DC Offset drops when I attach my circuit *SOLVED* sig gen resistance - Page 1
Forum section: electronics / beginners

author: NoFilter
author_rank: Contributor
timestamp: October 08, 2018, 12:05:59 am
content: 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

author: rstofer
author_rank: Super Contributor
timestamp: October 08, 2018, 01:03:59 am
content: 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.

author: NoFilter
author_rank: Contributor
timestamp: October 08, 2018, 01:20:52 am
content: 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?

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: October 08, 2018, 02:00:36 am
content: Quote from: NoFilter on October 08, 2018, 12:05:59 amNo, 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 7.7Vwith 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.

author: NoFilter
author_rank: Contributor
timestamp: October 08, 2018, 02:06:48 am
content: Quote from: basinstreetdesign on October 08, 2018, 02:00:36 amHmm 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.

author: Zero999
author_rank: Super Contributor
timestamp: October 08, 2018, 08:24:43 am
content: Quote from: NoFilter on October 08, 2018, 01:20:52 amYes, 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.

author: NoFilter
author_rank: Contributor
timestamp: October 08, 2018, 11:36:41 am
content: Quote from: Hero999 on October 08, 2018, 08:24:43 amWoooooooow, it even says 50 Ohms on my signal generator....Thank you so much, that accounts for the drop to 7.4 from 9V lol

author: rstofer
author_rank: Super Contributor
timestamp: October 08, 2018, 01:57:01 pm
content: 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.

author: NoFilter
author_rank: Contributor
timestamp: October 08, 2018, 04:20:17 pm
content: Quote from: rstofer on October 08, 2018, 01:57:01 pmI 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?).

author: rstofer
author_rank: Super Contributor
timestamp: October 08, 2018, 06:23:37 pm
content: 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 willalwaysbe 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.