So I'm using my homemade power supply to do some bread boarding and out of no where BAM! my 6800uf capacitor blows up for no reason. Luckily there was no fire or anything but any ideas on what causes this? Before I replace it I want to make sure it wasn't something I did.

PS the only load at the time was a single led so I doubt that was the cause

Thanks guys

 

BAM! my 6800uf capacitor blows up for no reason.

Usually there is a reason, e.g. an electrolytic with either AC or over voltage?
Or an Electrolytic that had been stored unused for some time, for large voltage/size they often require reinitializing with a small voltage for a time to reinitialize the electrolyte.
Max.

 

thanks for the quick reply. No AC voltage the cap was after the rectifier here is the circuit I used

minus caps c4 and c5. Based off what you said I'm guessing the cap was just old. C1 would by the cap in question and yes I know one of the diodes is wrong in the rectifier that is just on the picture and not how I actually wired it

 

Although the symbol does not indicate it, I assume it is electrolytic C1?
what is the voltage and is it one that has been on the shelf for a while?
Max.

 

Which diode is wrong? The picture seems to be right. If one diode is connected differently from the picture, you will get AC across the cap.

 

Its rated for 25v and I'm not sure how long it was on the shelf. My transformer is 120VAC to 24VAC so was I wrong to assume the capacitor would never see voltages higher than 24VDC? New to electronics

 

Which diode is wrong? The picture seems to be right. If one diode is connected differently from the picture, you will get AC across the cap.

Sorry neither is wrong. I pulled that picture from a website I found a while ago and they must have corrected the error that was there when I first made the power supply

 

Its rated for 25v and I'm not sure how long it was on the shelf. My transformer is 120VAC to 24VAC so was I wrong to assume the capacitor would never see voltages higher than 24VDC? New to electronics

With electronics go above the voltage with caps for some room for error and voltage spikes .... I am also new and still learning too

 

Its rated for 25v and I'm not sure how long it was on the shelf. My transformer is 120VAC to 24VAC so was I wrong to assume the capacitor would never see voltages higher than 24VDC? New to electronics

Your DC would be 24 x 1.414 = 34v, less a couple of volts for the diode drop.
Max.

 

Instead of starting a new thread I guess I'll post this here. I obviously need to replace this capacitor now and I'm unsure of the correct formula to use to find a smoothing capacitor. I would like to find a smaller capacitor with a higher voltage rating this time around if possible.

 

The μf sizing is dependent on what percentage ripple you are aiming for and what your current demand is?
As to voltage rating, obviously you need to go with the maximum DC and allow a % margin as the cost in not that greater.
Max.

 

My transformer is 120VAC to 24VAC so was I wrong to assume the capacitor would never see voltages higher than 24VDC? New to electronics

Unfortunately for the cap, Very wrong. :-(

The AC voltage out of a transformer is usually specified at the maximum load current. Without a load, the voltage will be higher. For small transformers, the no-load voltage will be about 25% higher than the full load voltage. In your case, the unloaded transformer voltage would be something like 30 volts AC.

Another problem is that the filter capacitor charges up to the peak voltage from the rectified AC. The peak voltage is about 1.4 times the RMS AC voltage. So, in your circuit, the capacitor has at least 1.4 times 30 volts RMS which is about 42 volts. (
The diode bridge drops about 1.5 to 2 volts which reduces the voltage somewhat).

Another problem is that the input voltage of the 5-volt regulator is only specified for an absolute maximum of 35 volts so, your 5-volt regulator may also have been damaged. To be safe you should replace this regulator at the same time you replace the capacitor.

I would recommend changing the transformer to one with lower voltage. For a 12-volt regulator a transformer outputting 15 to 18 volts AC under load is a better choice. The lower voltage will also reduce the heat sink requirements for the regulators -- especially the 5-volt regulator.

 

The μf sizing is dependent on what percentage ripple you are aiming for and what your current demand is?
As to voltage rating, obviously you need to go with the maximum DC and allow a % margin as the cost in not that greater.
Max.

This is just a supply I use for breadboarding. 5VDC and 12VDC outputs, so I'm assuming that a 10% ripple would be ok? As far as current goes the max I'll ever pull is about 1A MAYBE 1.5A

 

If you feel ambitious you could always take a few turns off the secondary to lower the voltage, you could generally reckon on around 2t/v.
100μf should be ample.
Max.

 

If you feel ambitious you could always take a few turns off the secondary to lower the voltage, you could generally reckon on around 2t/v.
100μf should be ample.
Max.

Its a premade transformer from radioshack so I wouldn't be able to do that. Could you expand on " you could generally reckon on around 2t/v.
100μf should be ample." I don't quite understand, especially 2t/v don't let my join date fool you I've always been a lurker and casual reader up until recently.

Thanks a lot

 

Your DC would be 24 x 1.414 = 34v, less a couple of volts for the diode drop.
Max.

Not to threadjack but where does the 1.414 come from to get 34v ???

 

1.414 is sqrt(2). The AC voltage is rated as RMS, not peak voltage. For example 120V power line across a heater will generate as much heat as 120V DC, but the peak voltage of the sine wave will be 1.414 times higher.

 

"BAM! my 6800uf capacitor blows up"

I have seen that happen if the cap wired with wrong polarity. Take my word for it - it doesn't happen for no reason.

 

Not to threadjack but where does the 1.414 come from to get 34v ???

The OP stated he has a 24vac transformer secondary, the resultant DC is AC 24v x 1.414 for a peak DC voltage after rectification and smoothing resulting in 34vdc.

To OP, removing turns can sometimes be a bit tricky, especially on a small transformer, but the 2t/v is a reference to taking around 2 turns off will equal a reduction of 1v for each pair removed or unwound.
I have done this countless times to tailor the voltage as needed, the same applies to adding turns.
Toroidal type transformers are easier to modify.
Max.

 

Get a 1000uF 50v capacitor. Make sure it's a low-ESR type and is rated for a high temperature (105* C) as power supplies tend to heat up fairly quickly. Can't really go wrong with these values, provided you don't connect it backwards