# Current Limiting Modification

*by Jean-Charles Vachon
<vachonjc@intlaurentides.qc.ca>*

*May, 1999*

Jean-Charles has designed two current limiting circuits for the
PCB.

(the following is Jean-Charles' explanatory text, slightly edited)

In the depicted version, the 240 ohms resistor (R3) is connected
to a common point with the TIP127 and diode 1N5408. Since there was
no ground connection available, the motor itself was used as a return
to ground. This is not as good as directly returning to ground, but
there was no choice. On a new board this circuit, or a similar
one, can be incorporated on the main PC board, or a ground connection
can be added on the DB-9 connector.

The other version, not depicted, this resistor (R3) is 1000 ohms
and connected directly to ground. This version is more
effective.

Now, the current limit value is determined by R2, in my case 0.15
ohms for 4 amperes. This resistor should be a 5 watter.
Any current can be chosen and it is calculated the folowing way:
R2 = 0.6/ desired current. For example in my case I
wanted 4 amperes maximum, so R2 = 0.6/ 4 = 0.15 ohms. In a
similar way, if you desire 3 ampere limitation the calculations are:
0.6 / 3 = 0.2 ohms, and so on.

If you use 24 volts I would raise R3 to 470 ohms in the first
circuit and to 2200 ohms in circuit 2.

Transistor TIP127 does not heat, but in pure precaution I attached
on the alluminium box, I used, a medium size heatsink. I
think it is better if used at 24 volts. This depends on the motors of
course.

What the circuit does, is to limit the maximum current to the
value presetted no mater what is the load, so as I discussed earlier,
you will not end with astronomic currents near the 20 ampere mark.

page by Mel Bartels