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Posted: Fri Jul 21, 2006 12:45 pm
by Bob Scott
No, you should not rely on only a GFCI for protection. It in itself does not provide isolation A GFCI is just a safety device that trips if there is a fault or error in wiring. You should not design a piece of equipment with a built-in error, like connecting ground to one side of the AC line.

When you are finished your design, there should be no conductive path from the ground on your computer, or any signal from your computer to either hot or neutral of the AC line. When you connect a the emitter of a transistor or source lead of a power MOSFET to the neutral AC line in order to drive the motor, this neutral should have no path to ground in your design, even though neutral and ground are connected together at the breaker panel or fuse box.

So you either need to use a 1 HP isolation transformer (746 Watts means a BIG transformer) for the motor or build-in some other form of isolation between either the computer's control signal (including it's ground) and the PWM circuitry or the PWM circuitry and the motor driver transistor. Usually this means adding an opto-isolator because this is cheaper and lighter than using a huge transformer.

A GFCI-only method just doesn't cut it, and would not be approved by UL, CSA, or any other industry safety organisation.

Do I sound cranky? It shouldn't be. This hobby is supposed to be fun. IF giving advice to a newbie it should be as accurate as possible. It should not be just plain wrong, like HACKLESUP suggesting that 12V "is not enough to turn off a MOSFET". Or it should not be dangerous that the guy can electrocute himself.

I think newbies should be encouraged to only experiment with electronics that is powered by battery or wall wart, unless they at least get themselves familiar with AC power, like from a federal/state/provincial electrical code book from a hardware place like Home Depot or the local library.

One time I was buying something at Radio Shack. The guy behind me was going to buy a bridge rectifier. I asked him what he was making. He said he was going to directly rectify AC from a wall receptacle, install a large resistor in series, and use it to charge his car battery. I little knowledge is dangerous. I wonder if he survived.

Good luck,


Posted: Fri Jul 21, 2006 5:16 pm
by Robert Reed
I see a lot of "don't go near the water" warnings about so-called high voltage on this post and many others. It was as though anything above 12 volts was taboo and some dark forboding area to be avoided. Voltage is voltage and comes in all different sizes with all different stiffnesses. Every electronic item/project has to be treated uniquely and voltage is only one parameter of design. I started my career with vaccuum tubes and corresponding power supplies of 150-600 volts dc. Some of these were regulated and quite stiff. In my school days,I also serviced radio & TV (tube type) like so many other students. Some of these were AC-DC chassis. All it took to be safe was a little common sense and circuit analyzation before starting out, much like everytime you plop your self down behind the steering wheel of your car. I will admit, that even with precautions I have had a few minor jolts over the years. But let me tell you this - the worst scenario I ever encountered was working on a 12 volt supplied circuit. It was late in a particularly tough day and my concentration wasn't what it should have been. I inadvertently hooked up a large tantalum `lytic reverse polarized and upon power up it let go like a hand grenade spewing aluminum and whatever else everywhere. It narrowly missed my eyes and left me shaking.
All this from a seemingly harmless 12 volt power supply. The cause - lack of concentration and this was certainly just as "dangerous as working with hi-voltage". Bottom line is no matter what your project is, perils lurk - just pay attention and use common sense.

Posted: Fri Jul 21, 2006 7:55 pm
by Chris Smith
Just follow the WOOD’s guide lines.

Quality, high voltage insulation, high quality parts and matched pair parts, ......and the rest is what we live for, doing it right.

There is nothing complex about a WOOD’s controller, and precaution should exist at every turn, anyway.

Precise parts, quality parts, and matched parts is their success story.

Cost about 5 bucks more.

Posted: Fri Jul 21, 2006 9:12 pm
by Bob Scott
Robert Reed wrote:I see a lot of "don't go near the water" warnings about so-called high voltage on this post and many others. It was as though anything above 12 volts was taboo and some dark forboding area to be avoided.
Robert, that is why there are warning labels stuck on all kinds of equipment to warn people of hazards. Surely you are not criticizing warning labels?

Caution, Hazardous voltage. No user servicable parts inside.
Caution, fan.
Do not look into laser with remaining eye. ;-)

Hazard warnings are a good thing and in most cases required by law. They are used to prevent injury or death, which is a lot worse than being bored by warnings. I have never seen a warning label like "HIGH VOLTAGE, NO PROBLEM. JUST USE COMMON SENSE."

Warning a newbie about directly connecting a grounded piece of equipment to a non-isolated AC source is a good idea. Not doing so would be irresponsible.


Posted: Fri Jul 21, 2006 9:29 pm
by Robert Reed
Doesn't reading warning labels fall under the heading of "Common Sense".

Posted: Tue Jul 25, 2006 8:27 pm
by connect21
Why not use IGBT's?
I built a circuit to "try" to control a large transformer using IGBT's and so far I was able to control 120 VAC rectified into 170 VDC, filtered and then chopped using an IGBT bridge and I tested the circuit drawing 10 Amps. and it worked. I have the control part of the circuit that works at 5 V. and 15 V. and the power part of the circuit at 170 Vdc is optically isolated from the low voltage part.
I also did fry many IGBTs, many parts from the control circuit, a motherboard and a power supply on my computer.

Posted: Tue Jul 25, 2006 9:46 pm
by dyarker
It's not so much "don't go near the water", as "don't go near the water with a cement block tied to your neck". The reason we're still here is we used proper precautions. Working on a hot chasis while standing on a wet floor is not an evolutionary trait. Those that do that kind of thing aren't here any more.

With new folks coming on the forum all the time, I think repeating warnings is a good thing, even if it bores the experienced hands. An out-right "don't do that" should be reserved for a project that will be dangerous to use after it is completed.

Looks connect21 learned the hard way, and got lucky, only killing a computer and some transistors. It is good that his project works. I just hope that all the live parts (like the transistor cases with 170V on them) are enclosed so they can't be touched by accident.

Projects involving 120VAC and/or high DC voltages are some of the most useful. Not explaining the hazards to beginners is NOT being helpful, even if you come off sounding negative.


Posted: Wed Jul 26, 2006 6:03 pm
by Chris Smith
IGBTs are great if you understand whats going on.

They are protections for what WE do wrong.

Posted: Thu Jul 27, 2006 9:46 pm
by dyarker
An IGBT (Insulated Gate Bipolar Transistor) is a low power FET combined with a power BJT in Darlington type arrangement. There is NO isolation (relative to 120VAC safety) between Gate and Emitter/Collector. The signal to the Gate has to share a common with power on BJT side.

Posted: Mon Jul 31, 2006 4:46 pm
by Chris Smith
"Insulated Gate" tells the whole story of what WE do wrong.

The same reason most electronics fail.

Zap and its all over.

The history of the gate is easy to follow, first it was just a gate, then they diode protected it one way, then they double protected the gate both ways, and finally after a long time the IG was formed.

Same goes for the "Gate Driver" now offered on many FETS, people just don’t understand enough to make them run properly.

Were losing our technological edge.

Posted: Fri Aug 04, 2006 2:27 pm
by Deal
Most PWM controlled fractional DC brush motors are controlled by potentiometers over gated thyristor boards. These are usually small brush fractional horsepowers like something controlling the speed of your grill rotisseri. Middle Powered 90 VDC motors are usually four wire Field armature types and are often geared. Start with the design of the motor and then work backwards to what your computer needs to control. Lots of sources. Check Grainger for tons of info on the different motors. I'm guessing your treadmill motor has four wires (two armature, two field). The cadillac of fract horse motors is Bodine (ball bearings, bronze -not plastic or zink- gearing). Check their site for some circuits. I have delt with this family of DC motors for many years suspect you have an easy soultuon.

90 volt dc pwm

Posted: Tue Aug 08, 2006 9:26 am
by zotdoc
There was a good article in home shop machinist, May/June 1996 issue on a shop built controller by a man named Charles Eyer. I have a copy and eventually will scan the darn thing. I can send you a copy if you would like.

Posted: Thu Aug 17, 2006 3:47 pm
by jwax
icestorm- How's your project coming?
I found a similar device- 90 VDC, 10 amp max motor and the PWM control board with the MOSFET toasted. I have a replacement on the way, but I wonder if you figured out how the touchpad on these things control the motor speed.
"Treadmill Duty" is another marketing mangle- this one is stamped "2 HP". If they get that out of 90 VDC and 10 amps, I believe they've broke some new bullshit barrier.

Hi - got similiar problem

Posted: Tue Aug 22, 2006 6:05 pm
by seker
about yr ago got a motor controller (also with transformer and 90vdc treadmill moto) they all worked - I recently got a 130vdc DC motor (and wonder if that would even work - not work properly or harm motor since diff on voltage)
Unfortunately dont have schematic on how is to be set up to try out - and getting majorly confused on proper 'connection'
- there is cord - 15 volt breaker - transformer - motor controller - and motor (motor is low rpm- 139 high torque-31 in lbs)
What is really throwing me is the order should be
- seems like should be more than 4 blades on motor controller - AC 1 AC 2 and A+ A- (and looks like A+ is more connected on board to A1 & A2 than A-)
seems like it was power from swich for ac - and transformer (?) for A+ tho cant be other end of transformer for A- since is same blade type (not to mention motor is 'left out')
And where I am at no one knows either (like to at least know if
motor works)

Somehow there must be 'sharing' of wires as in a series(another brand had same ones but also had blades on controller for transformer - have more things needing to be hooked up than seem to have places to put
- makes sense is that needs to be a series on at least 2 of em
(know that I had one of transformer on A+ blade tho other blade's are both male)
without knowing the 'order' that should go am lost (may be missing some wires esp for switch since when do try motor ends up 'left out'

May need to get motor controller that is made for 130 volt motor anyway -

Apprecite any assitance since I am getting more lost more I mess with it (been real nice if had left everything marked way back when)

Posted: Tue Aug 22, 2006 6:40 pm
by jwax
Are there any identifying numbers on the motor or controller?
Controllers will have a pair of "B" terminals (Battery, + & -), and a pair of "A" terminals for the motor (Armature, + & -). Can you submit a picture of what you have to help us identify it?