Saturday, June 27, 2015

Teardown of Mastech MS8268 multimeter

The advertising for this multimeter stated that it has "warning when incorrect banana jacks are used relative to function switch setting".  I got curious about how the meter detected the banana plugs.  At first, I thought that there is an optical proximity sensors under each jacks, but that was an incorrect guess.  Each jack has a momentary switch at the bottom.  The switch has an plastic plunger, which insulates the detection switch from the banana contacts.

Light pipes for ring indicators around banana jacks visible.
One of the light pipes, which forms the ring around a banana jack, is removed and shown separately on the right.
Red and black plastic parts inside of the banana jacks are the plungers which activate the detection switches.
The IC in the middle is a PIC microcontroller.  I suspect that it's main purpose is to detect leads, and to control the LEDs under the light pipes.

Mastech MS8268 multimeter with back cover removed. Fuse visible.

ICs in this meter, which I was able to identify
I bought this MS8268 multimeter to measure low currents on the order of units of microamps.  The miltimeter didn't disappoint, considering especially a relatively low price.


Mastech MS8268 review
Teardown video (Dec 15)

Thursday, June 11, 2015

Teardown of a paddle wheel flow meter

I'm setting up a control system for a small scale biofuel processing rig. Part of the task is to monitor the flow rate of feedstock. Due to budget constraints, I could not simply buy and industrial grade sensor. So, I'm evaluating this low cost sensor for low flow rate and more viscous fluid [than water].

This type of low-cost paddle wheel flow meter is sold by Adafruit among other places.

Assembled flow sensor before teardown

Cavity is open.  Top of the paddle wheel with the magnet visible inside the cavity.
The gap between the tips of the paddles and the cavity is about 2.5mm .  If the gap were smaller, this flow meter would be more sensitive at low speeds.

The static o-ring, which seals the paddle wheel cavity, is visible on the lid.  The latter also serves as the bottom of the PCB compartment.

At the end of the paddle wheel sits the magnet.  The outside diameter of the magnet is 9.7mm .

PCB compartment is open.  Component side of the PCB visible.

PCB with Hall effect sensor removed from its compartment

On the left side of the PCB stands the Hall effect sensor.  The writing on the IC is "W130".  I'm guessing it's Winson WSH130 (datasheet).  The Hall effect sensor sits 16.4mm from the axis.

The o-ring in this photo prevents liquid ingress into PCB compartment.  This is the 2nd o-ring in this device, and not the main one that seals the paddle cavity.


After this teardown, the reassembled sensor continued to work like new. 


Sensors Magazine article on paddlewheel flow sensors
Higher precision paddle wheel flow sensors made by Omega Engineering


I'm not affiliated with any of the companies mentioned in this post.

Saturday, June 6, 2015

How to make an educated guess about power ratings of common throughole resistors when datasheet is not available

This write-up is intended for beginners, perhaps for those who have logged less than 20 hours of building circuits.  You have a resistor in your hand, and you wonder: "Will it burn in my circuit? How much power can it dissipate?"

Maybe you are in a lab with a kit of resistors.  Somebody was kind enough to put the kit together, but the technical references for the resistors aren't readily available.  The maker of the kit knows the power ratings, but it's after hours, and he's not available.

I remember that in the first practicum of the introductory EE course we were instructed to "sacrifice" a quarter-watt resistor to see and smell how components burn.  The instructor would tell us matter-of-factly: "dial your power supply to 12V, take a 10Ω resistor, now connect the resistor to power supply with alligator clips."  It was a surprise for most students that the resistor immediately burned and smoked.

How much power will the resistor have to dissipate?

P = I2 R = V2 / R, where R is the value of your resistor.  Figure out a way to estimate the current through the resistor, or voltage across it.  Resistance is known.  Calculate power.

(To be more exact, it's VRMS or IRMS.  If the voltage and current approach DC, then the RMS value approaches the DC value.)

Attempt to look-up the power ratings.  Avoid guesswork if you can.

Even though this write-up is about educated guessing; guesswork should be last resort, not first.  Make an effort to find ratings provided by the curator of the kit or the supplier of resistors.  The raring may be written somewhere on the kit.  If the part number of the resistor is known, then you can look up the power rating in the datasheet.

Power rating of the resistors written on the package of this kit
Label of an 1/8-watt resistor kit. The information is there, if you look carefully.

How to make an educated guess about the power rating.

The larger the mechanical size of the resistor, the more power it can dissipate, the greater the power rating is.  So, it is possible to estimate the power rating of common** throughole based on outside dimensions. Measure the length and diameter and look-up the power rating in the table.

Power ratingBody length, lBody diameter, d
wattsinch [mm]inch [mm]
0.125 (1/8)0.130 ± 0.012 [3.30 ± 0.30]0.067 ± 0.012 [1.70 ± 0.30]
0.25 (1/4)0.236 ± 0.012 [6.00 ± 0.30]0.091 ± 0.012 [2.30 ± 0.30]
0.5 (1/2)0.335 ± 0.039 [8.50 ± 1.00]0.106 ± 0.020 [2.70 ± 0.50]

These dimensions come from the resistors' datasheets.  Since these axial throughole resistors are standardized, the datasheets from different manufacturers agree with each-other.

eighth-watt, quarter-watt, half-watt resistors side by side
Top to bottom: 1/8 watt, 1/4 watt, 1/2 watt resistors.
** Note, we are talking only about "garden variety" throughole resistors like the ones in the photo above.  We are not talking about power resistors like this or this.

What if you don't have the resistor with the required power rating on hand? 

1/4-watt is the most common size of a resistor that you will come across in hacker spaces, practicum labs in schools, and such.  1/4-watt is more than enough for small-signal circuits.  What if you calculations show that you need to dissipate more than 1/4 watt?  You have several options:
  • Procure a beefier resistor
  • Connect multiple resistors in series or parallel, such that their effective resistance has the desired value.  The combined power which they can dissipate will be equal to the sum of their power ratings.
  • Change your circuit to lower the power that the resistor would have to dissipate (perhaps temporarily until you procure a resistor with a sufficient power rating).


[1]  Datasheets for the "garden variety" axial throughole resistors: Stackpole Electronics, Philips, Panasonic, generic Chinese.  The datasheets agree with each-other, despite different manufacturers.  Common MIL standards were driving the design of resistors for a long time.

[2]  Resistor Sizes and Packages