CCS chip setting resistors.

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pre65
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#1 CCS chip setting resistors.

Post by pre65 »

The Ixys 10M45s chip needs a resistor to set the current.

I have a formula . Resistor (ohms) = 3000 divided by current (in ma)

So, for 35ma 3000/35 = 86R

BUT

on the data sheet there is a graph for the resistor values, and as I read that 35ma would be about 70R

What would the experts select for 35ma ?

I selected 75R cos it was the nearest I had.

http://ixdev.ixys.com/DataSheet/98704.pdf
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Nick
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#2

Post by Nick »

Hmm, I wondered about that formula when you first posted it, The graph shows 70R, I would use a preset and a bench supply to see what actually worked.

If you plot the equation on the graph you will see the difference.
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pre65
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#3

Post by pre65 »

Thanks Nick.

So, the formula (not of my making :wink:) is inaccurate, but gets one in the right area.
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#4

Post by Nick »

Yes, like anything you find on the internet, its worth checking against reality.
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#5

Post by little eddy »

When I tested the cascode 10M45S, the relationship was linear but not as per your suggested formula, (tested with a battery and 1k resistor as recommended by Nick).

Typical points I had were:

3.4mA 810R
5.0mA 520R
8.4mA 63R

Don't know whether this would rule out the cascode pairs for the current you are thinking about. Looks like if you extrapolated these points, with zero set resistance you would get less than 9mA!!!
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#6

Post by Mike H »

Actually that reminds me, I think I found a discrepancy between the formula and the graph on the thing's actual data sheet!
 
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#7

Post by Paul Barker »

Mike H wrote:Actually that reminds me, I think I found a discrepancy between the formula and the graph on the thing's actual data sheet!
I often find that with valves. To no insignificant difference.
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#8

Post by Nick »

Mike H wrote:Actually that reminds me, I think I found a discrepancy between the formula and the graph on the thing's actual data sheet!
That was what the image I posted was. the red dots are the formula, the line is the spec sheet.
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#9

Post by jack »

Nick wrote:
Mike H wrote:Actually that reminds me, I think I found a discrepancy between the formula and the graph on the thing's actual data sheet!
That was what the image I posted was. the red dots are the formula, the line is the spec sheet.
Doesn't look too bad until you realise its a log-scaled graph...

I'd always go with a multi-turn trimpot (say about 100R) - no two devices are the same and even using 1% resistors (unlikely) matching currents can be tricky...
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