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I wanted to confirm that I wasn't doing anything stupid. No load except for the DMM. Both Zeners came from separate suppliers. In fact, as I increase and decrease the resistor value, the Zener voltage increases and decreases along with it.
It's like the Zener resistance isn't decreasing as the current increases, which is not what I'd expect. What's odd is that the BZX55C is showing more-or-less the same behaviour although with different resistors.
Is this normal? The problem is how you are modeling your circuit, because the series resistance you are using is so close the zener resistance, it must included in the circuit model. As shown in the circuit below which models the behavior that you are seeing of a voltage reading of about 4V from the zener diode. The ones with voltage of 5. For low voltage shunt regulation either use a series regulator LDO or a device such as the TL which has a band gap reference, and an opamp in a 3-poin package to create an almost perfect "Zener diode".
If you want decent accuracy throw the zeners away and use some low dropout 3. Zeners are notoriously power inefficient even at no load. At least at no load or very small load the LDO regulator will be way more efficient.
Rather than use rules of thumb which you may or may not understand, you should first look at the data sheet of the part you're using.
Since you are driving more than 10 times that amount of current through it, it is no wonder that your voltage is high. Because it rises as per the quote you must have an unstable power source of 5V and this is rising as you increase the load resistance. Double check this supply. The next problem is that at 70mA and 4 volts, the power is 0. I'm not sure what the temperature coefficient is for this device and the data sheets are lacking in that respect. The 3. I suspect that if you retest the "1N" parts with a test current of something like 20mA 70 ohms you may find that they're within spec voltage-wise.
You don't say what you saw there. If you include the dynamic resistance in the model, you must use a lower voltage for the ideal zener. It's incorrect to add a voltage at the test current because of the dynamic resistance.
The dynamic resistance describes the small-signal behavior near the test current. Typically the dynamic resistance of the 1NA is about 5 ohms, so you could approximate it with an ideal 3. Zeners with nominal voltages less than 5V or so have very soft 'knees' poor regulation and are not really that useful as usually better alternative are available.
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Asked 4 years, 11 months ago. Active 4 years, 11 months ago. Viewed 2k times. I set up the standard test circuit shown below. Null 5, 10 10 gold badges 24 24 silver badges 36 36 bronze badges. Buck8pe Buck8pe 1, 11 11 silver badges 30 30 bronze badges. The dynamic resistance of the Zener may not be able to drop low enough, even with 70mA available to it, to make the proper resistor divider equivalent for the node to be equal to 3.
In my experience, low-voltage zeners anything below 5V all seem to have a very soft 'knee' and not particularly 'vertical' current-vs-voltage curve. See Fig 2 in diodes. Active Oldest Votes. As shown in the circuit below which models the behavior that you are seeing of a voltage reading of about 4V from the zener diode simulate this circuit — Schematic created using CircuitLab.
Kvegaoro Kvegaoro 3, 12 12 silver badges 14 14 bronze badges. Is that the dynamic resistance of the zener? I suppose the point is, at 70mA, according to the datasheet, the voltage across the zener should be 3.
It's not. Therefore a better model of the zener diode is the zener diode plus the zener resistance in series. You do not see 3. Even so I expect that the diode you have does match its data sheet. Kevin White Kevin White 17k 1 1 gold badge 22 22 silver badges 29 29 bronze badges. This query was simply to highlight my dismay at how poor these zeners really are! Can I still get a reliable voltage if Vcc dips below 5V?
How much current will th load require? The closer to the minimum input voltage the more difficult it will be to avoid large changes in current flowing through the TL If you want to get 3. Michael Karas Michael Karas It isn't anymore:- It seems to be all knee. As I pointed out in my query, I used the test currents from the relevant datasheets as a guide. First this stated in the question: - In fact, as I increase and decrease the resistor value, the Zener voltage increases and decreases along with it.
If you increase the resistor the voltage across the zener should go down. Andy aka Andy aka k 15 15 gold badges silver badges bronze badges. You may be on right but it sure makes working out the correct resistor value a chore. Spehro Pefhany Spehro Pefhany k 9 9 gold badges silver badges bronze badges.
Can you expand on the statement: "It's incorrect to add a voltage at the test current because of the dynamic resistance". Does this conflict with what Kvegaoro is saying? That part of kveagaro's answer is not correct. Sign up or log in Sign up using Google. Sign up using Facebook. Sign up using Email and Password. Post as a guest Name. Email Required, but never shown.
1N4729A Diode. Datasheet pdf. Equivalent
1N4729A: Zener 3.6V 1W 5%