Re: Servo/Spindle motor ammeters?

Re: lm3914

Good, much higher currents than I was thinking.

15V * 0.05 ohm = 0.75 V full scale. The voltage reference is 1.25
already. The input resistor stack is 10K.

The series resistor betweek the voltage reference out and reference in pins is:

Rseries = Rin * (Vref - Vmax) / Vmax = 6.67Kohm. Closest standard
value is R=6.65.

I'm assuming that the sense resistor is on the low side of the
mosfets. If so, then connect the SIG IN pin to the sense resistor.
Make sure you check!

The picture on rutex website doesn't show an output inductor, so
you'll need to filter it before you put the signal into the
comparitor. It really depends on how quick you want the response to
be. There'll be a visually noticable lag after about 10 to 200 ms,
which is roughly a time constant of 5 to 100. Go for maximal
smoothness and
R * C = 100 ms
With R = 10K, C should be 10 uF. You can probably find a tantalum that
size. Or you could go 1 uF for a zippy response.

On 7/27/06, afogassa <afogassa@...> wrote:
>
> I'm using rutex drives, it's 0.05 ohm resistor and I'm expecting
> about 15 amps full scale.
> I'm wondering if lm3914 has a high impedance input to not change
> the sense resistor value.
>
> Fogassa