Bridge diode - which one for my motor project?

Discussion in 'The Projects Forum' started by cazksboy, Apr 22, 2012.

  1. cazksboy

    Thread Starter Active Member

    Nov 9, 2009
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    Hello, I need a recommendation to which bridge diode would be appropriate to replace a shorted one from my motor project. I bought THIS MOTOR to use with my watchmaker's lathe. In case you don't want to click on that link, here are the motor's specs:
    SPECIFICATIONS
    • Power 0.115 HP
    • Voltage 120 AC
    • Amperage 1.5 Amps
    • Speed 3620 RPM
    • Rotation CCW
    • Torque 32 in.-oz.

    • Duty Intermittent
    • Mount Face
    • Shaft 5/16" diam. x 1-3/16" w/ flat
    • Size 3" diam. x 4-1/4"
    • Shpg. 4 lbs.
    Since the motor does not come with a switch to reverse direction, I removed the bridge diode and mounted it externally in an enclosure along with a DPDT center-off switch exactly per the attached diagram. I am attaching pics of the bridge diode both before removal from the motor's housing and after placing it in my external enclosure. Anyway, somehow in the build process the diode must have become shorted, because my circuit breaker trips just from plugging it in with the switch in the center-off position! I used my multitester to check continuity across the male plug's hot/neutral prongs - it reads less than one ohm, so it must be the bridge diode that has shorted because the plug and cord are fine.

    I need a recommendation to a good replacement bridge diode - preferably one of the ones sold by Radio Shack since there is one nearby. I would like to buy a replacement and get back to work with my lathe, but I am totally confused by the wide range of choices on Radio Shack's inventory.

    Thanks in advance.
     
    Last edited: Apr 22, 2012
  2. panic mode

    Senior Member

    Oct 10, 2011
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    assuming motor is DC, your circuit should be ok.
    but the bridge has to be rated in both current and voltage.
    if the voltage rating is lower than 120VAC*sqrt(2)= some 170V, the rectifier is likely to short instantaneously.
    radio shack seem to carry many rectifier models.
    just pick something like 3-4A or more, 400V or higher and you are good.

    http://www.radioshack.com/product/index.jsp?productId=2062583

    http://www.radioshack.com/product/index.jsp?productId=12673811

    http://www.radioshack.com/product/index.jsp?productId=12625303
     
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  3. wayneh

    Expert

    Sep 9, 2010
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    It looks like you've done a fine job assembling, but I'd double and triple check the switch wiring to be extra sure it's doing what you expect. Your diagram doesn't address that ground wire I see lurking. Might be worth adding that detail.
     
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  4. cazksboy

    Thread Starter Active Member

    Nov 9, 2009
    40
    1
    Yes, the motor runs on DC, apparently the bridge diode is just there to allow the user to use it with normal 117 VAC from a common outlet. OK, thanks for the specific recommendations - I know just enough to be dangerous! I was confused by all the different ratings.

    wayneh, thanks for the remarks. You are right, the diagram does NOT show the ground wire. That diagram was provided to me by a friend to show me how to connect the DPDT center-off switch. What I did was to connect my wires exactly per the diagram, but I added the ground wire already existing in the motor housing and connected it directly to the ground of the line cord. In retrospect, I wish I had used male/female quick-disconnect connectors so it would be easier to troubleshoot or replace components.
     
  5. wayneh

    Expert

    Sep 9, 2010
    12,100
    3,034
    Ah, so the green just passes thru and doesn't touch the other posts, good.

    Since you're pulling the bridge out anyway, use your ohmmeter on the open lines to confirm your switch behavior. While you're at it, and going to the Shack, you might want to think about putting a fuse in there. Never hurts.
     
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  6. cazksboy

    Thread Starter Active Member

    Nov 9, 2009
    40
    1
    Excellent idea...after I remove the bridge diode, I'll check the switch. Nothing like double- and triple-checking everything.

    Good grief - the best idea yet - a fuse! DUH! It's so obvious I never thought of it. But, what rating do you think I should use in view of the specific motor specs I'm using? And, I've never installed a fuse before...does it go inline with the hot or neutral prong? (see, I'm not afraid to ask dumb questions)
     
  7. wayneh

    Expert

    Sep 9, 2010
    12,100
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    Hah, well no question is apt to get more discussion here than which leg to put the fuse on! I think the convention of fusing the "hot" line is more common, probably to prevent over-current into a ground somewhere else not on the return line.

    As for size, you want it as "small" as possible without blowing too often. I'd guess something like a slow-blo 2A would be fine but maybe others here have other ideas. If you can get a physically smaller one, that might help you squeeze it in. An in-line might be handy - you could stuff it in there without having to mount it.
     
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  8. cazksboy

    Thread Starter Active Member

    Nov 9, 2009
    40
    1
    wayneh, thanks so much for helping me. I think I'll just get one of Radio Shack's NTE 53004's just because it has a high current capacity (10 amps).

    Thank you as well, for the pointer toward the fuse. If anyone else would like to add to the discussion, by all means, I'm here to learn!

    Your suggestion of adding a fuse made me think of just one more thing, and it's completely frivolous, but I actually think it might be kind of cool, and that would be to add a little pilot light. So I was thinking I could tap off one of the leads from the center terminals of the switch (the ones that carry the post-diode DC current to the motor brushes), and insert a little DC panel-mount LED so it would light up when the motor is on. Yeah, I know, if I can't tell when the motor is on then I'm a dummy! But I wonder, is there such a thing as a LED that glows two different colors depending on which way the current is running? In other words, if I'm running the motor one way, the LED light would show a given color, but if I'm running the motor the opposite way, it would light up another color? Is there such a thing or am I way off here?
     
  9. panic mode

    Senior Member

    Oct 10, 2011
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    there are LEDs that look like regular 3 or 5mm leds, have two wires and depending on polarity illuminate red or green for example.
     
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  10. panic mode

    Senior Member

    Oct 10, 2011
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  11. panic mode

    Senior Member

    Oct 10, 2011
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    in fact you could use standard 3-wire bi-color led as well but...

    you would need one more resistor and two diodes (using 2-pin bi-color LED, you just need one resistor).

    see attached:
    A - connects to one side of the DC motor (after DPDT switch)
    B - connects to other side of the DC motor (after DPDT switch)
    C - connects to negative terminal of the bridge (before DPDT switch)

    resistors need to be 22k / 1W for 5mA (should be bright enough)
    or 12k/2W for 10mA
    or 6k2/3W for 20mA

    the other two diodes are any diodes. they are needed to protect LED in case of reversed polarity (not needed whwn using 2-pin bi-color led)
     
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  12. cazksboy

    Thread Starter Active Member

    Nov 9, 2009
    40
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    Hey Mr. panic mode, thanks for that. I just got back from Radio Shack picking up a bridge diode rectifier (one of the ones suggested earlier) and fuses. While I was there I looked at their selection of LED's and noticed one of them called a bi-color LED. It had two leads coming out of it - it's this one:

    http://www.radioshack.com/product/index.jsp?productId=2062549&numProdsPerPage=60

    But I didn't get it because nothing on the package told me what made it change color. Polarity reversal? Voltage drop? I didn't know, and the one guy in the store didn't know either (grrrrrr....:mad:). So I figured no big deal, I can deal with it later. 3-wire LED's look too complicated for me, I'd rather stick with whatever is easier (2 wires, and no diodes!).

    So...would the LED referenced above work? If so, how would I wire it up? The working voltage from the bridge diode is 110 VDC and the LED in question works from 2.0 to 2.8 VDC at 30 mA (max), so I'll obviously need a resistor to drop the voltage & current to the LED. But what value, and where in the circuit? In parallel with the motor brushes or series?

    Thanks guys, I'm clearly not as knowledgeable as you folks.
     
  13. panic mode

    Senior Member

    Oct 10, 2011
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    that's the one...

    reversal of polarity changes color, just add resistor as mentioned above.

    human eye is not very linear, personally i see little brightness difference while changing LED current between 25% and 100%. the link says max 30mA. I'd probably meet it halfway and settle on 10-15mA.
     
  14. cazksboy

    Thread Starter Active Member

    Nov 9, 2009
    40
    1
    OK, but what value resistor? The above note seems to be about a 3-wire LED. I would like to use the 2-wire LED referred to in my previous post. So, what value of resistor should I use and where should I place it relating to the motor brushes? In parallel or series with the motor brushes?
     
  15. panic mode

    Senior Member

    Oct 10, 2011
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    it is same (regardless of configuration and number of pins, it is always ONE P-N junction that lights up).

    1. pick current (more current should give more brightness and needs lower resistance but larger power resistor). i would prefer lower power (less heat given off resistors). larger currents do not make significant brightness increase. suppose you pick 10mA...
    2. pick resistor that was calculated (estimated) for 10mA (12k/2W; you can use 12k with higher power such as 5W, no problem). note: it is an estimate (pretty close one) because we don't know exact voltage and duty cycle from controller.
     
  16. cazksboy

    Thread Starter Active Member

    Nov 9, 2009
    40
    1
    I'm really sorry and I'm not trying to be difficult, but I am not understanding your answer. Can anyone here advise me how to wire THIS BI-COLOR LED into the post-bridge diode side of the the circuit depicted in the attached diagram? For the bridge diode, I settled on THIS ONE from Radio Shack.

    I'm not much of an electronics technician, although I am familiar with the terminology and can handle a soldering iron just fine. So what I need is simple language describing what value resistor (ohms & watt rating) needs to be used with the LED, and the following information:

    1. Should the resistor be used in series or parallel with the LED?

    2. Should the LED/resistor pair be wired in series or parallel with the motor brushes?

    Thanks in advance...
     
  17. panic mode

    Senior Member

    Oct 10, 2011
    1,320
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    connect LED and resistor in series.

    connect that resistor-LED circuit in parallel with motor (after switch).

    calculate resistor using Ohms Law:

    R=V/I

    where

    V= DC voltage from bridge (in worst case 170VC) and LED voltage drop is neglected because it is much smaller (some 2V).

    I is LED current (in Amps). for example 20mA =0.020A

    you may pick any current your LED light up nicely. if you are interested to keep the heat loss from resistor down, current should be smaller. you can measure current and (and adjust it using potentiometer) to be sure if you like.

    then calculate power of resistor:

    P=I^2*R

    or

    P=V*I

    or

    P=V^2/R

    and that should get you in 2-3W range.
     
  18. cazksboy

    Thread Starter Active Member

    Nov 9, 2009
    40
    1
    Many thanks, panic mode. Yesterday I got the motor working just great with the replacement bridge diode, and I also installed a panel-mounted fuse holder, using a 2-amp slo-blo in it. If those don't work out, I picked up some 2.5-amp and some 3.0-amp fuses as well, but I suspect the 2-amp one will suffice.

    I also picked up the bi-color LED and a suitable panel-mounted LED holder. I didn't install any of it because RS didn't have a suitable resistor - I'll have to hunt one down online. But since it's non-essential, it's fine for now - the important part is getting my lathe motor working properly.

    Again, MANY THANKS!!
     
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