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| DC Offset drops when I attach my circuit *SOLVED* sig gen resistance |
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| Zero999:
--- Quote from: NoFilter on October 08, 2018, 01:20:52 am ---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? --- End quote --- Yes, I suspect your signal generator has an output impedance of 50 Ohms. I = (11.1-5.6)/(68+50) = 5.5/118 = 46.6mA, which is quite close to what the simulation gives. |
| NoFilter:
--- Quote from: Hero999 on October 08, 2018, 08:24:43 am --- --- Quote from: NoFilter on October 08, 2018, 01:20:52 am ---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? --- End quote --- Yes, I suspect your signal generator has an output impedance of 50 Ohms. I = (11.1-5.6)/(68+50) = 5.5/118 = 46.6mA, which is quite close to what the simulation gives. --- End quote --- Woooooooow, it even says 50 Ohms on my signal generator....Thank you so much, that accounts for the drop to 7.4 from 9V lol |
| rstofer:
Now you know what is happening for a single operating point but the idea of the zener diode regulator is that the load can change and the zener current varies to make up the difference. From the source, looking into the circuit, the current is the same. The difference is in the split between load current and zener current. Maybe the load current is 30 mA and the zener current 20 mA and then the load changes to 25 mA and the zener current increases to 25 mA. The source sees 50 mA either way. The input resistor drops the same amount of voltage based on 50 mA so the output voltage stays constant. |
| NoFilter:
--- Quote from: rstofer on October 08, 2018, 01:57:01 pm ---Now you know what is happening for a single operating point but the idea of the zener diode regulator is that the load can change and the zener current varies to make up the difference. From the source, looking into the circuit, the current is the same. The difference is in the split between load current and zener current. Maybe the load current is 30 mA and the zener current 20 mA and then the load changes to 25 mA and the zener current increases to 25 mA. The source sees 50 mA either way. The input resistor drops the same amount of voltage based on 50 mA so the output voltage stays constant. --- End quote --- I think I witnessed this, but I think the zener has a minimum current requirement of 20mA to provide the full zener voltage (please correct me if I am wrong..still learning. Iz on the sheet was 20mA with a max of 65)? When the input voltage dropped to 7.4V, the drop across the zener was 5.3V, but the current through the load was still ~30mA (little less). The current through the zener was ~6mA which is why I think the voltage was lacking (or perhaps the voltage across the zener was less than needed to allow full current to flow through based on the I-V curve?). |
| rstofer:
You would design the zener current to be near max when the load is disconnected. There really isn't much else you can do because it is likely that at some point the load will be disconnected. If you absolutely know there is some minimum load current that will always be present, you can assume this load when you calculate for max zener current. You want the zener current to be near max when the load is minimum. I would expect the zener current to be mid-range when the load was near mid-range, if such a thing exists. Shunt regulators aren't really the best way to go because you are always wasting zener current and it may be as much as the load current. Maybe even more under some operating conditions. |
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