Timeline for What is the purpose of this bias resistor?
Current License: CC BY-SA 3.0
9 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Dec 27, 2016 at 14:49 | vote | accept | efox29 | ||
| Jun 24, 2016 at 5:29 | answer | added | Jim Fischer | timeline score: 0 | |
| Jun 24, 2016 at 4:22 | history | tweeted | twitter.com/StackElectronix/status/746196875632488449 | ||
| Jun 24, 2016 at 3:34 | comment | added | Jim Fischer | I guess I should've looked at Q1's datasheet before commenting. According to the datasheet Q1's Miller plateau occurs at around VGS=-3.2 VDC; so if VGS=-4.25 VDC this would likely be sufficient to drive Q1 into its ohmic region. However, when Zener diode D2 is conducting (ON) it provides a very low-impedance path between Q1's source and gate, allowing Q1's gate voltage to quickly track Q1's source voltage. Maybe that's what the circuit designer had in mind here. | |
| Jun 24, 2016 at 3:25 | answer | added | Spehro 'speff' Pefhany | timeline score: 4 | |
| Jun 24, 2016 at 3:21 | comment | added | Jim Fischer | I think R5's value needs to be large relative to R6; otherwise with the nominal 12 VDC input (assuming a car battery), Q1's gate voltage will be around 8.25 VDC, its source voltage 12 VDC, and VGS=-4.25 VDC, which (a) does not turn on Zener diode D2, and (b) is probably not enough voltage to drive Q1 into its ohmic region (switched ON). With R5 = 100k, the resistor divider R5 and R6 try to drive Q1 gate to 2.16 VDC, but now Zener diode D2 turns on, locking Q1 VGS=-6.8 VDC, which is (presumably) sufficient to drive Q1 into its ohmic region (switched ON). | |
| Jun 24, 2016 at 1:54 | comment | added | Sam | 50uA of Ids would cause ~5V drop across R5 which would be enough to keep Q1 on permanently, although why they didn't just drop R5 to 10K I've no idea. it's not like it's going to save power or something as R4'll be seeing the full 60V across it when Q2 turns on. | |
| Jun 24, 2016 at 1:49 | history | edited | efox29 | CC BY-SA 3.0 |
added 6 characters in body
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| Jun 24, 2016 at 1:37 | history | asked | efox29 | CC BY-SA 3.0 |