Dell NPS-250KB caps

[stevo1210]

Hello Everyone,

I have a Dell NPS-250KB PSU with a 200V 560uf cap that has been squashed from an impact with something. The cap is a Nichicon LU(M) series. I have unsoldered the cap, only to find that I haven't got any caps close to 560uf.
The only caps I have are Nippon Chemicon 470uf 200V and Tosin 200V 470uf.
Both of these caps are from dead PSUs. Will they be suitable as a replacement for the 560uf cap or will I expect an explosion if I tried??

Is this Nichicon a General purpose cap or a Low-ESR cap?? I have no idea why that cap has tape wrapped around the bottom of it....

All the other caps in the PSU are Ltec and still look fine so I'll leave them alone. I think there is one Nichicon 1000uf 16V though.

Thanks.

[Rainbow]

Re: Dell NPS-250KB caps

The 470uF caps should work. Replace both original caps!

[stevo1210]

Re: Dell NPS-250KB caps

The 470uF caps should work. Replace both original caps!

What do you mean by replace both original caps?? The second Nichicon cap is undamaged.... or is it better to replace both because the 560uf cap won't like the 470uf cap and create some form of an explosion??

Thanks.

[gonzo0815]

Re: Dell NPS-250KB caps

Depending on the mains voltage, both caps could be wired in series, to make a virtual 200uF /400v capacitor (if switch is in 220v position).
But this is not a preferred operation mode of electrolytic caps.
So it is strongly recommended, to use caps with the closest difference in spec, usually from the same batch to avoid early degeneration.
I do not know, if this is an issue at 110v mains, but even there it is not good to have unbalanced caps, as this would limit the voltage double.

[starfury1]

Re: Dell NPS-250KB caps

We are 240V AC here Gonzo so it would be in the 220 Volt position
(side point and not sure I think they have changed us to 230 VAC inline with the rest of the world)

If the caps are in series and are just hung across the rectified mains DC
Is what your saying Gonzo,
different caps will have different specs and one may tend to do more work then the other?

so the advice is,
to use two caps which come from the same batch so the specs will be a lot closer and they will share the work load more evenly ?

Agree replace both caps with same type dont use two odd balls

I think thats what you mean.

There is a question in my mind about the switch setup 110/220...thats bugging me

[starfury1]

Re: Dell NPS-250KB caps

I am not familiar with the supply you are posting on but to add my first post

To your original question yes 470 uf would work (slightly degraded specs)

Just not really a good idea to mix the capacitors used here
for the reasons pointed out by Gonzo

There is a method of forcing more even sharing by using to same value resistors in series (known as a voltage divider) then in parallel with the caps.
(basically you end up with same value resistors in parallel with each cap)
but they must be suitable to this application voltage wise.
(I think and not totally sure they would be like 270K 1 Watt types)

You may find 2 resistors doing this job or possibly unfilled positions for them if memory serves me.

Anyway to expend on the above, here is the circuit

Original page source Notes on the Troubleshooting and Repair of Small Switchmode Power Supplies

Notes on the Troubleshooting and Repair of
Small Switchmode Power Supplies
Version 2.84 (11-Feb-07)
Copyright © 1994-2007
Samuel M. Goldwasser
--- All Rights Reserved ---

For contact info, please see the Sci.Electronics.Repair FAQ Email Links Page.

Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied:

1. This notice is included in its entirety at the beginning.
2. There is no charge except to cover the costs of copying.

just to keep them happy

Please note this only applies to older PSU's that have a Voltage selector switch on the PSU for 115V / 230V



Gif image Here

Assuming it is not a wide compliance 'universal type', a common way to do this is with a jumper (or switch) in the line input circuitry below

With the jumper, J1, installed, the circuit is a voltage doubler for use on 115 VAC. (D3 and D4 never actually conduct because they are always reverse biased.)

With the jumper, J1, removed, the circuit is a simple bridge rectifier for use on 230 VAC.

As a Voltage doubler



As a bridge rectifier circuit



So the Total original capacitance would have been more like 280 uf @ 400V

With 470 uf in series it would be 235 uf @ 400V

Here's a page on Voltage Multipliers

I should add that these type of caps aren't that cheap to buy so can understand why you want to use the 2 you got to hand

Its up to you but not recommended

HTH

Cheers

[starfury1]

Re: Dell NPS-250KB caps

Just to add a bit further to this

(had to find PeteS post)

1. Description of PSU Sections

Rectifier & Filter
The rectifier and filter convert the AC input voltage to DC. The input voltage selector switch is part of this circuit. In the “115V” position, the rectifier circuit is configured as a voltage doubler. In the “230V” position, the rectifier circuit is configured as a full wave bridge. In either case, the unregulated high voltage bus is about 300VDC. If the switch is in the “115V” position in Europe, the metal oxide varistors (MOVs) in parallel with the two input electrolytic capacitors will fire, blowing the fuse. If the MOVs fail (usually blowing up) before the fuse blows, the two input electrolytic capacitors, usually rated 250V, will vent. If the P/S is operated in the US with the switch in the “230V” position, the P/S may seem to operate OK in a system that isn't fully loaded. In a fully loaded system the P/S may not operate, or it may perform erratically. Either way, the P/S will run hotter and its life may be reduced.

Here is a photo of a blown cap in a junk supply...notice how anything that maybe used for safety etc has been mostly not used or probably cheap or sub standard.
and there is worse then this out there.
Think this was set as a 240 Volt only model
(thats if J1 is the J1) Will check that

( you will find this supply in the picture thread page 7 )

Anyway that should be more then enough info on whats happening at the front end of these psu's ...well the ones with 115/230V type switches

Cheers

Look at whats not there between the caps to start with

[stevo1210]

Re: Dell NPS-250KB caps

Thanks for the information on the PSUs starfury1, gonzo0815 and Rainbow,

Since 470uf caps will work with degraded specs of course, I guess I'll have to use what I've got here since I cannot find 560uf 200V caps that are the correct size. The diameter of the current caps is 20mm wide. The ones that are stocked at the electronics store is 25mm wide.... another problem is that they are quite expensive.... the PSu only costed me $7. A 200V cap is around $5-6AUD by my estimates.
So I'll just use the 470uf 200V Tosin caps in my dead 200W Delta PSU. After the recap, hopefully the PSU will turn on properly and not create some kind of deadly explosion.

Thanks.

[Rainbow]

Re: Dell NPS-250KB caps

Check that two resistors (that are connected in parallel with the caps) too. At least one of them is probably open. Once I repaired a Compaq PSU with one cap blown and that resistors had different values (different color codes)! Don't know if it was a mistake (design/manufacture?) or was it intentional? Anyway, replaced both caps and resistors (with equal value) and it worked.

[starfury1]

Re: Dell NPS-250KB caps

Was it working when you got it and then failed or did you buy it failed ?
Replacing those caps it will probably fire up

Do check the bridge diodes fuse etc Thought if you haven't ready.

Remember it hangs across the mains so be careful and double check your work.

Did you look to see if there were positions for those resistors I was talking about above ?
or if they were fitted
since your using used caps(not really a good idea but understand why)
if there are resistors, they should make the series capacitor arrangement less at the wim of the caps specs and real condition by forcing equal voltage across them

Anyway thats the theory....

Here is what that stage should really have in it...well mostly



Resistor R2 R3 at the end
Bit on "Voltage sharing" half way down this page
A pdf see here page 4

Anyway Not the guru here hopefully someone will offer their advice on this

Someone DID thanks Rainbow


As to the Tape on them ...no idea maybe as a little extra insulation to keep the cans better spaced?
it seems to be used a lot on the smaller dia caps ????
Anyone know for sure?

As to ESR not really overly important with the mains filter Caps
ripple maybe a more important spec to worry about
At this point it behaves as an unregulated DC supply
with a voltage of 1.414 X 240Vac = 340 Volts DC
(rounding a bit and barring any losses)


I couldn't find 560uf 200v at RS or or Farnells...470uf or 680uf 200V was about it (here's a possible 470 uf 22mm dia)

Anyway good luck with it and again be careful

[starfury1]

Re: Dell NPS-250KB caps

There is a fair chance what Rainbow said is the case

if you have a look at my photo of the stuffed psu you will note no R2 but possibly R3
I have yet to look a the photos I took to confirm these are in fact those resistor positions on it.

I'll post back when I know

BTW I was in the middle of doing the above post when rainbow must have posted
So I didn't see his till I posted mine but had time to edit mine with a thank you

Cheers

[starfury1]

Re: Dell NPS-250KB caps

Ok I checked
from the photos, most definitely check (replace) those resistors

No R2 which should have been across stuffed cap
R3 is there (right next to the heat sink) was across the OK cap

So when they build that supply it got left out
(along with a lot of other things)
and thats why that ruby failed most likely


let us know what you find

Cheers

[stevo1210]

Re: Dell NPS-250KB caps

Was it working when you got it and then failed or did you buy it failed ?
Replacing those caps it will probably fire up

I didnt even test it when i bought it home. It was like that when I bought it. The PSU was against something and dented the casing of the PSU, taking with it an internal capacitor. It worked well before the corner of the PSU was crushed, but ever since that, I'm too scared to turn it on.

Thanks.

[starfury1]

Re: Dell NPS-250KB caps

Well not being able to see the damage and from the photo of the cap, to me anyway ....that looks like "use" type damage...but I could be wrong


I would give the whole thing a good visual look over looking for any cracks etc in the pcb if it has been hit with something

I should have added that in this case.......Disclaimer first

General Disclaimer follows;
Please Bear in mind you are working with Mains Rated Voltages here and they are deadly so if you are unsure of your abilities or the legalities in this area please seek the services of a suitably trades qualified person
Disclaimer: ...In some states or countries it my be illegal for a non qualified persons to work on Electrical Mains or mains rated equipment


Generaly speaking ;
In short, You kill yourself don't blame me you have been warned!!!

Since you are replacing mains side components

The series light bulb trick (link)

It might be an idea to use the tV tec's trick of a lamp in series with the mains side if the psu.
(this trick does not work in all cases see link above)


What this mean is you put a 240 Volt 40W lamp in the active line to the psu
(you literally cut the active line and put one side of the lamp to the power point and the other to the psu)

Active---------------lamp------------PSU Active (Brown)

Neutral------------------------------psu Neutral (Blue)

Earth -------------------------------psu Earth (Green yellow)

The lamp acts like a big power resistor and if you supply does have a short (Active to nuetral or earth) on the mains side anywhere the lamp will light up and stay lite up.
This should limit current and hopefully save the psu from self destruction.

I would put a light load on the 5 volt DC rail of the psu with the lamp as above
(old HDD whatever you have that will load the psu a bit)

What should happen is if all is well,
it will briefly light up and go out or be very dim
as the mains caps charge up..
then if thats the case check the DC voltages on the supply

If all the voltages checks out ok you are probably up for a test run in the box.
(or further load testing if you like)

As I said, this side is lethal so do be careful OK

HTH

Cheers

[starfury1]

Re: Dell NPS-250KB caps

Should add its also a good idea to have a 1:1 (240V / 240V) transformer as well
but these are not commonly available from what I am aware of

If you don't understand whats written here and above the best option is leave it alone
Get it done by a professional service Centre

[stevo1210]

Re: Dell NPS-250KB caps

Well, since this is quite Risky, i'd better just trash this PSU and buy a new one. The casing on this one even needs a respray because it is rusted in some parts.
I wish they still sold FSP PSUs here. I had one the other day and traded it for a Jetway motherboard + AMD Duron 750.

[starfury1]

Re: Dell NPS-250KB caps

Well really just pointing out that anything that hangs off the mains directly does carry a potential for electrocution.

It should be treat with caution and the up most respect

So really the warnings are to emphasize the potential risks involved
(for the forums protection, forum members and casual readers of this post)

Also there are possible legal issues depending on where you are,
So you would have to check your state, country regulations in regards to this.

Like in QLD I think anything now to do with mains (even if on a 3 pin plug top) is the domain of lic sparks
There was a big thing on this in our last standing hobbyist mag SC on this issue
(more so with kit construction and mains side of this with wiring mains transformers etc)

The fact it has sustain some sort of physical damage is of concern too.
(since I cant see it to evaluate it)

If you got a mate thats an Electrician they should be able to help you with this.
As the chances are its reparable.

[PeteS in CA]

Re: Dell NPS-250KB caps

That power supply was made by a division of Delta Products, so it may be worth some effort.

[Spacedye69]

Re: Dell NPS-250KB caps

Or I could sell you a used FSP or two.

[stevo1210]

Re: Dell NPS-250KB caps

If you got a mate thats an Electrician they should be able to help you with this.
As the chances are its reparable.

I talked to my father, who is a qualified electrician, but when I showed him the PSU.... he had no idea what I was on about....
But he did say that the resistor next to the cap cant be damaged if the cap was physically damaged while there was no power....

Well, I thought i'd include some images of the damage to the PSU.... In the first picture, you can see where the damage took place.... theres a deep dent on the casing.