Help me resolve a heat problem of IC75176

[freedelphi]

I am designing an RS232 - RS422 converter using TI 1 pcs max232 + 2 pcs 75176B. I put 120 ohm load on 75176 output (for termination)...

The circuit is operational, but, is it normal that one 75176 (TX) is getting so hot?

TIA.

 

[mr_ghz]

75176

Hi freedelphi

I think something with your converter or with your 75176 is wrong, even
if the circuit is working.
If you have a load of 120ohm, the differential voltage will be about 3V
and the current below 30mA. The chip itself will heat with P=(VCC-3V)*30mA,
a normal DIL-IC (even SMD) has a tmperature resistance of about 90°/W,
so it will heat less than 10° over the ambient temperature.
If you're talking of a 'hot' device, I think the temperature is over 60°C
and this will say that you perhaps have a short on 1 output or that the
device ist 'slightly dead'

Greets

 

[J_expoler1]

75176 price

I have same problem too But i used 8Pin DIP package how i doing

 

[mr_ghz]

2 75176

Hi J_expoler1

can you upload or mail a schematic?

 

[freedelphi]

75176 hot

Hi mr_ghz,

Thanks for replying..

I just realised that, I put 120 ohm on both 75176 (one for TX, one for RX) I used.. So the load will be 60 ohm when I connect two converter, or loop-it back.. Do you think it's the problem? I'll try to remove one of it...

Is it true that I only need to put one Resistor on the receiver end?

Thanks and best regards,
freedelphi

 

[mr_ghz]

75176 circuit schematic

Hi freedelphi,

With a load of 60Ohms instead of 120Ohm the differential volatage will
drop to about 2.5Volts and the current will be about 45mA. The dissipation
is around 120mW. This will heat the chip about 20° over the ambient.
So if it get really hot (>60°C), this was not the reason.
In higher frequencies it is necessary that the sender, the cable and the
receiver has the same input-resistance or you will get reflections and
transmission-errors. In the case of a 75176 the transmitter is already
mached for this resistance, so only the receiver uses a (parallel) resistor
of about 120Ohms.

Greets

 

[Elvis P.]

75176 max232

Hi Folks!

Its very unusual that the ti transceiver gets that hot. i used that part in different designs. The temperature are about 20°C higher (40°C absolute) than the ambient temp. (~20°C absolute)

I found the National parts are working better for my needs. (DS8921 for single transceicer)

regards
Elvis P.

 

[mr_ghz]

rs232 to rs422 schematic|circuit 75176

If you have the chance to change the 75176, it's perhaps a good idea. This
device is old and consumes a lot of power. Its ESD stability is poor
compared with National or Maxim. The advantage is its low price...

Greets