Opamp Voltage Doubler Unstable (After Extended Operation)

Question

I've been working on a circuit which would drive an industry standard pneumatic regulator (E/P transducer). The transducer takes a standard 0-10V command signal with a typical input impedance of 6.5 kOhms. The DAC that I've selected (ADI 12bit 8 channel) has an internal 2.5V reference and can output 0-5V.

After researching extensively, an opamp voltage doubler seemed like a good fit. This worked and was rock solid for about two weeks, but failed after a ~2 day period of continuous operation without warning. Now the opamp output is close to a correct 2.0 gain but constantly fluctuates as if the output is barely stable and poorly damped. Has anyone ever seen an opamp fail this way?

We're using a UA7812 linear regulator to supply 12V (from 24V) to the opamp quad (LM324AN) and admittedly it may not be sufficiently heatsinked (gets wicked hot to touch). Still, the 12V supply is solid at 11.83V and voltages supplied by the DAC appear correct and stable. The actual resistors on the board are 10k with 0.1% accuracy and each measure 9.95k precisely. Any advice in solving the problem is tremendously appreciated! It's essential that this be a robust circuit.

Below is our final opamp configuration (and the part number):

Answer 1

I presume that your 12V is also supplying the E-P transducer power (and perhaps other stuff) and that's why the regulator is getting hot. You should heat sink it so that the case temperature is reasonable if you want reliable operation, or use a switching regulator. The exact numbers depend on your ambient range and desired reliability, but "wicked hot" sounds like it exceeds the 60-70°C range that I'd like to see for general purpose applications.

The 'fluctuation' in op-amp output could be caused by oscillations. The LM324 is fairly robust with small capacitive loads (such as could be caused by cable to the transducer), especially for gain > 1, but you may still be having problems. I suggest this kind of output circuit:

^{simulate this circuit – Schematic created using CircuitLab}

The input filter R4/C2 keeps high frequencies out of the input and matches the impedances looking out of the op-amp inputs so as to minimize the effect of the relatively large bias current on the LM324. The resistor R1 isolates the capacitive loading on the output from the internal output impedance of the op-amp. C1 forms a tight loop for AC to stabilize the op-amp. The feedback (R2/R3) is taken directly from the output so that R1 does not affect the output voltage when sourcing current (note that when sinking current the op-amp cannot get close to the negative rail when sinking more than 50uA or so, so the 0V output can't be approached even without R1). In most cases, the input of your transducer looks like a resistor to 0V so it's not an issue.

D1 and D2 conduct any transients to the power supply, saving U1 from damage due to ESD. R4/C2 and D1/D2 are optional as far as your immediate problem goes, R1 and C1 do the work. I strongly suggest you get that regulator temperature under control as well.

Answer 2

Your circuit should work, but there are some things that you should check:

First: Why is your regulator getting hot? Your OpAmp has a supply current of 3mA and the current into your pneumatic regulator should be less than 2mA. Even with your regulator dropping 12V at a worst case current of 10mA that only makes 0.12 Watt of dissipated power on the regulator. It should not get hot from this.

Are you driving any other things with it?

If not, check why you have a higher current than expected. You'll likely find the culprit if you go down this road.

If you're driving other things with the regulator try a dedicated regulator for the OpAmp to see if the other loads influence the supply. You've mentioned a pneumatic regulator. Is this a inductive load, e.g. a motor of some sort? If so the inductance can do some crazy voltage swings when switched due to inductive kickback. This could be enough to kill the OpAmp over time due to high supply voltage spikes. A beefly flyback diode may be needed to suppress these voltage spikes.

Second: How far away from the OpAmp is the pneumatic regulator? If you have it connected with a long cable you may have a to high capacitive load at the output of your OpAmp. This can lead to instabilities of all sorts.

Also long cables are excellent at picking up static electricity and may kill your OpAmp via ESD spikes into the output.

You can prevent most of these problems by adding a series resistor with the output. A simple transistor driver that isolates the OpAmp output from the long cable may also help.

Answer 3

I'm just guessing but your description makes me think that the pneumatic regulator is requiring a lot more current that you think. I base this on you saying that your 7812 regulator is running so hot.

Couple of things:

1) You don't need the regulator. The LM324 is good for a maximum supply voltage of 34 (36?) Vdc.

2) Add an emitter follower to the output of the op-amp. I'd be tempted to use a physically-large transistor just because of dissipation issues. NPN, TO-220 package, gain (Hfe) of at least 100. Collector goes to +24V, Base comes from output of op-amp, Emitter feeds both the pneumatic regulator and the right-hand end of R34. Note that the transistor is inside the feedback loop and doesn't introduce any error.

Question: is your regulator a voltage-input device or current-input? My company makes interface cards like this for a variety of different actuators and the ones that we work with are all current-input devices. We drive them with a low-frequency PWM signal where the output current is proportional to the control input. This eliminates temperature effects on the actuator's solenoid coil and the vibration that the low-frequency PWM produces reduces stiction in the actuator.