I posted the video of the burning 1N4004 diode on youtube.

I short circuited the diode across a variable power supply.
at about .8 volts you will see the pn junction begin to glow.
I fluctuate the voltage around a bit untill at 27 seconds I turn it up to 16 volts.
It lights pretty bright and watch the leads at about 31 seconds!
after that I drop the volts and let the smoke out from the lens of the microscope and reapply the volts to the diode.
It takes one more valiant effort and then gives up the breath of life at about 55 seconds.

R.I.P. 4004, R.I.P.
http://www.youtube.com/watch?v=x-mD5tjs_XE

 

It would be cool to do it again, but use a fan to keep the smoke out of the lens as the diode's burning up.

Notice the tinned leads "flash" at about 0:30 - 0:31

 

Seeing smoke is not so bad.

The bad thing thing is when there is no smoke but a strong burning smell coming from the project. Its worse because after you have killed the supply and have a detail look over, you can't see, find or smell any sight of overheating.

It has done its job and now in hiding.

This does not mean there isn't any and it will definitely going to get you sometimes in the future when you least expected it.

 

This is not a goof up or anything but I was just curios.
I can't actually remember how old I was but I do remember my very first experiment.

I use to light small 3V bulbs connected to 2 D size 1.5V cells.
then we call it battery, and I got quite thrilled seeing sparks coming out of the electrodes when I accidentally shorted the battery. It was fun, and kept doing it, creating itsy bitsy sparks.
Now I know that battery is used to drive motors and light bulbs and such.
so one day I wondered how much would a spark be if I shorted the fan socket.
I remember my dad sleeping next room and no one else was around, I climb up the chair, pull out the fan plug and twisted two wires to the plug. Now those days there is no earth and pretty much every house hold items have just two plugs. (live and neutral)
I donno wire I wasn't electrocuted then, anyways I held the wire, which has insulation peeled off at the ends so I can see the spark when I touch them.
Plugged the fan in and taking the two wires to both hands, looking closely as the wires touched and "BAAAAAAAAAAAAAAANG"
Next thing I knew was that I was on the floor and dad was standing beside me......well...u can guess what comes after..

Best part is no one was hurt and dad did not no why the main cutoff tripped. But he knew that I had something to do with the big bang.

I never told this to any one till now.
I still have one more mishap shorting 440VAC, would you like to hear it

Rifaa

 

I did not follow that schematic exactly as I stated. I do have resistors on the bases of each of those transistors. I figures that part out on my own but I think he shows them in his "selector" circuit ) he had a couple options I think.

And yes I am also aware that the fuse is wrong.

The only thing I missed was the "default" resistor. What is the downside of this (other than the supply running to it's max)?


I think my problem came from a bad 5VDC supply connection (not shown on schematic). The second time it fried my PIC, I noticed a spark coming from the 5VDC while I was measuring the voltage.

It turns out it was a bad solder joint (everything is perf boarded). Could this spark caused a momentary high voltage spike that fried the chip?

 

I never told this to any one till now.
I still have one more mishap shorting 440VAC, would you like to hear it

Rifaa

yes indeed!

 

Two fuses, one in each of my multimeters, the normal beginner gafe of measuring current, then forgetting to move the leads back and putting some voltage through it. One relay, flashlight bulb and an LED. Not too bad. but I haven't finished my project.

 

I did not follow that schematic exactly as I stated. I do have resistors on the bases of each of those transistors. I figures that part out on my own but I think he shows them in his "selector" circuit ) he had a couple options I think.

And yes I am also aware that the fuse is wrong.

OK, good so far.

The only thing I missed was the "default" resistor. What is the downside of this (other than the supply running to it's max)?

With no "default" resistor, you will have unregulated output with ripple on it, that follows the voltage at the main filter caps, less about 1.7v - depending on load and the temp of the regulator.

I think my problem came from a bad 5VDC supply connection (not shown on schematic). The second time it fried my PIC, I noticed a spark coming from the 5VDC while I was measuring the voltage.

It turns out it was a bad solder joint (everything is perf boarded). Could this spark caused a momentary high voltage spike that fried the chip?

You haven't posted the actual circuit you are using, so it's hard to say.

If you are using something like a 7805 regulator and the ground to the regulator was not good, yes - that would've fried it.

 

This is not a goof up or anything but I was just curios.
I can't actually remember how old I was but I do remember my very first experiment.

I use to light small 3V bulbs connected to 2 D size 1.5V cells.
then we call it battery, and I got quite thrilled seeing sparks coming out of the electrodes when I accidentally shorted the battery. It was fun, and kept doing it, creating itsy bitsy sparks.
Now I know that battery is used to drive motors and light bulbs and such.
so one day I wondered how much would a spark be if I shorted the fan socket.
I remember my dad sleeping next room and no one else was around, I climb up the chair, pull out the fan plug and twisted two wires to the plug. Now those days there is no earth and pretty much every house hold items have just two plugs. (live and neutral)
I donno wire I wasn't electrocuted then, anyways I held the wire, which has insulation peeled off at the ends so I can see the spark when I touch them.
Plugged the fan in and taking the two wires to both hands, looking closely as the wires touched and "BAAAAAAAAAAAAAAANG"
Next thing I knew was that I was on the floor and dad was standing beside me......well...u can guess what comes after..

Best part is no one was hurt and dad did not no why the main cutoff tripped. But he knew that I had something to do with the big bang.

I never told this to any one till now.
I still have one more mishap shorting 440VAC, would you like to hear it

Rifaa

Isn't it amazing that we are all still alive today.


Reminds me of a similar incident of years ago whern I was in high school. I was working on a television with what I suspected a a HV problem. Once again working late at night, so tired I should have been in bed.

I could not afford an HV meter so I did the old trick of placing a screw driver under the HV cap. Well you are supposed to make sure the screw driver is properly grounded first.

Anyway I became the ground and bang, same thing I was on the ground. I remember not being able to see for a few seconds. I guess that is what is meant by seeing stars.

I did eventually get the TV fixed.

 

OK, good so far.


With no "default" resistor, you will have unregulated output with ripple on it, that follows the voltage at the main filter caps, less about 1.7v - depending on load and the temp of the regulator.

Maybe this should be in it's own thread but in this particular circuit, how would you recommending fixing it? I could probably add a 100 ohm resistor but can't figure out how to do it and still have my selectable voltages.

I was thinking I could have a transistor one transistor that is always on then turn it off when I wanted to select the other voltages. Is there an easier and simpler way?

Maybe I just figured it out. Something like a 5K resistor inn parallel should do it? I would need to readjust the trim pots but that is not to big a deal.

You haven't posted the actual circuit you are using, so it's hard to say.

If you are using something like a 7805 regulator and the ground to the regulator was not good, yes - that would've fried it.

Yes I was using a simple 7805 (not shown in schematic). But I think I the ground was good. and it was the PIC and LCD that fried. I think I some how got way more than 5VDC from that regulator .

 

Keep in mind that the 7805 regulator is rated for a maximum input of 35v. If the output at your rectifier/filter is near that, a small line surge/transient could easily put you over the maximum.

 

Keep in mind that the 7805 regulator is rated for a maximum input of 35v. If the output at your rectifier/filter is near that, a small line surge/transient could easily put you over the maximum.

Anyway to protect from this? Belt and suspenders maybe?

Should I have another regulator in front of it? Or maybe have a special rectifier / filter connected to a center tap of the transfomer?

 

What is the secondary voltage?

 

What is the secondary voltage?

24 VAC.


Once I rectify and filter it, I am getting in the neighborhood of 36VDC.

 

Well, that solves the mystery as to why you're getting things zapped.

Replace the 7805 regulator, as it's probably damaged.

You could use just a single diode from the transformer's CT and a cap to feed your 7805, something like this:

D1, C1, and U2 represent your existing bridge rectifier, filter cap(s) and regulator.
D2, C2 are added between the transformer center tap and your 7805.

It isn't really obvious how this works, but the existing bridge provides the ground side for the ends of the windings. D2/C2 get 1/2 the secondary voltage. You should wind up with around 18v across C2.

 

Well, that solves the mystery as to why you're getting things zapped.

Replace the 7805 regulator, as it's probably damaged.

You could use just a single diode from the transformer's CT and a cap to feed your 7805, something like this:

D1, C1, and U2 represent your existing bridge rectifier, filter cap(s) and regulator.
D2, C2 are added between the transformer center tap and your 7805.

It isn't really obvious how this works, but the existing bridge provides the ground side for the ends of the windings. D2/C2 get 1/2 the secondary voltage. You should wind up with around 18v across C2.

Thanks! So I really don't need all that filtering I am doing with a the 2200 mf caps?

 

You DO need large filter caps for the main regulator. I didn't strictly go by the original specifications; it was just as an example.

You don't need really large caps for the PIC supply though; it doesn't require much current. Unless, of course, you're thinking of powering other things off this 5V supply.

 

You DO need large filter caps for the main regulator. I didn't strictly go by the original specifications; it was just as an example.

You don't need really large caps for the PIC supply though; it doesn't require much current. Unless, of course, you're thinking of powering other things off this 5V supply.

In addition to the PIC a backlit LCD display, 4 LEDs and a 74HCT238. Should I place a cap for the 5VDC supply?

If a previous thread you said use 3,000uF per ampere. So that is 3uf per ma.


So here is what I have

1 PIC 95ma max
4 Leds 100ma (25ma each do I factor them in since the 74HCT238 is driving them? I will guess yes).
1 74HCT238 .008ma (probably not enough to worry about.)
1 LCD panel 1.5ma max
1 LCD led 100ma max


so that is a total of 297ma (rounding up with the 74HCT238).

That would be 891uf and I assume I would probably need to round up to something like 1000uf.

Did I figure this correctly? Should I add the 1000uf cap for the 5vdc?


Wow I am really hijacking my own thread!

 

Cut down your LED current. Look at the source/sink rating for the 74HCT238. Keep below maximum current. 15mA LED current will probably still be brighter than you need.

You should have 0.1uF bypass caps across the Vcc/GND terminals of your PIC, LCD and 74 series IC's.

It won't hurt to have a bit more filter capacitance than you absolutely need.

Sure, go ahead with the 1,000uF cap. Use a 0.33uF cap right at the input of the 7805, and follow the datasheet for the output cap.

 

Cut down your LED current. Look at the source/sink rating for the 74HCT238. Keep below maximum current. 15mA LED current will probably still be brighter than you need.

You should have 0.1uF bypass caps across the Vcc/GND terminals of your PIC, LCD and 74 series IC's.

It won't hurt to have a bit more filter capacitance than you absolutely need.

Sure, go ahead with the 1,000uF cap. Use a 0.33uF cap right at the input of the 7805, and follow the datasheet for the output cap.

Yep I just calculated the LEDs at their max. I already had current limiters planned.

Already have a bypass for the pic. Did not know about the LCD panel and the 74 series. Thanks for the tip and a big thank you again for all of the help.