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Old 05-23-2012, 11:36 PM #1
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Default 445nm test loads

I have 1n4004 diode's laying around the house which have a foward voltage drop (Vf) of 1.6V

Since 445nm Vf is 4.0-4.5V, I could use 3 of these. for 4.8V

The question I have:

1N4001's have a different Vf. How much of a difference?

What effect will a LARGER Vf have on the mV reading?

Similarly, SMALLER Vf?

Also, what is the Vf of the 4001?



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Old 05-24-2012, 12:46 AM #2
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Default Re: 445nm test loads

One thing that I have noticed is the voltage drop changes as the current increases. Here is my red/blue selectable test load. I always use red to set 445 as it matches up a lot better over 1A than the blue setting.




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Old 05-24-2012, 03:02 PM #3
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Default Re: 445nm test loads

It's not the voltage drop of the diodes that change. Those are relatively constant. It's just the voltage drop across the 1 Ohm resistor that increases linearly with the increase in current.

V = IR. So the voltage drop when you are pumping 2A through your load is 2V across the resistor plus whatever else the diodes are dropping.
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Old 05-24-2012, 03:21 PM #4
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Default Re: 445nm test loads

Quote:
Originally Posted by cmg5461 View Post
I have 1n4004 diode's laying around the house which have a foward voltage drop (Vf) of 1.6V
Either you're measuring them wrong, or they're broken. They have <1.1V according to the datasheet.
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Old 05-24-2012, 03:23 PM #5
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Default Re: 445nm test loads

Here is a datasheet that shows 1.1V drop @ 1A for the diodes.
http://www.vishay.com/docs/88503/1n4001.pdf

Quote:
Originally Posted by Wolfman29 View Post
It's not the voltage drop of the diodes that change. Those are relatively constant. It's just the voltage drop across the 1 Ohm resistor that increases linearly with the increase in current.

V = IR. So the voltage drop when you are pumping 2A through your load is 2V across the resistor plus whatever else the diodes are dropping.

Yea I don't know where the increasing drop comes from but when you build a test load and use it as a completed device it does drop more voltage as the current is increased which make what I was saying a valid point which is if you use the red setting it will be accurate simulation of a 445 instead of the blue setting.


I think when people build test loads they try to get a standard voltage drop rating with their diodes for the red and blue settings that is the same as the ones in this picture below are. But I would have to get Jeff(Flaminpyro) who built this test load for me to confirm.


Last edited by DTR; 05-24-2012 at 03:28 PM.
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Old 07-11-2012, 04:19 PM #6
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Default Re: 445nm test loads

I know this is an old-ish thread. But I was always surprised why people don't take into account the drop across the resistor when they set the current into consideration.
Similar issue raised here:
Dummy load for high current - 2A test load and 3A
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