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Smoke Tester


Here is the Smoke Tester (wiki). It's a board designed to safely power you prototypes, and check if they have a short somewhere. Also included is a current measuring feature on which output the current could be read using a voltmeter. Here's the full feature list.

    5V and 3.3v voltage regulators
    7V-15V input
    3 possible input sources: USB, 2mm DC barrel jack, and screw terminals
    3 possible outputs: USB, Banana Jacks, and screw terminals
    Polyfuzes with fuse blown LED indicators on both rails
    Current to voltage converters - aloes you to read the current simply with a voltmeter
    Jumpers to bypass the current measurement shunts
    Jumper to bypass the 5V voltage regulator, allowing you to power the 5V output rail directly from the USB input

Design files are available in our SVN.

This design was though up as something we'd like to use in the workshop, but if enough people like it, it might go into production as well. We were inspired by Schazamp's "USB Power Supply with Resettable PTC", we strongly suggest you check it out.

Prototype boards will go out soon with any updates. If you have any suggestions please leave a comment below, or check out the forum thread about it.
best regards FIlip.

Re: Smoke Tester

Reply #1
It looks like eagle.  I guess you haven't been converted to Kicad yet :)

I like it.  I made some comments on the blog about options to power it from laptop supplies (more than 15v) and adding panel meters to read the current.  I think it is a good prototype.  I may be wrong about the laptop thing, maybe everyone has a 9-12v 1A wall wart laying around.  I just think that they are become less common as most things charge run off 5V wall warts these days. But on the other end of the scale I think most laptops are powered off 16v-20v  adapters.

In many ways I think for most of my projects this would me more useful than the ATX breakout because I rarely need 12v and  it would be nice to have a smaller footprint on the bench. Not to mention the current measurement.

Re: Smoke Tester

Reply #2
I thought of another thing.  What value fuses do you plan to fit?  It say 800ma max, but that would be over 500mA for USB.

Re: Smoke Tester

Reply #3
That looks great, I like the way you put the ok/blown labels together, and I like the multiple inputs and outputs.

Re: Smoke Tester

Reply #4

VUSB is directly connected to barrel connector?! that's a nono
It's better to connect VUSB to one side of J6 and barrel (+J2) directly to Vreg1 that's connected to other side of J6
Vreg2 better take input from middle pin on J6
J7 and J8 better be SIL3 and not SIL2 so you can chose
 - in 1-2 position the rsense is shortened and you can put ampermeter between 2-3
 - in 2-3 position current go trough rsense

And finally, I know you love this fuse circuit but I'd prefer a variable current limiter there, a latching one that will turn off output if current go over set value (set by simple pot)

Re: Smoke Tester

Reply #5
Instead of INA138, an LM358 based design would have saved some cost. Making a HS-CSA using LM358 requires 5 precise resistors, and thus the cost of the equivalent circuit would been downed to 0$20c instead of 0$80c as in INA138.
There's more, the LM358 consists of two independent, high gain op-amps which means add 5 more resistors and both INA138s replaced in the design, overall 0$30c instead of 1$60c.

What do you think?
Still learning

Re: Smoke Tester

Reply #6
I think that on a small scale (like this is) saving 1-2$ by complicating device makes no sense, especially as it would increase population cost so it would be less then 1$ ..

Re: Smoke Tester

Reply #7
I'm sharing Vimark's calculations for the Smoke tester curretn measurment circuit INA138. This was taken out of a development process.

Starting bellow this line is Vimarks original post
Here's my calculation, Kindly see schematic too for you to follow on :)
Finding Rsense value
The INA138 chip specifies a 50mV - 100mV full scale shunt voltage range in order achieve accurate reading.
In our circuit, given the 800mA Isense, I've chosen an Rsense value of 0.075 ohm which gives us a Vdiff/Vsense of 60mV (V=IR). 60mV are on the 50 - 100mV accurate range :)

Finding the wattage of Rsense using P=I^2 x R yields 48mW handling. worst case LD1117 can give 1.3A max, for this Rsense will dissipate more than 1/8W. A 1/2 W 1206 1% sense resistor will do.

Finding Load resistor on the INA138 output
The datasheet provides us formula for the output voltage

Vo = (Is)(Rs)(gm)(Rload)

Is = 800mA, Rs = 0.075R, gm = 200uA/V (transconductance value given on datasheet)
for Vo, we assume 800mV to give us a 800mA/mV one to one reading
solving Rload given the values above give us ~66.667K

Luckily, 66.667K can be accurately achieved by 3 parallel 200K resistors

In measuring the voltage, I've check google and few forums regarding the input impedance of DMMs cause this might affect the voltage reading.
A typical 10M impedance for fluke meters, this paralleled with the 66.6667K would give 0.06% error (if I'm correct)

Checking for Output voltage allowance
To check if the sense chip will allow us to output a full 800mV. 2 formula were given from the datasheet
Vout(max) = (V+) - 0.7V - (Vin+ - Vin-)
Vout(max) = Vin- - 0.5V
which ever is the lowest that would be the allowable output voltage

For 5V
Vout(max) = (5V) - 0.7V - (5V - 4.94V) = 4.24V
Vout(max) = 4.94V - 0.5V = 4.44V

4.24V > 800mV (still has a lots of room)

For 3.3V
Vout(max) = (3.3V) - 0.7V - (3.3V - 3.24V) = 2.54V
Vout(max) = 3.24V - 0.5V = 2.74V

2.54V > 800mV (same, still ok)
Thanks to Dave for the uCurrent, I've used as reference
best regards FIlip.

Re: Smoke Tester

Reply #8
Pointing out a boo-boo on the schematic - you want R9,R15,R16 and R8,R7,R12 to be 66.66kohm in parallel, so they need to be 200k resistors (as mentioned in the text above) and NOT 100k as marked on the schematic.

Re: Smoke Tester

Reply #9
Yo, first of all, good call on that VUSB and barrel connector thing, Arhi.  Beat me to it, but for me, that brings up a concern about J2.  Is that a connector to put 5V in or 7-15V?  I thought it was the latter, and if so, shouldn't that be connected to the same net as the barrel connector as opposed to VUSB?  I believe Arhi already mentioned this when he said:

"It's better to connect VUSB to one side of J6 and barrel (+J2) directly to Vreg1 that's connected to other side of J6."

Also, I've always wanted to use that third pin on a barrel connector for something, but I've never had the opportunity.  Wouldn't it be cool if you had some sort of MOSFET's gate (p-channel) hooked up to that middle pin of the barrel connector, then pull it up.  Finally, the source connected to J2 (positive pin) and the drain connected to the positive pin of the barrel connector (maybe with a diode or something).  MOSFET would switch J2 so when no barrel connector was in it would allow voltage from J2, but when a barrel connector is plugged in it cuts off J2.

Anyway, I'm just spit ballin, it's not a complete idea.  It's 2:30 in the morning and I'm on here to procrastinate studying for an exam, so forgive me if my ideas are nonsense.  I've just always wanted to see that little switch thing in barrel connectors get used because I thought it was the coolest thing when I learned that was an actual switch, and then I subsequently got real salty when I noticed that it's actually a pretty useless/unused feature.

Re: Smoke Tester

Reply #10
Speaking of barrel connectors - are the SMD versions as securely soldered to the pcb as the thru-hole versions?  For a lab-tool that might get shuffled around a lot and plugged/unplugged numerous times it would be good to have all the connectors as stable as possible...

Re: Smoke Tester

Reply #11
[quote author="matseng"]Speaking of barrel connectors - are the SMD versions as securely soldered to the pcb as the thru-hole versions?  For a lab-tool that might get shuffled around a lot and plugged/unplugged numerous times it would be good to have all the connectors as stable as possible...[/quote]

good question .. I use some on some of devices I made and I never had a failure but I always treat them as they are about to break as I'm not so confident about them :( .. and I use leaded solder (those I usually solder with leaded solder with copper in it as it's even stronger), with unleaded it's even worse .. I'm not sure I'd be confident at all, but it's just a feeling, they are probably good :D

Re: Smoke Tester

Reply #12
Well most usb connectors are also smd soldered, so It is ok ;) I had only once a connector came loose, which is prolly due unleaded solder or bad solderjoint.

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Reply #13
I've had a couple of SMD CR2032 battery holder to come loose.  The stainless steel (or maybe they are just  nickel plated?) tabs are a bitch so solder properly even with aggressive flux.

Re: Smoke Tester

Reply #14
I remember seeing complaints about one of sparkfun's arduino clones that had a surface mount switch.  They came off too easily.  I don't know if they switched to through hole or not.  I see that the barrel connector has pins through the board,  I bet that helps a lot to prevent it from twisting side to side when it is moved around on the bench.