pulse width modulation circuit (PWM)

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labview1958
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pulse width modulation circuit (PWM)

Post by labview1958 » Fri Aug 26, 2005 1:44 am

I have designed a circuit to contol the speed of a DC motor using a NI-6025E DAQ card and a PC. My program generates a TTL signal to control to control a TIP 41 transistor. By controlling the duty cycle with a program from my PC, I can control the speed of the motor. Is my circuit correct? Here is the schematics![img][URL=http://img397.imageshack.us/my.php?imag ... uit2wz.png][img]http://img397.imageshack.us/img397/1829 ... 2wz.th.png[/img][/URL ][/IMG]<p>I have added a diode for safety.

labview1958
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Re: pulse width modulation circuit (PWM)

Post by labview1958 » Fri Aug 26, 2005 1:52 am

ooops! No diagram. I try again!<p>Image

Bernius1
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Re: pulse width modulation circuit (PWM)

Post by Bernius1 » Fri Aug 26, 2005 8:47 am

Rudimentary, but it should work. But you'll also probably have problems getting it to work right. Most serial/parallel ports can handle 40mA at up to 12V, so driving the ckt should be enough. But is it a true step motor, or are you just pulsing a regular motor ? Is it brushless? It seems that the 'off' time will incur a subtantial power loss, with the collapsing field, and that the inrush current may be subsequently high, especially for a non-stepper motor. Not 'smoking'high, but high. I think you should add a lower power transistor as a buffer/driver, and bias it properly, with .01uf caps to cut noise. And I don't know if you want/need feedback for RPM control. An interesting twist would be to count the sine pulses from the motor during the 'off' cycle. Remember "resistor"="cheap insurance"="less smoke" !
Can't we end all posts with a comical quip?

labview1958
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Re: pulse width modulation circuit (PWM)

Post by labview1958 » Fri Aug 26, 2005 11:29 am

I am using a NI-6025E Daq card. Also I am using a normal PCB drill motor. Is the diode connected correctly?

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sofaspud
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Re: pulse width modulation circuit (PWM)

Post by sofaspud » Fri Aug 26, 2005 2:14 pm

I believe the diode should be across the motor instead of the transistor. The polarity is correct.

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jwax
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Re: pulse width modulation circuit (PWM)

Post by jwax » Fri Aug 26, 2005 2:52 pm

Hey lab! Once you get it working, how about switching (no pun intended) to a MOSFET stage in place of the bipolar?
I'd be curious as to what are the trade-offs may be in your application.
What's the current specs on the "normal PCB drill motor"?
Thanks!

Gorgon
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Re: pulse width modulation circuit (PWM)

Post by Gorgon » Fri Aug 26, 2005 3:05 pm

Hi Labview1158,
Your diode is correct placed for protection of the transistor. The resistor for driving the transistor is to high. If I read the value correct to 22k? 2k2 (or 1k) is better if the output can take it, If its standard TTL its no problem. This is depending on how much current your motor draws? The hFE of TIP41 is not very high.<p>To get rid of the noise from the motor, use a small capacitor over it.<p>TOK ;)
Gorgon the Caretaker - Character in a childrens TV-show from 1968. ;)

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sofaspud
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Re: pulse width modulation circuit (PWM)

Post by sofaspud » Fri Aug 26, 2005 3:33 pm

Can we get a 2nd opinion (3rd? 4th?). I understood that the protection diode or snubber was placed across the inductor, not the switch.
If the kickback forces the voltage at the cathode/collector to rise, the diode will remain reverse-biased. What am I missing?

Robert Reed
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Re: pulse width modulation circuit (PWM)

Post by Robert Reed » Fri Aug 26, 2005 5:09 pm

Jwax
Actually you and gorgan are both right, as in many circuits ,two diodes will be used. One for suppression, one for protection.

rstofer
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Re: pulse width modulation circuit (PWM)

Post by rstofer » Fri Aug 26, 2005 8:19 pm

Nobody can tell you that the circuit will work because it depends on the current requirement of the motor. You really have to specify this or everyone is guessing.<p>If you are thinking that you will use a 2N3055 (big TO3 device) because it can handle the current you will be surprised when you look at the current gain with high collector current. It is very low.<p>Your PC output shouldn't be required to drive more than a very few milliamps. In fact, I would put an opto isolator in that circuit before I fried the PC.<p>That's the bad news! Maybe it will work, probably not.<p>Look at the IRL540N MOSFET. This thing can handle a bunch of current and pretty high voltage and be switched with logic levels. I would still use an opto isolator to protect the PC. Then again, you may have that built in to the DAQ board.<p>In the case of the MOSFET, there is internal protecton for the device. All you need is a reverse biased diode across the motor. You also need 0.1 ufd ceramic capacitors between each motor lead and ground and a 3d capacitor across the motor leads. All 3 as close to the motor as possible.<p>Did I mention the opto isolator?

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Re: pulse width modulation circuit (PWM)

Post by Gorgon » Sun Aug 28, 2005 1:32 pm

<blockquote><font size="1" face="Verdana, Helvetica, sans-serif">quote:</font><hr>Originally posted by sofaspud:
Can we get a 2nd opinion (3rd? 4th?). I understood that the protection diode or snubber was placed across the inductor, not the switch.
If the kickback forces the voltage at the cathode/collector to rise, the diode will remain reverse-biased. What am I missing?
<hr></blockquote><p>Hi Sofaspud,
I agree with you that the positive kickback need a diode over the motor, but you also need a diode over the transistor for the possible negative peaks you get from a rotating motor. So the best is to have both in this case.
No arguing intended in my replay.<p>TOK ;)
Gorgon the Caretaker - Character in a childrens TV-show from 1968. ;)

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