View Full Version : mini lathe power supply DC voltage

04-24-2006, 02:14 PM
Mine is burnt up. Not the original one, it was burned up when I purchased the lathe. I think it's 90 volt DC, but it may be 120.

What I am thinking about doing is just using a light dimmer and a bridge rectifier. How bad of an idea is this?

This is one of the harbor freight 7x10 or 7x14, I can't remember.


04-24-2006, 03:06 PM
Contact Rabid Wolf on the minilathe Yahoo group. He repairs and sells these boards, or he'll tell you how to fix yours.

04-24-2006, 04:02 PM
The voltage could be 90 or 100 depending on which board/motor you have. As Rex says, just get another board or have your's fixed. I've seen them on ebay too ... Cheap! In fact, one that I sold late last year went of about $20. :( I could have sold the thing to Uncle Rabid for $25. I have two more but I have plans for those.

Here's the link to Uncle Rabid's site: www.unclerabid.com

04-24-2006, 04:06 PM
www.littlemachineshop.com (http://www.littlemachineshop.com) has both the most common parts as well as this document on troubleshooting:


It's been years since I purchased from them, but the service was excellent.....a small businessman whose little business has grown.


04-24-2006, 04:41 PM
Ok, so lets go back to my original question. Do you think it will work to use a light dimmer and a bridge rectifier.

04-24-2006, 05:22 PM
I was hoping you would read a bit about how the one you have works, come to the conclusion that the pulse width of the DC to the motor is what is varied and the light bulb would come on suggesting that you may be better off (and cheaper than a dimmer and bridge rectifier) just fixing the one you have or replacing it rather than risking burning up the motor you have. Gerbils on a wheel may work too, but why?:)

I know that several guys have replaced the motor with one from a treadmill and the respective controller (similar controller but handles more current for the larger motor), but since the original item is available so cheap, why not fix it the right way and spend less time rewiring ?

I would ask this question: Since most of the light dimmers chop the AC sine wave, what do you get when you put that through a bridge rectifier?


J Tiers
04-24-2006, 05:53 PM
chopped pulsating DC.... Some of the treadmill controllers do just about that.

Forrest Addy
04-24-2006, 06:29 PM
Snowman. Yes bridge/fan dimmer will work but the speed regulation may not be so hot. Use afan dimmer not a light dimmer. There's a difference in their circuitry that makes it important. A better choice than a fan dimmer would be a varaible transformer/variac.

A good DC drive from eBay would be better yet. Here's one of the right kind but it's over-rated for your applicatiion:

http://cgi.ebay.com/Dart-DC-Drive-253G-200C-2hp_W0QQitemZ7612659148QQcategoryZ78190QQrdZ1QQcmd ZViewItem

Dayton, KB, Dart, are but a few to choose from.

J Tiers
04-24-2006, 06:42 PM
Yes, a fan dimmer would be rated for inductive loads. A light dimmer, probably not.

Difference is important for one thing because it affects the ability to withstand the spike voltages resulting from the inductive "flyback" as the wave is chopped.

Also, the characteristics of the inductive load may confuse the phase-shift circuitry that controls the SCR / triac.

04-24-2006, 07:20 PM
I dont feel that replacing it with the "proper" controller is any better fix than what I have suggested...short of replacing it with a controller such as the one that forrest suggested.

When I recieved the mini lathe from Ebay, the original supply was shorted out. Every time you plugged it in, it threw the breaker...I didn't care to even think about fixing it, I had this dandy little Dayton controller that I used anyway.

Well, the dayton was crushed when I stepped on it...oops. I then resoldered the tabs going to the onboard SCR's and Diodes, well, one of those resoldered tabs broke, and when it did, it fried the board. Simply replacing the SCR's and diodes did not fix the board.

I would love to fix it with the proper item, but in all honesty...it's not cheaper. I have about five different 90 volt DC motors that are just sitting around waiting for projects....and capital. I am broke right now, and need to make something on the lathe. If I can get the lathe to work well enough with stuff that i have lying around, I will...if I burn the motor up, so be it. I wont really be at all behind where I am right now.

So, in short...I need the lathe to work, if I can get the lathe to work and get this job out, I'll have money to purchase the proper part...if not, well, then I'm screwed until about five pay day's from now (and I get paid biweekly).


04-24-2006, 07:52 PM
I used a 90v dc motor and "ran" it on my Taig lathe with a setup as you describe along with about 800mfd worth of capacitors. It worked quite well but the biggest problem is that the turn-on (trigger?) voltage is typically about 30v. This means that you cannot run the motor at low speeds.

As a result of this I abandoned this system and made up a PWM style controller. It also is not the cat's meow as there isn't any feedback circuitry to control the rpm. My next move is to get the correct style of controller, probably from the surplus center.



04-24-2006, 08:41 PM
Well, if you need it right NOW, why not put some car batteries laying around into series and drive it direct with on/off switch until things get quieter for a proper fix?

04-24-2006, 09:24 PM
Two light bulb sockets screwed to a board, wired in series, then the bridge rectifier fed from the junction between the sockets- that will get you running without the risk of having full rectified line voltage feeding the motor. Of course you have to put light bulbs in the sockets. Put a 100 watt in the socket that gets the line voltage hot wire, and maybe a 60 w in the other one. Reverse the order if the motor runs too fast.
You'll have to use 'real' light bulbs for this, not the fluorescent kind. This is the basic resistor voltage divider circuit, and aside from being almost the poorest regulator around, second only to the single light bulb in series with the load, it will act as a current limiter as well. It sounds like you have a bridge rectifier on hand, and not too many shops would be without a light bulb socket or two, so this could be the zero cost way to get the lathe running, even if it has poor speed regulation. (and it will). Experiment with different wattage bulbs and you'll probably find a combo that will run the motor at a reasonable speed under load, and get your job done.

04-25-2006, 11:49 AM
Jacob-- the additional information you provided may be really useful. You indicated that it trips the breaker when plugged in (implying even when it is turned off). If this is the case, your problem may be a lot simpler than you think. Something is shorted before the power switch. You might end up stumbling on this even if you wanted to implement your solution using an AC motor controller and a bridge rectifier as you will have the thing apart then. I would still encourage you to take it apart and look.

I second the motion that you should not use a light dimmer, but rather a fan speed control as they are designed for inductive loads. Most light dimmer packages even indicate right on them that they are not for use with motors. Even then, you may find fan controls non-linear enough in their control that they are less than adequate in the long run. The last one of these I bought was nearly $7. I know that in my own circumstances, I have often tried to save a few bucks and then didn't have the money I sunk in the cheap but non-functional solution when I had to turn around and buy the right item anyway, later.

Do also bear in mind that the controller that comes with the lathe has some auto re-setting overload protection. If you have plenty of motors and don't mind risking that one, then this may not be important to you. The mini-lathe is best suited to taking reasonably light cuts anyway. I have always been impressed with mine, but they do have their limits.

04-25-2006, 12:22 PM

I threw out the original...this was before I found out I could sell it!

I'll probably just end up throwing together a PWM, I think I have all the components.

I tried the lightbulb to diode bridge last night, I think I might have a short in my motor wires...I hope it's not that stupid.

After working on it for five minutes, I gave up and tore the garage apart. I can see the floor in 20% of it now!