-1.25 volts from positive supply. - Electronic Circuits Projects Diagrams Free

yafch · 1st November 2009, 10:45 AM

Hi,
can any one suggest me an easy way to get -1.25v(20mA) from a -5v power supply.
regards.

Sceadwian · 1st November 2009, 11:04 AM

A resistor.... What are you trying to power?

yafch · 1st November 2009, 12:48 PM

i need to use an operational amplifier in an application. supply voltages for the amplifier are +5v and -1.25. 20mA is just enough.i can use a little more voltage like -3 to -3.5, but for precision -1.25v is required.

Sceadwian · 1st November 2009, 12:51 PM

Can you link the amplifier you're using?

yafch · 1st November 2009, 01:56 PM

here it is.

KMoffett · 1st November 2009, 03:36 PM

Google for: 1.25V shunt regulator

Ken

Sceadwian · 1st November 2009, 03:46 PM

yafch I meant the opamp itself, there are a lot of unconnected points in that circuit, the reason I'm asking is a opamp's performance should improve with a lower negative voltage, unless it's required for some other portion of the circuit to function.

audioguru · 1st November 2009, 04:06 PM

If you want your LM317 adjustable regulator to go down to 0V then why not use the simple circuit in its datasheet?
Change the values of the resistors for an LM317 instead of the LM117 shown, change them for a different max output voltage and change the 680 ohm value for a different negative supply voltage which does not need regulation.

KMoffett · 1st November 2009, 04:06 PM

I agree. I would change the point that's labled as -1.25V to -5V. Use a resistor and 1.25v shunt regulator/reference in series from the -5V to common. And move the bottom for R2 to the junction of the resistor and shunt regulator.

yafch · 1st November 2009, 04:27 PM

op-amp itself is common LM358. see the diagram i attached with the second post. it has a current limiting feature.

it may work without a negative supply for op-amp. but don,t you think that current limiting feature may not work properly under short circuit conditions.

audioguru · 1st November 2009, 04:43 PM

Oh. The opamp makes an adjustable current-limiting circuit for the LM317 regulator.

If the circuit does not have a -1.25V supply then its minimum output voltage is +1.25V which might cause a much higher output current into a short circuit than its setting.

Sceadwian · 1st November 2009, 05:12 PM

You said you had a -5 volt supply, use that instead of the 1.25....

KMoffett · 1st November 2009, 05:22 PM

The -5V would be for the opamp and transistor, the -1.25 is needed to provide a negative reference to the regulator's divider R1/R2 so the output can be brought to 0.0V.

ken

yafch · 1st November 2009, 05:23 PM

ok let me try -5v and see what happens.

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1st November 2009, 10:45 AM	#1
yafch	-1.25 volts from positive supply. Hi, can any one suggest me an easy way to get -1.25v(20mA) from a -5v power supply. regards.

1st November 2009, 11:04 AM	#2
Sceadwian	A resistor.... What are you trying to power? __________________ "Because I be what I be. I would tell you what you want to know if I could, mum, but I be a cat, and no cat anywhere ever gave anyone a straight answer, har har."

1st November 2009, 12:48 PM	#3
yafch	i need to use an operational amplifier in an application. supply voltages for the amplifier are +5v and -1.25. 20mA is just enough.i can use a little more voltage like -3 to -3.5, but for precision -1.25v is required.

1st November 2009, 12:51 PM	#4
Sceadwian	Can you link the amplifier you're using? __________________ "Because I be what I be. I would tell you what you want to know if I could, mum, but I be a cat, and no cat anywhere ever gave anyone a straight answer, har har."

1st November 2009, 01:56 PM	#5
yafch	here it is. Attached Thumbnails

1st November 2009, 03:36 PM	#6
KMoffett	Google for: 1.25V shunt regulator Ken __________________ "To invent, you need a good imagination and a pile of junk." Thomas A. Edison (1847 - 1931)

1st November 2009, 03:46 PM	#7
Sceadwian	yafch I meant the opamp itself, there are a lot of unconnected points in that circuit, the reason I'm asking is a opamp's performance should improve with a lower negative voltage, unless it's required for some other portion of the circuit to function. __________________ "Because I be what I be. I would tell you what you want to know if I could, mum, but I be a cat, and no cat anywhere ever gave anyone a straight answer, har har."

1st November 2009, 04:06 PM	#8
audioguru	If you want your LM317 adjustable regulator to go down to 0V then why not use the simple circuit in its datasheet? Change the values of the resistors for an LM317 instead of the LM117 shown, change them for a different max output voltage and change the 680 ohm value for a different negative supply voltage which does not need regulation. Attached Thumbnails __________________ Uncle $crooge Last edited by audioguru; 1st November 2009 at 04:07 PM.

1st November 2009, 04:27 PM	#10
yafch	op-amp itself is common LM358. see the diagram i attached with the second post. it has a current limiting feature. it may work without a negative supply for op-amp. but don,t you think that current limiting feature may not work properly under short circuit conditions. Attached Thumbnails

1st November 2009, 04:43 PM	#11
audioguru	Oh. The opamp makes an adjustable current-limiting circuit for the LM317 regulator. If the circuit does not have a -1.25V supply then its minimum output voltage is +1.25V which might cause a much higher output current into a short circuit than its setting. __________________ Uncle $crooge

1st November 2009, 05:12 PM	#12
Sceadwian	You said you had a -5 volt supply, use that instead of the 1.25.... __________________ "Because I be what I be. I would tell you what you want to know if I could, mum, but I be a cat, and no cat anywhere ever gave anyone a straight answer, har har."

1st November 2009, 05:22 PM	#13
KMoffett	The -5V would be for the opamp and transistor, the -1.25 is needed to provide a negative reference to the regulator's divider R1/R2 so the output can be brought to 0.0V. ken __________________ "To invent, you need a good imagination and a pile of junk." Thomas A. Edison (1847 - 1931)

1st November 2009, 05:23 PM	#14
yafch	ok let me try -5v and see what happens.