Cheap Chinese Tachometer problem

[bulgie]

I added a cheap Hall-effect tach to my benchtop mill-drill, and it has worked properly for a year of light hobbyist usage. It was mounted right next to the Fwd-Off-Rev switch, so I just ran a couple leads right from the hot side of the switch to power it, so it was on whenever the breaker to the mill was on. The mill uses 220V and this tach says 120-240V on it. It worked fine on 220V.


(The LED display is actually bright and easy to read in real life, but the camera didn't capture it.)

Now it doesn't work, and the only thing that's changed is I'm powering it with its own cord plugged into 110 V. Well also the glued-on magnet flew off, who knows where, so I replaced it, but it's a good strong rare-earth magnet — getting two of them unstuck from each other is pretty difficult. I think it's the same kind of magnet used previously.

Now it reads HHHH which I think means the signal is above the High threashold, but then it blinks some random and way-too-high number like in the 6000s to 9000s briefly, never the same number twice, then goes back to HHHH.

Any ideas? I think the answer is "don't buy cheap Chinese crap" but unfortunately I have modified the front cover of the mill to fit this tach, and I don't want a big empty space where it was. Plus I want a tach!

I tried the magnet both up and down, in case polarity matters (I think it does) but same results. I tried tweaking the air gap, from actually ticking as it goes around, to a big gap like 5 mm, symptoms don't change.

Anything else to try before I ditch this junk? Oh putting it back on 220V isn't practical, the mill is now on a VFD, which is mounted pretty far away. The motor is 3-phase now of course.

Got any suggestions for a decent tach that'll run on 110V? I prefer that over the ubiquitous ones that need a 12V wall-wart, but I'd settle for 12V if it'll fit in the hole I made in the front cover of the mill. Honestly I'll probably just buy another identical one since it'll fit. Much as I hate to reward companies that make disposable crap.

Thanks,
Mark in Seattle

 

[markba633csi]

The hall effect sensor probably failed- a momentary voltage surge might have taken it out
This stuff is so delicate it's hard to depend on it. They use the cheapest parts available.
Eat 'em and smile as Diamond Dave Roth would say

 

[455dan]

Since you already tried reversing the magnet I see you tried changing the clearance to the sensor and watch the led on the sensor to make sure it appears to be triggering consistently.

I had troubles with mine 120-240 VAC model and then ordered a 12 v DC version from another company. Finally got it to work fairly reliably but once in a while it still miss triggers and prints up a number much higher than the spindle is rotating.
Thinking of changing the sensor and power wiring to shielded cabling which I ordered and have received.
My power supply is one of the DC treadmill motor set ups and thinking like a VFD it may be producing some interference to the circuitry.

Good Luck if shielding does not completely cure my china RPM display I may upgrade to something pricier and more reliable as well.

 

[bulgie]

The hall effect sensor probably failed- a momentary voltage surge might have taken it out
This stuff is so delicate it's hard to depend on it. They use the cheapest parts available.
Eat 'em and smile as Diamond Dave Roth would say

Thanks, I have already ordered a replacement, so cheap it's not worth crying over.

Thinking of changing the sensor and power wiring to shielded cabling which I ordered and have received.

Interesting, will look into that if the new one gives problems

 

[mksj]

That is a bad brand, when I used those I had about a 50% DOA, was not the hall sensor it was clearly the display. Hall sensors are magnet polarity specific, so only one way works well and the LED light on the hall sensor will light when powered and you run the magnet across the face of the sensor at about 1/4" away. I use ones by Digiten, but under different brand an sellers but has the same face.

Amazon.com

 

[RJSakowski]

Since this symptom began when you change to a VFD and 3 phase motor, I would suspect electrical interference. The Hall effect sensors srequire the correct magnet orientation and since you experienced the same symptoms with both magnet orientations, it points to outside interference. The extraordinarily high readings along with the HHHH display would be consistent with electrical interference. With the motor off and tach on,pass a magnet close to the sensor to see if you are getting a reading. One pass/sec should give you a 60 rpm reading.

Eliminating electrical interference can be tricky. Shielding wires, installing a choke, separating wiring, etc. can help. It is possible that you are experiencing interference through the power lines. Another possibility is magnetic interference since Hall effect sensors are fairly sensitive.

 

[silence dogood]

Does the power strip that your tach have a surge protector built in? If not, get one. A lot of cheap electronics fail because of voltage spikes. Every time you turn the motor on your drill press, there is a voltage surge. It was obvious with the old incandescent light bulbs. There would be a quick dimming and then the lights go back to normal brightness. What you don't see is a voltage spike after the dimming that could be double the normal voltage.

 

[bulgie]

That is a bad brand, [snip] I use ones by Digiten, but under different brand an sellers but has the same face.

Amazon.com

Good to know. Unfortunately that one you bought has smaller outside dimensions, won't fit the larger "window" I put in the front panel.

 

[bulgie]

Since this symptom began when you change to a VFD and 3 phase motor, I would suspect electrical interference. The Hall effect sensors srequire the correct magnet orientation and since you experienced the same symptoms with both magnet orientations, it points to outside interference. The extraordinarily high readings along with the HHHH display would be consistent with electrical interference. With the motor off and tach on,pass a magnet close to the sensor to see if you are getting a reading. One pass/sec should give you a 60 rpm reading.

Eliminating electrical interference can be tricky. Shielding wires, installing a choke, separating wiring, etc. can help. It is possible that you are experiencing interference through the power lines. Another possibility is magnetic interference since Hall effect sensors are fairly sensitive.

Unless the interference caused a permanent malfuntion, I doubt that's it. The head reads HHHH even when the mill and VFD are powered off.

 

[bulgie]

Does the power strip that your tach have a surge protector built in? If not, get one.

Good advice. The unit was not on a power strip back when it worked, but then I put a 120V cord on it and plugged it into a surge protector. It was never on 110 V without a surge protector. So if it failed from a surge, I would think it had to be back when it was on 220V. Unless this surge protector is no good — I know they get "used up" and I don't remember how old this one is, though it's a quality brand.