I used to work for a guy that kept all his schematics in his head....either that or he kept them under lock and key an committed them all to memory. Either way, the day I started tracing out one of his circuit boards, he told me that: "it wasn't necessary".
I hate to break it but it is.....it really, really is. To this day I had my suspicions about why he didn't want them tracing...possible fear of being found out it was a copy? I don't know?
But if you want to design something, then you have to draw it out, for your sanity and everyone else's!
Draw your schematic!
I always get asked to help with somebodies circuit. Usually they tell me what it is (e.g. an inverting op-amp) and I can usually work it out.
But sometimes I get some really big circuit, which in a student lab are on breadboard and are a complete rats-nest! When I ask to see the schematic, and they say they haven't got one (through that cheesy grin and that slight chuckle), I feel my shoulders drop.
It is a fundamental part of being an engineer - learning how to draw a schematic.
Be it a full blue-print of the Saturn five rocket, or the stick in the sand that was the first sketches of the land rover, it was drawn out and others could look at it and say: "Yeh that'll work!" or "if you swap this...".
It means you get your fellow engineers, colleagues, prospective investors on the same page. And for when it all goes wrong, somebody (like me) can help out. And for when it all falls apart, at least you have something to refer back to. And for improvements; something to scratch those ideas onto and mark test findings on.
Yes, having a schematic for development is:
VITAL!
But I can't draw...
You don't have to be an artist and draw out Da' Vinci type sketches. I have zero artistic skill yet I can still bash out a simple schematic.
Impressive, but for drawing a inverting amp based on a 741...nah!
For the prototype stage; you don't have to be a master drafts man to draw it either or use complicated CAD packages. All you need is a pen, a piece of paper big enough, a basic knowledge of electronic symbols and that's it!
Dave Jones fully utilising his copy of Dave CAD
Even Dave Jones on on the +EEVblog uses pen and a post-it note. Hell its not professional looking but its still a schematic.
OK, but I want to do a more professional looking schematic...
OK, that's good, a clear, professional schematic or drawing not only shows what your trying to build in better definition, but it requires some training and practise on the CAD package of choice.
There are a number of CAD packages available to draw a schematic:
EAGLE CAD - this is the CAD package I cut my teeth on. The tutorials are comprehensive and it offers a freeware user version. This version does only allow one sheet schematics though, but that schematic can be as big as you like! I'd put the user skill at mid-range, plentiful parts library and also Farnell/Onecall support EAGLE library parts with some of their catalogue.
Fritzing - Fritzing is a free schematic editor that you can draw up on breadboard as well - this is good for prototyping for those new to electronics as you can pull out images of what it should look like on breadboard and translate that your prototype design! it is a bit noddy though and the parts range is not the most extensive, for that I put the user skill at beginner.
KiCAD - For Linux users. This is suite of programs aimed squarely at electronics and PCB production. From the brief introduction to it, I find that its a bit more complicated and more involved (like applying your own component naming), than the other 2 and is a bit more like ISIS (Part of the proteus suite). Not the most up to date tutorial package either. This ones mid to experienced user skill.
Proteus - This is the Rolls Royce of the 4. Full schematic editor (ISIS), PCB design (ARES), BOM Editor, Firmware Editor, circuit simulation, 3D layout view, its got the LOT! However, it comes with a Rolls Royce price tag! There is a demo version you can download. This one is the toughie to learn!
Whichever one you choose, its a skill worth having! Being able to generate CAD files makes you not only more employable, but more efficient and productive.
But for development, a simple sketch in your note book is just as powerful!
Holidays! Great! Till you do something bloody moronic like go for a swim with your phone in your swim shorts pocket!! Good job there was free beer where we were staying! Fortunately pool water is not as corrosive as sea water, sea water...forget it, that thing is toast! If the water doesn't damage it, the salt on any electronics will get uber corroded and short out!
I soon as I realised what I had done, I pulled the back off it and let it dry out in the baking Turkish heat. That didn't do the trick, which was a surprise (24DegC outside! toasty!), tried to power and vibrated constantly, had to yank the battery out!
So managed to unscrew the screws holding the thing together (about 6x PH000 heads), and rather than straining my extremely poor Turkish for: "Please can I have a blow of rice grains?" I got one of the kids nappies (unused of course, which are rich in silica crystals to keep arse's dry all night long!) and wrapped the whole thing in it and left it for the rest of the holiday, only putting it all back to together and bagging all the loose bits till we went back.
Damage Report
Went to power on after a week in a nappy and it booted! Audio notification could be heard but no screen. Screen was already cracked from dropping it on gravel, so water must have ingresses through the cracks and penetrated the screen.
I looked on youtube for some video's on dismantling the phone (HTC 310 desire - see below). Managed to pull back cover off and remove the touch screen connector and the display connections - both had minor charring where they had clear short cos of the water and some tracks on the flexi pcb had come off! bad times
Old & Busted, New Hotness!
Thanks to our Ruski friend and an ebay find, I managed to fit the new screen for about £20!
The only issue I had was that the adhesive you fit the screen back on: as you can see from the video, its all gungy and very tacky! The only thing I could find was some equally tacky double backed tape, which I stuck in place and use a scalpel to remove the unwanted parts and cut holes for connections and the camera. It seemed to do the trick!
The only oversight was I didn't cut the hole out of my gasket big enough for the front facing camera!
Its crap anyway and never use it!
I tested the screen first by part assembling the phone and using the battery powering it and it worked! Display and touch screen, so my worries of the display and touch drivers on the motherboard being borked were removed!
The only down side - one single dead pixel....well for £20, I can live with that....till I get my upgrade!
So all in all for a £20 safe gamble I managed to save the ball ache of either forking out for a full repair, or worse, going back to vodaphone and trying to sort out a new phone or even worse a brand new contract!
Conversation I recently had: ME: "so whats the problem?" Student: "OK, so I'm running 5V to my logic circuit using a potential divider and......" ME: "....pardon?" Student: "yeh...can you do it that way?" ME: "errr no...not really" Student: "oh, ok.....why not?"
<Chris Proceeds to whiteboard and beckons student over.>
Why not to use a potential divider as a power supply
What is a potential Divider?
A typical PD circuit
A potential divider or voltage divider, produces an output voltage (Vout) which is fraction or ratio of the input voltage (Vin). This doesn't have to be 2 resistors, it could be as many as you like (look up ADC's and R-2R ladder networks), or even have capacitor or inductive elements, but for now, We'll just look at simple resistors.
Assuming that there is no load at Vout (i.e. no current flowing to Vout), the Current (I) flowing in this circuit can be calcualted using ohms law as the total resistance of the circuit divided by the input voltage:
I = Vin / (R1 + R2) -eqn 1.
So we can work out the voltage drop across R2, which is Vout:
Vout = I x R2 -eqn 2.
Substituting eqn 1 into eqn 2 gives:
Vout = Vin R2 / R1+R2 -eqn 3.
This is the equation to work out the output voltage of the potential divider. Here is an online calculator to do the leg work for you.
So why can't I use it as a voltage reg?
Take Kirchoff's Current law, the sum of the currents flowing into a point equals the sum of the currents flowing out. So if you hook a load to Vout and draw current you can derive the total current as:
I = I1 + I2 -eqn 4.
Case study
So say you want 5V as Vout and you input voltage is 12V. This means you can use a 7k for R1 and a 5k for R2 (not E-series, but it makes the maths work) - that's ok. But what happens when you specify a current for I1?
For this I will use SIMetrix (a Free circuit simulator, worth a look, definitely read the manual and tutorials) to simulate what happens rather than explain:
the PD circuit (Ignore X1 its for the POP calculations)
Probe1 = Vout, IPROBE1 = Total circuit Current, IPROBE2 = Current in R2. I1 is a constant current source, this simulates the circuit that would be connected.
I1 is set to 0.5mA - a fractional amount of current, yet when I run the simulation:
PROBE1 is the red line, IPROBE1 is the blue and IPROBE2 is green
Though you can't see it, the IPROBE2 reports the current through R2 is about 700uA and the total current in the circuit is about1.2mA, but also notice how the voltage Vout is now only just about 3.54V!
If I increase I1 to 1mA, it gets worse for your "regulated" supply!
Now Vout is only about 2V! So why is this? Well your pulling more current though R1, which gives rise to a greater voltage drop across it. Given that:
Vout = Vin - VR1
Then Vout will get smaller the greater VR1 is!
I did try ramping the current I1 up and did get some silly results.
To conclude
Don't use a potential divider as a voltage regulator - it just doesn't work! Leave their applications to reducing large voltage to a smaller voltage for measurement and setting ratioed voltages & references.
At work I get alot of students asking me questions about how to power their projects, why there power supply has blown or is not working, and how to power their projects off a battery.
So I thought I'd do some brief post about different ways of powering your project, the pro's and con's of each type and off the self solutions I would recommend for the new electronics hobbyist.
First off:
Linear Regulated Supplies
Linear supplies means the unregulated DC input to the regulator is - well - regulated to a stable constant voltage, without using any switching action, providing the input to the regulator is greater than that of the output.
A definition of an unregulated supply: the output of a rectifier, a battery, alternator or generator output - all of these will have a voltage but will not be constant over time.
A voltage Regulator can be analogous to a regulator on a divers air supply. The pressure in the tank is far greater than the diver would need to take a breath, otherwise it would blow their lungs! so the pressure regulator reduces the pressure to something sensible so the diver can take a breath without turning into a balloon! Likewise if the pressure in tank is lower than the pressure outside, no air will flow out of the tank (bear this in mind)
Why Linear
They are cheaper than Switch mode
less noisy - no switching frequency to be coupled onto the supply
simple
no inductive components
Why not linear
not very efficient - lots of power loss
very hard to handle high currents (as above), without having to add more components
not very good for battery applications (except maybe LDO's)
Cheap and Easy
The simplest linear regulator is a resistor-Zener network:
I don't particularly like using these, but they are stupidly simple, uber cheap and zeners and resistors are always available. However you must check the Iz for your zener as incorrectly calculating R will mean your zener will not behave to spec. Usually the physical size of a zener can dictate how much power the circuit can handle. For low power, cheap and easy applications, this one is a winner.
if you can't be bothered working out the values, then this online calculator takes the leg work out of it.
A Little more refined
A better way of regulating your supply is to use a voltage regulator:
This is a typical circuit for one. It does look more complex than the Resistor Zener network, but its pretty elementary: the op-amp basically drives the transistor till its inputs are equally balanced. The inverting input in this case is connected to a resistor zener network as before, but only this time there is no load current (assuming infinite input impedance one the op-amp). When the voltage at the non-inverting input reaches the same as that of Vref, the op-amp stops driving the transistor, till the supply drops. the process is very quick and linear. However you don't have to build this circuit each time as it comes in neat, cheap little packages.
A typical regulator circuit is shown below:
Linear regulators are the easiest to implement. Take the humble 78xx (where xx is the voltage it regulates the output at) series of voltage regulators: these come in the common TO-220 package (for beefier application) or TO-92 (for lower power applications, Usually noted by the L suffix):
TO-220 package 7805 (+5V) with pinout
TO-92 78L05, similar package to that of transistors, with associated pin out.
The 78xx series is the most common, cheap and readily available type of voltage regulator, so a good place to start for a simple DC-DC step down, but there a few things to consider:
Considerations
As with the diver air-tank analogy: the input voltage must be greater than the regulated voltage.
All linear voltage regulators have something called Vdo or Vdrop (or the drop out voltage). This is voltage drop across the regulators output to input. If you have a 12V-ish input and the output of your regulator is 5V (7805) then your Vdo = 12v - 5v = 7V.
If you look at the datasheet for a 78 series regulator, You will find that the minimum drop voltage is 2V - so for a 5V regulator you MUST have a minimum input voltage of 7V
With Vdo in mind you must also think about the power dissipated in the regulator itself
So say you building a USB charger for your car using a 7805 (which is a valid design) so you require a current of 500mA max on the 5V output, and your input voltage is max of 13.4V (charger running off alternator)
Pulling 500mA at 13.4V means the Power dissipated in 7805 is going to be: (13.4 - 5) x 0.5 = 4.2W! Your regulator is going to get a bit toasty and will probably reduce its life expectancy.
So how to get round it? Well you could screw the body of the 7805 to a heatsink, the car chassis in this case is ok (providing its a negative earth chassis), or you can reduce Vdo or your output current. A cheats way of reducing Vdo over the regulator is to put a resistor in series with the input of the regulator to dissipate some of the power in the resistor instead of the regulator, thus saving it from a quick death! This resistor will need to be rated correctly.
Alternatively you could opt for a LDO or low drop out regulator
If you have inductive or large capacitive loads on the output of your regulator, you might want to consider using a flyback diode across the regulator:
Inductive loads like relays and motors can cause spikes which give rise to reverse voltages. Regulators don't like these much.
Same applies with large capacitive loads, the fly-back diode can direct current from the capacitors back to source when the source is turned off.
Low Drop Regulators (LDO)
Low drop out regulators have a lower Vdo which means that you can afford to have a smaller input voltage before the regulator stops working. This makes it good for battery applications, typically where a 1.2V or 3.3V supply is required (so your not using an obscene amount of batteries).
An example of a LDO I've come across is an LD1117V33, where its Vdo is 1.1V, nearly half that of the 78 series. Some can go even lower (0.8V).
So bear this in mind for your application.
POOOWW-EEERRRR!
OKAY! ok! so you want more power out of your regulator, yeh? ok well going to have to add some more components and pay attention!
Typically what you can do is add a by-pass transistor to essentially divert some of the current around the regulator:
Typical by-pass transistor on a regulator circuit.
The short of it in this case is once the current through R1 gives rise to a large enough voltage to turn on Q1 (basically: Ireg R3 > Vbeq1)a current path opens up from the input to the output through Q1. However your transistor must be beefy enough to manage your power requirements: a TIP32 could handle decent amount of current, but a MJ4502 or equivalent could handle a helluva lot more.
When Q1 is on, the output remains regulated by the Vreg. When I first studied this circuit a while ago, I had trouble understanding how it worked as I though the transistor would just pass the input tot the output.
Ideally R1 should be calculated for your regulators max current rating and your by-pass transistors Vbe. Also you have to rate your transistors Vce correctly, no point in using one with a Vce of 20V if your input is 30V and your output is 5V, it'll just die!
So as a case study:
Say you have 30V coming from a rectifier and want a 5V, 3A and your using a 7805, rated for 1A, and you selected a suitable transistor to handle the extra 2Amps and calculated your value for R1, but what about the power disappated in your transistor: (30V-5V) * 2A = 50W! that thing is gonna get hot, so you are also going to need a decent heatsink.
If you look at the back of old Linear bench PSU's, you'll see that they have these massive heatsinks to dissapate the heat from the by-pass transistors:
An old Farnell TOPS2 PSU, massive heatsink on rear - these things are oldskool and tough!
This kinda drives home why linear PSU's are very inefficient!
Alternatively, you could revert to building your own regulator from scratch with an op-amp, reference, a transistor (or MOSFET) and couple of resistors, and pick a heavy duty rated transistor or FET.
Bit of a Boost?
If you want to boost the voltage output of your regulator slightly, you can place a diode in on the GND pin to GND. This will raise the output by 0.6-0.7V.
D2 in series with the GND pin to GND will raise the output by about 0.7V
This is good if you require a little more than the rated output to say transmit power of a long cable say.
Adjustable & Negative Regulation
The final thing I'll hit on is adjustable linear regulators and Negative voltage regulators.
So say you want 7.2V regulated from an unregulated supply. Not many 7.2V regulators out there are there? Not to worry: the LM317 is a glorious device that allows you to specify your own regulated voltage output! Not only is it an adjustable regulator, if you read section 9 of the datasheet linked above, there are loads of different applications you can use this chip for!
Here is a typical adjustable regulator circuit using an LM317 with pinout:
R2 here is a variable resistor which means the output can be adjusted on the fly between 1.2V and 25V. R2 can be a fixed value if you want a fixed output.
The typical formula to set the voltage output:
Vout = Vref (1 + R2/R1) + (Iadj R2)
Where Vref is typically 1.25V. The second term in this equation can be ignored as Iadj is in the region of 50uA, really small.
R1 is usually recommended to be 240R, not really sure why, but it works!
As with the other regulators, you can employ a by-pass transistor to carry some extra current to the output and as with other regulators, it has its limitations (Vdo and load current). A flyback diode to protect against inductive spikes and heavy discharges can also be employed into this circuit.
The down side with the LM317 is the minimum output voltage is 1.25V, not very good if you want to go below this, but still a cheap variable supply project can be made out of one of these.
The LM317 has some heavy duty cousins: the LM350 & the LM338 - rated at higher current capabilities and also available in TO-3 (Big Metal Can) package.
TO-3 metal package, pretty good for dissipating heat and bolting to heatsinks
All of the above regulators have their negative voltage counter-parts: the 78xx has the 79xx negative series, and the LM317 has the LM337 negative voltage regulator and work in a similar way.
Right, that's all I know for linear supplies, hope this is of some help to someone and thanks to all source for the pictures.
So I managed to breadboard up a simple constant current source from a schematic and CAD up my enclosure (whilst at the same time learn how to use Autodesk inventor) and make some end covers for it (also at the same time train up on works laser cutter).
The initial test proved my design did need some tweaking, and thanks to ease of CAD, these where amended on the schematic easily. Also to weeded out some crap components!
The Circuit
So the circuit is pretty basic but enough to constitute a constant current source with a little overload alarm/indicator.
Brief Run-down of the specs
Must be mostly constructed of recycled, scavenged or spare parts found lying around in spares boxes/collections.
Must be mostly THP components and populated on a single sided PCB
Must manage up to 7A load current at 30V maximum input voltage. Current must be fully adjustable from 0 to 7A.
There must be a display showing the set and driven current to a resolution of 0.1A (100mA)
must have over-temp warning for the driver circuit either audible or visible
must have an set/enable switch for turning the load on and off prior to adjustment or for quick disengage of load (in those situations where you realised you overloaded your DUT!)
driver electronics must run off a minimum of 12V supply, with input protection against reverse polarity supply and fused
DUT input must be protected against over-voltage (greater than 30V DC)
Spend no more than £20 on any parts that must be bought.
First iteration
I got the inspiration for this design from PauloRenato - its a pretty good design though, and also employs the same ethos as the design I want to construct.
Below is the first schematic revision and highlighted circuits are fitted to the breadboard:
So I used an adjustable reference source (TL431 adjustable voltage reference) to set a voltage of about 7.0V to the Current Set pot(goes off board on J2). The reason for a voltage ref? well why not, if you have it use it! I could have used a 9V1 Zener, but that would require potting it down and then there's the voltage/Temp issues with a Zener (as with any semiconductor junction), so with a voltage ref, there's more stability.
J3 & J4 are for the SET/ENABLE switch, but for testing on the bread board I just connected the wiper of the set pot to the non-inverting input of U1:A (first op-amp on U1). U1 is a LM324: cheap, readily available and are pretty much here to stay.
U1:B is a buffer for the LCD DMM. The LCD I chose was the DVM-210, main cos it had no units on the display meaning the reading could be translated into amps without confusion, it was small and had a backlight, which I intended to use as a over-temp indicator. It required a 0-200mV input for the DMM and required 9V to operate. So the output of the buffer had to be potted down to give a 0-200mV output from a 0-7V output, with the ability to trim/zero the meter reading.
I omitted VR2 and connected a 1R 5W resistor where J1 pins 2 and 3 are. This acts as the load resistor. Q1 & Q2 are the driver MOSFET's. They are FDP6670AL N-channel MOSFETS with some pretty good specs, albeit are they are becoming harder to find. The reason for the MOSFET pair is that the current load is split between the 2, thus halving the amount of power each FET is trying to dissipate.
U4 is a 555 timer which flashes the back light on the LCD when the over-temp detector circuit activates it. Its a slow flash at about 3Hz.
Also the input power protection was not needed and I also hit my first design snag. I had no 7809 voltage regulators (D'oh!) so I had to tweak the design and utilise an LM317 and a few resistors to get 9V. Below is the second revision of the schematic:
Rev02 of schematic - 9V reg from LM317 highlighted in green
Testing
Testing the voltage reference output was the easy bit. Turn on power and stick a volt meter across its output and GND: 6.99V - Spot on! Job's a good 'un!
Next I tested the 9V out of the LM317: 9.05V - Good enough!
I tested the balance of load through each MOSFET to ensure each one was getting their equal share of the load. I placed an ammeter in series with the drain of Q2 and hooked it up to load a bench PSU. I set the PSU voltage to about 3V and tweaked the set-point pot till the ammeter on the PSU read about 1A, which means you would expect 0.5A through the ammeter. And you would be right, except after a while the current started to go down slowly! After some head scratching I put this down the the combination of the voltage lost over the meter impedance and the Vds of Q2, along with the affects of temperature due to power dissipated in Q2. So it did work, but it's worth noting the affect of this measurement.
Then things started to go a little awry from this point! I hooked 9V power the display and tried out the alarm circuit by connecting the reset pin on the 555 to Vcc & GND and sure enough the backlight flashed.
However I could not get the blasted thing to give a sensible reading from the DMM, I kept getting negative values. At full scale on the set-point I got -7.00 on the display. As I approached 0A on the setpoint I got 0.56?! I tried zero it using RV1 and trying again but still negative, I thought I must have hooked it up the wrong way, but swapping the leads made things worse, and the voltage between the + and - DMM terminals remained at a steady 1.0V! WTF?! In the end I ditched it, not before dismantling it to see if there was anything stupid going on and then chucking it. I still have no idea what was going on there, the manual for these DMM's are pants and offered no detailed explanation of the internal circuitry, so I went on amazon and found me a china special 0-99V display for the low low price of £2.17! - and guess what? With VR1 & R6 removed, along with removing the LM317 circuit and hooking the display directly to Vcc, the bloody thing worked! up yours VoltCraft!
All I had to do to get it to flash was connect the power for the display to the output of the 555 timer to turn it on and off. simple and effective, the 555 can easily source the current required for this display. It did flash a bit fast now so I had increase C7 to slow it down to a little under 1Hz. There is a very small delay when the display is powered on and showing the set current, something I can live with.
I then proceeded to test the accuracy of the display against a Fluke 77 (old fluke!). I set a bench supply to a low-ish voltage to minimise heat from power dissipation (about 2V). I set the setpoint to 200mA on the display and took a reading of the voltage across the 1R shunt resistor and the current off the bench PSU:
OK so it's about 37mA out, but with the 0.1 degree of resolution and a single turn set pot I can't conclusively say that it's bang-on. Another at 0.5A (500mA):
This looks promising! The needle is pretty much half its full deflection of 1A and the Fluke says 526mV across the shunt! One more at an 1A:
That's pretty good! For now I'm satisfied that the display is not so far out for 100mA resolution. It's not uber precise or accurate, but best I could have hoped for from mostly scavenged and collected bits!
CADding!
With the basic circuit done, it needs something to live in. I found an old extruded aluminium enclosure in my spares box. It used to house some control electronics for a pump. its perfect for PCB's as its has guide slots. All it needs is some end panels for the front and back.
New job new things to learn so I decided to CAD up the enclosure shell into Autodesk Inventor CAD:
The best and simplest way to CAD this was to measure up all the dimensions of the profile of the enclosure, and then extrude them out. This way if I were to use the same enclosure I can just call up the outline and extrude to whatever length I want! Be useful for work related projects!
The end plates were a simple case of drawing them directly as part of an assembly so they fit the contour of the enclosure and then cut through the holes for fixtures and engravings:
From these parts I exported some DXF's and managed to find some scrap 3mm acrylic and cut them out on the laser cutter, plus some self tapping screws to bolt them to the enclosure and hey pesto:
I know the rating on the front is wrong, but I was between projects and got my spec's mixed up! The grill on the front allows ample air flow for the fan on the rear. there is a section of the fan grill where the centre of the fan blocks the centre of the grill and also doesn't match the CAD image, this is cos I amended the CAD after I realised the redundant cut outs and didn't bother re-cutting the panel.
What's next
Well the rest of the circuit! Testing the Fan control circuit, the temp control circuit and sourcing some more parts as well as getting the gerbers for the PCB done and milled out on the isolation router.
Amongst some of the parts I found these heatsinks, which'll be perfect for this design and enclosure.
