Odp: [sdiy] OpAmp Output Current Question

[Scott Gravenhorst]

Thanks Roman,

Just what I needed.  The application is a linear voltage to current
converter meant to supply drive current to LEDs (series) in H11F1 FET
optoisolators for a VC Sallen-Key experiment.  I also looked at LM324,
but that's about the same range.  The H11F1 spec says that 16 ma is
needed to illuminate the FETs to minimum Ron.

So, it looks like rather than the summing node solution, I'll need to
make a circuit using an opamp to drive a transistor base such that C-E,
the LEDS and a resistor in series provide the LED drive current from +V.
 The resistor then provides a way to sense current as a voltage which is
connected to the inverting input of the opamp.  This way, the opamp only
supplies base current, the transistor supplies the gross current for the
LEDs.  Right?

I'm assuming the transistor can be something unspecial like a 2N3904.

Something like:

                                     +V
                                     |
                                     V   LED
                                    ---
                                     |
                                     V   LED
                                    ---
                 |\                  |
                 | \                /
       CV--------|+ \          B  |/  C
                 |   >------------|      NPN
            +----|- /             |\  E
            |    | /                \
            |    |/                  |
            +------------------------+
                                     |
                                     R
                                     |
                                    gnd


"Roman" <modular at go2.pl> wrote:
>I was surprised first, but you're right. I couldn't find such parameter
>in TI or Motorola datasheet. But there is a nice graph showing
>maximum peak voltage vs load resistance. It drops below 1k, so
>we can assume usable output current is about 13mA.
>At 100 ohms Motorola shows 5Vpp, so it's 25mA, Texas drops to 0
>which is probably mistake. But at 200 ohms they say about +/-6.5V
>so it's over 30mA! Strange...
>But anyway, at 20mA poor TL072 will get hot.
>Don't know what is your application, but I'd use something different,
>or add transistor at output
>
>Roman
>
>----- Original Message ----- 
>From: Scott Gravenhorst <music.maker at gte.net>
>To: <synth-diy at dropmix.xs4all.nl>
>Sent: Saturday, January 10, 2004 7:02 PM
>Subject: [sdiy] OpAmp Output Current Question
>
>
>> I was looking at the spec sheet for a TL072 opamp today
>> wanting to know what the device's maximum usable output
>> current can be.  The only spec given is no-load supply
>> current.  I need to be able to supply up to about 20ma.  Is
>> this reasonable for a TL072 or should I choose a different part?
>> 
>> Thanks.
>> 
>> =========================================================  
>> - Where merit is not rewarded, excellence fades.
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>> 
>> -- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
>> -- Linux Rex         | RedWebMail by RedStarWare
>> -- FatMan: home1.gte.net/res0658s/FatMan/
>> -- NonFatMan: home1.gte.net/res0658s/electronics/
>> -- Master of Arcane and Hidden Knowledge.
>> 
>

=========================================================  
- Where merit is not rewarded, excellence fades.
- Hydrogen is pointless without solar.
- What good are laws that only lawyers understand?
- The media's credibility should always be questioned.
- Governments do nothing well, save collect taxes.

-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
-- Linux Rex         | RedWebMail by RedStarWare
-- FatMan: home1.gte.net/res0658s/FatMan/
-- NonFatMan: home1.gte.net/res0658s/electronics/
-- Master of Arcane and Hidden Knowledge.