OpAmp current sourcing

[axro]

The datasheet for the LM358 states that it typically sources 20mA. Lets say I want to power a single LED on the output.

Do I have to put a resistor inline or since it can only source 20mA I'm good?

 

[Dx3]

Is the data sheet saying that 20 milliamps is the guaranteed minimum value, or the guaranteed maximum value?

 

[Vizier87]

I have never been analyzing that part that much. But it is always a good practice to put a 100-1000 ohm resistor series with the LED for the output. If the supply current/voltage is high enough the output may burn the LED. The ones I built all have resistors to limit the output current to the LED.

Cheers.

 

[Torben]

The datasheet for the LM358 states that it typically sources 20mA. Lets say I want to power a single LED on the output.

Do I have to put a resistor inline or since it can only source 20mA I'm good?

A good rule of thumb if you don't know for sure is to assume that the amperage output is what the device can safely supply, and not that the device will limit the output at that value. Put a resistor inline.


Torben

 

[audioguru]

Why didn't anybody look at the datasheet for the LM358?

Different manufacturers have different spec's.
Its minimum sourcing output current is 10mA or 20mA.
Its typical current is 20mA or 40mA.
Its maximum output current is unknown or is 60mA.
But the max allowed continuous current for many LEDs is 30mA, so use a current-limiting resistor.

 

[axro]

Ok, so this is prob a dumb question. If you have a 9v supply lets say. And you have 1 red LED with a 1.5V forward voltage and you want 20mA current.

Do you then put a 390ohm resistor like you would if there were know IC between VCC and the LED?

Or do you use less of a resistor because the IC can't support as much current?

I guess what I'm saying is that for argument sake an IC can only output 20mA max. If you start putting resistors in series, does that lower the current?

 

[Sceadwian]

It depends on the output impedance of the IC in question axro. Somewhere in the datasheet it should tell you the conditions under which you can get 20ma out or otherwise specifically state the output impedance, regardless of the device you're going to need a current limit resistor, an LED will try to draw more than the 20ma that the IC is rated for and could damage the I/O line by trying to draw more than it's rated for. In general it's a good bet to assume the output impedance of any cmos based device is very low even if it can't provide a lot of current. The current limit is the physical limitation of the size of the output drivers.

 

[axro]

It depends on the output impedance of the IC in question axro. Somewhere in the datasheet it should tell you the conditions under which you can get 20ma out or otherwise specifically state the output impedance, regardless of the device you're going to need a current limit resistor, an LED will try to draw more than the 20ma that the IC is rated for and could damage the I/O line by trying to draw more than it's rated for. In general it's a good bet to assume the output impedance of any cmos based device is very low even if it can't provide a lot of current. The current limit is the physical limitation of the size of the output drivers.

This is what I found for the LM358

It doesn't show anything about resistance, so am I to assume it's just the IO lines can only handle 30mA typ or what? And I just treat the line as if it has no resistance and select a resistor value like you normally would if there were no IC in the way?

 

[audioguru]

You posted the minimum and typical max output current but the maximum current is not shown.
Other manufacturers list 60mA as the max current. Then with a 15V supply and a 2V LED the lowest output resistance is (15V - 2V)/60mA= 217 ohms. If you want a max LED current of 25mA then the total resistance must be (15V - 2V)/25mA= 520 ohms and the added resistance is 520 - 217= 303 ohms. Use 300 ohms. If the opamp has the minimum current of only 10mA then the added resistor is not needed and the LED will not be very bright.

There is such a huge range of output current from the opamp so I think a transistor should drive the LED.

 

[axro]

Ok thanks, that's what I was looking for, if you have to take into account the resistance of the opamp(and how you find it).

 

[Sceadwian]

You can't rely on the current limits as a fixed value they'll change with temperature and VCC voltage, opamps do not have fixed output resistance that can be calculated simply. Most opamps aren't meant to drive output resistances under 1000ohms so you generally wouldn't directly drive an LED with an opamp, as AG said you would feed the opamp output into a transistor to drive the LED.

 

[ronv]

358

Do you need to cut it that close?
My databook says 2 volts at 20 ma. Allow another 1.8 or so for the led. Put a 750 ohm in series and have at least 15 ma. (assumes a 15 volt supply) Is there a reason you are using an op amp for this job?

 

[audioguru]

Do you need to cut it that close?
My databook says 2 volts at 20 ma. Allow another 1.8 or so for the led.

A red LED is 1.8V to 2.0V. A blue or white LED is 3.5V.

Put a 750 ohm in series and have at least 15 ma. (assumes a 15 volt supply)

Not if the opamp has a max output of only 10mA, then its output resistance is (15V - 2V)/10mA= 1.3k ohms. If you add 750 ohms in series then the current is (15V - 2V)/750 ohms + 1.3k ohms= only 6mA.


Is there a reason you are using an op amp for this job?[/QUOTE]