Hi All, I have a project that will need level conversion between 3.3V and 5 V logic, so I picked up a logic level converter at the local Jaycar. I expected to find the documentation on line on their site, but what they provide is truly abysmal so I set out to do a reverse engineering job on it. What started me on this was the need to figure out what all the pins did, since there seemed to be more than should be needed for a two channel bidirectional level shifter.
What I found is a bit of a problem for anyone who might be tempted to use one of these. The basic circuit is the same as the Phillips application note AN97055. The extra pins are accounted for by the fact that as well as the two channels with a MOSFET each, there are two voltage dividers built in. These are just a divide by two, with the input on the high voltage side and the output on the low voltage side. They should work OK provided your 3.3 Volt logic will accept about 2.5 Volts as a logic high. They are of course unidirectional,OK for logic but not I2C
But the wasp in the jampot that I found was that the two converters with the MOSFET are both wired differently. One i exactly as per the application note, the other is not, and actually connects the low voltage input/ output pin directly to the low voltage VDD supply. This may lead to some unhappiness for your low voltage logic, when it tries to pull the power supply rail down to logic low. I've made a sketch of what I have found, I hope it will be readable. These converters are sold in Australia and New Zealand by Jaycar, presumably since they are made in China they could show up in other places. Hopefully this might help prevent someone letting the magic smoke out, luckily I am cautious enough to not want to connect it to anything precious until I knew exactly what all the pins do. I suppose in theory it should be possible to rework the board, but being surface mount that could be tricky.
regards, John
The details of the project may help us find a solution.
These consumer units can take various forms and are broadly classified into unidirectional and bidirectional. Electrical configuration is similar to:
https://learn.sparkfun.com/tutorials/bi-directional-logic-level-converter-hookup-guide
I have used numerous models, all have been successful.
Isn't it possible that the MOSFETs on one have a different pinout, with gate and source reversed?
Ok, Thanks for the replies.
The details of the particular project don't actually matter here, since all that will be needed is I2C level conversion, and it is not a case of a project not working since it hasn't been built yet.
I wondered if the MOSFETs were supposed to be mirror image, but was able to check the source drain diode, they are both the same and conform to the usual surface mount MOSFET connection, and even if the one could be flipped over the input pin would still be connected to the low voltage VDD. I think the cause of the problem is that someone has made a mistake with the circuit board design.
The proper solution of course would be for the supplier to replace it with something that works properly, I might try that although I don't see much chance of finding someone who actually understands the problem, and for a device that is only a few dollars I am not going to worry too much.
The main reason to post here is that I would hate to see someone use one of these and find that they had cooked some more expensive 3.3 Volt device. As Mrburnette points out, there are plenty of suppliers of this sort of thing, the main downside is that most of them are two or three weeks away from here by mail order.
regards
John
Hi
Is this your inverter?
https://www.jaycar.com.au/arduino-compatible-logic-level-converter-module/p/XC4486?pos=1&queryId=24ee6a189485b4312a11a2bf2ddb9283
But then it's possible that it was actually designed for a different MOSFET and they just threw in the common ones they had on hand. Athough I have to admit, I haven't actually seen any MOSFETs with the reversed pinout.
Details? Do they look the same? Are they both from the same supplier? Same screening etc.? Can we see some images?
The converter referred to by Ruilviana is indeed the one.
There is only one actual board involved, with one converter on it that works correctly, and one that will not since it is incorrectly connected by the circuit board design.
An inverted version of the MOSFET would not fix it since the input/output pin would still be connected to the 3.3V VDD, not a desirable state of affairs since any device on the bus that tried to assert a low state would be trying to short out the 3.3 Volt supply.
I doubt if it is worth trying to fix this device, too small and fiddly , I'll just order one from Sparkfun or someone else.
John
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