>Someone recently posted a picture of a motor controller board, wondering where to look for 12 volts to run a tachometer; I can't seem to find the post now. However I did take a good look at the picture earlier.
>
>In the upper right area of the board is a 14 pin chip. Clean it off and look for the chip number, (an example number would be LM3900).
>
>Google the chip number and data sheet. A data sheet shows chip pin functions,including the positive and negative supply pins (aka "rails"). Careful use of a voltmeter will tell you what voltage is on these pins, compared to each other (for a single suppply, e.g. +12 volts), or compared to ground for a two sided supply (plus or minus 12 volts, for example).
>
>I would be very cautious in doing this. I didn't see a transformer which would supply the chip voltage(s), which would typically be maximum 15 volts for a single sided supply.
>
>Some boards rectify the AC line voltage of 110 volts, and regulate this to lower voltages with zener diodes. This can cause serious hazards (electrical shock) because power is not properly grounded. If there is no transformer, do not go poking around the board, get advice from an electrician or electrical engineer.
>
>If there is a transformer, low voltage AC is rectified into pulsating DC, and an electrolytic capacitor smooths out the pulses. If this is 12 volts, you are in business. If it is higher you would likely need a 12 volt voltage regulator and a few other bits to get your tach going.
>
>I am not an electrician or engineer, and there may be errors in the above, so proceed at your own risk.
If you're going to do this, then the best way is to use a small wall
wart power supply, that will give you 12 volts and an isolated 12
volts. They aren't very expensive any more, and can often be picked
up at a thrift store (take voltmeter and try to test before buying)
for anything from 50 cents to 2 dollars. You'd like an amp or so
depending on what the circuit draws.
The advantage is that you have a completely isolated power supply, you
don't need to worry about modifications or grounding, and the entire
assembly can be swapped out or replaced without difficulty.
I did a lathe tachometer that way, and I have an automatic lathe stop
for threading that works the same way, although I did have to modify
the controller with a relay.
You never know what the manufacturer of the controller has done, and
you really want to play it as safe as possible.
The computer controlled speed controls have isolation between the
computer and the controller (when done as an add-on to the existing
board).
Harvey
>
>Grant
>
>
>
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