So, two diodes spontaneously burnt up when I increased the capacitance on my bread board (by going from 15 micro farads to 100 micro farads), I had ~20 Volts through the circuit..... it dumbfounds me because i had a 425 micro farad capacitor on there before..... any suggestions as to wtf happened?

I think I should have increased the resistance above 2.2 kilo ohms, which was in parallel to the capacitor, but too late now to test....

side note, this was in senior physics lab, I dont do this just for shits and giggles

2/4/2009 8:52:55 PM

schematic, component specs

2/4/2009 9:19:46 PM

yeah that description was about on par with a freshman in EE, can't exactly visualize the problem without a circuit, or at least more than an 'oh by the way' description of it

2/4/2009 11:22:21 PM

yeah..... with my knowledge of EE, A freshman might whoop me.... E&M is not my strong suit




[Edited on February 5, 2009 at 12:14 AM. Reason : ....]

2/5/2009 12:11:25 AM

well fuck it, pic wont show up...

2/5/2009 12:15:28 AM

Is this for your ECE 200 project?

Do you realize there's no capacitor in that circuit diagram?

And if you were just grounding a capacitor on the output there, you're going to want to put a resistor in series with your capacitor, to stop the current from spiking and blowing out the diodes.

I=V/R

If your R is 0, your I is infinite (or whatever the maximum capacity of your power supply is-- likely more than your other circuit elements can handle)

[Edited on February 5, 2009 at 12:25 AM. Reason : ]

2/5/2009 12:18:39 AM

First up, if you're getting 20V out of that circuit then something is seriously wrong, because you should only be getting 6.3 if you're set up according to that schematic.

Second, like moron said, there is no capacitor in that schematic. If you're adding one, keep in mind that capacitors not only have capacitance ratings, but VOLTAGE ratings. If you're adding a 425mfd 16V cap, for instance, and your circuit is putting out 20V . . . . then yeah, things are gonna go wrong. Also, I'm guessing you're using electrolytic caps, which are usually polarized. AKA they have to be installed in the right direction, or Bad Things will happen. Check that the big black strip down one side is pointing towards ground.

2/5/2009 3:31:57 PM

The Capacitor is in parallel with the resistor, it was added in a later step, and we are measuring the voltage across the resistor on an oscilliscope. 20V is going into the circuit, not out of it. It is a full wave rectifier, it is just an experiment in my senior physics lab, like I said, PY452. It is supposed to teach us how to wire up experiments and shit so one day we can work at places like the LHC or somewhere. I think the bread board just had a short in it or the capacitor was bad as in someone dropped it or something. I have to redo the lab tomorrow so I think I'm just going to boost the resistor from 2.2k to around 100 and see what happens, on a different apparatus of course.

2/5/2009 3:52:39 PM

and the 4 diodes were in the right way, the TA checked it about 3 times after the first one blew, then after we lit it up the second time I just called it a day and went home. It was the diode on the bottom right of the square if that matters to anyone

2/5/2009 3:54:42 PM

haven't seen the schematic - but sounds like a full-wave rectifier to me.
some of your diodes are backwards or you're not grounded correctly.

But since your input is 20VAC, your VDC is going to be ~9.8, i doubt you're going to blow a cap with 10V

Are your resistor and cap grounded to a separate DC ground ?

2/5/2009 4:03:58 PM

^^ what are you trying to measure?

Increasing that resistor value isn't going to solve your problem. As the capacitor reaches its maximum charge, it essentially becomes a break in the circuit, which means that the cap there has practically NO effect after it charges.

You have to put a resistor in line with the capacitor (in series) or put one just after the output of the bridge rectifier, before the resistor/capacitor branches, that V/R<I-max of your diodes (where V is the rectified voltage level).




[Edited on February 5, 2009 at 4:22 PM. Reason : wut??]

2/5/2009 4:04:12 PM

Quote :

"As the capacitor reaches its maximum charge, it essentially becomes a short in the circuit"

lolwut?

2/5/2009 4:15:10 PM

^^ i think you mean open circuit there chief

2/5/2009 4:16:36 PM

if you are still building the pwr supply circuit....

20 volts is wayy to high. try 15. and its a full wave. make sure you have all diodes facing the correct way, and also the leads off and on correct to the other components. The caps had nothing to do. you did something else to fry the diodes. best the caps did was finish them off by dropping the voltage while they charged.

2/5/2009 5:35:36 PM

i would imagine the non repetitive peak surge current of any diode someone with this little knowledge could actually physically solder/breadboard would handle the I=C DV/DT inrush in charging a 100uF capacitor. how much current could you even source in a few cycles. the ESR in the cap you're using (big electrolytic?) will limit your currents as well.



But what do i know?

2/5/2009 6:41:30 PM

Quote :

"But what do i know?"

nothing

2/5/2009 10:57:03 PM

ive been missing your kind words

2/6/2009 8:09:09 AM

Quinn is spot on, there's pretty much no way the diodes are blowing due to inrush...

it seems more likely that either the capacitor orientation, diode orientation, or AC supply / DC output connections are incorrect

if you put your AC supply on the DC- output it would probably blow 2 diodes real quick and the capacitor a bit later if you left it on

Quote :

"But since your input is 20VAC, your VDC is going to be ~9.8, i doubt you're going to blow a cap with 10V"

more like 20 * 1.414 - Vf = ~27

[Edited on February 6, 2009 at 12:43 PM. Reason : lol]

2/6/2009 12:33:59 PM

Hyundai Tiberon and I cant wait to figure out what happened to these diodes.

The suspense

2/7/2009 5:05:42 PM