I am having issues with Zener diodes. First time, I assumed I must have a dud zener, but I have bought two more and they are doing the same thing. I'm trying to get a reference voltage for a voltage comparitor circuit, but the zener is not giving the correct reference voltage. I've take the zener out of circuit to figure out what's going on...
My test circuit is pretty simple - V+ (11.5V), connected through resistor, then 3.3v Zener (reversed, with black line toward V+), then to gnd. At first, I was using a high resistor of 1Mohm, to get the reference voltage without drawing much current, but the zener voltage was very low, like 0.9V. I wondered whether the zener needed more current to achieve the required voltage, so I tried decreasing the resistor, and tried two different battery supplies, and got a big range of results...
R zener v zener v
(7.7V batt) (12.2V batt)
1.2M 0.93v 1.01v
400K 0.93v 1.01v
10k 1.7v 1.8v
150 3.07v
50 3.5v
So, I had to drop the resistor right down to get close to the right voltage, and at 50ohms, the zener voltage exceeded the spec voltage of 3.3v!
Out of curiosity, I reversed the zener to be fwd biased, and repeated some of the tests, and the voltage varied between 5.5 to 6.5 through the same range, so fwd biasing seems to give a better result, as reasonably expected, but the reverse biased zener should be giving a more consistent 3.3V, right?
The batteries were freshly charged, and I measured the battery voltages in circuit, so that should not be an issue. I have tested diodes from two different stores, and they are two different wattages (400mW, and 1W), and they are giving consistent results.
What is going on?
My test circuit is pretty simple - V+ (11.5V), connected through resistor, then 3.3v Zener (reversed, with black line toward V+), then to gnd. At first, I was using a high resistor of 1Mohm, to get the reference voltage without drawing much current, but the zener voltage was very low, like 0.9V. I wondered whether the zener needed more current to achieve the required voltage, so I tried decreasing the resistor, and tried two different battery supplies, and got a big range of results...
R zener v zener v
(7.7V batt) (12.2V batt)
1.2M 0.93v 1.01v
400K 0.93v 1.01v
10k 1.7v 1.8v
150 3.07v
50 3.5v
So, I had to drop the resistor right down to get close to the right voltage, and at 50ohms, the zener voltage exceeded the spec voltage of 3.3v!
Out of curiosity, I reversed the zener to be fwd biased, and repeated some of the tests, and the voltage varied between 5.5 to 6.5 through the same range, so fwd biasing seems to give a better result, as reasonably expected, but the reverse biased zener should be giving a more consistent 3.3V, right?
The batteries were freshly charged, and I measured the battery voltages in circuit, so that should not be an issue. I have tested diodes from two different stores, and they are two different wattages (400mW, and 1W), and they are giving consistent results.
What is going on?