UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Mini-drill; Speed Controller Problem

orbanp1 · 27th Nov 2017, 5:16 pm

You missed the point in my reply that the LM1458 is not a rail-to-rail op-amp!
Powered like that, it will not amplify the current information from the parallel of R1 and R2.
The input to IC1A will be outside of its operating range! Do lookup the datasheet!
You need an op-amp like the LM358.

Linear has its own version of the LM1084, called as the LT1084, and LTspice has the model built in, no need to use the third party model from TI.

The other "anomaly" that comes to attention, is the value of the R5 resistor. In applications suggested by the manufacturers that resistor is in the 200 Ohm range, here it is 1 kOhm, which looks high!

EDIT: That value, given the load of the motor, seems OK. It would not be right without any external load!

Regards, Peter

27th Nov 2017, 5:16 pm	#4
orbanp1 Heptode Join Date: Nov 2009 Location: London, Ontario, Canada Posts: 675	Re: Mini-drill; Speed Controller Problem You missed the point in my reply that the LM1458 is not a rail-to-rail op-amp! Powered like that, it will not amplify the current information from the parallel of R1 and R2. The input to IC1A will be outside of its operating range! Do lookup the datasheet! You need an op-amp like the LM358. Linear has its own version of the LM1084, called as the LT1084, and LTspice has the model built in, no need to use the third party model from TI. The other "anomaly" that comes to attention, is the value of the R5 resistor. In applications suggested by the manufacturers that resistor is in the 200 Ohm range, here it is 1 kOhm, which looks high! EDIT: That value, given the load of the motor, seems OK. It would not be right without any external load! Regards, Peter Last edited by orbanp1; 27th Nov 2017 at 5:42 pm.