The perils of.. using breadboards (was.. of using any old schematic off the web)

[smjcuk]

I just knocked up a negative voltage generator with a 555 and simple capacitive voltage inverter on the output. To be honest I blindly copied the design off the web.

So with a 1M load (the multimeter), yay -12v as expected. Not bad for a "I can't be bothered to design something properly" job.

Let's try 20mA load which is a respectable amount of load so on goes a 560 ohm resistor. 21.4mA but close enough.



Moral of the story: don't just copy anything off the web and assume it works nicely.

EDIT: See later. The schematic is fine...

[SeanB]

I never got a 555 to go like that. Either it detonated and blows itself to bits or just sits there and does nothing. I did try running a ultrasonic generator using a 555, and used the TO100 can versions, so I could put a heatsink on it, I knew it was going to get hot driving essentially a 100n capacitor at 40kHz. It did, but survived.

[smjcuk]

Indeed same. It's why I decided to photograph it. Amazingly while it was smoking away it was delivering 9v @ 20mA so it was actually working. Going to stick a big emitter follower on a fresh one later.

The smoked 555 still works as a timer as well which is impressive. Binning it though as the case is cracked. Not sire how much magic it lost.

This is interim while I wait for some tl7660 devices to turn up.

[Zero999]

Care to post a link to the website?

Was the breadboard damaged by the heat?

[wagon]

I had one of those blow up like that.  Lump of charcoal....

[Halvmand]

I had the almost exact same experience yesterday.
555 timer for negative rail. While testing it with different loads, the 555 gave up it's smoke.

Turned out to be a misplaced 3,3 ohm resistor instead of the 330 ohm my box indicated  Don't always trust your parts boxes.

What's wierd is your's crapped it self at 20 mA. Mine delivers 50mA without ripple and a 470 uF output cap while the temp is <50 deg C.

[smjcuk]

Care to post a link to the website?

Was the breadboard damaged by the heat?

http://www.csgnetwork.com/ne555c1.html

Breadboard was a little melted and burned. I tend to do all my first attempts at stuff on a cheapy breadboard rather than my nice wisher one as I have no intention of nuking that

[smjcuk]

I had the almost exact same experience yesterday.
555 timer for negative rail. While testing it with different loads, the 555 gave up it's smoke.

Turned out to be a misplaced 3,3 ohm resistor instead of the 330 ohm my box indicated  Don't always trust your parts boxes.

What's wierd is your's crapped it self at 20 mA. Mine delivers 50mA without ripple and a 470 uF output cap while the temp is <50 deg C.

Yeah thats what I was hoping for. I need a simple -/+9v supply to run an op amp circuit off. I dont have the luxury of a working dual rail supply (well I do have an HP6236b but the tracking isn't working well and the caps are bad). Plus I'm trying to standardise on 12v sources due to a potential move to a boat.

I checked the whole thing three times over before I powered up and checked all values. I've got a few 555s left - will attempt another go later.

I've drafted a design with just transistors using a multivibrator, follower, diode-cap inverter and two simple regulators with zeners so will try that as well.

Its all fun even if the remainder of the family are complianing about smoke inhalation

[Zero999]

That circuit doesn't look too bad. I certainly would not have expected it to fail like that. Perhaps the 555 had already been damaged?

Use lower value capacitors (try 10µF), add some decoupling and a 10nF capacitor from pin 5 to 0V. There should be no need for booster transistors at this low current.

[smjcuk]

Was a fresh 555 straight out of a tube. Thanks for the suggestions - will look at those too.

[smjcuk]

Cool thanks for suggestion.

[Howardlong]

Moral: always use an appropriate current limit on your supply.

Too many tales of my own that I now do that by default nowadays.

I still have these little bugger ups though, particularly on power supplies.

It works the other way too. You set current limit to what you think is appropriate on an MCU project, then spend two hours trying to figure out why it doesn't work when you turn the clock up to 11.

[smjcuk]

Agree entirely. This was powered off 8xAA batteries which can shift a ton of current. I usually stick a 1W 470 ohm resistor in series with the supply but I didn't today

Problem I've got is I've got the kit (I have a working TTi PL330 supply) but no space to set up a bench at the moment.

[alsetalokin4017]

That's pretty strange. By an odd coincidence I happen to have the exact same circuit, from the exact same source, that I built up some weeks ago. Here it is with a 330 ohm resistor as load, 27 mA. No sign of misbehaviour from the 555, cool to the touch. (NE555P flavor)
Powered by my bench supply at 12 volts.

Is it possible that you made some mistake in your breadboarded circuit? Have you tried another 555 from the same batch?

[smjcuk]

I checked the circuit a few times. Plan is to rebuilt it with different components later and see what happens. I've bagged this entire BOM as suspect. I've got some different 555s and plenty of stock parts left.

Human error is always possible. However it did work fine with no load which is weird.

[alsetalokin4017]

Here is mine with 13.5 V input, 200 ohm load. Perhaps the 555 is now running slightly warm... but maybe it's just warmed up from my finger. It is definitely not overheating. The load resistor is definitely heating but the 555 is not.

[smjcuk]

Interesting. Will have another go later.

Thanks for the test cases

[SeanB]

Next time run it with a 1R to 10R resistor connected in series with pin 3, to reduce the current charge into the capacitor. As well place 1000uF directly across the 555 ( pins 1 and 8 ) to decouple the rather massive current spike when switching. The series resistor increases the output impedance of the supply but will protect the 555 output stage from the high current charging of the capacitor.

edit to fix the SMF engine from thinking the close parenthesis was an emoticon.

[alsetalokin4017]

You're welcome. I've now tried several other flavors of 555:

NE555P
NE555N
SA555P
These all work about the same, no sign of undue heating, output current through 200R load 47-48 mA (13.5 VDC in)

TLC555CP
Output current dropped to 28 mA through 200R load (13.5 VDC in) but still no heating.

I want to emphasize that I am using the _exact same_ circuit from the _exact same_ website as linked above.

[Zero999]

Was a fresh 555 straight out of a tube. Thanks for the suggestions - will look at those too.

Where did you get them from?

[atferrari]

Old schematic does not mean necessarily "wrong".

How did you managed to get it still smoking and have time to take the picture. Quick reflexes, man!! 

[paulie]

I have also used a similar circuit many times with no hint of failure. Besides 555 a variety of other drivers from gates to MCU pins. Assuming no wiring error then almost certainly caused by intermittent plug-board contact. There are several points in this circuit that will cause self destruct if opened even briefly.

I stopped using those years ago. IMO strictly for the beginner/tarduino crowd who need the education (generally what NOT to do). Contrary to urban myth, solder/deadbug is actually quicker and more convenient in addition to being a cheaper and infinitely more reliable method.

[smjcuk]

Purchased from CPC/Farnell so not dodgy eBay ones.

My other half was taking eBay sales photos so I snagged the camera off her quickly.

Thanks for the further suggestions. Incorporating them into mk2 smoke machine

Edit: this could be breadboard actually. Some of the contacts are knackered in this one. Will build mk2 Manhattan style. Recent experiment with building in that style is holding up nicely (AF gen)

[paulie]

One alternative is to use CD4069 or similar part. A bit less rugged than 555 but fewer external components and gets you more voltage and current.

[T3sl4co1l]

Circuit's smoking... I know, let's take a picture of it first, before shutting off the power or anything...

Moral of the story, RTFDS.

Tim