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I am designing an electric veichle controller for a University project. The motor driver receives an analog signal (0-12V) to adjust the speed. I use a digipot followed by a buffer to convert the digital signal of the controller to ana analog one. Now I need some way to enable an disable each motor independently. I thought I can achieve this by cutting the supply power off the Op-Amp, but I have no idea how. I think there is Op-Amp with a disable pin, but I cannot what the state if the output is when the Op-Amp is disabled and I tried to simulate it using proteus software but it does not seem to work at all, further more I couldn't find this kind of Op-Amp locally to actually try it out. I am interested to know what you guys think. I will attach the schematic below.enter image description here

Edit: I also thought about adding a comparator and supplying power to the buffer from the output of the comparator. But think this could be unnecessarily complicated.

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  • \$\begingroup\$ Hello, I'm an applications engineer at Maxim Integrated; there are many op-amps that have a shutdown pin. Two that I'm using in a demo project right now are MAX40108 and MAX44260, both of which have a shutdown pin. \$\endgroup\$ – MarkU Apr 4 at 21:36
  • \$\begingroup\$ Does the motor controller not have an enable/disable? \$\endgroup\$ – Transistor Apr 4 at 21:37
  • \$\begingroup\$ About the op amp output state when in shutdown: unless otherwise stated in the datasheet, since the output is not driven it will "float", however it still has to stay between the power supply pins or else the internal ESD protection diodes would become forward-biased. \$\endgroup\$ – MarkU Apr 4 at 21:41
  • \$\begingroup\$ Can I may be add a pulldown resistor to the output to pull it to ground when it is floating? \$\endgroup\$ – Seif_1999 Apr 4 at 21:44
  • \$\begingroup\$ No the motor controller does not have an enable/disable pin unfortunately. \$\endgroup\$ – Seif_1999 Apr 4 at 21:47
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I have never seen an op-amp with a disable pin. Comparators, yes, op-amps no. Trying using an analog multiplexer to select the input between the op-amp output and a resistive divider (to set the disable voltage). You could also use an analog switch with a pull-up, pull-up, or bias divider to set the disable voltage.

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    \$\begingroup\$ um, they do exist. I don't want to post an answer since I work at Maxim and I'm not a shill, but there are a lot of op amps with SHDN pins. Our competition has this too. Trying to hack a 741 by disconnecting its power supply isn't as good a solution. \$\endgroup\$ – MarkU Apr 4 at 21:38
  • \$\begingroup\$ Honestly I did not know that op-amps and comparator are different things, I thought that a comparator is one configuration of the op-amp. Can you explain a bit more please the difference? \$\endgroup\$ – Seif_1999 Apr 4 at 21:45
  • \$\begingroup\$ @Seif_1999 Are you able to read this article? circuitcellar.com/resources/ee-tips/… Sometimes it is open for free viewing and other times it needs to be paid. Also, electronics.stackexchange.com/questions/72967/… and analog.com/en/analog-dialogue/raqs/raq-issue-11.html \$\endgroup\$ – DKNguyen Apr 4 at 21:47
  • \$\begingroup\$ @DKNguyen yes I can read it thank you. \$\endgroup\$ – Seif_1999 Apr 4 at 21:49
  • \$\begingroup\$ While this suggestion adds complexity, +1 for mention of resistor divider as a set point for disabled state. The question did not specify how 0-12V maps to the output. For example if there is reverse control then "stop" could be at 6V, in which case this answer makes good workable solution. \$\endgroup\$ – Maple Apr 5 at 3:35
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What you’re asking for is a ‘servo disable’ or ‘check’ function. A way to do this is to add a series resistor and a pull down FET to force the drive signal to ground. The FET would be controlled by some kind of safety supervisory circuit, which includes all the interlocks needed such as a power ok, key switch, locked-rotor detect and a watchdog timer.

Experience: daisy wheel printers.

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  • \$\begingroup\$ I tried this but the output only decreases by 0.5V would you draw a quick schematic for me to follow, maybe I am connecting the FET in a wrong way. thanks \$\endgroup\$ – Seif_1999 Apr 4 at 23:30
  • \$\begingroup\$ The motor driver backend should have its own enable. That serves the same function as the solution I outlined. \$\endgroup\$ – hacktastical Apr 5 at 1:19
  • \$\begingroup\$ @Seif_1999 The reason it did not work for you is that you still had OP Amp active. There is just so much the FET can do without increasing resistor too much. The combination of the OP Amp with shutdown control and grounding FET is your best bet. Assuming, of course, that you want output to be at 0, which doesn't seem to be a case \$\endgroup\$ – Maple Apr 5 at 3:40
  • \$\begingroup\$ I do want the output to be at zero when the op-amp is disabled, but I can't find an op-amp with shutdown pin locally so I was looking for an alternative method. \$\endgroup\$ – Seif_1999 Apr 5 at 5:39
  • \$\begingroup\$ @hacktastical yes it should, but if it had this function I wouldn't have asked this question. Thanks \$\endgroup\$ – Seif_1999 Apr 5 at 5:46
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OK, as I pointed out in comments, the question is ambiguous as stated. Nevertheless, I'll try to answer the way I understand it.

First, you do not really want to "disable" motors. Disable in a context of motor control means freewheeling. What you seem to want is stop it, by sending 0V to the driver.

The reason it does not work for you is that LM324N has up to 100 mV low minimum @ 5V supply, probably more @ 12V. So, even if you set you digital pot output to 0 you will not get 0 from OP Amp buffer.

There are several ways to deal with this.

  • You can try different OP Amp with rail-to-rail output; You may have to reconfigure it from unity gain to something slightly above 1 to get full swing though;
  • You can follow @hacktastical advice and use FET to ground output, with large enough resistor in between so that OP Amp won't affect output much; This method works best if combined with shutdown-capable OP Amp, in which you only need small resistor for safety;
  • You can follow @DKNguyen advice and use analog switch to choose between OP Amp output and ground, no resistor necessary.

Note that the latter two options have one thing in common - they do not have smooth continuous transition in 0~100mV range, which might affect motor operation.

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