MAX232 voltage supply - need help in a circuit

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Bateman

Joined: 27 Jun 2009
Posts: 3

27 Jun 2009 2:17 MAX232 voltage supply

Hi There.

I have a battery operated circuit where i need to convert TTL 0-3V to -4V low and +4V high. I was going to use a MAX232, however I'm unsure of the voltage supplies I wire to the MAX to output these voltages. How do I get -4V and +4V on the Vs- and Vs+ terminals? (pins 2 and 6) My circuit power supply is simply a +12V battery so I can't just wire it to the voltage rails.

MAX232 voltage supply - need help in a circuit

MAX232 voltage supply - need help in a circuit

Any help would be much appreciated.

GSM Man

Joined: 15 Apr 2009
Posts: 354
Helped: 46
Location: New Jersey, USA

27 Jun 2009 2:24 MAX232 voltage supply

You only have to connect power to Vcc - the chip generates the other voltages. Refer to the datasheet for the correct capacitor values and connecitons.

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Bateman

Joined: 27 Jun 2009
Posts: 3

27 Jun 2009 2:51 MAX232 voltage supply

I don't understand, the MAX232 can generate voltages HIGHER than its supply?

mfarooq1

Joined: 09 Jan 2009
Posts: 87
Helped: 4

27 Jun 2009 10:31 MAX232 voltage supply

see datasheet

GSM Man

Joined: 15 Apr 2009
Posts: 354
Helped: 46
Location: New Jersey, USA

27 Jun 2009 13:49 Re: MAX232 voltage supply

Bateman wrote:

I don't understand, the MAX232 can generate voltages HIGHER than its supply?

Yeah, it uses charge pumps! From the Datasheet:

Quote:

The MAX220–MAX249 have two internal charge-pumps
that convert +5V to ±10V (unloaded) for RS-232 driver
operation. The first converter uses capacitor C1 to double
the +5V input to +10V on C3 at the V+ output. The
second converter uses capacitor C2 to invert +10V to
-10V on C4 at the V- output.

There's a reason that manufacturers put the time and money into creating datasheets. You really should try reading them - it will help you understand how the device works.

elrayes

Joined: 03 Jun 2008
Posts: 145
Helped: 7
Location: Egypt

28 Jun 2009 9:13 Re: MAX232 voltage supply

the matter isn't about voltage higher than the supply voltage, yes the IC can generate higher voltage than the supply, the output will be 12 and -12 volts use either LM317 voltage regulator to get 4 volts or potentiometer to get 4 & -4 volts.
but take care the maximum output current is very low (nearly 30mA), refer to the datasheet to make sure.
Good luck

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