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| DC Offset drops when I attach my circuit *SOLVED* sig gen resistance |
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| NoFilter:
Hi all, This is a lab assignment for an electronics course. The part I am stuck on is using a Zener diode (1N752A Vz = 5.6V) to regulate a voltage. We're using a 0.9V peak sin wave on top of a 9Vdc offset as if it was a ripple from the supply. The current through the diode has to be close to 20mA and the current through the load (resistor) needs to be 30mA. I've solved for the resistor values (see attached image) and the simulation is fine, but when I apply the DC offset from the signal generator and measure it using a DMM, it only reads 7.7V. When I remove the circuit, the offset reads 9V as expected. Is this a sign of a bad offset? Is the DC offset supposed to drop? The output voltage on the build circuit is only 5.3V. Any help would be greatly appreciated! Edit: I can force the input voltage to be 9V if I set the supply to 11Vish...Is that what I need to do? Is that normal? Edit: Thank you Hero999, it was the resistance from the sig gen 50 ohms! Thank you, Tyler |
| rstofer:
I'm assuming the problem comes with real hardware and not the simulation... Your simulated source doesn't have any internal resistance so the voltage doesn't drop when the circuit is applied. That doesn't happen in the real world. For example, what would happen if the signal generator produced 10V open-circuit and had 50 Ohms of impedance and you connected a 50 Ohm load? The voltage would divide in half and the output would be 5V. Of course, I may be answering the wrong question. |
| NoFilter:
If this is the case, then it would be correct to set the supply voltage to ~11.1V to achieve the 9V on the circuit? When I do this, I get the expected results all around (except the display on the supply). When I remove the sin wave from the circuit, the 9V reading on the supply matches what the DMM reads...Maybe there is a large resistance in the signal generator? |
| basinstreetdesign:
--- Quote from: NoFilter on October 08, 2018, 12:05:59 am --- Edit: I can force the input voltage to be 9V if I set the supply to 11Vish...Is that what I need to do? Is that normal? Thank you, Tyler --- End quote --- No, that is not normal. Shunt regulators like zener diodes regulate by maintaining the total current through both the zener and the load as a constant. That means that if the zener current is to be 20 mA when the load draws 30 mA then the dropping resistor, R2, must be calculated to drop the required voltage at that sum current. Then, should the load draw less current for some reason the zener will immediately take up that slack and draw up to the full 50 mA to maintain its regulation voltage. Should the load take more, but not more than 50 mA, the zener will take less current, down to zero. So R2 needs to be (9V - 5.6V)/50 mA value. Shunt regulators are one of the most wasteful types of regulator as the current drawn from the supply is ALWAYS maximum regardless what the load is doing. EDIT: Sigh, I just re-read what you are seeing on the bench and it looks like your signal generator has non-zero output impedance. In fact we can calculate just what output impedance it has since it drops from 9V with no load to 7.7V with load of (7.7 - 5.6)/68 = 30.9 mA so is (9 - 7.7)/0.0309 = 42 Ohms. One way to fix this is to get a better source. Or another way is to incorporate the generator source impedance into the total value of R2. That is, subtract the generators source impedance value from the 68 Ohms you calculated and put in a resistor for R2 at that resulting value. Then the sum of R2 and the Gens source impedance will be the required 68 Ohms and the circuit should operate as expected. |
| NoFilter:
--- Quote from: basinstreetdesign on October 08, 2018, 02:00:36 am ---So R2 needs to be (9V - 5.6V)/50 mA value. Shunt regulators are one of the most wasteful types of regulator as the current drawn from the supply is ALWAYS maximum regardless what the load is doing. --- End quote --- Hmm the simulation agrees with this as well ((9 - 5.6)/50mA = 68 ohms as shown in the diagram). The issue comes up when I attach the physical circuit. The offset is set to 9Vdc and when there is no circuit attached, my meter reads 9Vdc just fine, but when I attach the circuit the meter reads 7.4Vdc even though the offset value still displays 9Vdc. Hopefully we'll learn about some better regulators soon! This is the first lab/exposure to diodes so I'm still trying to grasp them. |
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