SN75176/Max485 get hot when gnd disconnected

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haohaodk46

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Hi Everyone,
I use 485 circuit as below but Max485 IC get hot when GND disconnected
The issue just occurred when It connected to a other 485 ( receiver ) and the both circuit connnected to a power supply.




The both 485 node I ususe power jack as below



When I connect this jack to 485 node , If GND pin is connected last then IC 485 get hot and stop working and if GND is connected first then It still is good

I try to use good IC MAX485 that expensive price then It work well and not getting hot .

If I use a node is common IC max 485 and another node is good MAX485 then It still work well but It just work well when good MAX485 IC side should removed TVS diode protectioncircuit . If IT not removed then IT still get hot from common MAX 485 side

The everyone can explain me ?

Thank you so much !
 

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Incomplete schematic. Where does 24V go? Is it actually used? How's the circuit supplied? Has it other external connections beside JP3?
 

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Also:
What is VCC?
How do you expect current to flow from the 24V through a reverse-biased diode?
Those TVS devices are rated for 6.8V. RS485 can go -7/+12 volts common mode, so those could be conducting.
 

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Hi,

simply saying: Don´t disconnect GND.
As long as you don´t use isolated (optocoupler) signals you need GND as reference.

Maybe use connectors with "early connecting" GND or use a 5 pin connector with two GNDs ... one at each end.

Klaus
 

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FvM said:
Incomplete schematic. Where does 24V go? Is it actually used? How's the circuit supplied? Has it other external connections beside JP3?
Click to expand...
The 24V will path to a LM2576 circuit down to 5V of VCC net . It go directly LM2576 IC not external connections
--- Updated ---

KlausST said:
Hi,

simply saying: Don´t disconnect GND.
As long as you don´t use isolated (optocoupler) signals you need GND as reference.

Maybe use connectors with "early connecting" GND or use a 5 pin connector with two GNDs ... one at each end.

Klaus
Click to expand...
Good idea for using 5 pin connector but I want to find why when It connected to good IC part then It is not get hot . Seem good IC part have a circuit protect that It will protect for that IC part ( bad IC part)
--- Updated ---

barry said:
so those could be conducting.
Click to expand...
I try to with SMAJ5.0CA but the issue still occur
 
Last edited:

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In other words, with GND disconnected, supply current is flowing through data lines. Surely the circuit can't work, also RS-485 driver outputs are pulled outside allowed voltage range.
 

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haohaodk46 said:
Good idea for using 5 pin connector but I want to find why when It connected to good IC part then It is not get hot . Seem good IC part have a circuit protect that It will protect for that IC part ( bad IC part)
Click to expand...
Hi,

we definitely need the whole schematic to be sure,
Otherwise we just can guess.

I´m with the others. I guess
24V --> 24V_load --> 24V_load_return on PCB (broken GND) --> 24V return via signal lines (wires)

Klaus
 

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5.0 volts is LESS than 6.8v. Why would you expect that to make it better? You need at least a 12 volt part.

And as Klaus says: don’t disconnect ground. It’s not designed to work that way, its an invalid operating condition. It’s like pouring water in your car‘s gas tank and then complaining that your car is making a strange noise.
 
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I don't have intend to disconnect GND but I have many node that use 4 pin connector so when plug connector to each node then unfortunately gnd is conntected last to gnd lead to IC Max 485 get hot.
Why should use 12V part, Could you explain for me ? I think that only 5V enought to keep voltage on A , B signal alway 5v if have a transient on A, B.
 

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Max485 requires 5V maximum. How you create the VCC in your schematic from 24V input of power jack?
 

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Ok, I'll repeat myself:

The common-mode voltage range of RS-485 is -7V to +12V.

If the common-mode voltage exceeds the breakdown voltage of your TVSs, they will conduct current to ground.

And why are you plugging devices in with the power on??? Unless you've specifically designed your system for hot-swapping (which you haven't) it's a very bad idea.
 

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KApostol said:
Max485 requires 5V maximum. How you create the VCC in your schematic from 24V input of power jack?
Click to expand...
1. Check and verify that the voltage at VCC/GND pins of MAX485 IC is 5V +/-10%
2. Verify that the RS485_A is really connected to RS485_A among ALL nodes and similar for RS485_B.
3. If you use termination resistors, remember that ONLY the two far-ends must be terminated.
4. RS485 Nodes connecction MUST NOT be of STAR topology.
5. If the Nodes of connected RS485 devices are in large distance, then, maybe, there is LARGE DIFFERENCE in GROUND VOLTAGE LEVEL between RS485 NODES.
You can check this by inserting in-series an current meter in the GND conductor. In thgis case, if Ground voltage level between nodes is More than 7V, the A/B pins of RS485 driver feed a current via the SMA6.8CA protection devices to Ground, making the driver so HOT.
 

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