MOOLY, HELP HERE!!!

[abeteer]

a few clarification referring to your schematic

1) could I just place a jumper wire in place of R8 and put a V-trimmer instead, then what would be the safer value (with a margin)? It came to my mind that this would be a much safer approach especially when working with the power first. Alex MM pcb design was great and will suit me fine

2) was there any replacement for TL071D? the LM series abound in my area.

3) D1 is BZV85-C12VO is this a 0.5w 12v zener?

4) C5 is 10uf/25v, in the schematic it was a non-polar, is this correct?

5) can you please convert L1(6uH) into-size of wire, no. of turns over 10ohm/2w resistor. I could use some formulas...oh boy! I really did miss a lot of things

Pardon my inquisitive mind I'm new to MOSFET, I just came here full of clinging schematics in my head I'm still stuck in my ancient AnalogMM, no scope no sophisticated test equipment and such. The shop I've been working on had a DMM that'd been blown into oblivion long time ago (A careless tech did that)


Regards

[AndrewT]

2.) the opamp is supplied with +0v and -12V. Check the opamp output voltage. The output must not be too close to the supply voltages. I wonder if there is a Zener missing from the +45V supply rail?
Many FET input opamps can replace tl071. look for low output offset and low output offset drift. These are the two most important parameters that determine the accuracy of the DC servo to hold near zero output offset.

3.) a 500mW 12V Zener can pass upto 40mA long term.
The 10k resistor limits the total current to Zener and opamp to ~[45v-12v]/10k ~ 3.3mA.
I think this is too low. Could be increased 2 to 4times by using R=2k2 to 4k7

4.) the capacitor has 12V across it. It should be a normal (polarised) electrolytic.

[Mooly]

Hi abeteer,
1. Yes, fit a jumper for R8 to initially test, then replace with preset, or just start with the preset adjusted for zero ohms (short). Read ALL the posts in the original thread as all this is covered.

2. Answered in other your other thread.

3. An small 12 volt zener.

4. C5 is electroylitic, positive end to GROUND.

5. This is a non critical part. The 0.22 ohm resistor in series with the output (R26) ensures stability for any real world load. The coils were around 1.5 to 2cm diameter with 1.5mm copper wire... totally non critical though... and about 8 turns, airspaced.

[Mooly]

Hi Andrew, the TLO71 works on a single negative rail without problem, the output of the opamp is around -6 volts, nicely in the middle. Other devices will not operate in this manner without providing a positive rail as well. The TL071 typically draws around 1 milliamp so 10k feed is fine.

[Mooly]

abeteer... the opamp MUST be a FET device which has essentially zero input currents. Bipolar devices must NOT be used for DC servos. I saw you mention LM devices

[abeteer]

K!, thanks a lot