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dalee100
02-27-2018, 10:50 AM
Hi,

I have a home built wiper motor table power feed on my G0704 mill. It has worked perfectly for the last 4 years. Suddenly, I was getting intermittent failures from the 12VDC/6A power brick I was using. Turns out, on inspection of the internals, there is a cracked solder trace on the out going power. So I tossed it and replaced with a new power brick of the exact same specs.

New power supply, but still no go. So I grab a meter and start looking. I have proper 12VDC power going into the PWM control, (and through the on board 10A fuse), I use to control feed rates. But no power showing through the PWM as checked at the Motor out terminals. So I grab another brand new PWM off the shelf, (all spec'ed the same). Same issue. I have tried 3 different brand new PWMs all with the same exact results. Power in - no power out............

As a simple test, I ground hopped the motor by wiring directly to the power brick. Works perfectly.

I believe the flakey power supply killed the original PWM. Having a replacement one be bad right away? Yep. Second one? OK, fits my luck. But three in a row bad? Even my luck can't be that bad, can it?

I assume that my meter should show voltage coming out of the PWM as I turn the Pot up and down right? Is there something else I'm missing and should check?

RichR
02-27-2018, 12:00 PM
But no power showing through the PWM as checked at the Motor out terminals.
Is the speed set using an external pot? If so, is it supplying a control voltage?

Paul Alciatore
02-27-2018, 12:23 PM
OK, three active things in the circuit (power supply, PWM control, and motor); the rest is wires and switches. But anything can go bad: I have seen it all.

Power supply is OK if it will run the motor.

Motor is OK if it runs. Assuming no smoke or excessive heat.

That leaves the PWM control and the wires, switches, and that speed control pot. You have tried three (four?) PWM circuits. Were they all the same model? Did you buy them at the same time? If so, they could all have the same problem.

Check the wires. Are all the joints good? Solder joints? Screw joints? Crimp on connectors or terminals? You could wire around just the switches, one at a time if there are more than one. You could try replacing the control pot. You say there is no output from the PWM circuit. Where did you measure that? I would have been on the board's output terminals.

Last thought, it could be a short in the motor that causes it to draw more current than the PWM circuit can provide. So the PWM circuit may be shutting down as it is designed to do with excessive current. You could measure the current that the motor draws with your direct connection to it. Any VOM should be able to do this. Compare that to the motor's specs.

It's gotta be something.

BTW, why throw out a circuit with one bad solder joint? I would have whipped out the soldering iron and had it back in service in a minute or two.

I learned a long time ago that electronic troubleshooting is 99% persistence.

Dave C
02-27-2018, 01:42 PM
I'd be looking at the pot, depending on where it is in the circuit. Especially if it controls input to the PWM.

MaxHeadRoom
02-27-2018, 04:40 PM
One way of checking the PWM is substitute the motor for a suitable lamp load.
To check the motor, run it on an automotive battery.
What is the origin of the PWM controllers?
They can vary from $5.00 ebay to KB and T.M. types.
These have current limit pots and circuitry.
Max.

J Tiers
02-27-2018, 08:34 PM
Put your meter on the ground and the "wiper" terminals. Turn pot. See if you get a variable DC coming out of the pot/

If not, pot may be the problem, check again with no other connection to the wiper terminal.

If that is good, the PWM may have a problem. Check by connecting the input terminal directly to the power that goes to the CW terminal of the pot. The PWM should be at max. If not, then problem is either the PWM, or the power coming in (not coming in, really).

dalee100
02-27-2018, 09:09 PM
Hi,

Ok, got some more time after work here to do a bit more looking.

First, the PWMs were purchased over a space of a year or two off of eBay. These are one piece units with the pot as part of the PWM. They are rated/fused at 10A. They are the finest of Chinesium - as a lot of this stuff is. To my eye, they are pretty neatly assembled, but that doesn't mean much for quality I know. They are all similar to this unit https://www.ebay.com/itm/12V-40V-10A-PWM-DC-Motor-Speed-Controller-with-Knob-LW/182304262165?epid=1175122397&hash=item2a722e4815:g:94sAAOSwPCVX9dHY

Paul A, I have checked and rechecked my connections. My first thought too. Tried with crimp terminals and bare wire and even new wire. I checked the current draw as you suggested. I got 1/2A with no load. Well under what the power supply and PWM is supposed to handle. I checked the voltage out directly at the motor terminals on the PWM, it reads zero voltage while the power in has 12 volts. The only switch in the system is mounted after the PWM and is a three-way for change of direction and the motor off. Switch checks out OK with the ohm meter. In any case, the is no power making it that far. I tossed the old power brick because the cracked trace is under a translucent sheeting covering the bottom of the board. I never have any luck fixing that.

J Tiers, I don't get any power to the pot. It doesn't seem to make it that far into the PWM. The ohm meter shows the pots are working correctly though as I turn the knobs up and down. I can trace 12VDC going through the 10A glass fuse and that's it. But, I don't really have a good grasp of how it goes through the rest of the board. Perhaps I really do have 3 bad PWMs in a row.

J Tiers
02-27-2018, 09:40 PM
If there is no DC voltage across the pot, then there is a problem with the power in or the PWM.

It appears there is a constant current device that is used as a limiter going to the zener voltage regulator for the chip power supply. That is the device on the PWB marked "2H1002". You might try to measure voltage from that (try both ends) to the circuit negative (the non-fused connection).

If no voltage on one of the ends, then there is a problem in the PWM board. The other end goes to the incoming power, and should have voltage.

MaxHeadRoom
02-27-2018, 11:16 PM
Those Ebay versions are fairly simple, circuit wise and are based on the popular 555 IC.
Max.

dalee100
02-28-2018, 03:41 PM
Hi,

OK, after rummaging up yet a 4th PWM of used and uncertain lineage, my power feed motor now works. It's a different style with a remote pot so I will need to make a new enclosure for it.

Either something was bad in the circuit boards of the other 3 I tried, or they are not happy with the power supply I used.

danlb
02-28-2018, 04:01 PM
Just for grins, did you have a way to check the DC supply on a scope of any kind? Many power bricks are poorly filtered. If you have a lot of ripple I can see it causing a problem for the PWM circuit.

Dan

dalee100
02-28-2018, 04:08 PM
Hi,

No I don't have access to a scope. And I'm not smart enough to use one anyway. Well, there was that time I use one at work as a brain wave tester...............:p

I suspect you might be one to something. I do know cheap generic power bricks are amazingly crude.

danlb
02-28-2018, 04:14 PM
You can always use a car battery as a temporary test supply. You KNOW that's a nice clean DC source. :)

bob_s
02-28-2018, 06:45 PM
You've got a computer.

Does it have a sound card?

If so download a free copy of WINSCOPE. Should be sufficient capability to check out a DC power supply and PWM controller!

dalee100
03-01-2018, 11:53 AM
You can always use a car battery as a temporary test supply. You KNOW that's a nice clean DC source. :)

Hi,

We have a winner!!!! And I have learned something valuable, (even better yet!).

I ran a quick test as you suggested this morning. Dug out my 12V battery I use for my clay target thrower, (lawn mower type). And low and behold, all three PWMs work just fine with "proper" 12VDC power. Motor runs smooth and easy.

I guess they hate my cheap power brick. I do wonder what makes the old crusty one tolerant of the cheap brick?

MaxHeadRoom
03-01-2018, 01:42 PM
wrong edit

danlb
03-01-2018, 02:38 PM
Hi,

We have a winner!!!! And I have learned something valuable, (even better yet!).

I ran a quick test as you suggested this morning. Dug out my 12V battery I use for my clay target thrower, (lawn mower type). And low and behold, all three PWMs work just fine with "proper" 12VDC power. Motor runs smooth and easy.

I guess they hate my cheap power brick. I do wonder what makes the old crusty one tolerant of the cheap brick?

Good to hear that that worked. Ripple can wreak havoc on simple timer circuits if the voltage dips too low.

Your older PWM controller may have capacitors (and possibly coils) that function as a ripple filter. If you look at a laptop motherboard you will often find capacitors right next to the power jack.

J Tiers
03-01-2018, 02:50 PM
Odd that it worked before, but power problems certainly cause issues.

Now you get to see what the problem is.

Quite possibly there is something about the new ones that trips the shutdown on the "Power brick". If the new PWMs have less capacitance, then power pulses to the motor can trigger a short circuit protection event. Might be as simple as adding capacitance.

Tale a look at the old and new to see if there is a large difference. The pictures in the link seem to show 330 uF, which is not a lot for a device rated for several amps current.