RPM of small DC motors

[John Riley]

I have an automotive DMM with an inductive clamp to clamp a spark plug lead
to measure RPM of an internal combustion engine.
I want to measure the RPM of some small DC motors with and without
gearboxes. Range up to about 3000 rpm.
I was wondering what would be the simplest way to do this. Perhaps a tiny
strong magnet attached to a tooth of the sprocket I want to measure?
I was thinking about a small coil near the spinning magnet and clamping the
wire in this continuous loop, but then I wondered if I set the sprocket up
to rotate between the open jaws of my DMM's induction clamp, would this
likely give me a result? I realise that I should try this to see, but as I'm
not able to get to an electronics shop to get magnets etc for several days,
I want to try to find out more of what I should buy when there. Someone
suggested a Hall Effect switch, but I have no idea how to set this up for my
meter to read RPM. Thanks in advance for any help in this area.

 

[BobG]

If you have a scope (most new pc usb scopes have a freq button!) the
opto technique cant be beat. Even with an old scope, the period is div
x time/cm

 

[Chris]

John Riley wrote:

I have an automotive DMM with an inductive clamp to clamp a spark plug lead
to measure RPM of an internal combustion engine.
I want to measure the RPM of some small DC motors with and without
gearboxes. Range up to about 3000 rpm.
I was wondering what would be the simplest way to do this. Perhaps a tiny
strong magnet attached to a tooth of the sprocket I want to measure?
I was thinking about a small coil near the spinning magnet and clamping the
wire in this continuous loop, but then I wondered if I set the sprocket up
to rotate between the open jaws of my DMM's induction clamp, would this
likely give me a result? I realise that I should try this to see, but as I'm
not able to get to an electronics shop to get magnets etc for several days,
I want to try to find out more of what I should buy when there. Someone
suggested a Hall Effect switch, but I have no idea how to set this up for my
meter to read RPM. Thanks in advance for any help in this area.

Hi, John. If you need to measure with an inductive clamp, you're
probably going to need at least 100mA, and possibly twice that, going
through that clamp in order to get a reading.

Also, a magnet will be an eccentric load, which might not be great for
your motor bearings.

The best way to do this might be to make a small card stock disk with a
good sized slot in it, mount it to the motor shaft, and use an
opto-interrupter to drive a high current transistor switch. You can
then put the inductive clamp around the high current wire, and you're
good to go, without having to wait for the magnets. Of course, you
might be able to get a fast hall effect switch to drive a high current
transistor switch, too.

To make it simple, order an H21LOI opto-interrupter (Mouser 512-H21LOI,
$1.89 in stock) and a TIP105 PNP Darlington transistor (Mouser
511-TIP105, $094 in stock) and use the 9VDC@0.5A or greater wall wart
you've avoided throwing out. Put together something like this (view in
fixed font or M$ Notepad):

|
| VCC
| +
| |
| .-. VCC
|470| | +
| | | |
| '-' .-. VCC
| | VCC 1K| | +
| '-------. + | | |
| 1| |3 '-' |
| | | \ ___ | |<
| V ~ | H )--|___|-o--| TIP105 (PNP Darlington)
| - ~ |__/ 4 1K |\
| 2| |5 |
| | === |
| | GND | Inductive
| === |<---Pick-off
| GND | Here
| |
| .-.
| | | 33 ohm
| | | 2 watt
| '-'
| |
| ===
| GND
|
|
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

Just have the wire extend off your protoboard, and clamp around it.
Start out with a small slot in the disk, and open it up if it's too
brief a pulse for your meter at higher speeds.

The H11 and H21 series of optocouplers is a great choice for contrivers
of contrivances, because they have built-in hysteresis to square up
iffy signals, and open collector outputs that can easily sink 10mA if
you need it. Be sure to use a pullup if you require logic level, just
like with any open collector output. These ICs work on a 5 to 15V
supply.

Good luck
Chris

 

[John Duffus]

Hi John,
My first inclination would be to use a stroboscope if you can get your
hands on one. I see some on eBay.

 

[D. Akers]

John wrote:
"I have an automotive DMM with an inductive clamp to clamp a spark plug
lead to measure RPM of an internal combustion engine. I want to measure
the RPM of some small DC motors with and without gearboxes. Range up to
about 3000 rpm." <snip>
______________________________________
Re;
Just a reminder: if you do get the automotive "inductive-spark" tach to
work, don't forget you'll need to divide the indicated RPM by two to get
the actual motor RPM since 4 cycle engines only spark on every other
revolution.

Good Luck,
Dan Akers

 

[Phil Allison]

"D. Akers"

Just a reminder: if you do get the automotive "inductive-spark" tach to
work, don't forget you'll need to divide the indicated RPM by two to get
the actual motor RPM since 4 cycle engines only spark on every other
revolution.

** Err - that would be *multiply* the indicated rpm by 2.

Plus beware - some 4 cycle engines spark every revolution.



........ Phil

 

Sell:
China Shenzhen ZHAOWEI Machinery & Electronics Co. Ltd engages in designing, manufacturing and marketing all kinds of electric motors. They are mainly suitable for the following applications: smart home application used in smart kitchen and laundry, medical instrument for personal care, smart E-transmission applied in automobile, industry automation applied in telecommunication and a great variety of plastic/metal planetary gearbox in different sizes.
In order to develop the oversea market, we are current seeking new partners around the world to create a bright future together. ZhaoWei is a right choice and excellent partnership with sincere services.

Company: Shenzhen ZHAOWEI Machinery & Electronics Co., Ltd
URL: http://www.zwgearbox.com/
Contact: Anny Liu
Tel：+86-755-27322652
Fax：+86-755-27323949
E-mail:sales@zwgearbox.com
Add: Blk. 18, Longwangmiao Industry Park, Fuyong Tn., Bao’an Dist., Shenzhen 518103, Guangdong, China