So, I had another thread on this same topic, and while I hate to start another, my situation has changed, and that thread turned into a bit of a confusing learning exercise (much to my benefit in the end - sincere thanks to everyone). I simplified my circuit from that thread, no longer have multiple voltage sources, and am just powering everything with 5V. Here's what I've got. I found the following circuit on EDN:
http://www.edn.com/design/analog/4314544/Integrator-ramps-up-down-holds-output-level (article)
http://m.eet.com/media/1126960/12071-figure.pdf (actual circuit image)
Using a 5V source, I want to take a 5V motor (0,5A nominal, 2A stall) from off to full power using the ramp output signal of this circuit. I built the circuit using the LMC6484 op amp suggested by the article (omitting IC1c since I want to use the ramp signal to automatically ramp the motor's speed and not control the speed directly with a PWM). The circuit works beautifully and ramps up linearly from 0V to 5V in about 10-15s. However, the problem I'm having is that I don't know what to connect to the output of IC1b to power the motor. Here is where my mind has gone:
Problem: LMC6484 can only supply ~10-15mA with a 5V supply. This isn't really enough to drive a power transistor.
Solution: Pick a different rail-to-rail op amp with a higher output current.
...but:
Problem: The circuit is really designed to provide a voltage ramp, not exactly a current ramp. Maybe I'm going down the wrong path.
Solution: Perhaps this is a job for a MOSFET instead of a BJT?
Problem: 5V isn't enough to power most MOSFETs unless I get something logic-level/really low threshold.
Solution: Get a MOSFET with a really low threshold.
Problem: I'm not finding many MOSFETs that have a <1V threshold.
Solution: Use a logic-level MOSFET and supply and pre-bias the gate voltage to the threshold.
Problem 1: Despite sounding like I know what I'm talking about, I have zero idea how to properly pre-bias a MOSFET and then input the ramp voltage to the gate.
Problem 2: The output of a MOSFET is is rather logarithmic, not linear, correct? Is this really going to do what I'm wanting, or is there something else that would better accomplish what I'm looking for?
Thank you all again for your help. If you've been following my inane posts, I hope it's clear that you've managed to teach me a thing or two, at least...
http://www.edn.com/design/analog/4314544/Integrator-ramps-up-down-holds-output-level (article)
http://m.eet.com/media/1126960/12071-figure.pdf (actual circuit image)
Using a 5V source, I want to take a 5V motor (0,5A nominal, 2A stall) from off to full power using the ramp output signal of this circuit. I built the circuit using the LMC6484 op amp suggested by the article (omitting IC1c since I want to use the ramp signal to automatically ramp the motor's speed and not control the speed directly with a PWM). The circuit works beautifully and ramps up linearly from 0V to 5V in about 10-15s. However, the problem I'm having is that I don't know what to connect to the output of IC1b to power the motor. Here is where my mind has gone:
Problem: LMC6484 can only supply ~10-15mA with a 5V supply. This isn't really enough to drive a power transistor.
Solution: Pick a different rail-to-rail op amp with a higher output current.
...but:
Problem: The circuit is really designed to provide a voltage ramp, not exactly a current ramp. Maybe I'm going down the wrong path.
Solution: Perhaps this is a job for a MOSFET instead of a BJT?
Problem: 5V isn't enough to power most MOSFETs unless I get something logic-level/really low threshold.
Solution: Get a MOSFET with a really low threshold.
Problem: I'm not finding many MOSFETs that have a <1V threshold.
Solution: Use a logic-level MOSFET and supply and pre-bias the gate voltage to the threshold.
Problem 1: Despite sounding like I know what I'm talking about, I have zero idea how to properly pre-bias a MOSFET and then input the ramp voltage to the gate.
Problem 2: The output of a MOSFET is is rather logarithmic, not linear, correct? Is this really going to do what I'm wanting, or is there something else that would better accomplish what I'm looking for?
Thank you all again for your help. If you've been following my inane posts, I hope it's clear that you've managed to teach me a thing or two, at least...