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Full-Wave Bridge Rectifier With Capacitor Filter

  1. May 25, 2004 #1
    I am putting together a control circuit for a DC motor. This motor requires 110 volts DC. It must be variable speed.

    I happen to have a nice continuously variable transformer box that goes smoothly from 0 to 130 volts. This should take care of the speed variations.

    I hooked this up to a rectifier and the DC that comes out seems to operate the motor pretty well.

    In reading, however, I discovered that by adding a capacitor across the DC output lines I should be able to smooth the ripples in the DC quite a bit more.

    There is a schematic of this simple circuit at this site:

    PowerVolt AN01
    Address:http://www.powervolt.com/techan01.html [Broken]

    Scroll down a little bit to the diagram labeled "Full-Wave Bridge" which is to the right of another diagram.

    The only clue I have to the value of this capacitor is that it should be "large".

    The motor, as I mentioned, will be running on voltage from 0 to 110, depending on the particular speed I need. The watts, therefore will also vary, but it is a pretty torquey motor for its size so I expect peak watts will be up there.

    Can anyone give me an idea what value of capacitor I should try in this circuit?



    P.S. The motor is 3/4 horsepower.
    Last edited by a moderator: May 1, 2017
  2. jcsd
  3. May 27, 2004 #2


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    If this is for some serious application then I would just go for one of those caps that you can buy for your car stereo. They have 'em in the 1000 F (that is not a typo, that is 1 kF) range. They are the size of a large beer can. Unfortunately, I don't know off the top of my head what the voltage rating is. The larger the better, though, so as large as you want without making a big cost or physical size issue out of it.
  4. May 27, 2004 #3
    Thanks for your response, turin.

    This motor operates a bench lathe.

    The voltage rating of the cap I need doesn't baffle me since it just has to be in excess of the 120 volts that come out of the wall.

    A 1kF cap would certainly cover the situation with much room to spare. Generally, the bigger the cap the more expensive it is. This is why I'm interested in limiting it to what I realistically need.

    I happened to have a 250v 65 microfarad cap that is physically large ("beer can" size) and I hooked that up. It doesn't seem to improve the operation of the motor at all.

    At very low speeds, such as I will use for cutting threads, the torque is not good. I can stop the chuck with my hands. I am hoping the right cap will inprove this.
  5. May 27, 2004 #4


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    I don't think your problem is the cap. I suspect that your problem is probably the diode bridge. Did you purchase it or build it yourself? For the amount of power you're talking about, you need on the order of 100 A, which must get to the motor through the diodes. Check the specifications for the diodes; they may be giving you excessive output compression.

    Now that I think of it, I imagine that the transformer can saturate at this current as well.
    Last edited: May 27, 2004
  6. May 27, 2004 #5
    You can do it simpler by running the rectified DC through the primary of and open circuited transformer. With the addition of a "fly-back" diode at the input of this choke should work quite nicely. And BTW, did you say 1 kF cap.....????, if that is 1000F, I do think you need a reality check.
  7. May 27, 2004 #6
    I bought the rectifier. It is rated 600v and 10 amps. This was the largest amperage they had in stock at the time. I am just assuming that if the motor were drawing more than this it would have ruined the rectifier by now. My guess is that there are variable resistors inside the transformer between the taps to create a smooth transition when you turn the knob, and that is what is ultimately limiting the current.

    O.K., I just checked the transformer and found it had "5 amp max" stamped on the I.D. plate. Thanks for your prompt. This is no doubt the problem.

  8. May 27, 2004 #7
    Let's make sure I'm following here. You mean that this would be a better filter to smooth out the ripples in the rectified DC than the cap?
    I have no idea what a "flyback" diode is. What would be its purpose here?

    If you read my post above you'll see that my problem is most likely the transformer, but this alternative you are suggesting to the cap is interesting.
  9. May 27, 2004 #8
    it is very common in the switched mode variable speed drives, the transformer is a biginductance (ie. choke), the flyback diode is shunted between the motor return and the input of the choke for the transient reponse. You can add a cap across the output of the choke to form a L-C lowpass, but this is not generally neccessary. A cap will short out high frquency, the inductor blocks the high frequency. same effect, except the iron core inductors a bullet proof, more reliable.
  10. May 27, 2004 #9
    BTW, the flyback diode allowes current to keep flowing in the large inductance between the rectified pulses. Ie, when the current is flowing in a big inductor, it does not want to stop, the diode allowes current to be sucked out of the motor by the EMF stored in the inductor. make any sense??
  11. May 27, 2004 #10


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    There are some new 100F capacitors intended for car audio use on the market now, but nothing close to 1000F and the voltage here.

    120V is RMS, better have a cap over 170V rating.

    A good solution would be to use a PWM (chopper) drive circuit to drive the electric motor. The chopper is able to maintain a constant level of current in the motor but at full voltage to overcome the back EMF. The amount of current is variable according to pulse width and maintains torque far better than dropping the drive voltage which instead causes an exponential drop in power.

  12. May 27, 2004 #11
    What does PWM stand for?
    This sounds like it would be what I'm looking for. Any links to one of these circuits?

    As far as the cap goes, I think I like wwtogs idea of an inductor as a filter better.
  13. May 27, 2004 #12
    I think I understand how this would work. I'm a little unsure about the diode, though. It should be oriented to allow the current to flow from the motor back into the imput of the inductor, correct?

    Just to be sure, lets say there is a section of wire going from the - (negative) side of the rectifier to the inductor. Lets call this section A. Then the current goes through the inductor and into the motor. It comes out of the motor through a section of wire we'll designate as section B which goes to the + (positive) side of the rectifier. You are saying to connect the diode between section B and A oriented such that the current flows from B into A, correct? This is what I got from your description.
  14. May 28, 2004 #13
    Pulse Width Modulation (PWM).

    Google H-Bridge motor controller. You can build a very simple control circuit using an AVR/PIC/8051 microcontroller. Set the PWM frequency to about 8KHz. Using PWM to control a DC motor improves efficiency substantially by allowing the motor to make use of the flyback current. Also, turning the voltage on/off allows the motor to run cooler. Uhmm, as far as filter caps go: I've seen my fair share of underrated caps go boom when least desired. Try to select a voltage rating 3 times greater than input voltage when using a filter cap in conjunction with a motor. In your case I'd go with a Big Fat electrolytic 500V 1000[itex]\mu[/itex]F cap. Also, as far as H-Bridges go you can get pre-made packages that will handle your current requirements without a problem. They're pretty inexpensive too.

    You could probably build a fairly useful motor control using a 555 timer swithing the afore mentiond H. You just have to figure out a way to keep the overall time constant the same while varying the charge and discharge TC independently (and in opposite directions) If you're industrious you could come up with something using a dual pot and a couple of resistors to balance the total resistance. It'd be just about impossible to get a stable PWM frequency from a 55 timer, but a workable frequency could be made.

    This is how you orient a flyback diode.
    http://mechatronics.mech.nwu.edu/mechatronics/design_ref/actuators/fly3.jpg [Broken]

    My 2 cents.

    Good Luck.
    Last edited by a moderator: May 1, 2017
  15. May 28, 2004 #14
    This sounds like what I need, but when I googled all those circuits look very complex. I'm sure I have no idea how to put something like that together.
  16. May 28, 2004 #15
    Thanks for the diode orientation diagram. What are the two things that look like switches inside circles?
  17. May 28, 2004 #16
    representations of transistors. You can buy pre-made H-Bridge circuits. I'll scratch a couple of links up for ya. I've used Vishay H's for small DC motor apps. They have good docs if you want to learn a little more about how they work.

    http://www.dprg.org/tutorials/1998-04a/ [Broken] a good start

    Go to National semi, Dallas, or Fairchild. They all make H-Bridge IC's.
    Last edited by a moderator: May 1, 2017
  18. May 28, 2004 #17
    The motor I'm trying to control is a 3/4 HP. Not exactly small.
    The H-Bridge itself doesn't look too baffling at this site. The one with the four transistors and four diodes is obviously something I could solder together. However, he leaves the question of the MCU unanswered. What is an MCU and how do I connect it to the H-Bridge? Are they expensive?
    The four transistors and four diodes seem only to be a rather complicated forward/reverse switch by themselves. There is no explanation of how to vary the speed. I assume that is what the MCU does?

    I also do not understand what the bases of the transistors are connected to. They seem to be just dangling, unconnected in his diagrams.

    In the end I want to be able to vary the speed just by turning a knob, and to change the motor direction just by flipping a switch. I don't need the braking capacity. The motor is already driving a belt which drives a shaft with the high/low gears which, in turn drives the lathe spindle. All that friction brings it to a stop pretty quickly when you shut the motor off.

    Thanks for your help. Sorry more questions keep arising.
    Last edited by a moderator: May 1, 2017
  19. May 28, 2004 #18


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    The bases are connected to the PWM output so the transistors can be 'pulsed' on and off by the output.

    This is a simple small motor circuit based off a 555.

    On this page are examples for circuits using FETs and a simple potentiometer to control speed along with explanations.

    Also a simple DPDT switch could be used to reverse directions and save the complexity of the H-bridge.

    If you had a univeral motor that could also run on AC, the easy answer would be a $35 wood router speed controller available at woodworking shops. But what fun would that be?

  20. May 28, 2004 #19
    You might want to add another diode in series with the choke in front of the shunt flyback diode to isolate your bridge from the fly-back circuit. In fact, you can try this with no bridge, and 1/2 wave rectification using the series diode alone. I have tryed this trick on an obscure voltage dc motor and a standard A/C triac based light dimmer, I would not recomend this for a lathe motor, however.
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