MrHeckles

Hi guys, This is part of a motor controller circuit. It is designed to function as an inhibitor from switching directions if the motor is still moving. It works MOSTLY as intended, ie if the motor is spinning forwards (even with no speed demand) and then is flicked into reverse, the circuit will sense the current in the motor, and inhibit the reverse PWM signal. Hence braking the motor, until such time as the voltage into "V in 1" or "V in 2" is less than that set by the 5k pot. It then allows the motor to change directions. Here is the problem. The negative voltage rail is sagging below the designed -9v (by sagging below, I actually mean it is higher). AND there is a difference in this drop between the forwards and reverse or the motor. It results in the voltage preset (from the 5k pot) being different for forwards and reverse. Now, I figure that this drop is due to there being too much load for the ICL7660 to handle, thus reducing it's negative voltage output. My question is this...... How can I keep the negative voltage rail at a steady -9v and stop there being variations between the forwards and reverse. Note: Audioguru is probably very familiar with this cursed circuit by now :-)

Aha, got it. I was running the transmission gate off the +9v supply. The PWM signal is between 0 and 12v. All fixed, the gate is now running off the 12v supply and shuts it all off nicely.

I have tried that now, but even with a direct connection to ground on pin 6, the bilateral switch is still letting through the signal. I am most confused.

Righto, clamp diode stays in. I am now having issues with the use of the transmission gate. The PWM signal for the motor is fed into pin 8 and then from pin 9 to the half bridge driver. Pin 6 is the control pin. When the comparator is on, pin 6 has a bit over 8v and everything is cool. The PWM pulse goes into the bilateral switch and comes out, and the motor works fine. However, when the comparator is off, pin 6 is low (-0.7v). The PWM pulse goes into pin 8, and then comes out from pin 9 slightly reduced. The motor will still operate, but at a slightly reduced rate. I was thinking that the bilateral switch should have stopped the PWM pulse completely when the control pin was low. Why is it still allowing the signal though?

I have put an RC filter in, and it seems to make it work fairly nicely. Two questions. 1. Is there an upper limit on values for the resistor and/or the capacitor in the RC filter? 2. Will NOT having the clamp diode result in the -ve output of the comparator damaging the base of the transistor?

Well, I have changed the circuit as per audioguru's diagram. Now it holds a nice +7.5v or so once turned on. Some more background. Vin is a feed from a sense resistor from one of the legs of a motor H-bridge, which goes through an absolute value amplifier before getting to the negative input of the 741 comparator. I find that with this new configuration, the comparator works fine when the motor is stalled, but as soon as it turns freely the comparator turns on, despite the fact that Vin is well above Vref. I presume that it is due to some voltage spiking somewhere, but how can I stop it?

I put the output clamp diode on to protect the transistor from having the base sitting at -9v. Does it not need this?

Hi guys, I have built the following circuit. The Vref is 0.01v. The Vin ranges from 0 to maybe 5v at most. What I was intending was a comparator that latched on once the Vin was less than Vref (and in which case the LED would illuminate). The trouble I have is that when it turns on, the voltage on the + input of the 741 only increases to 0.3v (from 0.01v initial reference). If Vin goes above this, it turns off again. If I reduce the 380 ohm resistor in value, the LED dims. If I increase the value, the latched + input voltage drops even more. If I remove the diode on the feedback loop, I cannot set the Vref properly. One point to note, is that the switch is in there as a reset. My question is how could this layout be changed to give a greater latched on voltage (so it wont turn off) and also why is the feedback resistor affecting the LED brightness? Thanks in advance.

Thanks again for all your help, i'll have a go at building it all and see how it works. Thats a whole lot of turkeys. I thought they were normally black anyway?

I keep getting confused by these bloody op-amps and comparators and what constitutes "on" and "off". I wanted this to have the transmission gate turn on when the voltage in was less than the reference voltage, and then latch on. And to have the transmission gate off when the voltage in was higher than the reference (assuming it hasnt already been latched on). How would I set this up? We've still got a day or so to go before christmas. Whats with you northerners and turkeys? We tend to stick with the variety of meats, chicken, ham, beef, maybe some lamb too.

Right then. One negative supply coming up. Can you have a +12v and a -5v supply into one op amp? or do they have to be equal? Is the comparator set up correctly so that it will latch on? Thanks for all your help audioguru, got big plans for christmas?

Hows this look I wasnt sure what values to put on the resistors you mentioned, so I took a punt on 1k for both. Are the op-amps going to be able to produce negative voltage output, even though there is no negative supply?

Whoops. Made a few mistakes in that last one. This one should be better If i'm correct, the switch should stay off while the absolute value of Vin is above Vref. Once Vin drops below Vref, the comparator should switch on, thus turning on the switch and lighting the LED. The comparator should stay latched on after that. Will a reset of the system be achieved by cutting power to it all? Does that look and sound correct now? I figured I dont really need to stop Vin from being too negative, since I will use an LM4558 dual op amp which has +/- 15v on the inputs. My only question is whether the circuit will allow a negative value on the 1st inverting op-amp since it has no negative supply rail. Should I perhaps connect PIN 4 of the dual opamp to a -ve supply?

How does that look. Removing the diode across the 1st op-amp isnt going to mess up it's gain is it? How does that transistor/diode setup look? I put the pull down resistor on the output to pull the CTRL of the Cmos to low when the comparator is off. I also put the feedback on the comparator to latch it on, but I'm not sure if I've done it properly. I know this sounds a bit dumb, but could you explain how you think this circuit will work please?

STOP THE PRESS! How about this setup? Would you care to pass your eyes over this please. According to my theory (which we've established is a bit shakey), this should act as an absolute value comparator, which switches high (and turns on the LED) when the absolute value of Vin is greater than V ref.

So if the inverting opamp receives a decent +ve input, then it will still only be close to ground for the output? Have you had any experience with absolute value circuits? I think thats what I need, so I can preserve the value of the input (regardless of whether its positive or negative). That way I can set up current limitations for forward, reverse, forward regen and reverse regen on the motor.

Wouldnt an inverting op-amp cause any positive voltage on the input to change to negative? The Vin for the circuit is from the sense resistor from one leg of the motor H-bridge. Depending on what the motor is doing, it could be positive or negative. If it's positive, it needs to stay positive. If it's negative, it needs to be inverted.

Hmm, it might be more sensible to ensure that the Vin is always going to be +ve, then I could get away with a normal comparator instead of a window comparator. Is there such a device that can leave a +ve voltage alone, but change a -ve voltage to a positive one?

Is it going to be alright to have a negative voltage as a reference on the window comparator? I vaguely remember seeing something in a datasheet saying inputs needed to not go below -0.3v.

Just to clarify.... Am I correct in thinking that your standard comparator can only sink current on the output (ie drag the output to ground), and that op-amps can either source or sink current on their output?

Hi guys, here is the latest incarnation of the inhibitor switch.... Hopefully, it works like this (please correct me if I am wrong).... When the window comparator is outside the range of say -0.5v to +0.5v then it will be off. That will mean that on the next comparator the - input (9v) is greater than the + input (6v) so it will be on, thus grounding the CD4066 and turning the switch off, inhibiting the signal (and turning on the LED). When the Vin of the window comparator enters the above range, it should turn on, thus grounding the - input of the next comparator making it less than the + input and turning the comparator off. When it turns off, the + input should now be raised to about 9.5v through the 2k resistor, the LED (hence the drop) and the feedback line from the output. This should also turn on the CD4066 enabling the signal. The LED should also get very very dim at this point. Now even if the window comparator turns off again (being outside the range) the + input (9.5v) should still be greater than the - input (9v) and keep the comparator off and the CD4066 on. The only way to reset everything should be to cut power to everything. How does that all sound?

HA! Maybe I should start all my posts with "is there such a device that does this ....." Okay, i'll look up transmission gates and redesign it. Is the window comparator (with the negative reference for the lower end) and the schmitt trigger to keep it on all kosher?

Yip, but we compensate by walking around on our hands alot of the time, and watching tv standing on our heads so we can understand our northern hemisphere brothers. That and the water swirls the opposite down the plug hole. Freaks alot of tourists out. They think its some kind of south pacific voodoo equivalent. If the PNP is going to turn on when the PWM pulse goes 0.6v above the voltage on the base of the transistor, wouldnt 12v with a current limiting resistor be better? Would that stop it from turning on? Would it be better to try and redesign the system to use an NPN transistor? If I could do that, is there a way to stop the voltage on the base of the NPN(to turn it on) from passing through and messing up the PWM pulse by making the IR2184 go high on the input? (or have I misunderstood things again with these bloody transistors)

Okay - the PWM signal is generated by an SG3524 chip. The output is the 2 open emitters of the chip joined together. The datasheet says the emitter output voltage is typically 18v, though since the chip is being driven by 12v (or close enough to) I would say that it would be at most 13v (since my battery doesnt tend to give more than that). I would say that 0v would be the lower limit, unless there are some negative spikes in the PWM signal. I dont have an o-scope so I cant confirm that. That output connects to the input of an IR2184 half bridge driver, and a connection to ground via a 1k resistor. BTW this is all from my previous schematics that you have helped me with. I hope that clears things a little? If not, tell me what I need to find out (and how to, because we can be quite stupid here in NZ sometimes).

Okay, how about this setup? It should have a much lower voltage going to the base of the PNP transistor. I must admit though, I dont really understand how this whole emitter-base voltage limitation works.