V570 PWM setup

[Ken]

Hi,

I am in the process of setting up a plc to output a 20khz with a duty cycle of 20%. I set up the ladder and setup the hardware for pwm. The duty cycle was setup as a MI and I set it to 200 for 20 Percent. My problem is figuring out the proper way to set up the frequency? I tried different MI for the frequency but cant get the signal to output at 20000hz. I am able to turn the pwm on and off but having difficulty acheiving higher frequencys.

I have V570 with V200-18-E3xb snap in I/O. I set up the dip switches for NPN. Can I get a diagram or just the proper steps for setting this up. Ive checked the manual and the help topics but none of them show clearly how to set this up. I am new to this software, but fluent in ladderlogic.

Thanks,

Ken

[Joe Tauser]

Setting up an -E3XB for PWM is pretty simple and it sounds like you've done it right. When you enter 20,000 in the frequency MI, what frequency do you actually get? Are you measuring it with a frequency counter or an oscilloscope? What type of load is connected to it? Have you tried measuring the frequency at 50% duty cycle?

If you're trying to measure the frequency with a frequency counter instrument, you may be running into problems with the harmonics of the square wave output.

You can also assign an ML to the frequency on a PWM output, and in NPN mode it should go to 50 kHz.

Joe T.

[Ken]

Setting up an -E3XB for PWM is pretty simple and it sounds like you've done it right. When you enter 20,000 in the frequency MI, what frequency do you actually get? Are you measuring it with a frequency counter or an oscilloscope? What type of load is connected to it? Have you tried measuring the frequency at 50% duty cycle?

If you're trying to measure the frequency with a frequency counter instrument, you may be running into problems with the harmonics of the square wave output.

You can also assign an ML to the frequency on a PWM output, and in NPN mode it should go to 50 kHz.

Joe T.

Hi , Thanks for the response. I am measuring the frequency with an osciliscope. The square wave comes up just not at the frequency specified. When i try and enter 50000 for frequency it will not let me enter that value. The max it will let me enter is 4 digits not 5. The duty cycle works correctly.

[Ken]

Can I also get a diagram of the proper way of hooking up to the output of the PWM. . I am not getting it to output correctly.

[Joe Tauser]

The max it will let me enter is 4 digits not 5. The duty cycle works correctly.

I set up a very basic configuration with an -E3XB module and downloaded it, with the frequency mapped to ML 0. I had no trouble entering 50,000. I did configure the display variable for six digits, which is not the default. Take a look.

What exactly is the load you are connecting to the module? Unitronics has a connection diagram on page 5 of the V200-18-E3XB specification sheet, but if you let me know what you are hooking up I can draw a more detailed schematic.

Joe T.

V570 ML PWM test.vlp

[Ken]

I set up a very basic configuration with an -E3XB module and downloaded it, with the frequency mapped to ML 0. I had no trouble entering 50,000. I did configure the display variable for six digits, which is not the default. Take a look.

What exactly is the load you are connecting to the module? Unitronics has a connection diagram on page 5 of the V200-18-E3XB specification sheet, but if you let me know what you are hooking up I can draw a more detailed schematic.

Joe T.

Thanks for the response.

I am hooking up to a High staic pressure fan for a heatsink dissipating 3000 watts. The PWM controls the fan speed. The lead for the Fan speed input is referenced to the ground of the fan. By connecting to ground the fan is off. When you connect the speed control line to a signal generator ( + of signal generator to speed control, - of sig generator to ground) The speed is controlled perfectly at 20khz with speed changing linear with duty cycle. When the fan speed signal is open the fan is at full rpm. Ive tried connecting directly to the input with no results. I then tried connecting through a highspeedfet using the output of the HSO to control the base. I had no luck with this either. I then tried hooking up the HSO to an oscilliscope, The output gave alot of noise when it was enabled. The speed input of the fan is a high impedance input.

[Joe Tauser]

Ken,

We're getting there....

It sounds like the speed input wire of the fan can't generate enough current to overcome the breakdown of the NPN output. You would normally need a need a pull-up resistor, but we don't know if putting 24V into the control wire will blow up the fan's control circuit. I'm guessing you already know that you have to run your logic in reverse - 0% duty cycle = full speed, 100% duty cycle = stop.

You didn't mention the voltage of the fan, the peak-to-peak voltage of the signal generator that worked, or the maximum input voltage the control wire will tolerate so I will hypothesize. Since you have an oscilloscope and know how to use it I'm also guessing that you know how a voltage divider works.

Let's assume that your power to the PLC is 24 VDC and the signal generator creates a 5 VDC PWM squarewave. The maximum current rating of the NPN output is 50 mA, so we'll size everything so the output is carrying 10 mA when it's on so we don't blow that up. The voltage drop when the output is on is 0.4 VDC.

We use two resistors to create a 5V signal when the output is off and limit the current to 10 mA when the output is on. The supply is 24V, so to limit the current through the output:

(24 - 0.4)/0.010 = R1 R1 = 2360

Use a 2.2K ohm 1/2w resistor

Now we need to size R2 to give 5V at the junction:

Vo = R2 /(R1 + R2) * Vi

or

R2 = R1 / ((Vi/Vo)- 1)

So R2 = 2200 / ((24/5) - 1) = 578

Use a 560 ohm 1/2w resistor

So here's our circuit:

This should give you a roughly 5 V PWM signal at the fan input. Note that the output voltage will never go all the way to zero; this may or may not be a problem. As you can see, the more specific details you provide when asking a question will give you a better answer. If the fan's input will take 24V, you can just use the 2.2K as a pull-up and be done with it. If this won't work then post more details on the fan and I'll have another go at it.

Joe T.

[Ken]

Ken,

We're getting there....

It sounds like the speed input wire of the fan can't generate enough current to overcome the breakdown of the NPN output. You would normally need a need a pull-up resistor, but we don't know if putting 24V into the control wire will blow up the fan's control circuit. I'm guessing you already know that you have to run your logic in reverse - 0% duty cycle = full speed, 100% duty cycle = stop.

You didn't mention the voltage of the fan, the peak-to-peak voltage of the signal generator that worked, or the maximum input voltage the control wire will tolerate so I will hypothesize. Since you have an oscilloscope and know how to use it I'm also guessing that you know how a voltage divider works.

Let's assume that your power to the PLC is 24 VDC and the signal generator creates a 5 VDC PWM squarewave. The maximum current rating of the NPN output is 50 mA, so we'll size everything so the output is carrying 10 mA when it's on so we don't blow that up. The voltage drop when the output is on is 0.4 VDC.

We use two resistors to create a 5V signal when the output is off and limit the current to 10 mA when the output is on. The supply is 24V, so to limit the current through the output:

(24 - 0.4)/0.010 = R1 R1 = 2360

Use a 2.2K ohm 1/2w resistor

Now we need to size R2 to give 5V at the junction:

Vo = R2 /(R1 + R2) * Vi

or

R2 = R1 / ((Vi/Vo)- 1)

So R2 = 2200 / ((24/5) - 1) = 578

Use a 560 ohm 1/2w resistor

So here's our circuit:

This should give you a roughly 5 V PWM signal at the fan input. Note that the output voltage will never go all the way to zero; this may or may not be a problem. As you can see, the more specific details you provide when asking a question will give you a better answer. If the fan's input will take 24V, you can just use the 2.2K as a pull-up and be done with it. If this won't work then post more details on the fan and I'll have another go at it.

Joe T.

Thanks for response.

The fan is a 12v fan with a dedicated powersupply. I used a voltage of 5v peak to peak to get the fan to work. I will try your circuit tomorrow. Thanks for your help.

Ken

[Joe Tauser]

Ken,

The resistors will change for a 12V supply. The formulas dictate a 1.2K for R1 and an 820 ohm for R2. I'd try using a 1K for both- the fan should tolerate a 6 Vpp signal and the current on the output will be 12 mA, which will be fine.

Joe T.