Opamp power supply

mixmonster · 20th March 2005, 06:08 PM

Hello All,
When designing a power supply for an opamp that requires + / - 15V at 5mA. Obviously the voltage doesn’t alter however would I be correct in saying for one opamp you would require just 5mA for two 10mA for three 15mA and so on… or does it operated the same way as voltage rails, (e.g.: remains the same)

I apologize if this seems very basic however I am new to electronics and any help would be appreciated

Thanks…

boholm · 20th March 2005, 06:27 PM

Yes. One amp = 5 mA, 2 amps = 10 mA . . . . .

Next thing to consider is noise and output impedance of supply - depending on job.

peranders · 20th March 2005, 06:33 PM

You must also add some for all passive parts like resistors which act like different kinds of loads and of course the real load. Don't forget that 78xx regulators themselves consumes power, see datasheet for this but it is a couple of milliamps. Notice also that 78xx and especially 79xx must have a minimum load in order to work. Too small load you will get more voltage out!

AndrewT · 21st March 2005, 07:35 AM

Hi,
opamps are usually ClassAB. The current quoted is nearly always the operating current for zero output current.
You should add at least 100% of output current +5mA.
Remember to allow for current in the feedback loop and an allowance for peak current into non resistive loads.
Your requirement for a single could now be 40ma and a dual 75ma.
Add in local decoupling.

peranders · 21st March 2005, 07:55 AM

Opamps have normally rather resistive loads but it is a good rule to have 20-50 mA per opamp as available current.

mixmonster · 22nd March 2005, 08:40 PM

Thanks for all your help guys!!!

One more question on your average opamp power supply how much supply current is too much??? (at what point does the IC fry!!!)

Thank again

Darren

richie00boy · 22nd March 2005, 09:11 PM

The limit of the LM317T is stated on the datasheet as 1.5A but this is dependent on a number of factors such as temperature and input-output voltage differential.

boholm · 23rd March 2005, 05:41 AM

The opamp itself regulates its own current, so there is no need to worry about too much current. As mentioned you must give it access to more - given by the power supply. So make your power supply to give around 10 times as much as you think it will use, then you are safe. Because the opamp will use more power, if the output of the opamp is connected to a low impedance/resistance and the signal swing is high. Calculations via Ohms law: U=R*I and P=U*I.

sivan_and · 23rd March 2005, 05:58 AM

Quote:

peranders

:

Quote:

Notice also that 78xx and especially 79xx must have a minimum load in order to work. Too small load you will get more voltage out

WRONG
For Linear regulators no min load is required!!!

AndrewT · 23rd March 2005, 07:09 AM

Hi,
I have not checked the datasheets but I seem to recall that some (all) linear regulators like to see a minimum load current to operate properly.

20th March 2005, 06:08 PM	#1
mixmonster diyAudio Member Join Date: Sep 2004 Location: London	Opamp power supply Hello All, When designing a power supply for an opamp that requires + / - 15V at 5mA. Obviously the voltage doesn’t alter however would I be correct in saying for one opamp you would require just 5mA for two 10mA for three 15mA and so on… or does it operated the same way as voltage rails, (e.g.: remains the same) I apologize if this seems very basic however I am new to electronics and any help would be appreciated Thanks…

20th March 2005, 06:27 PM	#2
boholm diyAudio Member Join Date: Apr 2004 Location: Copenhagen	Yes. One amp = 5 mA, 2 amps = 10 mA . . . . . Next thing to consider is noise and output impedance of supply - depending on job. __________________ Best regards Bo

20th March 2005, 06:33 PM	#3
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	You must also add some for all passive parts like resistors which act like different kinds of loads and of course the real load. Don't forget that 78xx regulators themselves consumes power, see datasheet for this but it is a couple of milliamps. Notice also that 78xx and especially 79xx must have a minimum load in order to work. Too small load you will get more voltage out! __________________ /Per-Anders (my first name) or P-A as my friends call me

21st March 2005, 07:35 AM	#4
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, opamps are usually ClassAB. The current quoted is nearly always the operating current for zero output current. You should add at least 100% of output current +5mA. Remember to allow for current in the feedback loop and an allowance for peak current into non resistive loads. Your requirement for a single could now be 40ma and a dual 75ma. Add in local decoupling. __________________ regards Andrew T.

21st March 2005, 07:55 AM	#5
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	Opamps have normally rather resistive loads but it is a good rule to have 20-50 mA per opamp as available current. __________________ /Per-Anders (my first name) or P-A as my friends call me


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

22nd March 2005, 08:40 PM	#6
mixmonster diyAudio Member Join Date: Sep 2004 Location: London	Opamp current Thanks for all your help guys!!! One more question on your average opamp power supply how much supply current is too much??? (at what point does the IC fry!!!) Thank again Darren

22nd March 2005, 09:11 PM	#7
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	The limit of the LM317T is stated on the datasheet as 1.5A but this is dependent on a number of factors such as temperature and input-output voltage differential. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more

23rd March 2005, 05:41 AM	#8
boholm diyAudio Member Join Date: Apr 2004 Location: Copenhagen	The opamp itself regulates its own current, so there is no need to worry about too much current. As mentioned you must give it access to more - given by the power supply. So make your power supply to give around 10 times as much as you think it will use, then you are safe. Because the opamp will use more power, if the output of the opamp is connected to a low impedance/resistance and the signal swing is high. Calculations via Ohms law: U=RI and P=UI. __________________ Best regards Bo

23rd March 2005, 07:09 AM	#10
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, I have not checked the datasheets but I seem to recall that some (all) linear regulators like to see a minimum load current to operate properly. __________________ regards Andrew T.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
SS Power Supply and generic opamp PCB.	Geek	Swap Meet	16	31st January 2009 09:01 AM
HV benchtop power supply, help on opamp setup	panos29	Tubes / Valves	17	20th April 2008 10:27 AM
Opamp power supply, super regulator alternatives?	imperfectcircle	Power Supplies	33	3rd May 2007 06:55 PM
Power supply to opamp. Ground plane?	argonrepublic	Parts	3	10th May 2005 12:37 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read