Capacitor popped even after voltage rrquirements met

[Diver300]

ok I did measure the DC voltage from the wart before the incident happened and it reported 18V which is far less than the 50V the blown cap was rated for.

I'm about ready to throw this adapter away and buy a new one, but when I measure ac coming from the adapter, what number am I looking for on the meter to indicate whether the 22uF/50V capacitor I'm about to connect to it is likely to blow up or not?

When measuring AC, some meters are distinctly better than others at measuring the AC voltage that is superimposed on a DC voltage, like in this case. Here is a method to check the smoothing without knowing how the meter reacts to DC.

I suggest you measure the voltage with no capacitor, and then with the 22 μF capacitor only, but with no load. There is little risk of it exploding if the polarity is correct.

If the voltage is larger with the capacitor than without, there is no smoothing inside the wall wart.

Also, if you measure the AC voltage, with the meter on AC, check with the leads both ways round. If the reading is the same or nearly so, and a lot smaller than the DC reading, it's probably correct. If the voltage shown is very different one way round from the other, your meter is no good at measuring mixed AC + DC voltages.

 

[Nigel Goodwin]

I'm about ready to throw this adapter away and buy a new one, but when I measure ac coming from the adapter, what number am I looking for on the meter to indicate whether the 22uF/50V capacitor I'm about to connect to it is likely to blow up or not?

There's a lot of rubbish been posted in this thread, mutterings about 'ripple current' etc. - such esoteric failure modes wouldn't destroy the capacitor in a couple of seconds, what would is the capacitor been connected the wrong way round. Just stick another capacitor in circuit, making absolutely sure it's the correct way round, and try it again.

 

[ci139]

, what number am I looking for on the meter to indicate whether the 22uF/50V capacitor I'm about to connect to it is likely to blow up or not?

i never seen capacitor connected to rectifier blowing up
what you must keep your eye on is that the rectified DC may exceed the reading of the AC voltmeter attached to transformer secondary terminals by a factor as high as 1.73 (i have not seen greater) e.g. if your TF II-ry is 2 x 15 then your schematic may output up to (depends on diodes and transformer) 1.73 · 15V = 25.95V or when coils in series the double of that as 51.9V dc from 30V ac
... this however usually loads up to capacitor after many seconds when no load is connected
and even a moderate load such as 2kΩ brings it down near or even below of 30V dc

. . . so the only case in your case is that your battery forces your 317 to a low conduction mode that enables such voltage to build up at the input of 317
to verify that put a 1.2kΩ to 1.5 kΩ resistor shunting the input (wear protective goggles , place 1.5A fast blowing fuse before 317 e.c.)

-------------------------------------------------------

. . . other thing that now reminds is that the switched supplies may develop "waving" on output capacitors - that should be fading but the time it fades it may many times exceed the rating of the "filter cap." - thefastest way to overcome this is to put a current limiter before your filter cap

keeping in mind the function of your circuit then everything that is an overkill - i assume you intend to charge the "7.2Ah ups battery" their charging current is (must be) limited = see datasheet or a datasheet of a similar capacity , technolgy , peak load , battery-full voltage battery that actually lists the allowed charging current . . . . . . . . . and setup different circuits for initial constant current and later constant voltage charging - it goes complex but likely avoids exploding components (easier would be to buy any charger designed for your specific target - it's because what was just mentioned - you actually'd need to match your end voltage to battery temperature know the actual safe and effective currents and voltages that do cahrge your battery correct and keep it healthy)

 

[dr pepper]

A simple way to check for ac without pulling the wart apart, put 2 leds in inverse parallel (anode tp cathode and cathode to anode) put a 1k resistor in series with them and connect them to the supply, only one led should light, if they both do or one lights bright and one dim you have Ac.
Its handy to have one of these made up with croc clips on the end, if you use a bicolour you only need one led.

 

[danielsoy]

There's a lot of rubbish been posted in this thread, mutterings about 'ripple current' etc. - such esoteric failure modes wouldn't destroy the capacitor in a couple of seconds, what would is the capacitor been connected the wrong way round. Just stick another capacitor in circuit, making absolutely sure it's the correct way round, and try it again.

yes, backward cap , partial/full AC from the Wall Wart .

 

[ADWSystems]

If the capacitor is radial leaded, it could be the plastic shrink wrapped cover got spun around. In 20 years it has happened to me three times I can confirm. In each case there was remnant of the wrapper still stuck to the capacitor based with the + or - on it. Sure enough when I tested the circuit with the blown capacitor in the circuit (now open, only two deadend leads remaining) I could see the polarity mark matching with the DVM lead and voltage reading. Confirming I had the cap in the right way around. After replacing the capacitor with another from the same bag, the circuit worked fine. The only conclusion that has been able to be determined is the label spun on the cap.

 

[ronsimpson]

Maybe the cap is bad. (was bad) before you started.

 

[dr pepper]

I seem to recall a while back comming across a bag of electrolytics, maybe at a fest, the long lead went to the - marking on the label so messups happen.

 

[ci139]

the first simulation shows the optimistic max. for the constant current from the 1.0 A 15-0-15 V TF using the LM317 as a regulator

the second shows microbe-hour comparator use to switch modes -- however . . .
the charging cycle likely should be as follows

1. safe current limit charging e.g. about some max. 15A for automotive battery -- untill safe long term current is reached
   • PS! -- the mode "1." assumes you use a charger that has limited output voltage so as the empty battery becomes more full (it's terminal voltage rises) the decreasing voltage difference will drop down charging current (that becomes about 7A for 70A·h battery at ? say 12V of terminal voltage) - the specific numerical values depend on your battery type/model/technology
2. constant current charging -- typically 1/10C and less (one tenth and less of the battery capacity in A·h) less-equal 7A for 70A·h battery -- until the "critical voltage" is reached
   • PS! -- the CC current may be orders of less for some battery types
3. speculating - ? wait the voltage dropping to "full voltage"
4. apply constant voltage hold at that v. -- monitor charging current
   
5. IF the CV-current exceeds some limit THEN enter mode 6.
6. about 1A CC charging - until (2. 's -- "critical voltage") voltage is reached THEN loop back to 3.
   • PS! -- again the 1A applies to a "conventional" 60 to 80 A·h automotive battery -- you must find the correct value for your battery

all this should be calibrated for the condition and temperature of the battery (?measured manually/automatically before/at charging - assumed to be in range by performing "battery valid" tests) - or - the window of assumed values -- e.g. -- by knowing the parameters range for operational battery >> find the optimum charging parameter values that suit and won't worsen a random operational battery (of the battery type)

otherwise you can't do fire (/ turn ON) and forget - type of battery charger - and must monitor and manually control and interrupt/end the charge cycle

 

[DerStrom8]

You should also have a 0.1uF capacitor on the input to the LM317 and a 1uF capacitor on the output for stability/transient response reasons. Probably not the cause of your problem though.

 

[Colin]

Electro's left lying around can lose some of their "forming."
They become a resistive element and just heat up when connected.
This heat generates gas and they blow up.
It's hard to re-form a 50v electro with 25v
Just put another electro in the circuit.
22u is quite useless.
You need about 1,000u.
You don't know the value of the electro in the wall wort, add 1,000u at 25v will solve the problem. If it blows up, the wall wort is faulty.