Re: International standards

I've got an old modem card here. I'd like to know what the various components do. Have you come across a site that identifies the electronic componets and their functions.

Thanks

SP.

PS. explanations for other boards/cards would also be welcome

X-No-Archive: Yes

The typical variable voltage power supply consist of a voltage control IC and multiple high power transisors such as 2N3055.

The problem with this design is that it's highly inefficient and requires large power supply for the input.

I'd like a design that can use a 12A ish 24V switch mode power supply for input and output stage made of PWM voltage controller much like CPU voltage controller that can drive external MOSFETs.

Is there such thing as one chip controller that will drive MOSFETs and change output voltage from potentiometer input?

Hi,I have a problem with this circuit

Where is the mistake?

Data:

Vi= 100mV;

Ii( AC current at the transistor's base ) = 794nA;

hfe = 83.6;

re = 42 Ohm;

Ri ( calculate ) = (hfe+1)*( re + RE ) = 84.6*1042 = 88153.2 Ohm;

Ri ( measured ) = Vi/Ii = 100m/794n = 125944.6 Ohm;

The difference between the two result is too much!

Help me to understand where is my error!

Thanks !

Francesco

Could someone suggest a simple circuit that would light an led for about 2-4 seconds when power is applied and then turn off and stay off until power is cut and applied again?

Thanks in advance JJ __________________________________________________________________

Figure about 15 watts per pound for conventional transformers, maybe

John

is

That's not a bad idea! I'll have to think about that... I still have quite a few technical books, though by no means every tech book I used to.

Based on the diagram, I'm assuming R5 is intended to be the output load. For an emitter follower, it's on the wrong transistor terminal. It should be on emitter for an emitter follower - note this would change the RE value to R3 || R5 ( '||' = parallel reduction ). Strictly, as drawn, C2/R5 do nothing right now - for small signal they are not even part of the circuit since all DC sources are effectively grounded for analysis.

Perhaps you are measuing the correctly wired circuit rather than the displayed circuit diagram: the change in RE is ~0.1% but 0.1% * (Hfe+1) is 8.4% difference in the formula. Not enough to explain the whole difference (formula deviates 29% from measured).

More likely: remember that the Ri/Zi formula you are using is only valid for "small signal".

Try measuring with an even lower input signal level, e.g. 1 mV or 100 uV. 0.1 V is probably not "small signal" enough. Roughly speaking with an 0.1V input and hfe of 83.6, your collector current is going to move significantly so your Q-point/bias point is not going to be where you thought it was.

For a 100 mV DC level you've shifted the Q-point, while for 100 mV AC level you're seeing an integral input impedance of all the traversed Q-points. Considering that, the difference of 88K vs 125K seems like a reasonable result.

Another source of error: is the Hfe measured or from the datasheet? If the latter, there is statistical variance between any actual transistior and what the data sheet says (which is only the average value of Hfe). If measured, how? In reality Hfe = Hfe(Ic) so it's Q-point dependent also. Again gets back to not being small signal. BTW the datasheet value of Hfe is always Hfe(Icmax) where Icmax is the Ic that gives maximum(Hfe) - obvious marketing prefers listing *that* value.

A remote possibility is Rb: the "full form" is Zin(CE/CC|s.s.) = (rb + RB) + (Hfe+1)*(re + RE). Usually Rb is ignorable but not always.

Small signal can be a tricky concept: small signal is small only in the sense of bias point shifting (as in Taylor expansions and the resulting shift of y (or in this case Ic) ), but *not ever* in the human intuition of when signal level seems "small" sense. Thus small-signal really means "*small enough* that I *feel* comfortable ignoring the *actual deviations* from my *assumption* of linear operation". It's an important subtlety.

MM

Namely, look for "one shot" or monostable 555 circuits.

Jon

Try this:

John

I have a Kenmore electric dryer that went on the blink this weekend. I set the timer (which I can hear running) and push the START button. I hear a humming sound whilst pushing the button but nothing happens. I took the back off and wiggled the wires. Still doesn't work. I couldn't see the belt very good from the back but it looked attached. I can move the drum one way manually.

I have limited tools (my trusty swiss army knife is alls I have used thus far if this gives you an idea). Should I just bite the dust and call a repair person, or are there a few things I can still check.

Thanks in advance.