Hmm. Well, I found that from EDN.com, so I figured it would be pretty decent. I could never get it to do what I needed, though. I'll go breadboard yours, but given my luck, I'm not hopeful . . . (not a comment toward your design, of course).

 

Well, that circuit takes my motor from on to off (and it is really choppy as it does so). This is really frustrating.

Can someone explain why my first simple capacitor charge circuit doesn't work? Or, maybe it's time to jump ship and go for a microcontroller (I haven't used one since the Altera Quartus when I was in college, and those are overkill for what I need).

 

Well, that circuit takes my motor from on to off (and it is really choppy as it does so). This is really frustrating.

Make sure that you got the "IN+" and "IN-" connections correct on the last opamp - if they are swapped, you will get an ON to OFF. To get rid of the choppiness, decrease the value of the first capacitor.

 

Are you referring to the voltage supply or the inverting/non-inverting inputs?

 

Are you referring to the voltage supply or the inverting/non-inverting inputs?

The inverting/non-inverting inputs. Looking at my diagram, it appears that I reversed them.

 

Well, that accomplished having run at barely turning for about 1s and then going to full voltage - not exactly a ramp. Perhaps it's because I don't have the exact model of FET, but I tried a few with the same result.

I'm considering the TMP815 by TI. It is similar to what @jpanhalt suggested, but it works at 6V minimum. I'll need a TSSOP breakout, though.

Either that, or I'll put a bounty up for this. I've been trying to make something on and off since March of this year with no success. Isn't there a section on this site for that?

 

The exact model of the FET does not matter - the one you selected will work fine. Do you have a back EMF diode across the motor?

Check your wiring on your last change. If the first effort worked, although backwards, then changing the inputs should have done what you wanted.

 

I have an EMF diode on the motor.

I checked some wires and had mixed things up, and it works. However, just like every other op-amp solution I've built, I can't get the voltage to the motor to go much higher than 3.75-4V. I powered the op-amp with 12V to increase the voltage to the gate and connected the motor between drain and a 6V supply. Still, it never got any higher. Why?

 

I checked some wires and had mixed things up, and it works. However, just like every other op-amp solution I've built, I can't get the voltage to the motor to go much higher than 3.75-4V. I powered the op-amp with 12V to increase the voltage to the gate and connected the motor between drain and a 6V supply. Still, it never got any higher. Why?

Which circuit are you using now? I got lost

 

Post #13 (except with the invert/non-invert inputs to the last op-amp swapped).

 

I can't get the voltage to the motor to go much higher than 3.75-4V. I powered the op-amp with 12V to increase the voltage to the gate and connected the motor between drain and a 6V supply. Still, it never got any higher. Why?

What voltage do you measure on the mosfet gate, drain, and on the 6V supply once the delay time has passed?

 

I grabbed a different MOSFET, and it's working much better. However, here are some measurements all measured from a common ground (I made sure to calibrate my power supply as closely as possible using my multimeter and with no load):
G: 12.1V
D: 0.5V
S: 0.5V
6V supply: 5.3V
12V supply: 12.1V

Recall that I have 12V powering the op amp and 6V to the motoe. The motor is getting 4.8V across it (way better than any other circuit, so I'm happy), but what is forcing the supply voltages up and down? Is that going to harm my step up converters?

 

Cn you give us a link to 6V DC-DC converter, or its model number?

 

It's one of those cheap XL6009-based boards like this:
https://www.amazon.com/Adjustable-Step-up-Converter-Module-Replace/dp/B00W94H41I/

 

I powered the op-amp with 12V to increase the voltage to the gate and connected the motor between drain and a 6V supply. Still, it never got any higher. Why?

The resistance across the FET is approximately zero when on, so my guess is that your power supply can't supply the needed current.

 

I think the 6V drops because that board doesn't work well at 3.7V input - it works best from 5V upwards.
From the Amazon page:
"Wide input voltage 3V ~ 32V, optimum operating voltage range is 5 ~ 32V"
The datasheet for the XL6009 says:

 

Hmm. Well, I'm not hooked up to that step up converter yet. I'm using an ancient analog power supply for the moment, and it only has a 0.5a output to each rail (which I'm maxing out, now that I check). Based on my other circuits (the ones that kinda worked), I never saw any blue smoke or got excessive heat, so I'm gonna say it will work with the converter. I'm gonna go put this on some perfboard and report back when I'm done.

 

Oh, and I made a few substations. I don't have any 500k, 16k, or 50k resistors, so I went 470k, 15k, and 50k resistors, respectively. I don't assume that makes a huge difference...?

 

Oh, and I made a few substations. I don't have any 500k, 16k, or 50k resistors, so I went 470k, 15k, and 50k resistors, respectively. I don't assume that makes a huge difference...?

The difference will be trivial, except for the 50k resistors, which will have NO difference!

 

This circuit ended up working well - thanks, @SLK001 !!! Did you find that schematic somewhere, or did you put that together on the fly?