Why doesn't this rail splitter circuit work?

Thread Starter

Cookieman10101

Joined Jul 18, 2019
22
Ok, so I'm using a TCA0372 op-amp to split a single ended voltage into two rails. 8V and -1V. The -1V rail is to give some other op-amps (not yet designed) a bit of a negative rail so they can swing to 0V. The rail split part works fine until I add a standard linear regulator (7805) and use the output of the op-amp as a virtual ground (like floating the ground with a voltage divider) then it tries to blow up, pulling way too much current (tested with a CC supply so it should be fine). I'm planning to use the virtual ground as common for an Arduino circuit so if I use battery ground for the 7805, the rest of the circuit would be seeing 4v instead of 5v. Obviously I could just use an LM317 with battery ground and set it to 6V, but I wanted to know why this didn't work.

Thanks.Capture.PNG
EDIT: Ignore the ground flag on the output of the op-amp, that goes nowhere.
 
Last edited:

Alec_t

Joined Sep 17, 2013
12,744
LTspice says it should work for load currents up to ~25mA. You can't pull much current from the opamp without affecting the negative rail voltage.
DualSupply.jpg
 

crutschow

Joined Mar 14, 2008
29,521
The rail split part works fine until I add a standard linear regulator (7805) and use the output of the op-amp as a virtual ground (like floating the ground with a voltage divider) then it tries to blow up, pulling way too much current
I don't see a path for high current so I suspect a wiring error.
Double check all your connections.
 
Last edited:

Thread Starter

Cookieman10101

Joined Jul 18, 2019
22
I don't see a path for high current so I suspect a wiring error.
Double check all your connections.
I looked over the wiring like 3 times (soldered to prototyping board) and couldn't find anything. I then built it again on a breadboard and all is well, so It most likely is wiring. I guess I'll study it again, thanks for humoring me.
 

Thread Starter

Cookieman10101

Joined Jul 18, 2019
22
As it turns out, the TCA0372 is not stable under capacitive load (surprise surprise) and the 0.33uF cap was causing it to oscillate about 1.3V p-p. i guess the option is to either omit the cap or use another opamp.
 

crutschow

Joined Mar 14, 2008
29,521
As it turns out, the TCA0372 is not stable under capacitive load (surprise surprise) and the 0.33uF cap was causing it to oscillate about 1.3V p-p. i guess the option is to either omit the cap or use another opamp.
Many op amps are unstable with a capacitive load.
Try a small resistor (e.g. 10-100Ω) in series with the op amp output but after the feedback connection and see if that helps.
 

Alec_t

Joined Sep 17, 2013
12,744
The TCA0372 the TS is using is good for up to 1A on the output.
Indeed but, regardless of opamp type, any current above 25mA produces the result shown in the above waveforms. I suspect the limitation is due to the current path being via the opamp bias components.
 
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