ON RC time delay to drive a transistor - Page 1
Forum section: electronics / projects

author: Nikos A.
author_rank: Regular Contributor
timestamp: July 25, 2023, 08:45:12 pm
content: Hi guys,I want to create an ON RC time delay circuit to drive the gate of a transistor. I will use the 2N3904 NPN. With the transistor, I want to drive a DC/DC converter module.https://www.robotaki.com/dc-dc-step-up-circuit-module-3v-to-5v-1aMy problem is that the capacitor does not charge above 0.75 - 0.8V and the transistor does not conduct. When disconnecting the ground from the transistor, the delay works fine and the capacitor charge in t=RC. What do I miss?Note: I tried to replace the dc/dc converter with an LED and the LED turns on based on the delay (however the voltage across the capacitor remains 0.75-0.8V)This is my circuit

author: Benta
author_rank: Super Contributor
timestamp: July 25, 2023, 09:22:27 pm
content: The circuit works exactly as it should.That it doesn't do what you expect is a different thing.For example:You list a 1 A DC/DC module. The 2N3904 will not carry that much current.The 2N3904 is a BJT. It needs base (not gate!) current. 100 kohm is far too high.In short: this concept will just not work. Look for a module with an "Enable" input instead.

author: Nikos A.
author_rank: Regular Contributor
timestamp: July 25, 2023, 10:02:58 pm
content: Thank you for your response Benta,Quote from: Benta on July 25, 2023, 09:22:27 pmThe maximum current I intend to draw from the boost converter is 50mA.However, the capacitor won't charge above 0.8V with the current RC network even without the boost converter connected.

author: Benta
author_rank: Super Contributor
timestamp: July 25, 2023, 10:27:17 pm
content: Quote from: Nikos A. on July 25, 2023, 10:02:58 pmYou're right, it won't. It behaves exactly as designed.

author: Nikos A.
author_rank: Regular Contributor
timestamp: July 25, 2023, 10:39:50 pm
content: Quote from: Benta on July 25, 2023, 10:27:17 pmCould you explain why?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 25, 2023, 10:51:17 pm
content: Quote from: Nikos A. on July 25, 2023, 08:45:12 pmOf course, it is clamped by the base.If you really want to use a NPN part, then work back from the 50mA. Ballpark might be 1v change and 1mA base current, to saturate reasonably at 50mA.That puts C as ~3mF or 3000uF, and R as ~ 2k2Or, you might want to change to a MOSFET - that has no base current limitation, so you can use higher R and smaller C.You need a lower threshold gate mosfet that will drive 50mA from 3V, so a FDV303N or similar part, that gives < 1 Ohm with VGS 2.7 or 2.5V

author: Nikos A.
author_rank: Regular Contributor
timestamp: July 25, 2023, 11:08:04 pm
content: Quote from: PCB.Wiz on July 25, 2023, 10:51:17 pmThank you a lot!! I will try both solutions!! Do you think the 555 timer is more suitable for this application?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: July 26, 2023, 12:22:26 am
content: Quote from: Nikos A. on July 25, 2023, 11:08:04 pmNot really, if you want more control and low current / voltage operation, maybe look at Voltage Monitors with user programmable delays.Parts like NCV308SN180 have a 1.8V threshold, and a cap sets the delay (~ 390nF for 3s) until the pin goes high (which then turns on a MOSFET)