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Need to control the supply to MAX3232 ( RS232_TO_TTL) IC using Arduino. The 3.3V supply is also tapped from the Arduino board. I have to keep the Vcc supply of MAX3232 around 3V. With the present circuit diagram, the voltage drop is around ( hence Vcc indented) is around 2.3V. Should I use a different Rc value or should I use driver level transistor like SL100 and avoid Rc altogether? Thanks.

enter image description here

seccpur
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  • Why do you need to keep Vcc around 3V? It's the minimum recommended supply voltage. – ocrdu Nov 05 '20 at 03:45
  • Observed with 5V and used case for 24x7, the MAX3232 heats up immensely. 3v works perfectly. – seccpur Nov 05 '20 at 03:48
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    The MAX3232 is specified to run with a supply of 3V to 5.5V. If it 'heats up immensely' then you're doing something wrong. – brhans Nov 05 '20 at 04:05
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    I wouldn't feed it through a resistor; it will drop voltage depending on the IC's power consumption, and waste energy when the transistor is conducting. Why not use a high-side switch? – ocrdu Nov 05 '20 at 04:19
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    Your circuit makes no sense. What is your goal? Why do you want to switch the power? – Chris Stratton Nov 05 '20 at 04:25
  • MAX3232 need to be started in disabled mode otherwise it doesn't sync with external device. Else the external device goes to remote mode and looses sync and control from the uP – seccpur Nov 05 '20 at 04:36
  • MAX3232 works well on 5v but heats up over long period of time. – seccpur Nov 05 '20 at 04:38
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    I second what brhans says. Plus, since it's not a good thing to apply a voltage to a chip's inputs when it's unpowered, you should also guarantee that the chip will not get any input from RS-232 when you cut its power. By the way, as others have already stated, the circuit shown in the post is not that efficient. – Rohat Kılıç Nov 05 '20 at 05:13
  • Using relay will solve, but I don't want to complicate – seccpur Nov 05 '20 at 05:33
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    The MAX3232 should not heat up so you have some other problem in the circuit. And the power supply switching circuit will cause even more problems, as supply is provided via resistor and turning chip off consumes 15mA via transistor, so it makes no sense. Also @RohatKılıç that is not true, it's a RS232 PHY, it is meant to hanle being connected unpowered to another powered RS232 PHY, or our modems and computers etc would have blown up if they weren't simultaneously turned on. It won't handle the TTL side though so it must have power when MCU outputs logic high. – Justme Nov 05 '20 at 05:35
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    MAX3232 has a SHDN pin for exactly this use case, please don't externally switch VCC... use the internal shutdown circuitry, that's what it's designed to support. – MarkU Nov 05 '20 at 08:18
  • @MarkU: That's brilliant idea. I will check. Please post as answer. – seccpur Nov 05 '20 at 09:07
  • @MarkU MAX3232 does not have a SHDN pin. Some other models do. – Justme Nov 05 '20 at 10:04
  • OP: @MarkU 's excellent idea is your way forward. Look yourself for other parts in the MAX3232 family or similar that do have a shutdown pin. There's a lot of variants of these part families. – TonyM Nov 05 '20 at 12:45

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