Determination of Voltages

Thread Starter

pujulde

Joined Jul 24, 2013
108
Can anyone help me to understand the basic thing. I want to know how much will be the voltage on the anode of diode D1 in case if we assume that on D1cathode we have -14v as it is on diagram. As i understand in this condition diode is forward biased and it will conduct the current. But what about the voltage on point of anode? The resistor values must be taken into account also. My main concern is if it will be positive relative to 0V or negative. Thanks 15007125550151354552797.jpg
 

dl324

Joined Mar 30, 2015
13,142
Ok, my question was about D1, but these circuits are similar. But what about +14V from R28,
For the convenience of others. Rotated, clipped, brightened schematic
rot.jpg
From what I can see, TP7 will be at about -1.6V.
\( \small I_{D11} = \frac{28V - 0.7V}{17.5k} = 1.56mA
{V_{TP7} ={14V - 1.56mA*10k} = -1.6V\)
 
Last edited:

ian field

Joined Oct 27, 2012
6,539
Yes it must be 0V on TP7, but what about my quetion?
There's no such thing as an ideal diode, so the other end will be Vf less than what you used to forward bias it.

Values are approximate - germanium and Shottky barrier are similar at around 0.2V, silicon is somewhere around 0.7V and silicon carbide usually rather higher.

Zener diodes usually read slightly more than regular on DMM diode test - there's many more I can't be bothered looking up.
 

dl324

Joined Mar 30, 2015
13,142
Now its easy to understand, its a simple circuit, thanks.
Actually, it's a little more complicated.

We have to assume that the diode is forward biased. Many will use 0.7V, some 0.6V; it depends on what you were taught.

Assuming "full on" voltage of 0.7V, I calculated a current of 1.56mA. From the graph below, a 1N4148/1N914 type diode:
upload_2017-7-22_10-31-43.png
We see that at 1.56mA, the forward voltage would typically be around 630mV. To get a more accurate current in the circuit, we would do the calculation again with a more accurate diode voltage. We would continue iterating until the values settled or we figured it was close enough.

In your case, the first estimate may have been sufficient.
 

Thread Starter

pujulde

Joined Jul 24, 2013
108
About the voltage drop everything is clear. It depends on type and substance its made. My doubt was about the polarity and amount of voltage relative to ground. But I got an excellent answer, thanks to dl324. So when the diode conducting its connecting the -14 Volt source to TP7. I was thinking whether the voltage is positive or negative on TP7.
 

Thread Starter

pujulde

Joined Jul 24, 2013
108
I know the basics and general concepts of electronic components. But when its time to make analyze of circuit I am stuck and its boring. I am sure that I need to make more analyze and more calculations. With time it must be easier to understand more complicated circuits. If you advice me how to get more knowledge in this direction I will be glad. Some source or type of activities.
 

dl324

Joined Mar 30, 2015
13,142
But when its time to make analyze of circuit I am stuck and its boring.
Are you getting stuck because you find the subject matter boring? Or are you getting stuck because you find it too difficult?

Electrical engineering is a subject that requires a lot of attention to details. If you're not a detail oriented person, you'll likely never be able to understand circuits well.
 

Thread Starter

pujulde

Joined Jul 24, 2013
108
Its not boring absolutely. It seems a bit complicated. I am repairing electrical staff and working in this field but I want to go deep and understand circuits. And some obstacles discouraging me. But I decided to analyze simple circuits to rich my goal(understand circuits in more details).
 

dl324

Joined Mar 30, 2015
13,142
I want to go deep and understand circuits.
I'm of the opinion that you should learn electronics from the bottom up so you have a good foundation in fundamentals. If you don't, you'll have holes in your knowledge that may make things seem more complicated than they are.
 
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