Current Limiting Modification

by Jean-Charles Vachon <>

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.

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