ON RC time delay to drive a transistor

[Nikos A.]

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-1a

My 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

[Benta]

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.

[Nikos A.]

Thank you for your response Benta,

You list a 1 A DC/DC module. The 2N3904 will not carry that much current.

The 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.

[Benta]

However, the capacitor won't charge above 0.8V with the current RC network even without the boost converter connected.

You're right, it won't. It behaves exactly as designed.

[Nikos A.]

You're right, it won't. It behaves exactly as designed.

Could you explain why?

[PCB.Wiz]

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)

Of 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 ~ 2k2
Or, 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

[Nikos A.]

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 ~ 2k2
Or, 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

Thank you a lot!! I will try both solutions!! Do you think the 555 timer is more suitable for this application?

[PCB.Wiz]

Do you think the 555 timer is more suitable for this application?

Not 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)