Electronics > Projects, Designs, and Technical Stuff
Dual channel 0-15V 0-500mA power supply
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kallek:
Thank you. Actually, I used that ti circuit to design my own. I just put current sense resistor below negative terminal, because my opamp can go to negative rail but not to positive. I will show my schematic when I have drawn it.
Zero999:

--- Quote from: David Hess on February 20, 2020, 06:00:30 pm ---
--- Quote from: kallek on February 18, 2020, 07:38:48 am ---
--- Quote from: David Hess on February 18, 2020, 05:44:50 am ---Why require low side current sensing?
--- End quote ---

I have these on hand and layout was easy to make. With high enough value current sense resistor accuracy is about 2mA and current can be set down to under 20mA. I choose low side sensing because then diff. amplifier is not deeded.
--- End quote ---

Nothing requires high side current sensing to use a difference amplifier; the error amplifier for controlling the current can operate at the high side directly if the control voltage is referenced to the high side which is trivial when poteniometers are used.  A simple example is shown below.


--- End quote ---
The LM101A is now expensive and hard to get hold of. Use the LM201A instead.

What's going on with the diode across C8? I thought polarised capacitors can withstand short reverse voltage transients, so I doubt it's to protect against that.
David Hess:

--- Quote from: Zero999 on February 20, 2020, 08:50:07 pm ---
--- Quote from: David Hess on February 20, 2020, 06:00:30 pm ---Nothing requires high side current sensing to use a difference amplifier; the error amplifier for controlling the current can operate at the high side directly if the control voltage is referenced to the high side which is trivial when poteniometers are used.  A simple example is shown below.
--- End quote ---

The LM101A is now expensive and hard to get hold of. Use the LM201A instead.
--- End quote ---

I just use that schematic as an example.  The unique thing about the LM301A at the time was that its input common mode range includes the positive supply which made it almost uniquely useful in high side sensing applications.  A lot of old JFET parts would also work.  Today many rail-to-rail input parts meet that requirement or a slightly higher supply voltage for the operational amplifier could be used.

Incidentally, there is some advantage to putting the current sense resistor directly adjacent to the output capacitor although this could be done on the low or high side.  If AC feedback is taken before the current sense resistor, then the increased ESR of the output capacitor as seen from the error amplifier provides extra phase lead to increase stability.  Here R15 provides the same function but at the cost of increasing the high frequency output impedance.  The old Tektronix PS503 type power supplies do this with the current sense resistor on the high side but without the AC feedback part.  Some modern low dropout integrated regulators which can use ceramic capacitors do this internally.


--- Quote ---What's going on with the diode across C8? I thought polarized capacitors can withstand short reverse voltage transients, so I doubt it's to protect against that.
--- End quote ---

Q2 could potentially sink 2 amps of current when pulling down a large capacitive load into the negative supply which is unlikely to be rated for that so the diode is required to avoid damaging C8.  Q2 and associated parts could be left out if that function is not required or undesirable.
exe:

--- Quote from: David Hess on February 21, 2020, 01:59:13 am ---Q2 could potentially sink 2 amps of current when pulling down a large capacitive load into the negative supply which is unlikely to be rated for that so the diode is required to avoid damaging C8.

--- End quote ---

Interesting, so a capacitor can be damage by too large current through it? I had this question is a long time ago, all I could find was that average current through the cap should be within its rating, or it may overheat. But no information that, say, shorting leads of a charged capacitor can damage it. Or charging it too quickly.
Kleinstein:
The problem is more that the negative supply may not be able to provide 2 A of current. So if the supply than goes into sinking mode this could bring the negative supply to positive values and thus reverse the capacitors.  This would be an unlikely scenario and many electrolytic caps do survive a little reverse voltage for a short time. The diode would just limit that voltage.

In simulations I found ot tricky with the shunt at the collector side of the power transistors. This kind of slows down the transistor and thus can help with instability. There is nothing really bad with a low side shunt, especially with an OP that works at the negative rail.
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