OPAMP circuit design help

Hi guys,

I have a problem I'm trying to solve. I have a C6 speed control hooked to a KBMM motor controller. I'm running the KBMM off of a 220V circuit and using a 90V motor, so I only need about 5V out of the C6 to get top speed on the motor. That end of the rpm range works great. The problem is that the low end needs about .5-.7V just to get the motor turning. I have played with all of the spindle settings in Mach3 but it just wants to put out a nice linear output. At 1000 rpm in Mach I get about 400rpm at the spindle.

I need to find a way to offset the output of the C6. I thought that a non inverting summing op amp would do the trick, but I can't seem to get it working. I'm using a LM741 running off the 12V supply for the C6. The problem is that the lowest voltage I can get on the output pin is 2V. I think I've either screwed up the circuit design or I need a different op amp. I've attached a PDF drawing of the schematic. Can one of you help me out?

Thanks,


Chris

Attachment 241526What about a LM358 setup as a comparator, if you run it on 5 volts it will never go higher on the output side.

Even simpler:
Use a 5-10k linear pot, using the wiper lead as the output would also limit the voltage and have some adjustment.

Hi Chris,

The "native" power supply scheme for analog Op Amps such as the 741 is a dual supplies of ±15V. The original uA741 (Fairchild), like it's numerous clones, should be able to work down to ±5V but it still need to be bipolar!
However, as I understand your message, you are trying to use the 741 with a single supply of 12V...
The quad LM324 would be a good choice, the LT1014 an even better one: both are specifically designed to work on a single supply (from 3 to 22V), their «common mode input range» extends to ground
and their output can also «sink» down to ground, well below the 0.5 V output you are looking for.
Good luck!
Jean

Jean,

You hit the nail on the head. I selected the wrong op amp for the job. I'm going to stop by the electronics store this week and see what they have in stock that has rail to rail switching.

Thanks for your help,


Chris

I built a new circuit tonight with the LM324. It works great. I was able to calibrate the spindle to within 50rpm over the entire 500-4500rpm range.

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