Timeline for Error in constant current sink circuit and current regulator
Current License: CC BY-SA 4.0
25 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Feb 5, 2021 at 18:20 | vote | accept | Clone Wars | ||
| Jan 11, 2021 at 5:26 | answer | added | Tony Stewart EE since 1975 | timeline score: 0 | |
| Jan 11, 2021 at 5:13 | comment | added | user57037 | It is a somewhat constant current source. The two diodes are used as a voltage reference. But as the input voltage changes, the diode voltage also changes a bit, thus the output current changes, too. Your approach of adding a zener gives a better result because the reference voltage doesn't change as much with Vin. | |
| Jan 11, 2021 at 3:47 | answer | added | jonk | timeline score: 2 | |
| Jan 10, 2021 at 21:30 | comment | added | jonk | @CloneWars But if all you are doing is simulating this circuit, I recommend that you not only vary the supply voltage but that you (1) vary the load's voltage drop from a minimal value to a maximal value; (2) significantly vary the temperature of operation; (3) significantly vary the BJT parameters -- especially IS (10:1 span), VA (40 to 120), BF (100 to 300), and perhaps to a small degree even NF (1 to 1.2). | |
| Jan 10, 2021 at 21:26 | comment | added | jonk | @CloneWars But increases do affect the collector current. In addition, the load matters too as the BJT has an Early Effect, as well. Ambient temperature significantly affects the collector current. Part variation will also significantly affects the collector current. Etc. If you want a fairly rock-solid current regulator then a TLV431 coupled with a BJT comes darned close, though a 2-BJT design at least removes the Early Effect (variation due to variation in voltage drop across the load.) I guess I'm just confused about where your head is at. Probably just me. | |
| Jan 10, 2021 at 20:36 | answer | added | Bruce Abbott | timeline score: 0 | |
| Jan 10, 2021 at 20:35 | comment | added | Clone Wars | What I mean is it regulates current.When Vin reaches the zener voltage any increase in voltage from that point on doesn't affect the collector current which is a current regulator. | |
| Jan 10, 2021 at 20:22 | comment | added | jonk | @CloneWars Define "correct" and I'll try and answer. If all you mean is that you have arranged a BJT such that it's base voltage is held relatively constant, that the BJT operates as an emitter follower such that the emitter resistor has a relatively constant across it, then yeah. If you mean that the collector current is a "good" current sink then you need to define "good." If you don't care much at all, then it is a current sink. It certainly falls into the general category. It's just not as often useful as many others are that are just as simple or simpler to make. | |
| Jan 10, 2021 at 20:20 | comment | added | Clone Wars | At least the 2nd circuit which is a current regulator is correct isn't it? | |
| Jan 10, 2021 at 20:06 | comment | added | jonk | @CloneWars Look here for an extended discussion with both theoretical results followed by simulation results, for comparison against theory. That discussion shows a circuit that addresses and largely solves issues related to the Early Effect. It's also possible to apply sensitivity analysis to any circuit to quantify the boundaries of behavior, as well. Also look up the Wyatt Cascode Peaking Current source for a particularly good one using only four BJTs. | |
| Jan 10, 2021 at 19:26 | comment | added | Bimpelrekkie | The collector current depends only on the base current That's not how this circuit works. If what you say is true then a different transistor (with a different \$\beta\$) would result in a different output current. The circuit would be highly unusable. You're trying to understand things for which you need a lot better understanding of transistors and how they behave in a circuit. | |
| Jan 10, 2021 at 19:22 | comment | added | Pete W | "The collector current depends only on the base current " -- look up Early Effect | |
| Jan 10, 2021 at 19:21 | review | Close votes | |||
| Jan 26, 2021 at 3:03 | |||||
| Jan 10, 2021 at 19:12 | comment | added | Clone Wars | Yes i put it on 5 and 10V | |
| Jan 10, 2021 at 19:10 | comment | added | glen_geek | This current sink is only valid for Vc > Vb. Have your tests ensured a Vcc high enough that this is true? | |
| Jan 10, 2021 at 18:59 | comment | added | Clone Wars | Bimpelrekkie why should it change ?The collector current depends only on the base current which in turn depends on the base voltage which is fixed for every value of V1(in the first circuit). | |
| Jan 10, 2021 at 18:52 | comment | added | Bimpelrekkie | I am getting a 1.2mA difference for if for 5V difference. You have to be much more precise and detailed that that. In the circuit, indicate all the voltages and currents for both situations. A "5 V difference" doesn't mean much. Is that 0 V / 5 V or 10 V / 15 V or 50 V / 55 V ? | |
| Jan 10, 2021 at 18:50 | comment | added | Bimpelrekkie | Also: when in your simulation the output current does not change when you change the supply voltage (V1 and/or V2) then your simulation is wrong because in the real world that current would change. | |
| Jan 10, 2021 at 18:48 | comment | added | Clone Wars | I am getting a 1.2mA difference for if for 5V difference.But that's big if you look at my resistors.And I know the wire has some resistance as well but I don't need that big detail. | |
| Jan 10, 2021 at 18:45 | history | edited | SamGibson♦ | CC BY-SA 4.0 | Improved tags. Fixed typo in title. Some grammar and spelling changes, to help readability. |
| Jan 10, 2021 at 18:45 | comment | added | Bimpelrekkie | You should not expect "no change at all", even the most fancy-complex-expensive constant current circuit in existence's output current will have some dependency on the supply voltage. For a simple circuit like yours, a 10% change in supply voltage could result in a 1% change in output current. You don't show any values. Maybe what you are getting is already very good for this simple circuit. | |
| Jan 10, 2021 at 18:45 | history | edited | SamGibson♦ | CC BY-SA 4.0 | Improved tags. Fixed typo in title. Some grammar and spelling changes, to help readability. |
| Jan 10, 2021 at 18:36 | review | First posts | |||
| Jan 11, 2021 at 1:20 | |||||
| Jan 10, 2021 at 18:36 | history | asked | Clone Wars | CC BY-SA 4.0 |