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I am using an op amp to amplify an input signal from a microcontroller, which in general is working fine.

For over voltage protection, I added the crowbar circuit taken directly from figure 32, page 27 of the TL431 data sheet and that added some undesirable behavior to the circuit I don't quite understand.

With the TL431 triggering at a voltage of 2.5 V and the voltage divider \$R_3\$/\$R_4\$ the crowbar should trigger at an op amp output voltage of 4.8 V and blow the fuse. But what I see is, that as soon as the output voltage reaches 3 V, the output drops to 0.75 V and stays at that level until the input voltage drops far enough, that the output should be below 0.75 V in normal operation. After that, it works as expected again, until 3 V or more output is reached.

I found in this discussion of this crowbar circuit, that the placement and size of the capacitor as depicted in the data sheet might not be ideal. Could that somehow cause my problem? If not, what else might be responsible for this behavior?

EDIT: For proper context for the added crowbar, I regulate the power of a laser with the op amp output. I have to make sure that the laser is not permanently turned on by a short circuit of the output to the 5V that is used as +Vcc for the op amp and for other parts on the pcb. Since I don't need more than 4.2V output and shouldn't get more that that during regular operation, blowing a fuse with the crowbar was the best I could come up with to protect against this case.

Datasheets:

Fuse: https://www.mouser.de/datasheet/2/358/typ_MGA-A-1388649.pdf

Op amp: https://www.mouser.de/datasheet/2/609/AD8605_8606_8608-877839.pdf

Triac: http://www.ween-semi.com/sites/default/files/2018-11/BT137S-600D.pdf

schematic

simulate this circuit – Schematic created using CircuitLab

Update: Removing C1 completely does not eliminate the described behavior, but increases the voltage at which it occurs to 3.3V

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  • \$\begingroup\$ Please provide links to the manufacturers' datasheets for the op-amp, the fuse, and the triac. It sounds like your op-amp is not supplying sufficient current to blow the fuse. \$\endgroup\$ Jul 12, 2019 at 14:59
  • \$\begingroup\$ I added the datasheets. Even if the current is to low to blow the fuse, that would not explain why something seems to happen at 3 V instead of 4.8 V, or would it? \$\endgroup\$ Jul 12, 2019 at 15:07
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    \$\begingroup\$ Measure the voltage at the TL431 REF pin (relative to ground) when there is a bit less than 3V at the op-amp output (before the circuit appears to trigger). \$\endgroup\$ Jul 12, 2019 at 15:12
  • \$\begingroup\$ I would try removing the triac to see if the TL431 alone is working properly. \$\endgroup\$ Jul 12, 2019 at 15:20
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    \$\begingroup\$ Why do you want to do this? Would a simple resistor in series not be sufficient enough to protect both the opamp and the MCU, perhaps with external clamping diodes after (perhaps also a zener)? No fuses to blow, and you could still detect this overvoltage condition in firmware. This may however present some issues when directly connected to the ADC; but those are easy to overcome with either an additional capacitor by the ADC pin, or a buffer. \$\endgroup\$ Jul 15, 2019 at 14:36

3 Answers 3

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In the circuit as you show it you require no C1.
As pointed out in the prior discussion, the capacitor may turn on the Triac on sudden rises in opamp output.

The TL431 is not really suitable for what you are trying to do since it requires a minimum Ik to set the reference (0.4mA). The strange conduction you are seeing is in all probability due to the impact of the internal reference generator.

However assuming you want to blow a fuse (and as already pointed out the fuse you selected is not suitable) I'd suggest the following change may solve your problems:

schematic

simulate this circuit – Schematic created using CircuitLab

R3 ensures the TL431 internal ref is always adequately serviced and is not dependent on the signal level.
M2 shorts the output of the opamp ….but here things are hazy. The opamp is only capable of 80mA, so I assume you are trying to blow the fuse when the opamp is dead (and the current is uncontrolled).
However if the opamp is ok and the signal just too high, then this circuit would clamp the output sinking the 80ma without problems. Getting a fuse to blow is hard work.

Update: What is the reason you want to limit the output swing to 4.8V when the rail-rail operation limits it to 5V already? Explain your needs more fully for a better hope of a viable answer.

Looking at the problem from a pure opamp perspective, is your specification as follows:

  1. Output of opamp must NEVER go above 4.8V with a 5V supply
  2. Input must be high impedance
  3. The opamp is not broken (so the output current limits work)
  4. Clamp the input rather than the output

This might be a viable approach to simply clamp the input signal:

schematic

simulate this circuit

The TLV3011 provides a very accurate reference voltage and R4/5/6 provides an adjustment for the output threshold.

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  • \$\begingroup\$ You're right, I didn't explain the use case properly. I have to ensure that the device that gets regulated with the op amp output is only turned on when there is the proper input signal. I was specifically told to consider the case of a short circuit between the 5V +Vcc and the op amp output. Since I need at most an output voltage of 4.2V and can regulate the input signal accordingly, this crowbar was the best I could come up with to be safe in this scenario. I didn't notice that the fuse current rating is too high for the op amp output, but maybe it serves it's purpose in the error case. \$\endgroup\$ Jul 15, 2019 at 9:03
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    \$\begingroup\$ @EyeofHorus Then the first schematic with the FET device will work in both error and the short to supply. You just need to select a suitable fuse. You might look her: datasheets.avx.com/AccuGuardLC_0402.pdf ….a fuse rating of 37.5 mA would seem appropriate for this application. Blowing a fuse is a very drastic action though, you'd be better creating a current limited 5v supply and simply shorting the opamp output. Let me know if you want to explore that. \$\endgroup\$ Jul 15, 2019 at 15:02
  • \$\begingroup\$ I've simulated your first schematic with LTSpice and it works fine if I just ramp the signal from 0V to 5V, but if I add my op amp with the feedback resistors, the output never goes above 4.7V, no matter how high the input. The TL431 ref is always a little below 2.5V, so the mosfets don't trigger. I don't necessarily have to blow a fuse, if I can make the design safe in an easier way. I have a 5V supply that I can't really change currently. If that's not a problem, I'd be grateful to hear your other suggestion. \$\endgroup\$ Jul 17, 2019 at 8:40
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The opamp does not provide sufficient current to blow the fuse. The fuse is rated for 200mA (for the lowest current rated fuse in the family), the opamp can only supply 80mA (if running at 5V at 2.7V it's only 30mA) , or less then half the current to blow the fuse.

Lets suppose that ground was attached to the other end of the fuse, instead of the crowbar circuit, only 80mA would flow through the fuse, and it still would not blow, even if you raised the voltage as high as the output of the AD8605 would allow for it's rating (6V).

Crowbar circuits are for voltage circuits that have low source impedances/high currents like a power supply.

EDIT:

There are a few options, one way would be to limit the opamp's output by changing Vcc of the opamp to 3.4V.

The other method would be to use a zener diode on the input, however this would sacrifice some linearity and the load impedance as seen from the Vin. The resistor could be raised to a higher value, but would also change the slope of the limiting curve and make the upper range of the Vin/Vout curve where the diode clamps inaccurate/less linear. This is not a good option, I prefer clamping the output or limiting the Vcc ( which would probably be the simplest and only add a regulator to the circuit).

enter image description here

The last option would be to use a series resistor and diodes on the output, also with some loss due to the series resistor and the leakage current from the diode.

ANOTHER EDIT:

If current limiting is what you want there are plenty of circuits that can accomplish this task. (there are also many IC's suited to this task). Most involve detecting the current with a current sense amplifier as the one shown below (the amplifier IC1 switches the optocoupler which in turn switchs a pmos high side switch):

enter image description here
Source: https://www.electronicdesign.com/power/current-limiter-offers-circuit-protection-low-voltage-drop

OR many circuits that are listed here

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  • \$\begingroup\$ @JackCreasey I had to read your comment several times and look at the schematic, because I think you mean that the schematic in the answer is correct, but it's inverting. When changed into a non-inverting configuration (which the OP needs) it wouldn't work anymore. Because -ve input in the given schematic is virtual ground, but in case of a non-inverting amplifier, -ve would follow Vin. Just clarifying for any other people that also might be confused... \$\endgroup\$
    – MartinF
    Jul 12, 2019 at 18:07
  • \$\begingroup\$ No, it wont work, not with single sided. I'll edit my answer and see if I can find a better option. The best thing would probably be an input limiter circuit \$\endgroup\$
    – Voltage Spike
    Jul 12, 2019 at 18:10
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    \$\begingroup\$ @MartinF The answer is ONLY correct for a dual power supply opamp (it's a classic configuration for clipping the output). Not what the OP has and so the answer is wrong and won't work based on the question. The TL431 is also the wrong type of device to try an achieve the OPs goal. \$\endgroup\$ Jul 12, 2019 at 18:15
  • \$\begingroup\$ I'm thinking: In the non-inverting config, if Vin=1.62V, Vout=4.8V. 4.8V-1.62V=3.18V over Rfeedback. Wouldn't it work with a zener of 3.1V? \$\endgroup\$
    – MartinF
    Jul 12, 2019 at 18:16
  • \$\begingroup\$ @JackCreasey Ah, thanks! I didn't consider the power supply indeed - the Opamp used has a single supply voltage of max. 5.5V. Good thinking! \$\endgroup\$
    – MartinF
    Jul 12, 2019 at 18:26
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I found the original reason for the unexpected behavior of the TL431.

It turned out that the Eagle library I was using had the SOT23 package for the TL432 under the TL431. Since the two have switched cathode and reference pins, my circuit didn't work properly with the TL431.

The original circuit still had some instabilities, which is why I accepted Jack Creasy's answer, since his alternative circuit works very well.

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