Constant Voltage Diodes

diode calculations?, finding the diode current?

i Q4(a)
The equation relating diode current id to diode voltage vd at temperature T is
given by
id = 10-14[exp(qvd/(mkT) – 1]
where q = 1.6 x 10-19 C , Boltzmann’s constant k = 1.38 x 10-23 J.K-1
and the diode ideality factor m = 1.2.

Calculate the diode current at
(i) a forward bias voltage of 0.7 V.
(ii) a reverse bias voltage of 2 V.

Calculate the diode voltage when the diode current is 50 mA.

Q4(b) Plot a graph showing loge(id) vs vd. Tabulate clearly its slope and intercept.

please cd someone show me how to calculate this

How do you do it? you 'plug' in the numbers and calculate the answer.

Id = 10^-14 * [exp(q*Vd/m*k*T) - 1]

Vd = 0.7
T = 270 Kelvin (room temperature)

Id = 10^-14 * [exp(1.6*10^-19 * 0.7/(1.2 * 1.38*10^-23 * 270) - 1]
Id = 10^-14 * [7.56*10^10]
Id = 0.000756 Amps
which is 756 uA

For problem (ii), set Vd = -2 and solve. You'll get some incredibly low number (10^-45 amps). That may not be reality -- the actual reverse current will be on the order of 10^-9 to 10^-6 Amps but, it does demonstrate that reverse currents are very very low.

for problem 3
0.05 = 10^-14 * [exp(q * Vd / m * k * T) - 1]

You will need to rearrange the formula. Since the -1 on the end is insignificant to the kinds of numbers produced by the e^x function (i.e. 10^14 - 1 is still 10^14 for 2 or 3 decimal-place accuracy) you can get rid of it. Then it is a matter of taking the natural logarithm (ln) of each side and using regular algebra so solve for Vd.

I think you can figure it out. If you still can't e-mail me (look on my profile).

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Do you have a (stalled? inefficient?) micro-hydro or solar/wind project?

Ripple Voltage equation is slightly incorrect?

We used diodes to build a DC voltage supply that ideally would have a constant DC voltage output of 12V for example. Assume we feed a diode rectifier with the output from a 25Vrms transformer and use a 100μF capacitor & a 10KΩ resistor. T=1/120s, V(ripple)/V(max)= T/RC....This formula uses the RC time constant of an RC circuit in decay. It is slightly incorrect. From the information given so far, can you determine if the actual ripple voltage will be less or more than calculated and explain why?

assuming that the transformer is 25 VCT and the diode is actually 2 diodes in a bridge configuration, and this is at 60 Hz. the DC output would be 12.5 x 1.4 – 0.7 volts, approximately, or about 17 volts.

assuming the 10k is the load, so the current is about 1.7mA
ripple will be about V=it/C = (0.0017)(0.008)/0.0001 = 0.14 volts P-P

what did you get?

The ripple equation is an approximation only, it does not consider the diode and transformer impedances, capacitor lead and internal resistances, or the exponential vs linear discharge of the cap. But which way the error lies depends on those unknown factors.

bottom line: slight error? no problem.
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