Anybody Know How To Wire A Bridge Rectifier?

[Tom LaRussa]

I'm trying to wire a DC motor for a prospective sharpening station-thingie. I'm using a bridge rectifier to change the AC current into DC.

Problem: The rectifier did not come with a wiring diagram.

So, I found this page on the internet, http://www.kpsec.freeuk.com/powersup.htm#rectifier which implies that the two wires from the AC current should be kitty-corner from one another and the two wires going to the motor should be kitty-corner from one another.

The only marking on the Rectifier is a + sign in one corner, so I hooked the black lead from the motor to the tab at that corner, put the white lead from the motor on the tab at the opposite corner, and hooked the two wires from the AC cord to the remaining tabs.

Results: A single "click" -- then nothing.

I am afraid that the click was something frying inside the rectifier. Since I only have one rectifier left, I thought it best to get some opinions before I continue with my experimentation.

I'm pasting in a picture of the rectifier, as well as a diagram of it and another diagram of how I wired it the first time.

Any help greatly appreciated!

[Carl Eyman]

I went to google and found this: http://www.tpub.com/neets/book7/27c.htm
I tried to copy it and attach it, but it was too big for the Creek.

Notice where th + sign is - on the input side. It looks like you're right on the opposite corner bit, but maybe th + is on the input side. I don't think you've fried the rectifier, but I don't think it'll do any harm to use the + corner for either the black or white line wire. See what happens. Let us know.

[Keith Christopher]

What you have is I believe a Fairchild semi or Internation rect BP 25-04, you hooked it up exactly opposite as you should have. input leads one to the + the other to the diagonal. the rectified output is on the remaining two pins. I believe this module is rated very high and doubt you've blown it. (you would know if you did -bad smell and and all. )

[Chuck Wintle]

One way to determine the orientation of the diodes is with an ohmmeter. You can see that in one direction it will be high resistance and by reversing the leads of the ohmmeter the R will be low corresponding to the anode and cathode of the diode

[Steve Stube]

You have wired it correctly except possibly for direction on rotation. The + indicates the positive DC output. On a two wire DC series or PM motor the direction of rotation is reversed by switching the two DC motor leads.

I'd have a question about the "DC motor" you are using and its voltage and amp requirements. Can you fill in the blanks?

[Kurt Aebi]

Tom,

You have a 25 AMP General Purpose Bridge Recifier, very similar to the one's we use at work for our Stepper Motor Drives. Carl has supplied you with a very good web site for understanding what the rectifier is doing and you have shown the correct wiring. If the motor runs choppy (herky-jerky, or if it is "cogging"), add a 1,700 to 17,000 microfarad, 200 Volt Electrolytic Capacitor between the Positive and Negative terminals of th eBridge Rectifier. What this does is hold the charge when the full-wave goes low and will make it a smoother curve, with less drop-off.

You are on the right track - remember to mount the Bridge Diode to a Heatsink with silicone heatsink grease for maximum efficiency and bridge life.

[Tom LaRussa]

I went to google and found this: http://www.tpub.com/neets/book7/27c.htm
I tried to copy it and attach it, but it was too big for the Creek.

Notice where th + sign is - on the input side. It looks like you're right on the opposite corner bit, but maybe th + is on the input side. I don't think you've fried the rectifier, but I don't think it'll do any harm to use the + corner for either the black or white line wire. See what happens. Let us know.

Hi Carl,

Thanks for your suggestions. They seemed quite reasonable, so I tried them out. I put the black wire from the power cord at the + sign, the white wire from the power cord at the opposite corner, and the two leads to the motor at the other corners.

This time nothing happened at all.

So I tried my backup rectifier in the same configuration, and still no result.

I think maybe this thing just doesn't like me.

[Tom LaRussa]

See what happens. Let us know.

Hi again Carl,

I just tried your configuration again -- after resetting the circuit breaker that I popped with my first attempt -- and it works!

Thanks!

[Tom LaRussa]

I'd have a question about the "DC motor" you are using and its voltage and amp requirements. Can you fill in the blanks?

No problemo.

It's a Pacific Scientific double-shafted treadmill motor.

Here are the ratings:

120 VDC
11.2 Amps
1.5 HP
4800 RPM

[Tom LaRussa]

If the motor runs choppy (herky-jerky, or if it is "cogging"), add a 1,700 to 17,000 microfarad, 200 Volt Electrolytic Capacitor between the Positive and Negative terminals of th eBridge Rectifier. What this does is hold the charge when the full-wave goes low and will make it a smoother curve, with less drop-off.

Thanks Kurt!

I've read a little bit about electrolytic capacitors, but I never found a reference that told me what size to use, so you just filled in a major blank for me.

Question though: When you say to put the capacitor between the positive and negative terminals of the rectifier, do you mean between the two that connect to the motor?

You are on the right track - remember to mount the Bridge Diode to a Heatsink with silicone heatsink grease for maximum efficiency and bridge life.

I think I've got that covered. I've got a spare CPU cooler that I think should do the trick. I'm planning to take the little fan off of it and cut the aluminum part in half to use for the two different rectifier/motor setups I'm making.

[Tom LaRussa]

Okay, here's another question for you guys.

I have another DC motor I want to wire to a rectifier. This time it's the motor itself that's confusing me.

It has one red lead, two blue leads, and one black lead sticking out of it.

Any idea what to do with all of these wires?

Here are a couple pics:

[Steve Stube]

The motor has a thermo overload switch - if the motor gets too hot the circuit is switched OPEN. If the two blue wires are connected to the internal thermo switch (my best guess) then to take advantage of this built in safety you would wire the sw. in series with the input 120 volt feed.

Tom, Are you saying that you now have an AC input going to the + terminal in post #1? That would sure defy convention if that is the case. The + and - signs in the link shown only refer to one 360 degree cycle of the AC signal, not polarity as in DC circuits. If you would show this bridge again with the wires connected as you have them arranged when the circuit works, I'd like to see that. Assumming it is different of coarse. TIA. Steve

[Kurt Aebi]

Tom,

That is exactly what I mean, connect the positive ("+") lead of the capacitor to the "+" Lead of th eMotor & Bridge and the ("-") lead of teh capacitor to the "-" Lead of the motor & bridge.

That heat sink setup should be plenty overkill and will therefore work just fine. You may actually find that you don't need to run the fan, the heatsink portion will work fine.

Not sure what the Blue leads are used for on the tradmill motor, they could be used for the thermal overload as Steve mentioned, or maybe they are just a feedback connection for measuring speed or torque. Most likely Steve has it right - I am just speculating.

[Tom LaRussa]

The motor has a thermo overload switch - if the motor gets too hot the circuit is switched OPEN. If the two blue wires are connected to the internal thermo switch (my best guess) then to take advantage of this built in safety you would wire the sw. in series with the input 120 volt feed.

Okay, but I'm not sure I understand what I'm supposed to do.

The two blue wires have different connectors -- one male, one female. Do I just plug them into one another?

Tom, Are you saying that you now have an AC input going to the + terminal in post #1? That would sure defy convention if that is the case. The + and - signs in the link shown only refer to one 360 degree cycle of the AC signal, not polarity as in DC circuits. If you would show this bridge again with the wires connected as you have them arranged when the circuit works, I'd like to see that. Assumming it is different of coarse. TIA. Steve

Yep. That's how it's now wired, and the motor runs.

Maybe my rectifier is just mis-labeled?

Here is a pic:

[Sparky Paessler]

Steve

I pulled up the datasheet on this part and the leg that is turned 90 degrees from the other three is the +dc output (not the leg that is near the + sign).
The -DC output is the leg on the opposite corner.

AC input _ _ DC- output
-
DC+ out | _ AC input

Don't know why that + is on the side of the part where it is.