MC33072 - Am I exceeding something?

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atferrari

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Today I tested the attached circuit which basically works as expected but after maybe one minute, the opamp becomes rather warm if not hot to touch.

Am I asking too much from it? The current, as shown is 11 mA (steady) through the whole range.By reading the datasheet I could not conclude if I am exceeding anything. Honestly, I do not know where to look at for this.

With the scope I can see noise everywhere, of up to 50 mV pk-pk. The whole PCB appears plagued by that noise.

Any comment, appreciated.

Had to use Paint for drawing instead of Corel. Sorry if it looks horrible.
 

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Don´t know where you get those +5V and -38v supplies from, but you might consider filtering those on the way to the opamp with something like 10ohm and 10uf RC filters + maybe even more small caps.
 
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You don't say what op amp you are using. Many can only use a maximum og 36 volts for the supply voltage. You have more.

Ooops, I guess it's in the title. Must be oscillating, but it shouldn't be.
 
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The drawing and the title says it - MC33072 with a maximum of 44V. I have 37.5 rail to rail

Datasheet could be found here
 
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Try slowing down the op-amps by adding a 1nF or 10nF cap from the negative to output.
 
The MC33072 short circuit current is a minimum of 10mA so you are operating the op amp near or at its limit. With 11mA current the maximum power dissipation in the op amp will be about .35W which would make it quite warm.
 
crutschow said:
The MC33072 short circuit current is a minimum of 10mA so you are operating the op amp near or at its limit. With 11mA current the maximum power dissipation in the op amp will be about .35W which would make it quite warm.
Click to expand...

For the LM337 I am sourcing current, right?

What could be the way to overcome the limit? A transistor I think, but I do not know how...
 
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You could add a buffer transistor to the output of the opamp; the transistor will get warm then, and not the opamp.

Alternately you could use a different negative regulator that doesn't have a minimum load current (or a large ground current).

Yet another option is to just use a transistor in place of the regulator as shown in the second attachment (please excuse the horrendous drawing).
 

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Do you have the recommended filter caps directly at the input and output of the LM337?
 
Yes crutschow, I have them.

I tested my idea of a 2N1711 (VceR = 50V - Ptot =0.8 W) inside the loop. The transistor is in charge of sourcing the 11 mA.

It works with the TO93 case hot to touch through the upper half of the range while the opamp stays barely warm now.

No matter what I use to take the 11 mA load for the LM337, I should expect the same. Power has to be disipated, right?

Maybe I should fit a heatsink and move on, I guess.
 

I found your post minutes after posting in another forum the same idea.

As I said, it works but cannot avoid the transistor being hot.

Thanks for replying.
 
Well something's gotta get hot either way. You can use a transistor with a larger package, that will get much less hot because larger suface area means lower thermal resistance.
 
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