How long does it take to charge up a capacitor

Question

I am using Esp32 to control a NRF24L01 with antenna

To power the solution I am using a HLK-PM03 which provides a 3.3V Output.

I was thinking of using 470uF capacitor (other suggestions are appreciated).

When writing the software, in the startup process I would like to know exactly how long I need to wait until the capacitor is fully charged, before sending any commands to the NRF24L01

The capacitor is connected to +Vo and -Vo to be able to give more stable power, because when sending commands from the Esp32 to the NRF24L01, the NRF24L01 will draw more power when sending.

The esp32 is getting commands via wireless lan so I am using the Wi-Fi which takes some power and I believe that needs to be taken in to account when choosing the size of the capacitor.

Is there a formula I can use? How do I decide on what size capacitor to use?

Answer 1

A couple of things, but the first one is: don't do this.

The question is: Why?

A) 470uF is a lot of capacitance and is more likely to hurt than help

1. 470uF can take a while to charge up. Most chips have a supply voltage specification that the voltage MUST rise at a certain rate or the reset circuitry will malfunction. That much capacitance could slow you down far enough that you might not meet the specification.

2. A 470uF capacitor will probably have quite a bit of series resistance and so won't supply current as quickly as you think it will.

3. A 470uF capacitor is probably pretty big and expensive.

4. The nrf24L01 only pulls 10mA MAX. There is no way you need a capacitor that big.

B) The inherent power on reset takes a non-deterministic amount of time. It can be up to 10.3ms for the nrf24L01 (see the datasheet)

C) A capacitor that large is rarely useful for RF circuits as the capacitor likely has a terrible high frequency response.

Given all this, the question becomes "What should you do?"

What you should do is read one of the nrf24L01 registers.

The nrf24L01 has several registers that come up with useful bit patterns so that they can be distinguished from a chip that hasn't powered up yet.

Register 0x0A defaults to 0xE7E7E7E7E7. Register 0x0B defaults to 0xC2C2C2C2C2.

Both of these bit patterns have transitions which means that you can distinguish them from a stuck bus. If you do a read and don't get the expected value, the chip hasn't come up yet and you should wait a bit and do a read a little later. If you do a read and do get the expected value, the chip is up and you're ready to go.

Hope this helps. Good luck.

Answer 2

How long does it take to charge up a capacitor

There is no formula you can use to tell you how long this capacitor will take to charge. The best you can do is provide an indication of how long it will take if the power supply does not go into shutdown.

Look at the datasheet for the HLK-PM03 power supply:

From this data you can see that:

1. The power supply can provide about 1.2A without shutting down.
2. There is no indication of the power supply risetime.
3. 200% rated current implies shutdown (and a time begore it restarts).

If you were to assume that the turn on current will be below 200% of rating, then you may be able to get between 1.2A and 2A during power up risetime.

Given that dV/dt = IC Then at 2A the risetime to 3.3V will be about 800us.

If this same supply is used for your WSP32, then when you come out of brownout and initialize, a 100ms wait would seem appropriate before you send commands.

You may also be limited by the risetime of the power supply itself, and the datasheet provides no information on this.