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diode

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boro3

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Helloo :)
have a question about diodes an lt spice,
lets say we have a zener diode (5V zener voltage, 10 mA zener current).
does this mean, that if the reverse current is smaler then the zener current, the reverse voltage could get higher then the zener voltage?

lets say i have a voltage soure with 100V Rms, R1 is 90k Ohm and R2 is 10 kOhm, ,

the current by the positive halve vave is going to be, (100-5)/R1 + 5/R2 right?
so if this is the case and 95/R1=.0.00005A wich is not high eneough, what will happen?

Thx.
 

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ronsimpson

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the current by the positive halve vave is going to be, (100-5)/R1 + 5/R2 right?
Wrong: I agree with the idea that R1 has 95 volts across it.
But: The current in R2 is subtracted from the current in D1. (not added to R1)
We know the current in R1. That current goes through R2 and D1. R2 current is 5V/10k The rest of the current is in D1.

0.00005A wich is not high eneough, what will happen?
You do not have to have 10mA in a "10mA diode".
A real diode: "5.1V 10mA" This is to say they measured the diode at 10mA. I looked up some diodes and they measure 5.1V at 10mA. 5.2V at 20mA and 5.0V at 1mA. It is a little hard to see on the graph in the data sheet. Because your current is much smaller than 10mA the voltage will be small. Maybe in the 3 or 4V range.
 

boro3

Member
yeah your right, ill try it again,
the cuurent through R1 is (100-5)/R1=0.00105A,
the current through R2 is 5/R2=0.0005A.
the current through D is the drifference of them, wich is 0.0005..A. but this current is to smal for the zener diode?
will it work anyway and hold the 5 V output?
 

ronsimpson

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upload_2018-7-9_6-53-55.png
100 rms = 141 peak
Your diode current is about 1mA which will work but is as low as you should go.
 

crutschow

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and what happen if it goes lower?
You mention Ltspice in your thread title.
Why not simulate the circuit and see what happens?
 

boro3

Member
i did it, but cant understand the results,
the voltage on the zener also goes lower...

i mean, the resistive divider schould make 10 Volt on R2, but with the working zener it schould be 5V,
but to get the zener to work, there schould flow a minimum zener current, in the simulation it also works with a zener current of 0.0000..A which is not right if you look at the curves in the datasheets
 

crutschow

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in the simulation it also works with a zener current of 0.0000.
In may be 0.0000 amps but it's not zero current.
My simulation below shows a Zener current of about 50μA with a Zener voltage of 4.8V (near the lower current limit for the Zener voltage break) for a 5.2V Zener diode.
So the zener voltage is maintained near is rated voltage over a wide range of current, as all Zeners do.

upload_2018-7-9_11-55-53.png
 
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