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Linear Power Supply based on HP/Agilent E3610A
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Dave:

--- Quote from: timsu on May 24, 2016, 08:07:51 pm ---
--- Quote from: Dave on May 23, 2016, 09:02:50 pm ---The serial interface isn't actually isolated.

--- End quote ---
TX has to use an external 5V source, right?

--- End quote ---
Yup.


--- Quote from: timsu on May 24, 2016, 08:07:51 pm ---Why did HP added resistors between GND and op amp input? (Example: R26 at the inverting input of the voltage error amp)

--- End quote ---
To cancel out the offset voltage caused by the input bias currents.
sync:

--- Quote from: Dave on May 24, 2016, 08:18:35 pm ---
--- Quote from: timsu on May 24, 2016, 08:07:51 pm ---Why did HP added resistors between GND and op amp input? (Example: R26 at the inverting input of the voltage error amp)

--- End quote ---
To cancel out the offset voltage caused by the input bias currents.

--- End quote ---
I don't think it's about the offset voltage. It's a LF411 JFET op amp in the HP design. I think it's for the op amp compensation. Without R26 the inverting input would be grounded and R21, C22 from the op amp output to the inverting input have no effect.
Dave:

--- Quote from: sync on May 24, 2016, 10:15:54 pm ---I don't think it's about the offset voltage. It's a LF411 JFET op amp in the HP design. I think it's for the op amp compensation. Without R26 the inverting input would be grounded and R21, C22 from the op amp output to the inverting input have no effect.

--- End quote ---
You're absolutely right. This is what happens when you just glance at schematics instead of taking the time to give it a proper look. :palm:

The bias current compensation resistor like I had in mind would be R33, for example (though it should be a higher value - around 50k).
StillTrying:

--- Quote from: timsu on May 24, 2016, 08:07:51 pm ---
--- Quote from: StillTrying on May 23, 2016, 08:13:26 pm ---You're asking too much of the LM393 to pull down 100R from 5V. (10mA max)
I'd try to invert the sense there and make the opto part of the pull up to 5V because I hate wasted milliamps.  :)

--- End quote ---
Can you explain this a little bit more?

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On the data sheet graphs the open collector output of the LM393 it can only sink about 10ma before its saturation voltage is 1V and rising. With 100R pull up it just won't be able to pull down enough. I think you'll have to invert it and use the opto as the pullup so that all the current goes through the opto.

I started to draw it and then decided to make it 2 working versions. Just to be awkward the simulations version of the LM393's output saturation seems to be about 4 times better than what's shown on the data sheets graph.


--- Quote ---Does 10K ensure fast enough switching of the MOSFET?
LTSpice shows about 2u seconds switching time with 10k.

--- End quote ---

I think a few uS is fine for this occasional switching of 12V at up to 2A, but there's much better expurts than me here!

StillTrying:

--- Quote from: Dave on May 24, 2016, 08:18:35 pm ---To cancel out the offset voltage caused by the input bias currents.

--- End quote ---
And especially their drift with temperature.
I'm surprised at how every everybody just sticks 10Ks, on op amp inputs, it seems like almost 50% of the time they're actually unbalancing the inputs!
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