# 4.1. Choosing the motor, power supply, and Jrk

The information in this section can help you select a motor, a power supply, and a Jrk G2 controller that will work well together.

The Jrk requires a DC power supply. The Jrk is designed to drive a brushed DC motor, either by itself or as part of a device like a linear actuator.

#### Voltage and current ratings

When selecting your motor, power supply, and Jrk controller, you must consider the voltage and current ratings of each.

The rated voltage of a DC motor is the voltage it was designed to run at, and this is the voltage at which it will draw its rated currents. It is fine to drive a motor with a lower voltage than what it is rated for, in which case its current draw will be proportionally lower, as well as its speed and torque. Conversely, using a higher-than-rated voltage will result in proportionally higher current draw, speed, and torque, and could negatively affect the lifetime of the motor. However, you can limit the PWM duty cycle used to drive the motor to keep its average current draw within rated limits. (For example, running a 6 V motor at 12 V but limiting its duty cycle to a maximum of 50% should generally be OK).

The stall current of a DC motor is how much current the motor will draw when power is applied but it is not spinning (for example, if the motor shaft is prevented from rotating), producing maximum torque and minimum (zero) speed. The stall current depends on the voltage that is applied to the motor, and the stall current is usually measured at the rated voltage of the motor. Most brushed DC motors are not designed to be stalled for extended periods and can be damaged if they are.

It is not unusual for the stall current of a motor to be an order of magnitude (10×) higher than its free-run current. When a motor is supplied with full power from rest, it briefly draws the full stall current, and it draws nearly twice the stall current if abruptly switched from full speed in one direction to full speed in the other direction.

The free-run current of a DC motor (or no-load current) is how much current the motor draws when it is running freely, producing maximum speed and minimum torque (since there is no external opposing torque). Like the stall current, the free-run current depends on the voltage that is applied to the motor, and is usually measured at the rated voltage of the motor.

The voltage range of your power supply is the range of voltages you expect your power supply to produce while operating. There is usually some variation in the output voltage so you should treat it as a range instead of just a single number. In particular, keep in mind that a fully-charged battery might have a voltage that is significantly higher than its nominal voltage.

The current limit of a power supply is how much current the power supply can provide. Note that the power supply will not force this amount of current through your system; the properties of the system and the voltage of the power supply determine how much current will flow, but there is a limit to how much current the power supply can provide.

The minimum operating voltage of a Jrk is the lowest voltage that is acceptable for the Jrk’s motor power supply. If you try to power the Jrk with a voltage lower than this, it might fail to deliver power to the motor, but it should not cause any permanent damage.

The absolute maximum operating voltage of a Jrk is the maximum voltage that can be tolerated by the Jrk. If the voltage of the power supply rises above this voltage, even for just a brief period of time, the Jrk could be permanently damaged.

The recommended maximum operating voltage of a Jrk is the maximum voltage we recommend using for the Jrk’s motor power supply. We have chosen this number to be significantly lower than the absolute max operating voltage so that if there is noise on the power supply it is unlikely to exceed the absolute max operating voltage.

The maximum nominal battery voltage of a Jrk is the maximum nominal voltage we recommend using for batteries that supply power to the Jrk. We have chosen this number to be significantly lower than the recommended maximum operating voltage because fully-charged batteries can have a voltage that is significantly higher than their nominal voltage.

The maximum continuous current of a Jrk indicates the motor current that it can continuously supply without overheating in typical conditions (at room temperature with no additional cooling). The Jrk’s MOSFETs can handle large current spikes for short durations (e.g. 100 A for a few milliseconds), and the Jrk’s hardware current limit can be configured to help it handle large transients, such as when starting a motor. However, note that the Jrk does not have an over-temperature shut-off. (The Jrk’s motor driver error can indicate an over-temperature fault, but the Jrk does not directly measure the temperature of the MOSFETs, which are usually the first components to overheat.) As a result, an over-temperature or over-current condition can still cause permanent damage.

The voltage and current ratings of the different Jrk G2 controllers are shown in the table below.

Jrk G2 21v3 Jrk G2 18v19 Jrk G2 24v13 Jrk G2 18v27 Jrk G2 24v21 4.5 V(1) 6.5 V 6.5 V 6.5 V 6.5 V 40 V(2) 30 V 40 V 30 V 40 V 28 V 24 V 34 V 24 V 34 V 24 V 18 V 28 V 18 V 28 V 2.6 A 19 A 13 A 27 A 21 A

1 The 5V logic voltage drops when powered from a supply below about 5.2 V.
2 Operation from 28 V to 40 V should be transient (< 500 ms).

These are the main constraints you should keep in mind when selecting your power supply, Jrk controller, and motor:

1. The voltage of your power supply should ideally be greater than or equal to the rated voltage of your motor so that you can get the full performance that the motor is capable of. It is OK for the power supply voltage to be higher than the rated voltage of the motor, but if it is significantly higher then you should consider configuring the Jrk’s maximum duty cycle limits in order to limit the voltage applied to the motor.
2. The voltage of your power supply should be within the operating voltage range of the Jrk. Otherwise, the Jrk could malfunction or (in the case of high voltages) be damaged.
3. The maximum continuous current of the Jrk and the current limit of the power supply should ideally be greater than or equal to the stall current of the motor. However, this might be impractical or unnecessary in some applications, especially with high-power motors that are not intended to be stalled for prolonged periods (and will be damaged if they are). If so, you should at least make sure that both current ratings are greater than the free-run current of the motor. The higher the stall current of your motor, the more important it is to consider using the Jrk’s acceleration limiting and hardware current limiting to prevent the motor from drawing too much current.
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