About AC-DC Adaptor–Heavier is better

So i heard from a long time ago – when choosing a AC-DC adaptor – heavier is better. Really?

For a circuit guy – like myself – I think what the saying is really trying to tell you that AC-DC adaptor with transformer built in is better. For this I can’t agree more... from low level signal perspective.

For a +5V AC adaptor without transformer – each and every node inside the adaptor circuit is considered “LIVE” which means it is dangerous – the AC outlet voltage 230V for example is directly switched by silicon IC, and filtered to give a output of +5V. See schematic below:

(picture from some online forum.. sorry that can’t remember where I got it exactly, been with me for some time)

For a +5V AC adaptor with transformer – generally the transformer will scale down the line voltage to much managed able 12VAC or so, before passing it through bridge rectifier to and further drop down to +5V. So the secondary side of the transformer is not “LIVE” and much safer to mess around. Note that since the switching is done at much lower voltage level – the electrical noise will be much less. As shown below:


There are considerations on which is better – costing, how “clean” it is..., eventually it is up to the application and individuals to decide. For me, when I build something that requires mV or nA level of accuracy – those with transformer are definitely my choice of adaptor. Other than that, it does not bother me much.

I guess at the end of the day, knowing the fundamental of different AC adaptors allow an engineer to decide what is good enough for the application, and this especially important when dealing with low level analogue signals.

Moments of my intimate contact with electricity:Telephone Line Voltage

I like wiring – since my childhood years – guess it is just the way i am.

One of my early experience as a teenager is to extend the telephone line – essentially i just strip the wires and twist the interconnect – with my bare hand. In the middle of doing that, muscle at my upper arm started twitching involuntary – it felt kind of funny – no pain but still… seeing part of your body behaving out of your control is kind of weird…

As it turns out, telephone line is typically at 40VDC, and when it rings it can go all the way up to 90VAC.. so i guess my twitching muscle act as the incoming call detector!!

Having your body as part of current path is never a good idea, few mili ampere of current is enough to mess up your heart beat under the right condition – so give electricity some respect is what we should do..

Side notes:

from http://en.wikipedia.org/wiki/High_voltage

IEC voltage range	AC	DC	defining risk
High voltage (supply system)	> 1000 Vrms	> 1500 V	electrical arcing
Low voltage (supply system)	50–1000 Vrms	120–1500 V	electrical shock
Extra-low voltage (supply system)	< 50 Vrms	< 120 V	low risk

< 120VDC is considered as low risk.

The Catches: Device Thermal Resistance

I been designing with power devices (BJT, Mosfet) for years – but I will always remember when I first started looking at their thermal resistance – by not reading through those fine prints,  I made gravely mistake of picking the wrong device for my prototype.


Take an example from the figure above – the device might be promoted to have less than 25degC per watt of junction-ambient thermal resistance (with fine print indicates test condition) – but without heatsink or large enough copper area, the thermal resistance easily go up to 40degC/Watt - as shown in the graph when copper area is less than 1 inches square.

Power dissipation specification at times shows best number - but only when mounted on large copper area - not practical for actual circuit – often heatsink is needed for low thermal resistance.

But it was the standard way for the industrial to compare using this method - too fresh and naive and some said silly to take it literally :)

The Catches – Microcontroller Embedded I2C Module: My First Encounter with Errata-sheet

 
When I first started doing design works, there was this particular project that required me to communicate through I2C. It was my virgin experience using Microcontroller with embedded I2C engine– and it did not works well – errors occur intermittently – I spent 2 week checked and rechecked my codes, bus waveforms – start , stop, acknowledge conditions – every single thing that I could think of…

There was so much frustration, finally I told myself that enough is enough and there must be something else going wrong instead of my code – and Google long enough – I found out that there’s something called “Errata sheet”. And true enough – looking for the errata of that silicon revision on manufacturer’s website it showed that there was a bug in the silicon – and it proposed some work-around, plugged the workaround in and everything works accordingly

Lesson learnt –
Before choosing a controller or similar devices – always read the errata (if it has one) for the stuffs that related to you, and newly released devices might be more happening that you would expect!

The Catches: Safety Valve of Electrolytic Capacitor

Whether one believes in luck or not, for me there is no deniable that at times luck play a crucial part in life.

I remember my lucky escape from getting injured dealing with a boost converter with electrolytic capacitor. It was a brand new design – a boost that generate 100V. Something was not right – it could not go up to 100V – so I was having this board vised up ( to see what a “vise” is, go to http://en.wikipedia.org/wiki/Vise) is  to ease probing on each side – with those cap facing me (bad bad choice).

While I my face was inches away from the board – one of the cap exploded and shooting up hot boiling oily substance right pass me, hitting my cubicle wall about 1 meter way – it turns out that wrong part was installed – it was a 48V part.

It could at best cause some burnt on my not so handsome face – or at worst blinded me… come to think about that – I was simply lucky enough to escape un-injured and extremely thankful. To show you what i meant, i managed to find just a similar end result from wiki (http://en.wikipedia.org/wiki/Electrolytic_capacitor) as shown below:

Lesson learnt – those weak points being built on top of the capacitor can is the “safety valve” – it is designed to open up (“explode”) under overheat – overvoltage condition – never ever face the valve toward you or anybody else.

The Catches: Voltage follower that rings

In my first year doing circuit design – I was evaluating a circuit that contains a voltage follower op-amp – something that I didn’t quite happy with is the amount of overshoot on the step input. Digging through all the information – I found out that not op-amp are suitable for being a voltage follower. And in fact – for a voltage feedback op-amp to be used as voltage follower – it will state “unity gain stable” in the datasheet.

 

 
Lesson learnt –
There are certain amp that are not meant to be unity gain stable – mainly for fast response – read the datasheet before using them.

The Catches: Boost Converter that don’t last

This happen about second year i started hands-on design – it was a 100V boost converter – most of the time the prototype works well – but every once a while – it stop working  after a large “blak” sound.

So tracing back on the switching waveforms – voltage, current I deduced that the inductance was not right – VL = L * (di/dt) relationship does not hold – drop in another piece – and it start working again.

Being curious as I am (luckily), I check the resistance of the inductor and compared to the new part (did not have LCR meter on hand). The resistance is much lower lower than the number stated in datasheet.

As the failure made me very un-comfortable – I went Google around – and it turns out that inductor does have voltage rating (although not mentioned in the datasheet of the one I was using), so the design was flawed in the first case – luckily it was found out earlier than later (it pays for being curious), else can’t even imagine what it would do in the field. Changing the part to high voltage part – and it work out solid!

For more details, refer to previous post: http://electroniccircuitdesignsharing.blogspot.com/2012/12/inductor-voltage-rating.html

Lesson –
Inductor does have voltage rating – due to the insulation coating of the windings, and the way wires being wound – and the inductor intended for high voltage operation will have working voltage stated in the datasheet – else don’t use it at high voltages.

Always look for the root-cause – it will save your ass

The Catches: My first encounter with Current Feedback Op-Amp

So to say, I was evaluating a circuit given in manufacturer’s reference circuit, being young and naïve (ignorant too), I just get the parts and put it in together – most of the time the circuit works – but when debugging one particular non-working circuit – something was not working accordingly.
So it was the time that I look at the datasheet of that amp – and found out that it was current feedback op-amp – not the assumed typical voltage amp – it turns out to be that current feedback op-amp are generally faster – and the behaviour differs as well.
for more details refer to wiki:
http://en.wikipedia.org/wiki/Current-feedback_operational_amplifier
Lesson Learnt:
Textbook op-amp circuit is generally voltage feedback op-amp – and there are much more in the market then what offered in text book.

Inductor Voltage Rating

Have you ever wonder that when you read through datasheet for inductors, you rarely see the voltage rating?
Shown in picture below are some common inductors seen.

The fact is, inductor does have voltage rating, the winding wire have a fix thickness of insulation coating, if high enough voltage is applied across the inductor, although momentary and the current is within the specification limit, the insulation will breakdown and short to adjacent wire. From personal experience, normally inductor voltage is about 60V, unless specifically stated in datasheet. Maximum voltage allowed is usually depends on insulation thickness of wire used, and whether the windings overlap.
If you does wonder whether your inductor has insulation breakdown, the easiest way is to measure DC resistance, depending of actual short, the resistance will be a lot or somewhat lower than a good one. Alternately, if you have a LCR meter, you can use it to measure the inductance instead, in this case, expect faulty inductor to have lower inductance (less winding over the magnetic core).

Real Life Inductor : When Does An Inductor Stop Behaving Like One

1. When current is too high – because of magnetic saturation
2. When signal of interest is of too high frequency – higher or near inductor self resonant frequency. There are parasitic capacitance across inductor winding  so in a way, we have parallel LC circuit, at high enough frequency, the impedance of C will gets low enough to shunt out the impedance of inductor.

Debug - Op amp virtual ground probing with DMM

Watch out when probing op amp virtual ground, as the DMM loading will modify the feedback element of the inverting op amp. See http://electroniccircuitdesignsharing.blogspot.com/2012/05/feedback-loop-element-identification.html on how to identify the feedback element of inverting op amp.

Why oscilloscope is needed for circuit debugging

As handy as a DMM can be, unfortunately it cannot shows you circuit oscillation. DMM has a very slow response, for example, if the circuit shown below oscillate at frequency above 1kHz or more, DMM will just give the average reading, in this case -1V. Since you are expecting -1V, you would think that the circuit is all fine. But the oscillation will give faulty result to other stages. And at times like these, you will make the wrong conclusion and spend days on debugging.

Debug - DMM loading

DMM are not ideal ( ideal cases rarely happen in real life :) )

Catches of Kelvin or 4 Wires Operation

Let’s face it, there’s no free meal, there are prices to pay if Kelvin or 4 Wires operations were to be used

1.       4 wires operation supplies are generally more costly, you pay more $$$ for this feature.

2.       2 additional wires are needed to sense the voltage

3.       There’s limitation on how much voltage the supply can source, if max voltage it is capable is 15V, and there’s 10V drop across the cables, which means only 5V is available across the load

4.       It works by sensing what is at the voltage across the load and adjust itself – which means it is a feedback control loop, and there’s limitation on how fast it can adjust itself, or how stable it can be. When very fast current transient occurs, capacitor across load is generally required.