continuity beeper. It could read in mV or Volts. One polarity will be OL or overload or whatever your meter uses, the other polarity around 0.6V for a silicon diode. 0.2 to 0.7 is probably OK.

This will not work for very high voltage diodes; Kv range for peak reverse voltage.
 
i use a function generator and oscope to test high voltage stack diodes. generally the forward voltage on these stacks is about 5 or 6 volts, but i doubt this is the case with the diodes the OP is testing.
 
Sorry for the blatant thread hi-jack:eek:

Uncle Jed said:
i use a function generator and oscope to test high voltage stack diodes. generally the forward voltage on these stacks is about 5 or 6 volts,

JimB asks:
Would you care to give us a bit more information about that?
I have never heard of that one before, what are the advantages over say a bench PSU and a DMM?

JimB
 
Most DMM's have a diode scale that measures the voltage drop. Usually it's associated with the continuity beeper. It could read in mV or Volts. One polarity will be OL or overload or whatever your meter uses, the other polarity around 0.6V for a silicon diode. 0.2 to 0.7 is probably OK.

This will not work for very high voltage diodes; Kv range for peak reverse voltage.

KeepItSimpleStupid,

Thanks very much for the info on the DMM diode scale, it appears mine reads mV.
I will do some testing when I can & see what I can find.

I knew an absolute genius who's favorite saying was the same as your user name, it's was good advice.
 
JimB:

UncleJed may be making a curve tracer out of his function generator and Oscope. Here is a simple curve tracer using a transformer:

A -+ Ramp from a function generator is a better waveform to use.

The Huntron Tracker https://www.huntron.com/products/2800.htm essentially uses an in-circuit I-V curve across components to COMPARE with functioning boards for troubleshooting.

Note there is a resistor embedded in the photo.
 
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I just tested the diodes using the DMM & the diode scale setting.

Three of the diodes give the following DMM readings when testing the different polarities:
OL & 635mV
OL & 623mV
OL & 641mV

The Fourth diode reads about 1.25v & 1v the otherway, it's quite difficult to get a precise reading with this diode as the multimeter keeps drifting radically.

It appears that one diode is dead.

Would anyone know where I could purchase these SKN 100/08 diodes in Australia as I have no idea?
 
KeepItSimpleStupid,

Thanks for that, although having to buy six at a time is a bit tough!

Can I use a higher voltage & current rated diode with the other three or will this cause issues?

Do they all need to be the same rating?
 
while similar in concept to a curve tracer, the method is very simple. turn the function generator up to max output (usually around 10-15Vp-p) with a sine wave output and connect the diode across the output, and look at the result with an oscope. the function generator's 50 ohm output impedance acts as an internal dropping resistor, so what you see on the scope is a sine wave with the peak on the conducting side truncated at the diode's conduction voltage.
 
turn the function generator up to max output (usually around 10-15Vp-p) with a sine wave output and connect the diode across the output, and look at the result with an oscope.

OK I see what you are doing.
Thank you.

JimB
 
It should be OK to use a higher PRV, although check the values of V(forward( in the datasheet. Higher currents, OK.

Thanks Again,

I have two Semikron SKN 240/04 Diodes & two SKR of the same value, the SKR I can't use for this.

I have to say that no, I don't really understand about the V(forward) on the datasheet as you mentioned, I used to be an engine builder & my electronics skills are virtually non existant.
I understand a little!
If you had the time to explain from the datasheet attached it won't fall on deaf ears so to speak, I would like to learn more now that I have nothing but time!

I found a place where I can buy the SKN 240/04 diodes at a reasonable price so I wouldn't hesitate to buy four new diodes for this power supply if they are suitable but they may be overkill for this?.

I would like to keep this is in very good working order as it means a lot to me, I wouldn't like to just patch it up roughly.
I can also buy SKN 130/08 Diodes if they are better suited.

Could you have a look at the link provided & see what the best choice of diode is compared to the SKN 100/08 diodes originally fitted to the power supply.

Maximum current draw is 196A.

I understand the thread sizes in the heatsinks may have to be re tapped to suit the new diodes.

If you click on Similar products the whole range of available diodes comes up, five pages or so.


**broken link removed**

Thank You
 
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the forward voltage is the voltage a diode begins conducting at, so with a silicon diode, this voltage is usually around 0.5 to 0.8V some higher voltage diodes have a forward voltage of about 1-1.4V (or in the case of microwave oven diodes between 5 and 7V) because there are actually two or more diodes in series. for any diode below a 1000V PIV (peak inverse voltage) rating, it's almost always 0.5-0.8V. you will also note from the data sheet, the diodes in the data sheet begin conducting at 0,85V and reach a forward voltage of 1.55V by the time you get to the diode's current limit. this is also normal in a diode, the forward voltage rises with current.
 
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Sorry, I took so long in getting back to you. The SKR 240/04 are both unsuitable.

If you look at the first table of the datasheet, there are Rows of V and columns with diodes pointing in different directions. SKN means that the anode is connected to the stud.

So, lets, look at SKR 240/04 vs SKN 100/08 (Original)

The peak reverse voltage is 1/2 of the original, but the current is more than double.
If these are on the mains then 240 * 1.414 = 339 is < 400 with some safety. The PRV or Peak Reverse Voltage would have to be > 339, so 400-600 is OK, where 600 is preferred.

It also looks like the suffix UNF is used for english studs, otherwise the studs are metric.

The Diode direction is the #1 issue. The thread size #2, unless you can re-tap. #3 is the current. #4 is reverse voltage
You should check the stud size.

If these diodes rectify the AC mains then 400 is the minimum. If they rectify a much lower voltage, then it's possible that the PRV was higher than it needed to be in the first place.

A schematic could be useful as well as the "specifications" of the charger. Current can be higher and PRV could be higher, but they could have been oversized to begin with due to availability.
 
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