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freedelphi
Joined: 07 Jun 2002
Posts: 23
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 08 May 2003 0:07 75176 Heat problem |
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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.
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mr_ghz
Joined: 01 Apr 2002
Posts: 294
Helped: 2
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| 08 May 2003 6:22 |
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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
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J_expoler1
Joined: 02 Apr 2003
Posts: 12
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| 08 May 2003 7:24 |
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| I have same problem too But i used 8Pin DIP package how i doing
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mr_ghz
Joined: 01 Apr 2002
Posts: 294
Helped: 2
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| 08 May 2003 7:41 |
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Hi J_expoler1
can you upload or mail a schematic?
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freedelphi
Joined: 07 Jun 2002
Posts: 23
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| 08 May 2003 14:09 |
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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
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mr_ghz
Joined: 01 Apr 2002
Posts: 294
Helped: 2
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| 08 May 2003 15:32 |
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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
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Elvis P.
Joined: 28 Aug 2001
Posts: 31
Location: Europe
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| 10 May 2003 10:35 75176 |
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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.
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mr_ghz
Joined: 01 Apr 2002
Posts: 294
Helped: 2
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| 10 May 2003 10:41 |
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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
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