Erm, voltage regulator exploded

moxhamj

New Member
Dippy - power adaptors catching fire. Yikes! Maybe that is what they call post production quality assurance testing - done by the consumer.

Diode works but there is a circuit that only uses two resistors, both connected in the usual way from leg 8 to 7 and leg 7 to 6 on the 555. Just a matter of picking the right values...
 
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steirny

Member
Square wave from a 555

Before the picaxe entered the scene, the 555 was king and had almost unlimited applications. Its only downfall was a lack of inverted output.

The easiest way to get a square wave is with one resistor from the cap to pin 3 instead of pin 7.

Pin 7 is an open collector NPN while pin 3 is a totem pole output. It will charge and discharge thru the same resistor.

Asymetry will depend on how close the output switches to either rail. The 7555 will perform better than the old bipolar 555.

You can possibly trim the duty cycle with one large value resistor or pot from pin 5 to either Vcc or GND to shift the target thresholds. The internal voltage divider is tapped at 2/3 Vcc at pin 5. For a 555 the internal divider uses 3 x 5k and for a 7555 it uses 3 x 75k.
 
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moxhamj

New Member
Addit: I am still really worried about components blowing up. I've just seen a patient who had a lead acid battery blow up in his face, and while electronics may be smaller, usually you have your face a lot closer too. Stuff blowing up is dangerous.

I've been pondering the schematic more. There are still a lot of things that need fixing, but for people (me included) who prefer to try things rather than draw them up on paper, the safe option is to use three cheap batteries to power this circuit. The cheaper the better, because cheap zinc batteries have a higher internal resistance and so are less likely to be able to deliver currents that can blow things up. I believe this is what manuka recommends and there is a good reason for choosing cheap zinc batteries over, say, alkalines or rechargeables.

The nice thing about the 4000 CMOS range is the wide working voltage eg 3-15V so picaxe and 4026 and 3 batteries would be perfect.

Re the 555 teaser, here is a little online calculator http://www.coolcircuit.com/tools/ne555_calculator/index.php?R1=1000&R2=10000000&C1=0.1&T1=&T2=&sub=

The cap charges up via R1 and R2, and discharges via just R2. So if we make R2 much bigger than R1, the charge and discharge times become very close to being equal. There are a few tricks though. If we make R1 equal to zero, it shorts out the discharge transistor and this would cook the chip. So the current through the discharge transistor on leg 7 needs to be kept low - eg 5mA, so this sets R1 at 1k.

R2 then has to be much bigger. Let's try 100k. Plug in 10uF into the calculator and we have a duty cycle of 50.25%. We can do better than that though. Let's try 1000 and 10,000,000 ohms. Now it is 50.0025%. That is pretty close to square.

But if you use big resistors for R2, this does mean that the capacitor needs to be fairly small. For longer time delays eg minutes, the next trap is that the self discharge of the capacitor can be an issue, so you need to use low leakage caps like tantalums or polystyrene or greencaps. The capacitor leakage would affect the duty cycle more than the 1k when the resistor is more than 100k or so.

If you really wanted 50.000% duty cycle, run it through a divide by 2 chip that clocks on the leading edge. But for 49% to 51%, a 1k, a 100k to 10M and a suitable cap will do the job.

Good old 555 and picaxe can go together in a very handy way. Say you want to turn on a pump, but you want to be absolutely certain that the pump was never turned on and off many times a second. There is a very small but not zero chance that a haywire picaxe with lots of nearby RF hash or mains hash (eg flouro tubes turning on) could fail in that way. But a 555 is very tolerant of power supply noise and hash, so use a picaxe to trigger a 555 monostable and use a 555 to drive the pump.

Oh, and steirny's square wave circuit is even better!
 
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Dippy

Moderator
Gosh that brings back memories. I think the 555 is older than me.
Well, I bothered. Ten seconds later I got:-
http://www.aldinc.com/pdf/time_15002.0.pdf

But, reminiscing aside and back to the plot.
I haven't seen any input from Blue Eagle as to whether he has progressed - or given up?
Has he blown himself up and floating in orbit? Has the Eagle landed?
It would be nice to know if the efforts of others has helped.
 

demonicpicaxeguy

Senior Member
i once had a 1 farad capacitor literally explode, it was part of an 300w amplifier i built from scratch, it had me quite stumped since i built two of these amplifiers and the other one worked fine i simply replaced the capacitor when i couldn't locate a fault it's still in perfect working order 3 years later

i'd put it down to quality control not doing their job in their little factory in china
 
Hey guys, sorry about the inactivity, just been busy with out stuff. And I totally agree with Dr Acula, exploding stuff is dangerous and that has made me rather reluctant to be standing over the circuit, measuring voltages and whatnot incase something goes off again. However I go back to school in two days, and I’ll have access to a better variable voltage psu, I’ve never had a component fail when plugged into them.

I’ll defo try and lay my hands on some cheapo batteries and a torch bulb (and maybe extend the length of the cable on the switch lol) I’ve made the updates to the schematics as you all suggested and will double check them against the real circuit.

”Dr Acula” said:
And just to add to evan's comment about Q3 - you have the wrong sort of transistor there. You have specified a PNP transistor (arrow going into the transistor) and you want an NPN. NPN transistors have the arrow going out of the transistor. Drawing it upside down doesn't fix the problem!
Yep, I know it shouldn’t be a PNP but I changed that transistor at the last minute (I was originally going to use a relay, but transistors are cheaper and smaller) and rather stupidly just went straight to the current rating part of the data sheet, and missed the fact it was a PNP, and ordered the wrong thing. It did work fine with my test setup though, which consisted of the picaxe, one 4026 and one display. I just connected the emitter to the resistors and the collector to ground, just meant that high on the picaxe turns the displays off, rather than on and vice versa (oh and the voltage regulation circuit worked fine in that setup as well)

”Dr Acula” said:
Another point re the circuit, is Q3 needed for the moment and are the current limiting resistors needed? http://pandorasbox.50webs.com/projec... counter.pdf
After seeing your link, it seems they might not be needed, but I think I may as well leave them there as they may reduce the current being drawn by the 4026 to drive the displays, which in turn helps the heating problem. And as Dippy said, yep, I’ll pull the blanking transistor for the time being......In fact, i could actually dispense with it completely, and use pin/leg 3, which blanks the displays when pulled low. I decided to use a transistor instead of this because it would have meant widening my pcb further, and it was much more space efficient to just add another component, but i guess it doesnt matter on a breadboard.

Thanks for all the responses guys, I’ve attached the updated schematics (in super widescreen format again lol) :)
 

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moxhamj

New Member
Looking great. Time to breadboard that and test it all out.

You mentioned a PCB - maybe just hold off on the PCB for the moment - till it is all debugged.

We look forward to some pictures of it working!
 
Heh, well unfortunatly this is a subsitute for the PCB. I designed a dual layer one, and gave the copper sheets to my friend who has access to a CNC router, but he found it virtually impossible to get both the side to line up. I got 3 sheets just incase, but 2 attempts went wrong and he doesnt rekon the 3rd will go any better.
So i guess i will just build it on breadboard and maybe put it in an ABS box when i am done, which is alot less elegant, but it would be extremely difficult to design on single layer board, and having one professionally made is too expensive since this is more of a learning experience, rather than an attempt to make a clock if you know what i mean.
 

Dippy

Moderator
I trust it is all working 100% before PCB?

Trouble with D/S board? Then use 2 x S/S and stick them b2b. You may have to use wire-wrap style chip sockets (lead length) unless you can get hold of the thin pcb sheet.
 
Hmm yea could do that if i got some more board and as you say, different chip sockets, but i think it would be wise to remain on breadboard until i can get my problems sorted. I am going out soon but should have a chance to experiment later today :)
 

moxhamj

New Member
Coz it is a fun distraction compared with getting a circuit working?

I know if I had a CNC router I'd probably end up playing with it rather than getting the original circuit working.

Keep breadboarding. I think you are so close now. I don't think things should be blowing up any more based on that last schematic. It should be possible to test things one at a time - eg test the reg gives out 5V, then connect the picaxe, test it can flash a led, then connect one 4026, then count that chip, then add the others.
 

Mycroft2152

Senior Member
Mmm... I have gone mental... why are you carving PCBs when your circuit isn't finalised?
Dispter,

Breadboading is of course the prper way to develop a circuit.

But, if you have a CNC milling machine set up, it is pretty trivial to "carve" a pcb.

You draw the board with a CAD program, (DIPTRACE works well), then let the software create the isolation millng tracks, convert to g-code, mount the blank pcb and let it go. It sounds more complicated than it is.

I'm puitting together a Fireball CNC kit right now. A relatively inexpensive CNC machine. yes, they ship ol over then world. There is a very active user group.

Myc
 

Dippy

Moderator
Well, it sounds like fun.
I wish I had one.

I also wish I had an unlimited supply of board...;)

(General comment, not specific to this thread): It just seems odd that when saving a few pennies is so important on this Forum that people are quite happy to trash ten quids worth of board..?? It's not just the cost but also having to wait another day or two to replenish the stock. Of course, if the board is free issue from work or supplied by the taxpayer from school then that's OK. I'm getting to sound like my Dad.
 
OK, in school atm and have a nice varible, current limited PSU to use, as well as a better multimeter.

I turned it on with an ammeter, which read 1.3A, so i turned it off quickly. I measured the resistance between the 5v and 0v rails, 4.7k. Which seems a bit precise, and a bit low, so i am wondering if that could be the problem, but i have removed the switches, transistors and the 4.7k pull up's on the i2c bus lines and the picaxe reset are in fact 47k's, woops but i guess they will do the same thing and dont get me any closer to the problem.

When i tried the continuity setting between the 5v and 0v rails, it beeps for a split second and then is silent. I dont know if this indicates anything or maybe it has just something to do with the capacitors in the voltage reg charging or discharging.

I may post back later after i am done a bit more experimenting.

p.s I dont have an unlimited supply of copper board, I got 3 sheets (for about £10) and now just have one left, so it, nor the components are coming from school/the taxpayer.

::Edit:: changed the 47k's to 4.7k's just for accuracy, still reads 4.7k between the 5v and 0v rails. Also i forgot to mention that i checked every pin/leg against the schematic at home and everything was fine (ticked them off as i went, so i know i got them all)
And is it a bad sign that two of the 68Ohm 1/2 resistors come up as having a resistance of 0? (the other two are fine) could these be a victim of the earier accidents, suggesting that lots of stuff is fried?
Also, the resistance between the positive and negative leg's of all the IC's is 4.7k...eek, have i fried all of them or something?
 
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BeanieBots

Moderator
Hmm, your findings are very strange.
You completely MISSED THE POINT about using a current limited power supply.

"I turned it on with an ammeter, which read 1.3A, so i turned it off quickly"

TOO LATE. Damage already done, however quick your reactions might be.

The whole point of using a current limited power supply is that you set it up BEFORE you connect anything so that it is IMPOSSIBLE for it to over-current any of your devices.

Measuring the resistance across the power rails of your circuit is only meaningful if it says 0ohms. Any other value is meaningless except to say it's not a dead short. For example, try measuring the resistance of a diode using your multimeter. Then turn it around and measure again. Then try two in series. Then do the same using a different multimeter.
Then take the reading you got and calculate the current it would draw if you applied 5v. Then (using your current limited power supply set to 100mA) apply 5v and see what current is actually drawn.

Your 680R resistors which read as 0R are connected wrong. Resistors don't go to 0R even when abused. When severley abused they go open circuit. Most semiconductors will go short circuit when 'cooked'.
 

Dippy

Moderator
I agree with BB.
Its all very well HAVING a Current Limited power supply, it's knowing how to USE it that provides the benefits.

Why didn't you set it to 100mA for example BEFORE connecting it?? - like BB suggested.

Do you really mean a Current Limited PSU? i.e. one with an adjustment knob on it?

Can you get any advice from teachers on this? As Beaniebots says (and as I tried to imply on the very first page of this thread) blindly measuring circuits which contain multiple semiconductor devices is open to huge errors. Try BB's example. Ask your teachers about semiconductor junctions. Even your History teacher may be able to help.

My only comment about the resistors is that in the past I have bought resistors which were open circuit. Maybe they are duff? Maybe from Ebay?(Just kidding).

BB:"Most semiconductors will go short circuit when 'cooked'." - unless you cook 'em really hard, at which point, like most things, will go very much OPEN circuit with the accompanying pop and puff!

Blue Eagle: Build up your circuit in segments. Check at each stage. You're doing it A/T.

You really should be building this circuit up a step at a time rather than just crossing your fingers and chucking a PSU on it and then pulling things out - esp as you may now have popped one or more things.

When you have capacitors, esp larger ones, you will see your multimeter reading change as the caps charge up.

".. the resistance between the positive and negative leg's of all the IC's is 4.7k...eek, have i fried all of them or something?"
- is that in-circuit or out? Is that with a Multimeter which has a high test voltage or sub semijunction voltage test voltage?
- is that a duff/cheapo multimeter or something good?
- I really don't know. It's of no use as a measurement on it's own anyway. Sorry.

Start again :)
 

evanh

Senior Member
One assumes you are still testing the previously fried circuit. As much as you don't want it to be true you really have to accept the highly likely case of the Picaxe and DS1307 are dead.

With that in mind, I'd leave everything else in place but remove those two ICs and test the circuit without them. You should see very little current draw. Just a few milliamps for the regulator.

Also, where is this 680R resistor in the circuit?
 
There are no 680's, i said 68Ohm, someone read it wrong and everyone copied ;)

And yea, I dont think it is a 'current limiting' psu in the way you think of one. There isnt a dial, it's just 1.5A. I just called it that because it will throw a circuit breaker if you exceed the limit, where as the other PSU i was using would just work as hard as possible until it blows up.

Gunna build it up in stages as everyone suggests, rather than rushing it.
 

boriz

Senior Member
Do you really need all those 4026’s? Can’t you drive the segments directly from the AXE and MUX them with 4 outputs + transistors? Circuit would be much smaller and simpler.
 

BeanieBots

Moderator
I'm the guilty one who misread your 68Ohm as 680R. Sorry.:eek:
It's for people like me that there is a convention of using R/K/M in place of the period when stating component values. (It's really because the "." can get lost when copying).
eg 12300 Ohms would be written as 12K3
6.8 Ohms would be written as 6R8
680 Ohms would be written as 680R

Anyway, the statement about it becoming 0R is still true. It simply can't/won't ever happen as a result of applying power to it. At worst it would become open circuit and even then, it would be very obvious visually.
 
Heh np, i'll try and adhere to that convention in the future.

Weird that they read as 0R, but anyway, it's probably not the biggest issue (and i think someone said earlier that they are not technically needed anyway) 4x studies tomorrow (2hr20mins) so should get looking at it then. Thanks again for everyone's quick replies and for not giving up on me lol.
 
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