My variable speed drill switches turn into ON/OFF switches :-(

[Bruce Ratcliffe]

TL;DR Summary

My Bosch 25Amp, 24 VDC, drill switches keep buring out. Is there a way to include a capacitor, or inductor in the circuit to smooth out voltage so switches last?

My students build "eCars" that use wheelchair motors and two 12 volt AGM lead acid batteries in series. A pair of Bosch 25Amp, 24 VDC, drill switches control power to each wheel. Tons of fun (see here):

The switches last in some cases for years, but quite a few have burned out the "variable" part of the circuit, and are now just ON/OFF switches. I imagine voltage spikes are involved. Is there a way to include a capacitor, or inductor in the circuit to smooth out voltage so switches last?

Reference: (reference link broken)

 

[jedishrfu]

Might find something in this video:



There was some suggestion to get a heftier mosfet for the circuit.

 

[Baluncore]

The motors are inductive, so will produce voltage spikes when the current is interrupted, probably due to the brushes in the motor bouncing. One or more diodes might be added to the circuit, to catch the inductive high voltage spikes, before they can damage the speed controller electronics.

 

[Bruce Ratcliffe]

AH! I've seen diagrams with a diode that does that exact job. That would be much easier than sticking a capacitor in there. My problem now: I'm not educated enough in diodes to know what size, what polirity, and where in the circuit it should go. If you could "educate" me in that regard, I'd be most grateful!

Bruce Ratcliffe

 

[Bruce Ratcliffe]

AH! I've seen diagrams with a diode that does that exact job. That would be much easier than sticking a capacitor in there. My problem now: I'm not educated enough in diodes to know what size, what polirity, and where in the circuit it should go. If you could "educate" me in that regard, I'd be most grateful!

Bruce Ratcliffe

Dear Baluncore, I went to my goto source (Youtube) and found a video that addresses my concern. The video what quite educational, but I'm still unsure exactly WHERE to solder in the diode. My guess would be to connect it across the wires going to the motor, with the negative pole of the diode connected to the positive lead going to the motor. I have a bunch of big, beefy-looking diodes (10A10) and some more modest (MUR4109427) diodes in my shop. Of course, considering what a cheap fix this would be (compared to$70 per switch--when I can even find some for sale), I'd be glad to purchase whatever you recommend. If so, that would be an easy "operation"--no need to pop open the switch, which is quite a trick. Want to correcet my exam answer, Teach'?

 

[Baluncore]

If you could "educate" me in that regard, I'd be most grateful!

We do not know what type of controller or motor you use.
For example, does your motor have brushes?
Can you drive the field and rotor currents independently?
We need makes, model numbers, and links to the web data.

 

[Bruce Ratcliffe]

We do not know what type of controller or motor you use.
For example, does your motor have brushes?
Can you drive the field and rotor currents independently?
We need makes, model numbers, and links to the web data.

AIE! I've been having my kids build these "eCars" for 20 years, but I never looked closely at tahe motors. I can tell you there are out of wheelchairs. There are large curved permanent magents on the insides of the motor housing and there are brushes on opposides sides of the motor housing with spring-loaded carbon brushes contacting the motor shaft on the inside. If this is not enough information, I'll dissect a motor to find out more detail.
Bruce

 

[Baluncore]

Do you use forward and reverse, or always the same polarity and rotation direction?

The diodes primarily need to be fast recovery; rated at two or more times the battery voltage; for the average motor current, the sort of thing you can take out of an old computer power supply. To catch spikes, the diodes do not need a heat sink as used in the computer PS.

Take a look at the specification plate on the motor. Maybe there is a part number and manufacturer's name.

 

[Bruce Ratcliffe]

Do you use forward and reverse, or always the same polarity and rotation direction?

The diodes primarily need to be fast recovery; rated at two or more times the battery voltage; for the average motor current, the sort of thing you can take out of an old computer power supply. To catch spikes, the diodes do not need a heat sink as used in the computer PS.

Take a look at the specification plate on the motor. Maybe there is a part number and manufacturer's name.

Here is the spec plate on the momtor. NOw that you mention it, it DOES have forward and reverwse modes. This would seem t make problems with a diode--wouldn't 1 of the 2 diodes always be condecting current AROUND the motor?

 

[Baluncore]

This would seem t make problems with a diode--wouldn't 1 of the 2 diodes always be condecting current AROUND the motor?

Find or make a 4-terminal rectifier bridge from 4 diodes.
Connect the bridge close to the controller.
The + terminal to battery+.
The - terminal to battery-.
The AC terminals to either side of the motor.
Maybe connect a ceramic capacitor across the B+ and B- terminals of the bridge.
Current will not flow in the bridge, except during overvoltage spikes, when it will recover the energy.

 

[Bruce Ratcliffe]

Excellent! I have a box full of big, beefy FWBR's! What value on the ceramic capacitor would you recommend?

 

[Baluncore]

0.1uF should catch and slow down the spike on the battery leads.

 

[BRUCE A RATCLIFFE]

This is great. After having soldered the connections for, oh, 15 years, your advice has prompted me to do a redesign that is much simpler and, I think, more elegant. I'll show you the BEFORE and AFTER pictures when I get a prototype completed. I do have one more question. What gauge wire should the connections between full wave bridge rectifier and motor be? I ask because the workspace is quite cramped inside the plastic SKIL drill handles that I use. If I can get by with a gauge lighter than 12 AWG stranded, that would make the assembly simpler. Thanks for all your help,
Bruce Ratcliffe

 

[Baluncore]

You can get away with lighter wire if you can keep the connections short. An alternative would be to place the bridge where the thicker wires are all together, closer to the motor, but outside the handle.

Have you forgotten your password?

 

[BRUCE A RATCLIFFE]

Thanks for your quick reply. The FWBR-to-motor distance is ~3 inches. For that distance, would 16 or 18 gauge stranded allow enough current during the voltage spike? Of course, the actual motors are a couple of feet away, along a 12 AWG stranded wire. The motor in the phrase FWBR-to-"motor" is actually the male spade connectors on the switch (see attached).
Bruce

 

[Baluncore]

Three inches is not a problem. Without all the layout details, it is difficult to decide. I think you will have to find the easiest way to keep the leads to the bridge short, and see if that makes a difference.

 

[BRUCE A RATCLIFFE]

Sounds good. I'm on it!
Bruce

 

[berkeman]

Thread closed temporarily for Moderation...

 

[berkeman]

Update -- Thread is reopened. Turns out that Bruce had a problem with his PF account, and the result is that he temporarily has two PF accounts with different capitalization of his name. Those accounts will be merged in the near future.

 

[Bruce Ratcliffe]

AH! Glad to hear the good news!
Thanks for your help.
Bruce

 

[Nik_2213]

Been a few years since I played with 'bots but, IIRC, my 'trad' motors with brushes just kept finding better ways to kill their control circuitry.

Directly across the brushes, I had well-rated uf capacitors...
Flanked by pf for their superior ESR speed...
Flanked by hefty back-to-back Zeners rated about 150% of supply voltage for spike-topping.
Plus, hefty, fast 'protective' diodes across the electronics.

One 'amusing' trick was to wire a small neon bulb ( >60 V ?) across a nominally suppressed 'brush' motor and watch it flicker...

Hence the need to have spare motor-control boards to hand...

D'you wonder that brushless / stepper motors were gleefully embraced, despite their drivers' complexity ??

 

[BRUCE A RATCLIFFE]

I've built the prototype but now realize a problem: Does it protect my switch? I thought I'd simply connect an oscilloscope across the motor and check for a spike on the trace, but can see nothing. I'm embarrassed to say I never bothered learning much about how to use oscilloscopes. Perhaps they can reveal a difference between old circuit and New! Improved now with FWBR Technology! version. I've attached 3 pictures, one of a really cool tool just recently invented called an oscilloscope : ), and two of the prototype. I'm also thinking of 2 other ways to detect spikes, and the lack thereof.
1) My clothes iron has a small, orange light to show power ON. I believe those neon bulbs light with ~70 volts. If the spike is that high, it might light that lamp.
2) I am a big fan of Dave Vernier's Lab Quest 2 for graphing voltage, pressure, temperature, CO2, sound, and a whole lot more. I'm going to call Vernier and see if its response time is quick enough to show the spike.

Learning lots, especially how much I don't know,
Bruce

Been a few years since I played with 'bots but, IIRC, my 'trad' motors with brushes just kept finding better ways to kill their control circuitry.

Directly across the brushes, I had well-rated uf capacitors...
Flanked by pf for their superior ESR speed...
Flanked by hefty back-to-back Zeners rated about 150% of supply voltage for spike-topping.
Plus, hefty, fast 'protective' diodes across the electronics.

One 'amusing' trick was to wire a small neon bulb ( >60 V ?) across a nominally suppressed 'brush' motor and watch it flicker...

Hence the need to have spare motor-control boards to hand...

D'you wonder that brushless / stepper motors were gleefully embraced, despite their drivers' complexity ??

Thanks for the thoughts, Nik.
I'm not well-enough educated to understand some of your terms (i.e. IIRD, pf for their superior ESR speed, fast protective diodes across the electronics) but I DID, at Baluncore's suggestion connect a 25 amp full wave bridge rectifier across the motor wires. I then thought to compare the light from one of those small neon bulbs rated, I hear at ~70. I put it across the motor wires and tried one test WITHOUT the FWBR, and one WITH the FWBR. I was able to get a light from the leon bulb hooked up to an inductor where I suddenly cut the power. It flashed at 12 volts, but actually flashed down at 3 volts. So I was prepared to see the improvement my FWBR provided. Imagine my puzzlement when the neon bulb NEVER FLASHED, with or without the FWBR. I'm wondering if it has nothing to do with induced EMF when the switches suddenly turn off. Maybe just too much power for the switch to handle? It says "24V, 25A" right on the switch. I've ordered these MOSFETs:

5Pcs Mosfet IRL2505PBF IRL2505 60V 100A TO-220 N-Channel Mosfet which are way more than the rating ("just" 60A on the MOSFET in the switch I have the keeps blowing. Do you think that might help?

Bruce

 

[Nik_2213]

Such spikes may be too narrow for scope to catch, especially if 'digitally sampling' type connected to PC...
Similarly, a neon bulb will require a combination of spike-width and height to light bright enough to spot...

 

[Baluncore]

1) My clothes iron has a small, orange light to show power ON. I believe those neon bulbs light with ~70 volts. If the spike is that high, it might light that lamp.

For fast pulses, a neon globe, NE-2, depending on Ar content, takes about 100 volts to strike, then continues to glow at about 70 volts. In general, flyback pulses will destroy semiconductors before the Ne+Ar gas conducts.
https://en.wikipedia.org/wiki/Neon_lamp#Voltage_surge_suppression

One problem with testing is that, worn or faulty brushes in the motor provide an intermittent arc or spark gap. Testing with a good motor will not demonstrate the situation that destroys controllers.

Maybe connect a small audio speaker across the motor, in series with say 0.1 uF. You will hear the bad brush contacts scream, hopefully before the controller is damaged.

 

[BRUCE A RATCLIFFE]

For fast pulses, a neon globe, NE-2, depending on Ar content, takes about 100 volts to strike, then continues to glow at about 70 volts. In general, flyback pulses will destroy semiconductors before the Ne+Ar gas conducts.
https://en.wikipedia.org/wiki/Neon_lamp#Voltage_surge_suppression

One problem with testing is that, worn or faulty brushes in the motor provide an intermittent arc or spark gap. Testing with a good motor will not demonstrate the situation that destroys controllers.

Maybe connect a small audio speaker across the motor, in series with say 0.1 uF. You will hear the bad brush contacts scream, hopefully before the controller is damaged.

For fast pulses, a neon globe, NE-2, depending on Ar content, takes about 100 volts to strike, then continues to glow at about 70 volts. In general, flyback pulses will destroy semiconductors before the Ne+Ar gas conducts.
https://en.wikipedia.org/wiki/Neon_lamp#Voltage_surge_suppression

One problem with testing is that, worn or faulty brushes in the motor provide an intermittent arc or spark gap. Testing with a good motor will not demonstrate the situation that destroys controllers.

Maybe connect a small audio speaker across the motor, in series with say 0.1 uF. You will hear the bad brush contacts scream, hopefully before the controller is damaged.

Interesting Idea, Balancore. I will brave the 110F heat on my bike to head down to my classroom to try it with the speaker. I'm assuming that if I can hear a click, it will be a quieter click when I test the FWBR-equipped switch.

 

[Bruce Ratcliffe]

Dear Baluncore,
I was daunted by the 110 F yesterday, so stayed home, which allowed me to finish creating a solution to TOO HOT:


Well, I've just completed the first FWBR addition to one of the drill swithes. Each eCar is operated by a driver that brakes, accelerates and steer by controlling each drive wheel separately using the repurposed Skil Drill:



(the eCar action starts at time stamp 0:29). So I was able to add the FWBR to just ONE drill and then check using your speaker spike detector. The click only occurred with the CONTROL--the TEST drill was silent. It summa: IT WORKS!!!!!!!!
I forgot to add the capacitor in series, and got a very loud click from a small speaker. I'll add that cap as I work through all 10 of the eCar double drills.. Thank you SO much for your patience and expertise. It can't tell you how good it feels to not have to worry about blowing out drills during the annual Race Around the School.
Bruce Ratcliffe

 

[Baluncore]

I forgot to add the capacitor in series, and got a very loud click from a small speaker. I'll add that cap as I work through all 10 of the eCar double drills.

A 24 volt DC click, to a small speaker, might remove the coil from the cone.

It can't tell you how good it feels to not have to worry about blowing out drills during the annual Race Around the School.

The placebo effect can be very powerful, if you know what you really want.
Experience will tell if the patch is sufficient.

 

[BRUCE A RATCLIFFE]

I tried the 0.1 uF (ceramic) cap in series with the speaker, but heard no click on the CONTROL switch. Perhaps a cap that small doesn't let ecnough current through to wiggle the speaker cone? I next tried a ~100uF cap. Silence. Perhaps the surge was so brief the cap could just be filling, but not full when the surge ended? IN any case, I have a LOT of little speakers around. Today I'm going to try using a ~2amp FWBR. the 25 A big boy FWBR is a pretty tight squeeze to fit in the drill handle. Always more to learn,
Bruce

 

[Baluncore]

I next tried a ~100uF cap. Silence.

Maybe the speaker failed the earlier 24 volt DC test.
Maybe the speaker is the perfect snubber.
You can test the speaker and cap value, by scratching the wires on the battery terminals, to simulate faulty brushes.

 

[Bruce Ratcliffe]

The speaker still works. I THOUGHT that I'd figured out how to install the big 25 A FWBR's. I tried a much smaller (~2 A FWBR) and it did NOT work. So I started a production run--retrofitting each of my swithces with the 25A FWBR. The first pair (2 per car) went fine. But when I did the first switch for the second car, it DIDN'T WORK : ) I've check wiring, checked the FWBR's to make sure all four diodes are working inside each unit and am flummoxed. I even tried changing out the FWBR. Here are two videos. The first shows my first success installing the FWBR. You can see the speaker move in and out, but it doesn so smoothly, silently, as one would expect with a variable voltage switch. The second video shows the second unit I worked on. There is the same "click" I got before I installed the FWBR, most easitly detected at the end of the video..
Puzzling, disappointing.

 

[Tom.G]

Your description of 'doesn't work' seems to describe the 80% chance of an 'operator errer.'

Open up the first one you did, the one that works.
Put it side by side to the one that does not work.
Trace and compare every wire, terminal label and part number on each.
80% chance that they are either not identical or not connnected.
Pay attention to polarities.

The other 20%? -- bad component.

(Sorry, no authoritative reference, just many decades of "Arrgh sh*t, what now!")

Cheers,
Tom