We're designing a DC power supply through the use of a transformer, a rectifier and a capacitor. We've finished that, and would now like to regulate the voltage to *almost* pure DC through the use of a zener diode in parallel with the capacitor.
The problem is that in PSPICE, the software we're using to model the circuit, the voltage waveforms we're getting from using a zener diode don't look like they "should." I say they don't look like they "should," because from what we've learned, the response doesn't make sense.
As far as I've learned, a zener will break down when it's in reverse bias at some specified voltage (the one in pspice was determined to be about 4.6 volts through the use of a simple DC power supply in series with a zener and a load resistance). Since the breakdown is so dramatic, the current can increase a lot more with very little change in voltage.
So, just for an example, this circuit:
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should just limit the voltage at 4.6 volts (as long as the current flowing through the zener is within specified power regulations), and once the input voltage goes to more than 4.6 volts, it shouldn't matter, and the voltage across the diode should still be about 4.6 volts. It produces this output:
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The red is what I thought it should do and the black is what it does.
Any information anyone could offer to me on zener diodes or why this isn't working would be greatly appreciated.
Take a "peek at the peak" current in the zener, then draw some conclusions ;-)
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Which is it, parallel or series? Obviously it's supposed to be parallel.
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Go back and look at figure 2, labeled Voutput. That is the correct graph you are looking for. Figure 3 looks like it's just current through the zener or something.
A zener does work as a shunt regulator (staying within the power limit of the zener). You are just getting graphs and/or terminology mixed up.
You can pay extra money and get PSpice "smoke" detection.
Personally I haven't had a circuit design of mine "smoke" now for perhaps 40 years.
But I do occasionally roll my own macro-based probes that test sensitive small-feature-size CMOS for SOAR violations.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
Very good reply. Thank you for the in depth explanation about using a zener in a regulated voltage supply. Thanks to martin for those links, especially the pdf.
I think I definitely have a better understanding of it now, and I'll probably finish the design tonight. I'll post again if I run into problems, and I'll post the final circuit to with output.
Thanks to everyone for their replies; this is really going to help.
John Popelish: Your reply was the one I was citing in the previous post.
The point of a class is to learn before you just go out and blow something up. I don't have the 20+ years of experience yet. We just learned what a diode was three weeks ago.
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
But there is certainly some voltage change with current increase. this is often modeled as a resistor in series with an ideal zener.
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This statement does not take the internal resistance of the zener into account.
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It looks about right to me. Check how high the current is going to drive those voltage peaks up to where they are. I think you will discover you are *way* overloading the zener at those peaks. And the voltage sags after the zener turns off each half cycle, because the capacitor is supplying current with no assistance from the transformer during those periods, so it must be discharging.
Zeners do a fair job regulating voltage only when their current stays fairly constant. Fairly constant means that the current never goes all the way to zero, and stays below the rated current for the device. If you don't know the rated current, divide the rated power by the zener voltage for a rough idea of what that is.
Now, to design a regulator with a zener, you need to have a supply that produces a voltage that is *always* higher than the zener voltage. This implys that you need to let the capacitor charge up as high as it can, without trying to clip the peaks with a zener. Then you connect a series resistor to the parallel combination of zener and load.
In this circuit, the series resistor and parallel combination of zener and load form a voltage divider with one leg (the zener) being a variable resistor whose value changes dramatically as voltage across it changes slightly.
The series resistor must be chosen such that at the peak capacitor voltage, the zener current (series resistor current minus load current) must not exceed its ratings, and at minimum capacitor voltage, the zener current must not go to zero. For best regulation, you may need to keep the zener minimum current well above zero, say, no less than 1/4 or 1/2 the peak current.
You should be able to design this by trial and error, by monitoring the zener current during a half cycle, and seeing if it is possible to pick the correct series resistor while keeping the zener current in bounds. Then you can go back and try variations in line voltage and see how it handles those cases. You may find that your zener needs to be a higher power rated device to do a reasonable job under all conditions. And you will find out that this type or regulator is very wasteful with power.
Interesting. We were pulsing 300 watt rated fets, bolted directly to a machined-flat copper block, and most of them blew up in under 100 milliseconds at 300 watts.
If you set your newsreader to post with quoted text, it will show who said what (as mine did, above), who you are talking to and what you are commenting on.
But delete everything but reminders of where the conversation is, to keep things brief. Adding markers for your edits keeps you from being accused of altering the meanings of someone else's words with your clipping.
Picking the best newsgroup also increases the chance of a helpful reply. This thread, for instance, was a better fit to sci.electronics.basics (for beginners). You are not quite a designer, yet. ;-)
You need to refresh your knowledge on zeners. When its above the Z voltage, it is very low impedance. you need to limit the current, otherwise its will look like a short to the transformer
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your pic#1 is almost there, but you take the load from the top of the zener, not from the unregulated supply.
This is the hot electron issue with very small feature size CMOS. Foundry physicists tend to be bird brains ;-) They don't want the device to enter the region AT ALL. Thus no "t" in the equation. This macro spits out a "1" at such occurrences, making it easy to spot in Probe.
A real-life BJT SOAR macro would have an integrator in there.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
I love to cook with wine. Sometimes I even put it in the food.
did you see the paper on TO-247 transient-related thermal failures in one of the IEEE trans. recently? same deal as big die, the attachment develops solder voids due to CTE mismatch and rapid dT/dt...
that URL doesn't point to an image, links can't find an obvious image on that page, mozilla unavailable due to a hardware fault during an upgrade, so I gave up.
I'm guessing you should put the zener after the resevoir capacitor (with a resistor between them) and don't draw so much current that the resistor drop is greater than the difference betwen the capacitor voltage and what you want.
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