nano PWM outputs

[cch1955]

I need to drive 2 small DC motors with the PWM outputs on a nano. No problem using the 24VDC output as shown in the manual, but I need more speed.

I would like to drive the motors using 48VDC. Is there any problem using the PWM output in the same way with the other side of the motor supplied by the +48VDC and the PWM output providing the low side?

Current would be less than 2 amps. I would tie the grounds for the to power supplies together.

I know I can do it several more complicated ways, but I have 16 motors to drive and want to keep it simple.

[garysdickinson]

I would be very careful with attempting to directly drve a load that is connected to a supply with a voltage different than the 24VDC running the PLC.

My concern is with the off state of the nano output pin.  Traditionally the TRI PLCs have used an output driver chip similar to the ULN2001. The nano doesn't, but I'll bet their circuitry is substantially similar.

The ULN2001 has protection diodes that are connected to each output pin and the the 24V supply.  This was done to prevent the flyback voltage that occurs when driving inductive loads from destroying the output transistors.

When the PLC output is OFF their is a path from the 48V supply though your load via the protection diode to the 24V supply.  The end result is that the motor will see 24V when the output is OFF and 48V when the output is ON.

I'd suggest using transistors external to the PLC to handle your 48V loads.  Because your DC motor is a highly inductive load, you would be wise to pick transistors that will not be damaged by voltages substantially higher than the 48V supply.

Gary d

[cch1955]

I am using flyback diodes across the motors to take care of the inductive feedback issue. The output is pulled up to +24 internally with a weak pull up resistor it says in the manual so I would think the current avalible is way below the threshold of the motors needs.

I changed direction to using the output to drive a DC-DC solid state relay which is working well. I could build a simple MOSFET driver board , but really want the solution to be off the shelf for longterm mainenance man type of support.

Thanks for your help on this.

Cheers

[garysdickinson]

Your approach is pretty well though out.

I took a look at one of my nano-10 boards and traced the output circuit.  They appear to use a desecrate power MOSFET with a 5.6K pull up with a red LED.  The interesting observation is that the pull up is not tied directly to the 24V input, but to a diode connected to the 24V input.

The single diode will prevent current from the 48V supply from flowing through your load to the 24V supply.  The motor won't turn and the output LEDs won't pop because of the reverse bias.

But, there is only one diode in the nano output circuit and it is shared by all 4 outputs.  If you have a load connected to 48V and the output is
 off, the other outputs will be pulled up to your 48V supply through your load.  This may cause some issuses (output LEDs may melt and/or the 5.6K resistors may momentarily emit light/smoke when the other outputs go active.  The current that flows thru the other 3 output's LEDs will be sourced through your motor load.

You still may want to consider an external solution.  If you are already using a SSR to handle the motor direction issue, you might want to use another SSR for the pwd issue.  You may want to look at something called an "h-bridge driver".


Gary d

[support]

Do not connect 48V load directly to the PLC output. The PLC outputs are mostly spec to handle no more than 30VDC (the components may have higher tolerance but we need to build in some safety factors). If you need to inteface to 48V or higher load then using optical isolation such as SSR is the right way to go.

[cch1955]

Thanks. Working well.