choice of capacitor

[fifahaywire]

i made a buck converter for an LED driver and i tried different types of output capacitors and found out that if i use the one on the right (refer picture), the LED will start to dim out on it's own even when i didn't off the supply, while if i use the one on the left (it's the blue one, polyester capacitor), the LED will light up like normal and turn off only after few seconds i turn off the power supply. why is this so?
View attachment 63700

also this website show the blue capacitor i'm using **broken link removed**

actually i'd prefer to use a smaller size capacitor (other than electrolytic) that is the reason why i tried the one on the right. but apparently the output isn't what i want. and just to add in, i did put in a current limiting resistor in series with the LEDs (just in case if it's the reason why but i'm not aware of it )

please advice

 

[alec_t]

Seems to me the cap on the right has excessive leakage current.

 

[fifahaywire]

Seems to me the cap on the right has excessive leakage current.

do you mean it's damaged or...?

 

[alec_t]

It could be damaged. Is it a polarised cap (with a + or - marking)? If so, that type can have much greater leakage current than a plastic film type (as on the left), especially if it has been inadvertently connected the wrong way round at any time.

 

[fifahaywire]

It could be damaged. Is it a polarised cap (with a + or - marking)? If so, that type can have much greater leakage current than a plastic film type (as on the left), especially if it has been inadvertently connected the wrong way round at any time.

i guess it is damaged because the first time it worked fine. after i tried it again, only then it started to produce the unstable output at the LEDs.
that's a ceramic capacitor i think because it doesn't have any + or - markings.
are you saying that plastic film capacitor type is better for this application?

 

[alec_t]

are you saying that plastic film capacitor type is better for this application?

I'm not saying that, but I think from your experience you've proved that the plastic film type works well .

 

[fifahaywire]

I'm not saying that, but I think from your experience you've proved that the plastic film type works well .

i did search about it. polypropylene and polyester cap has the lowest ESR values. i guess that's good?

 

[alec_t]

i guess that's good?

We were talking leakage; but low ESR is good to have too.

 

[unclejed613]

the yellow blob looks like a tantalum cap. you say it has no markings whatsoever? tantalum caps are polarized just like electrolytics, but are unreliable unless you use a cap rated for at least twice the circuit voltage. the yellow blob epoxy should be thermally sensitive, and that's how they mark them. laser etching leaves the lettering not only etched in the surface, but a dark brown color as well.

 

[fifahaywire]

the yellow blob looks like a tantalum cap. you say it has no markings whatsoever? tantalum caps are polarized just like electrolytics, but are unreliable unless you use a cap rated for at least twice the circuit voltage. the yellow blob epoxy should be thermally sensitive, and that's how they mark them. laser etching leaves the lettering not only etched in the surface, but a dark brown color as well.

is it a tantalum cap or ceramic? i'm actually unsure because i have it for quite some time already.

anyway, do you know if this capacitor is required for my bridge rectifier from 240Vac? View attachment 63891
or can i just use the blue one (it is rated 1kV)?View attachment 63892

 

[unclejed613]

this (from your picture) is a tantalum cap
View attachment 63895

the big black can shaped one is an aluminum electrolytic. you never mentioned that one. your picture in your first post was the tantalum and a film cap

 

[fifahaywire]

this (from your picture) is a tantalum cap
View attachment 63895

the big black can shaped one is an aluminum electrolytic. you never mentioned that one. your picture in your first post was the tantalum and a film cap

the first two (yellow blob and blue box) capacitor is for the output of my buck converter.

while, my second question, is for the second two (electrolytic and the small blue) capacitor is for the output of my bridge rectifier. i'm just wondering which from this both is more appropriate with a source of 240Vac