Confused By EBAY Level Convertor

Other Hardware General Electronics
1

Hi

I bought a couple of level converters on EBAY to convert from 5v to 3.3v

The item is here http://www.ebay.com/itm/331264801549?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

1013×735 97.9 KB

I have connected it as follows

Arduino Uno via USB
Arduino 5v to module 5v
Arduino GND to module GND

Tested the 3.3 connector on Module and it is running at 3.3v

Connected PIN2 of Arduino to H1 on Module
Measured voltage on L1 on Module and receive 4.91v

void setup() {                 
    pinMode(2, OUTPUT);     
}

void loop() {
  digitalWrite(2, HIGH);
}

Now on the ebay site it has this statement

Level conversion principle:
Between 5V and 3.3V level plus 470 ohm current limiting resistor, to avoid being burned of 3.3V port.
So, when the LOW side no-load, HIGH side plus 5V level,LOW side test is still at 5V level，vice versa.
This is a normal phenomenon, not the level conversion function does not work.

So this is what I am seeing, the low side is at 5v but I need 3.3v and I am confused by the above statement and the actual output of the module.

If I put a 3.3v device or IO to the pin and it is delivering 5v then dead 3.3v device!

Please help me understand.

2

OK

I put an led onto the L1 side with a 220 ohm resister and I do see about 3v output on the L1 side now.

So it must be working...

Just need to understand this more.

I am wanting to connect an ESP8266 WIFI module and do not want to blow it up as there is confusing things about the IO pins on the Internet. Some say RX/TX directly to 5v and some say that will blow the IO pins on the WIFI module.

3

Card seems to rely on input protection/clamp diodes of the receiving device to help out the level conversion.
The resistor limits the current flow, the clamp diodes keep the level within 0.5V of receiving device's Vcc and Gnd pins.
For example, 5V signal into 3.3V device:
(5V - 3.3V)/470 = 3.6mA of current flow
Atmel recommends 1mA of clamp diode current flow, so this would not be a good level converter.

Got a link to ESP8266 WIFI module data sheet?

4

Thanks for the advise.

Here is a "potential" datasheet for it

Chris

5

iisfaq:
I bought a couple of level converters on eBay to convert from 5v to 3.3v

Sorry. You have been ripped off. That is not a level converter, it is a useless piece of junk (unless perhaps the regulator is somehow usable).

You want an actual level converter.

6

Thanks for the feedback Paul

So the 471 chip just has 4 resisters in it.

So 5v in -> 5v out with less current.

Chris

7

The "471" chip is just four resistors connected side by side -- only for mounting convenience. The 471 is the resistor code for the value "47" followed by "1" zero (470 ohms).

8

Yes I will agree you have been done.

A 5V to 3v3 level converter is easy anyway a 510R series resistor and a 1K pull down. Got to be less than one cent.

I was going to report the item to eBay but it looks like it doesn't want to play with me anymore.

9

And, as if by magic, passing a 3.3v output through a 470 ohm resistor will bring it up to 5v ready to be input to your 5v device? Absolute Genius :slight_smile:

10

Measuring the floating secondary (3.3V ) side of a 470 ohm resistor with a DMM with a 10 Mohm input resistance and no return path for the current on the 3.3V side of the resistor is going to give you the same
voltage as the 5V side. If you had a 3.3V device connected to that side when you measured the voltage then there might be a problem but trying to measure it with nothing connected is not a valid test for a voltage divider that relies on a flow of current (which, in the case of a DMM is probably nanoamps.)

11

the seller has this in the auction:

Level conversion principle:
Between 5V and 3.3V level plus 470 ohm current limiting resistor, to avoid being burned of 3.3V port.
So, when the LOW side no-load, HIGH side plus 5V level,LOW side test is still at 5V level，vice versa.
This is a normal phenomenon, not the level conversion function does not work.

I can't quite grasp what it means except for the " the level conversion function does not work" part.

12

naut:
I can't quite grasp what it means except for the " the level conversion function does not work" part.

  • "NOT the level conversion function does not work"

The level conversion works perfectly. It's just that YOUR understanding of THEIR level conversion is wrong :slight_smile:

13

KenF:
The level conversion works perfectly. It's just that YOUR understanding of THEIR level conversion is wrong :slight_smile:

I am not sure we want to be brought down to their level in any way! :o

If it included a Schottky diode array to clip the outputs to 3.3V, I could believe it as a one-way converter, but it never was, nor will be, a useful I2C level converter.

Strangely enough, I can see that it will work in a manner, in some instances, where the pull-ups are at least 10k, but we should not give it the "benefit of the doubt".

Stick to the sort I cited.

14

The funny thing is that the seller provides an honest schematic that shows a circuit that is clearly not a level converter and still he has sold 44 of them. I am not sure who to blame, there are so many choices.