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Voltage Drop Issue across Diode Bank.

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uj1964

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Hi,
I have Diode Bank of 100nos of Diode (SF4007) are are connected in series. My aim is to test this diode bank on full load. SF4007 has iMax =1A, Peak reverse voltage is 1000V.
This Diode bank is connected to 30Ohm 300W resistance which is again in series. I am giving 100VDC to
Anode of bank and Cathode is connected to 30Ohm 300W resistance and another end of resistance is connected to ground. I am expacting 30V drop across 30Ohm resistance and 70Volt drop across Diode Bank (consists of 100nos x SF4007 diode), so that 1A current can flow. But in actual, its not happening.
I am getting 97V drop across Diode bank and 3 Volt drop across 30Ohm resistance.

Pls. let me know the reason and solution.

Regards
Joshi
 

Hi,

I assume you did not read the diode´s datasheet.

According your measurement you get 3V across 30 Ohms --> 0.1A.

Now use Figure 7 of the datasheet, 25°C chart at 0.1A --> it shows about 1V voltage drop.
This is close to your measured 0.97V.

--> Everything is fine.

Klaus
 

Everything is fine.
Click to expand...
More or less. There's no typical Vf specification in the datasheet, only maximal. Seeing actually about 1V Vf at 0.1A is a bit surprising, but yes, within specifications.
 

Hi,

Mind: it´s not (like expectable) "Max forward voltage vs forward current"
it says: "max. forward current vs forward voltage".

It is the other way round..

--> max current for a given voltage drop = min voltage drop for a given current.

Klaus
 

Mind: it´s not (like expectable) "Max forward voltage vs forward current"
it says: "max. forward current vs forward voltage".

It is the other way round..

--> max current for a given voltage drop = min voltage drop for a given current.
Click to expand...
The diagram title suggests so, but it makes no sense and I guess it's just a typo. At least the interpretation contradicts the max. forward volts point in the preceding table which is on the same "max. current" line.
 

Hi,
Thanks a lot for your valuable comments.
So if i increase the Total voltage(at present 100VDC), Vf will increase (till i get Max Fwd voltage) across each diode so the Current according to vF vs If graph.
 

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