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Old 20th March 2005, 07:08 PM   #1
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Default 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…
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Old 20th March 2005, 07:27 PM   #2
boholm is offline boholm  Denmark
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Yes. One amp = 5 mA, 2 amps = 10 mA . . . . .

Next thing to consider is noise and output impedance of supply - depending on job.
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Old 20th March 2005, 07:33 PM   #3
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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!
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Old 21st March 2005, 08:35 AM   #4
AndrewT is offline AndrewT  Scotland
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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.
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Old 21st March 2005, 08:55 AM   #5
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Opamps have normally rather resistive loads but it is a good rule to have 20-50 mA per opamp as available current.
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Old 22nd March 2005, 09:40 PM   #6
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Default 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
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Old 22nd March 2005, 10:11 PM   #7
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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.
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Old 23rd March 2005, 06:41 AM   #8
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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.
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Old 23rd March 2005, 06:58 AM   #9
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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!!!
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Old 23rd March 2005, 08:09 AM   #10
AndrewT is offline AndrewT  Scotland
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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.
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