Advice sought for new Phasor Display

[johnkelly1908]

I’m a Lecturer in Electrical Engineering, and I have been looking for many years for an affordable instrument for measuring and displaying phase relationships in AC circuits. I was going to use this instrument in Lab practicals to help students visualise these relationships, and help them gain a more intuitive understanding of the subject. I couldn’t find one, so I made one. I’m bringing it to market, but I realise that this is a very small market.
Would I be better making it an open source development platform for people to make their own instruments? Or is this an overcrowded market?
You can view the instrument here    www.actheory.com     and I would be grateful for any insights or advice.

[dmendesf]

Nice product.

[jbb]

This looks like a great product in the works, especially with differential inputs! When I was at university I was amazed how hard some people found the concept of ‘the scope probe is grounded.’

While my natural inclination on seeing it was “what about buttons and a bigger screen,” I think you have a great concept there for educational use; plug it in and it goes. Screen doesn’t need to be giant.

By not doing a battery you’ve saved yourself a ton of irritation (charger, safety, shipping, dead batteries).

Have you managed to get much battle testing in a real teaching lab? That will always find some surprises

Some suggestions:
- get some branding on there so visitors to the lab know where to buy their own

- get some labels on the measurement terminals. Especially safety concerns eg what’s the maximum allowed voltage

- you need some technical specs (eg signal levels, input impedance, allowed waveforms, operating and storage temperature range, power supply requirements)

- a spec sheet in printable/emailable format might be critical for getting sales approved

- consider isolating the external power input for safety. If some fool plugs in 415V and blows the guts up, you don’t want the students’ laptop pulled up to line voltage via the USB port. May not be required.

- will performance change if using an isolated USB source like a power bank?

- how’s your input protection?

- lab technicians may be more comfortable if you mention input protection (especially if self-protecting)

- check your power supply will work with crap USB supplies. I suggest trying 4.75V + 0.7 Ohm cable resistance

- check that hot-plugging a USB supply won’t damage the PSU (note you could get quite large spikes on the input due to cable L resonating with input C); 6.0V absolute max chips aren’t reliably up to it here, and a 6V TVS diode won’t reliably clamp below around 9V

[rstofer]

The Buy Now button leads me to Amazon.co.uk but that doesn't seem to go anywhere useful.  Bummer...

[rstofer]

The Analog Discovery 2 does something similar but without the neat phasor display.   For the attached plot, I used the Impedance Adapter (because it is handy, it is not required) with the Impedance tool and then selected a 100 Ohm internal (to the adapter) series resistor.  Given a 0.1 ufd capacitor, we expect the phase to start out at -90 degrees at 0 Hz and reach -45 degrees when the impedance of the capacitor equals the 100 Ohm resistor and this occurs about 16 kHz, mathematically.  Given real-world parts, it occurs somewhere around 18 kHz - close enough.

Note that, for some reason, I didn't start at 0 Hz but 1 kHz.  Maybe next time...

The AD2 is cool but the phasor display is much more interesting!  Nevertheless, were I an EE student, I would have the AD2 and a few attachments in my backpack at all times.

Sell it or give it away, I don't care which, but I would really like to get one!

[duak]

John K,

Most excellent!  This is a really interesting instrument that would have helped me visualize phasors back in the day.  I like it!

I think its market could be expanded by considering it to be a three channel autoscaling voltmeter with both a numerical and an analog readout for each quantity.  How many times I've wanted to monitor a number of test points but have only one meter.  With this instrument and a way to set options I can do so.  The analog readout could be pointers and/or thermometer style bargraphs.

I could also see using it to measure instantaneous voltage and current, calculate the Power Factor and display the Power Triangle.  This would be great for both educational and practical uses.  I could have used it when I was developing a programable AC complex load.

[johnkelly1908]

Thanks everybody for the input and advice.
JJB
Thanks for that, I have tried it quite extensively in Labs and it seems to work well. The only issue I’ve come across is that students sometimes don’t pay attention to polarity and hence can get 180 degree errors, but this can be a good thing as it reinforces their understanding.
The power supplies do worry me, I’ve tried it with a number of switch mode supplies and some can give noise at 50Hz. I’ve put a lot of capacitor decoupling in but it doesn’t make much difference. Once you move away from 50Hz it’s not a problem, I think it could be a voltage regulation issue. Battery banks and laptops do not give this problem, it could be  ground loop.
I’ve protected the inputs with 22meg 10kV resistors, and have performed 2.5kV flash tests on the inputs with no ill effects. I could protect the power inputs with fast blow fuses.
I am compiling a data sheet from the results of the testing performed for the CE mark and will publish this on the website.
The production run instruments will be digitally printed with safety instructions and some branding.
Any safety tips are always very welcome. Again thanks for the valuable input.

[duak]

Can you share an estimated price with us?  I wouldn't mind something like this on the bench, especially if it can be modified.

The problems of polarity and signs will always be with us.  To wit:

A professor of mathematics noticed that his kitchen sink at his home leaked. He called a plumber. The plumber came the next day and sealed a few screws, and everything was working as before.

The professor was delighted. However, when the plumber gave him the bill a minute later, he was shocked.

"This is one-third of my monthly salary!" he yelled.

Well, all the same he paid it and then the plumber said to him, "I understand your position as a professor. Why don't you come to our company and apply for a plumber position? You will earn three times as much as a professor. But remember, when you apply, tell them that you completed only seven elementary classes. They don't like educated people."

So it happened. The professor got a job as a plumber and his life significantly improved. He just had to seal a screw or two occasionally, and his salary went up significantly.

One day, the board of the plumbing company decided that every plumber had to go to evening classes to complete the eighth grade. So, our professor had to go there too. It just happened that the first class was math. The evening teacher, to check students' knowledge, asked for a formula for the area of a circle. The person asked was the professor. He jumped to the board, and then he realized that he had forgotten the formula. He started to reason it, and he filled the white board with integrals, differentials, and other advanced formulas to conclude the result he forgot. As a result, he got "minus pi times r square."

He didn't like the minus, so he started all over again. He got the minus again. No matter how many times he tried, he always got a minus. He was frustrated. He gave the class a frightened look and saw all the plumbers whisper: "Switch the limits of the integral!!"