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negative supply generation!!

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algilsan

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hello,
i have a make to shitch rs232 signals, without using any type of relays, so i have used an analog switcher, this chip need dual suplly to be powered, so to be able to conmutate negative an d positive voltages of rs232 signals. My problem is that i don't know the way to generate such negative voltage from my positive (12 V). I have seen the best solution is to use charge pumps circuits, but the problem is the low current they can supply.¿?.

Can anybody help me?
 

7rots51

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MAX232 or similar are chips that are used for TTL and RS232 signal conversions.(RS232 is peer to peer and the current supplied with these chips are sufficient )

For converting a positive voltage to negative voltage you can use switching power supply isolatec or nonisolated configuration.

see www.analog.com and www.linear-tech.com and www.st.com
and motorola for thier SMPS chips.

u can also use DC/DC converter modules, see www.newport.co.uk
:wink:
 

GrandAlf

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If you do not want to use a driver ic that generates its own -12, there are plenty plasic packaged + to - converter modules. Or u could transmit at logic level rs232 rx will will be ok down to about 3v. You may need an inverter to get the right polarity.
 

Gunship

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algilsan said:
hello,
i have a make to shitch rs232 signals, without using any type of relays, so i have used an analog switcher, this chip need dual suplly to be powered, so to be able to conmutate negative an d positive voltages of rs232 signals. My problem is that i don't know the way to generate such negative voltage from my positive (12 V). I have seen the best solution is to use charge pumps circuits, but the problem is the low current they can supply.¿?.

Can anybody help me?
Maxium has a simple charge pump chip that can put out 20-30mA. (If I remember correctly.) Since you are using for switching with analog switch, this should be more than enough. Check the current consumption of your analog switcher. If it consumed less than that, which I fully expect it will, you are better off drop one of those charge pump into it. They are cheap anyway.

Good luck

Gundam
 

algilsan

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Since this chip can only deliver 20 mA, i can't use it because rs232 specifies a nominal current value for each pin of 20 mA. But i succesfully find a chip called TC962, from Microchip, that can deliver current up to 80mA. And this is good (i think..)

Regards
 

evil

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don't switch the rs232 signals with the analog switch (+- supplay)!
use MAX 232 ICs and switch the TTL signals with analog switches with only 5V supply!
 

algilsan

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Ok!, evil,

This is good form to perform my task, but i really had in mind, the reason to don't do it is that max 232 are a bit expensive, and in this proyect money is very important, and i would need two 232 and a digital switcher like 4066.
 

cherrytart

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an analog switch usually will not be as robust as an RS232 driver or receiver, more susceptible to latchup, static and other perils of connecting to the outside world.
 

jodokus

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It may be that rs232 spec says that it must be capable of 20mA but in the real world this is almost never the case and you may settle for a 5mA supply. But if you really want to have more you can use the LT1054 from linear technology. Be aware to use the extra transistor as shown in the datasheet.
 

shanren

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i think the best way to produce a nagtive votalge is to use MC34063,from morotala or st.it is easy to use and can aford 200ma.you can use a max232 change the level to logic,then switch the logic level,it is easy.in my design,i use two max232,for 4 232 input,and use 2 74hc125 to switch them to a 8052 cpu,and it works well.if you do so ,no need of nagive voltage.you can also use cd4051 to switch the logic level signal instead of 74HC125,just like another design my friend have done,it works well too.
 

renato

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It's a stupidy solution and maybe could match our necessity, please ignore if you had already thought about ...
If you could isolated the circuit of switch you can use two 5 or 6 V voltage regulators in series.

GND +12

-5 "reg" GND "reg" +5

You will create a virtual ground so your switch power supply must be isolated from RS232 signal.
:oops:
 

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