# AC to DC conversion voltage regulators and zener diodes question

1. Feb 29, 2008

### O.J.

I am at the last stage of AC-DC conversion. I understood filtering all right. but when the filtered signal arrives at the regulator, isnt the regulator supposed to change that 'ripple' signal into a straight line??

Also, is there a derivation for the diode IV equation somewhere?

2. Feb 29, 2008

### O.J.

I also am having trouble understanding clippers. How come when the diode is forward biased it will limit the voltage of the load to the diode cut in voltage only? Doesnt the source voltage increase beyond that of the diode's cut-in voltage and so the voltage across the diode itself increases along with Vs? I thought diodes limited current not voltage..

3. Feb 29, 2008

### Staff: Mentor

Yes, a linear voltage regulator (like an LM7805) will regulate the output voltage to a "straight line" constant voltage. There are some limitations, though, like the minimum input voltage to the regulator must be met, in order for it to hold the ouput voltage in regulation. Look at the datasheet for the LM7805 +5V regulator, for example. IIRC, the minimum input voltage is around 7V, in order for the output to stay held a a constant 5V. If the ripple on the input voltage to the regulator causes Vin to dip below Vin,min at any point, then the output may also dip during that time.

As for diodes, the Diode Equation is generally derived in your first solid state physics class, and the derivation will be in your textbook. Alternately, you can start at wikipedia.org's page, and expand your reading through the links:

http://en.wikipedia.org/wiki/Diode_modelling

I'm not sure I understand your final questions about using diodes as clippers. After you read over the wikipedia.org page, could you maybe re-phrase that part of your question?

4. Feb 29, 2008

### O.J.

I really want to understand HOW can the diode LIMIT the voltage, since looking at its IV curve, the voltage can increase. So why can't a increase beyond 0.7 V (in the source voltage)for example force the diode voltage to increase accordingly??? doens't make sense.

And no, the derivation for the diode equation isn't in out textbook.

As for my first question,can you give me an example of a simple regulator at work?

5. Feb 29, 2008

### Staff: Mentor

For an intro to voltage regulators, back to wikipedia.org:

http://en.wikipedia.org/wiki/Voltage_regulator

That's a general introduction -- click on the "Linear Regulator" link in that page for a more detailed description of the basic linear voltage regulator. You can also look up the datasheet for the LM7805 at National Semiconductor, and look for application notes there or at Maxim or at Linear Technology, etc.

As for the diode and using it as a voltage limiter, you are using the variable impedance of the diode, as shown by the plot of the diode equation. For reverse bias, and for forward bias up to the breakpoint around 0.6V to 0.7V (depends on temperature, etc.), the impedance of the diode is high. But above the breakpoint, the impedance ($$Z = \frac{\Delta V}{\Delta I}$$) drops to a low value. This is what gives the "clamping" effect, when used as a forward-biased clamp or clipper.

For Zener diodes it is different. There you use them in reverse bias, and their impedance goes from high to low as the reverse bias voltage nears the Zener voltage rating of the diode (where the diode begins to reverse avalanche).

Hope that helps.

6. Feb 29, 2008

### O.J.

OKay, so in FB beyond the break point, their impedence falls to a low value, and at low impedence, voltage can still increase for large increase in current. so i dont see how this still LIMITS THE voltage at a certain value..

7. Feb 29, 2008

### Staff: Mentor

We need to have a circuit diagram to get much farther, I'm afraid. Can you please either post a sketch, or point to a diagram on the web that illustrates what is confusing you? Thanks.

8. Mar 1, 2008

### O.J.

imagine a simple diagram of an AC source with a resistor (in series) along with a diode in parallel as well as two output terminals ?? to the diode. Assume the diode pollarity is such that it is FB in the first voltage swing. Now, the voltage gra[h drawn in our text shows the voltage across the output normally increasing till it reaches the diode cut in voltage where it remains constant. My question, why doesnt the diode voltage increase as the source voltage increases, how does it LIMIT the voltage across itself to its cut-in voltage?

9. Mar 1, 2008

### mheslep

A voltage limiter or clamp is created with a diode by applying a fixed voltage (Vlimit) from some source to the cathode side of the diode and the anode is connected to the circuit w/ varying voltage. Then whenever the varying voltage of the anode attempts to rise more than Vlimit+ ~0.7 the diode is forwarded biased, turns 'on', an effectively 'shorts' the attempted increase. Commonly the next question is, well where to I get Vlimit? I only have my single supply voltage? Ans: voltage divider.