Diode identification

Thread Starter

BertyB

Joined Dec 18, 2020
4
Hi all. First time poster. I am currently rebuilding a Sega motor control board from G-Loc and I need to replace some diodes but I am not sure what types of diodes these might me. Below are some pictures and readings from my test gear. Any help greatly appreciated.

Diode 11
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Diode 8

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20201204_173715.jpg
 

dl324

Joined Mar 30, 2015
12,210
Why are you replacing diodes that test good?

To identify possible substitutes, we need to know secondary voltages and currents involved.
 

Thread Starter

BertyB

Joined Dec 18, 2020
4
Hi. Thanks for the reply. In regard to why, the short answer is that there are multiple instances of these diodes used and these were the only ones that I could remove in one piece due to corrosion damage caused by rat urine.

Im not sure how to determine the secondary voltage as I have minimal detail on these boards and the other board is also too damaged to test. Any suggestions?
 

Thread Starter

BertyB

Joined Dec 18, 2020
4
Sorry, my English must be sub-par as I have not explained my dilemma well. I do not know the secondary voltage and current so I need to know alternative strategies to determine this.

Can you please give me some guidance on how I may be able to determine the secondary voltage. For example, can I extrapolate by looking at the value of capacitors in the circuit after the diode?
 

dl324

Joined Mar 30, 2015
12,210
Sorry, my English must be sub-par as I have not explained my dilemma well. I do not know the secondary voltage and current so I need to know alternative strategies to determine this.

Can you please give me some guidance on how I may be able to determine the secondary voltage. For example, can I extrapolate by looking at the value of capacitors in the circuit after the diode?
The definitive way would be to measure it. You could use capacitor voltage, but that should be a conservative voltage.

For current rating, you could look at what's connected to the secondary.
 
How did I get there? It basically feeds a 78M05 which has a max current output. This http://www.hammondmfg.com/pdf/5c007.pdf guide is also helpful. The topology is a half wave rectifier with a capacitor filter.

We also know that the regulator needs at least 5+3 or 8V input to function. 50V is a usual PIV (Peak Inverse Voltage) specification. the 1n400x and 1n54xx are standard diodes for use at power line frequencies.

In 12V automotive applications, 50V isn't a good PIV number because of the alternator spikes. 200 is much better to use.
Exceeding the PIV value can be non-destructive if the current is limited.

There are other types of diodes such as Zener and avalanche which are usually put in the same category. There PIV is precisely controlled. Shockley diodes are a metal semiconductor junction and have a low voltage drop. There are variactor diodes where the capacitance can be changed. Switching power supplies need fast recovery diodes. Most diodes these days are made of silicon. Germanium was an early diode material and even an early transistor material. Selenium rectifiers are a bird of a very different color.
 
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