Voltage divider to reduce Ch340G Tx signal from 4v to 3.3V results in voltage drop on Tx

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I am trying to connect Ch340G USB to TTL with Esp32 for Serial communication.

on Ch340G I put jumper between VCC and 3.3V, expecting it to give me 3.3V output signals on Tx/Rx

However, when Ch340G is plugged into my macbook, and I measure voltage between its Rx and GND, and between Tx & GND its between 3.8V to 4V. And I am afraid it can damage Esp32

So I tried to reduce the voltage using voltage divider as shown in below image

circuit-2
circuit-2740×340 4.5 KB

However, as soon as put voltage divider, the voltage between Rx and GND drops to some thing around 2.3V. And i do not get enough voltage output from divider to Tx pin of Esp32.

I believe, this is happening, because voltage divider is drawing current and Ch340G is unable to provide the enough current and voltage drops.

I tried with 2200 + 10K resistors and voltage still drops.

Any help, whats going wrong here ?
Or, should i assume, that as soon as i connect Rx output from Ch340G to Esp32 Tx, it would automatically drop to tolerable level of voltage around 3.3V. I think, its risk, to assume it would happen, as my multimeter shows 3.8 to 4V between Rx and GND of Ch340G

2

Could you complete the schematic with VCC and all relevant pins? What do you mean with a jumper between VCC and 3.3V? So how are you powering it?

The voltage in the output pins should be about VCC - 0.5V, regarding the datasheet. Check which voltage you have in VCC, it should be 3.3V, right?

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@anon27210439 : Its usb to ttl, usb side is plugged into macbook, and macbook gives it power

This is the device : Buy CH340G Serial Converter USB 2.0 to TTL 6 Pin Module Online in India | Robocraze

Pulse_44_a60c1fcc-fea3-44d3-a5e3-b506412ce678.png
Pulse_44_a60c1fcc-fea3-44d3-a5e3-b506412ce678.png700×700 36.1 KB

Putting jumper between VCC and 3.3V is expected to give 3.3V ttl signals

So i think, only pin that is relevant as of now is GND and RX - I am not doing any thing with tx yet.

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Rx is an input, what should the voltage divider do?

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@DrDiettrich Tried same circuit between Tx of ch340G and GND and still as soon as i put voltage divider, the voltage drops. But if i directly measure voltage between its Tx and GND, thn there's 3.9V

Also, even when Rx of Ch340G is not connected to Esp Tx
I see 3.9V between Rx of ch340G and its GND - tht looks problematic too, not sure though

Below are the images i took from breadboard - Its show voltage between Tx and GND of ch340G (3.95V)

Unknown
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The new circuit using Tx

circuit-2
circuit-2740×340 4.5 KB

Here also, the voltage on V1 drops very low as soon as I have voltage divider.
But if i open the voltage divider circuit, voltage on V1 is 3.9V

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That is true but you are measuring 3.8 to 4V so I would say that the converter is no good

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That is true but you are measuring 3.8 to 4V so I would say that the converter is no good

@jim-p but it should be close enough. Eg in last screenshot, its 3.95V which is close enough to 4v
and devider should have given me around 3.3 (even if not exact)

The issue is, drastic voltage drop from 3.95V to <2V as soon as there's voltage divider

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My point is that if you set Vcc to 3.3V then the output should not be higher than 3.3V so the converter must be defective.That is why a voltage divider is not working.

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Ah, is there a way to "test it" - coz this is brand new, just arrived. But i agree, it could be defective

10

Basically you just did. If the output is supposed to be less than 3.3V and it's not then something is wrong.
However, maybe it can still work with 5V. Try putting the jumper on 5V and check the TX voltage.

11

Try putting the jumper on 5V and check the RX voltage.

in 5v case, the Voltage between Rx &GND is 5V
Btw, I believe, Ch340G has pull up resistor on its Rx, so when idle, Rx is high instead of floating

12

That is true and presents a problem because now you will need a voltage translator for the RX input, a voltage divider will not work

13

This Ch340G thing definitely looks damaged. I tried another USB to ttl (Cp2102)
and voltage looks good, with and without divider

Ch340G drops voltage to < 2V on Tx signal as soon as even microamp current is drawn from it.
Regardless of i have it on 5V mode or 3.3V mode

Thanks for helping out

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Glad I could help

1 Like
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