Theory problem

[Leezygeezer]

Apologies if this is in the wrong place in advance.

I am studying some basic theory but have encountered a problem that I can't seem to solve. It is about calculating current across R1 in series with a diode wired in parallel with a resistor R2. I have been given the values of Vs = 10V, R1 = 10K, R2 = 1k, and it is a silicon Diode (normally 0.7 Vd). However I have been advised that with a supply voltage of 10v or higher to assume that the diode has no effective voltage drop.

Following the equations of :-

Vd = R2 x I2 to find I2
Vr = Vs - Vd
Vr = It x R1 to find It
It = I2 - Id to find Id

I cannot get my answer to match the given one.

I need to rearrange Vd to I2 = Vd/R2 or 0.7/1000 or 0.7ma
Vr = 10 - .7 giving Vr = 9.3v
It = Vr/R1 giving 9.3/10000 giving It = .93ma
Id = It - I2 = .93 - .7 = .23ma

This is correct, but assuming no drop the calculation fails as Vd is zero, can anyone help? Im confused

 

[BrownOut]

I think you have it right and you're done. What's the problem?

Next time, put up a schematic with your quanties (I2, It, etc) clearly labeled.

 

[Dr_Doggy]

Maybe since Vd is insignificant after 10V, 0.7/10 = 07%

 

[RCinFLA]

You did everything correct except for a general case you should first check that the resistor divider ratio with the applied voltage yields a high enough voltage across R2 to cause diode conduction voltage to occur first and clamp the voltage across R2 at 0.7 vdc.

 

[Leezygeezer]

Ok, first of all thanks for the help and advise, secondly, I think i must be missing something simple because if assuming no drop across my diode I can't find I2 using this method as I have no value to fit in the equation

i.e. Vd = 0, so I2 = Vd/R2 is a fail
Vr = Vs so 10v (no drop)
It= Vr/R1 or 10/10000 = 1ma
Id = It - I2 which fails because I have no values for Id or I2 this is where I am stuck, basic current division in a 2 resistor parallel circuit uses I2 = (It)(R1)/(R1+R2), giving .9ma
This would then fit the equation giving Id as .1ma, the only trouble is the answer I am given is .3ma, can anyone point out where I have gone wrong?

Thanks in advance

 

[alec_t]

If I've understood your circuit correctly then the answer you have been given (Id = .3mA) is wrong! If you assume there is no voltage across the diode (i.e. diode short-circuit) then the diode current is 0 and the voltage across R2 must also be 0. So all you can calculate is the current through R1, which will be approximately the current you originally calculated with Vd = 0.7. In other words, the assumption simplifies the calculation of the R1 current at the expense of accuracy.

 

[Leezygeezer]

This is the diagram I have for reference

hmm image doesn't show up, ill have to work on this a bit more !

 

[Mike_2545]

**broken link removed**

One too many Http's

 

[Leezygeezer]

sorted by Mike, cheers

 

[BrownOut]

Assuming VD=0 makes the question trivial. Before banging my head any further, I'd get clarification from whomever told you that.

 

[alec_t]

Having seen your circuit confirms that I did understand it correctly. The answer, therefore, is as in post #6.