Legal to sell DC-DC Converter *Kits*?

OTOH, engineers in general have a much better grasp of civil law than most people-- aside from coming into contact with legal practitioners and process through creation of IP, expert witness, product liability and E&O concerns and such like, in many jurisdictions we are required to have and demonstrate such knowledge formally in order to get a license. The minority who are also business owners are generally well aware of business structures and commercial codes and practices from a legal as well as taxation and insurance pov.

So, it's not silly to ask for legal advice, but it's reckless for a non-lawyer to offer it without appropriate disclaimers and it would be pretty stupid to *RELY* upon such advice.

Given the high cost (and specialized scope) of good legal advice, it's not a bad idea to know what information to give a lawyer, what questions to ask, and which other professionals you will also have to consult.

Best regards, Spehro Pefhany

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It's legal to sell kits. Don't assemble it at all. Assuming you have a disclaimer of liability statement prominently displayed within the purchasing process, it is the person who builds the device who assumes liability.

Once you start preassembling them, UL listing or not it is you who assumes liability, a good insurance policy should be in place by that point.

Nothing is bulletproof, nor do you have any reasonable guarantee that even if your design is perfect, that someone won't improperly assemble it or spill a bottle of liquid inside, throw it in the bathtub, knock it off a shelf and then try to use it attached to a kite during an electrical storm.

All you can do in that regard is put thought into it's ruggedness and test it as much as your conscience demands. Spontaneous failure and fire can usually be prevented with basic steps like a good strain relief at the chassis, fuse(s), adequate clearance between parts and cementing down anything prone to flop around due to it's height vs weight (though in this latter case, it's part of the instructions for the assembler). Test it in high ambient temperature conditions, power cycle the heck out of it. Drop it on the floor a few times and see what goes wrong. At that point you have exceeded due diligence, most finished products you buy won't survive these conditions.

Make sure you clearly state there are no refunds once assembly has begun and emphasize safety during construction and use. At that point each individual customer assumes responsibility for their ability to understand the circuit and build it properly, but if you don't have an EE degree it would be good to have an EE validate it in writing just in case anyone ever tried to sue you for their own mistakes.

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m/electronics/ucurrent/

CUTE BUT IS IT PRACTICAL

I AM PROTEUS

Yes, unlike your multimeter at measuring current accurately in low voltage circuits.

Dave.

How is this shunt better than the shunt in the multimeter?

Mine has orders of magnitude lower burden voltage than a multimeter. Almost every multimeter on the market has a high enough burden voltage at high displayed currents (relative to the range) to be a REAL problem the lower your circuit voltage gets. It can even be a big problem with 5V supplies.

Dave.

In article , snipped-for-privacy@gmail.com says...>

You're going to have to explain this one.

The current passing through the shunt causes a voltage drop. This only occurs when the multimeter is in-circuit, so inserting the multimeter changes the behaviour of the circuit. If the voltage drop is significant, then the current displayed by the multimeter could bear little or no resemblance to the current in normal operation (without the multimeter present).

Obviously, since this is what is needed.

Sure (uncertainty, and all that).

So this box becomes a permanent fixture of the DUT? It is never removed, permanently changing the behavior of the DUT? Hardly seems like a reasonable explanation.

No problem. Best way is with a cut'n'paste example from next month Silicon Chip article that should explain it:

QUOTE: "Let's look at how the supply voltage can impact your current measurement, or vice-versa as the case may be:

Lets say you want to measure the supply current of a chip or circuit taking

200mA using say a 4000 count meter on the 400mA range. A not uncommon scenario, and one you would think would be pretty easy for any mutlimeter to handle right? - Hold on.

A typical high end "accurate" multimeter will have a "low" 1mV/mA burden voltage (about as low as it gets), so this means the meter will drop 200mV across its shunt resistor at 200mA. This represents an almost tolerable 4% (200mV / 5V * 100) of a 5V supply voltage. This may not be a big deal if your supply voltage is spot on 5V, as your chip will get 4.8V and still be within spec. But what if it's already say 4.8V?, your chip or circuit is now getting 4.6V and may be outside of its operational spec. This already shows the limitation of the current range on a typically mid to high end multimeter. Not to even mention the circuit current can differ when you lower the rail by 0.2V.

Let's now say you need to do the same thing on a modern circuit or chip with say a 1.2V power supply. That same 200mV burden voltage is now a whopping

17% (200mV / 1.2V * 100) of the supply voltage. Your circuit will likely fail to function correctly and this is clearly not acceptable, not to mention inaccurate.

Think this is only a problem with "cheap" meters? Think again. The Fluke

87-V, probably the most popular high-performance meter available has a burden voltage of 1.8mV/mA (which is still pretty good). So the above numbers are even worse - a 360mV drop for a 200mA current.

Sure, you can switch up a current range, using the 10A jack, with its burden voltage of say 10mV/A, giving you a very nice drop of only 2mV. But your display is now showing 0.200 or 0.20 instead of 200.0 - you've just lost a valuable digit or two of resolution. And the higher 10A current range is likely much less accurate than the mA range too!"

/QUOTE

And that's the same with virtually every multimeter on the market, regardless of price.

Some are a LOT worse. Try the above example on a Fluke 77 Series III or a Meterman 37XR for example.

They will drop so much your 1.2V circuit will get ZERO volts. The Fluke

79-III is so bad Fluke decided to remove the 400mA range entirely.

And that's of course not even mentioning that the current accuracy on almost all meters is a lot worse than the basic DC voltage accuracy. Try 5 times worse for an average.

So my uCurrent provides a much lower burden voltage and uses the more accurate voltage range to measure current. So it solves two rather annoying prolems in one hit.

Nothing tricky about it at all, it's just that no one has bothered to do it before as a multimeter add-on (that I am aware of). But it's carefully considered and done all the time in in-circuit current shunts.

I suspect the article will be a real eye-opener for some people.

Dave.

Correct, and the drop gets higher the closer you get to full-scale on your current range. Virtually every meter on the market will have between 1mV/mA and 10mV/mA burden voltage for the mA ranges, so do the math for the 200mA range and then ask if your circuit can handle (or be affected by) that drop. Similar but often not as bad for the uA ranges.

Dave.

What'd be useful is a momentary-action pushbutton that applies a dead short, so while your clip leads are in circuit and you're testing it, you can hit the button to see if the behaviour changes. That way you know if the meter matters.

Clifford Heath.

Nice idea, but it's not just the circuit behaviour that matters it's also the current accuracy of what you are measuring. Your circuit might still be within spec and fully functional, but when you put that meter in series it can change the actual circuit current. Makes an already inaccurate current range even less accurate. If you are not concerned with high accuracy then a regular multimeter will usually suffice of course.

Dave.

Nice idea, but it's not just the circuit behaviour that matters it's also the current accuracy of what you are measuring. Your circuit might still be within spec and fully functional, but when you put that meter in series it can change the actual circuit current. Makes an already inaccurate current range even less accurate. If you are not concerned with high accuracy then a regular multimeter will usually suffice of course.

Dave.

Also, don't ignore the volt drop in your test leads - unless you abandon your standard leads for something considerably thicker - and shorter. You will also have to replace the 4mm input sockets on your meter with adequately rated screw terminals.

Don't forget the power dissipated in the shunt either. If you're measuring

20A and your volt drop is 200mV, your shunt has to dissipate 4W. This is Ok with a properly designed external shunt but not if you're going to stick it inside the average multimeter case.

[snip]

Maybe you should elaborate on exactly what you want explaining. I took it as "why does it matter that it has orders of magnitude lower burden voltage than a multimeter?". If that wasn't what you were asking, then my reply is of no use to you.

It appears you don't know the UL/CSA safety specs. Without those, you are truly lost. And you won't learn them in a day or two. Do not sell anything with any voltages greater than 42V. Design to UL/CSA specs, advertise it thusly and be certain you actually did that correctly. Many labs around the country would do a test suite for you for a reasonable price, and then you could use those in your adverts too. Note that many cities, counties and town around the country won't allow a product to be sold unless the safety approval is in place, though. All of Ca, NYC, Chgo, DC and FL are a few of them. Also be certain that every component has a UL or CSA listing also. It doesn't guarantee you'll USE them correctly, but if you do, at least the components are listed.

Burn down my house and yes, you have a guaranteed law suit on your hands. But then, I would never purchase anything that was not UL approved, including kits. It's difficult to get approvals on kits, too. Lot of hoops to jump thru.

Go talk to UL; they will talk to you and give you some good info.

You're missing the point: What is its source? The output itself is almost irrelevant. Don't do this; you are not prepared to handle it.

It appears that you are nothing more than a mouthy little bitch.