Electronics → 7905 + bad caps

Can the output of a 7905 be dragged down by worn (leaky bulging) capacitors?

The expected output voltage of -5v is closer to -3.84v.

Yes, they short out.

no, leaky caps fail open.

can oscillate.

Leaky capacitors are called leaky because they pass current. When that condition occurs they will pull the voltage down, sometimes to the point where they can cause the component to fail or if fused it may cause the fuse to open.

I think the OP and Public are referring to leaky as in the electrolyte is coming out.

Could very well be. Leaking electrolyte the capacitor can still have a current leakage and cause the voltage to be pull down. At any rate the voltage is not where it should be and if electrolyte is leaking from the capacitor it should be replaced.

Assuming it's aluminum. What's the input voltage? Is the 7905 hot. It would have to be leaking a lot of current to drag it down like that unless it's supplying a lot of current to whatever it's powering. If the dielectric fails it can short. .

Yes there seems to be a small amount of black stuff on the top of one (electrolytic type) along the imprinted stamp marks, the other ones have bulged upwards (same type and make). No leakage of fluid out the bottom (the manufacture didn't mount them flat against the PCB for some reason).
The damaged ones ones are:
2x 330 uF (bulging) and 1x 56 uF (bulging and leaked out the top)

The 7905 input is powered by the output of a SMPS, and has no heat sink. According to the label for the whole PSU the output is 5v 1A and -5v 0.5A, Input is 120-250AC 50/60Hz. It powers a 8 port KVM, everything seemed fine, then the video would blank out for a second (never did this before), and did so at an increasing frequency until it totally crapped out.

Will scope it tonight and see whats going on.

Since the input is sourced from a SMPS, I would recommend using Low-ESR caps rated for high-frequency ripple current... (such as Rubycon YXG series or equivalent.)

Let us know what you find. See if it is oscillating.

The caps not flat on the board could be because what ever assembly technique they use they had problems cleaning under the cap.

and leaky caps fail closed.

Oh ya is oscillating all right. (forgot to mention coupling was set to DC)

I don't have any ESR caps as Turpiops_G recommends but do have a few 330uF's laying about, but no 56 uF, the closest I have is 47uF; I think I will try replacing them and see if that flattens the output.

78** series regulators do have a tendency to go into oscillation depending on the type and condition of the output capacitance and load.

For this reason a common practice is to parallel a small value capacitor (0.1uF most often used) with the larger electrolytic capacitor in order to suppress high frequency oscillations. In fact, it's even recommended in the datasheets.

Check to see if such a capacitor has been included in your device. If not, it would be a good idea to add one after you replace the faulty electrolytic. This will further help to ensure oscillations don't happen and may even prolong the life of the electrolytic.

In this usage the actual value of the cap doesn't matter that much -- a 47 uF should be a good enough replacement for the 56 uF as long as the voltage rating is equal or better.

Also, as it was already mentioned, adding a 100 nF ceramic is a very good idea. You can actually add one before and one after the 7905, if they are not already there.

Ok more information, Vin to the voltage regular is -7.72v

I have replaced the caps and success! I am now getting -5.01 volts and 5.08 volts on the output. There is a sizable (mV range) ripple on both outputs which seems to be coming from the SMPS system. My selection of ceramic caps is limited messing around with the ones I did have made no improvement or change to the -5 output, and also on the 5volt output. Could be a bad design, anyways I am going to do a circuit diagram for the secondary side.

Check with a scope the output from the SMPS.

If the caps on the output of the SMPS are drying up, it is possible that there is too much ripple for the linear regulators to handle.

The linear regulators won't get rid of all the ripple, in fact the higher the ripple frequency, the lower the ripple rejection of the regulator.

But, even at SMPS operating frequency of 100kHz I'd expect the output ripple of the linear regulator to be in the low single digits mV.

Rule of thumb, for a 8V SMPS, output ripple under worst case condition should be no more than 80mV. More than that could indicated dried up capacitors on the output of the SMPS, or a poorly designed SMPS.

I got here late, but yes the 79xx series regulators are very prone to oscillation (as they almost always will) of you don't have good caps close to the device. Preferably on both the input and output. 10uf is generally enough to keep them stable, more is better.

agree with TheMG . at 7.7 they are operating close to the minimum input voltage for the 7905. Any sizable ripple is going to find it's way through.

Attached is a PDF of the secondary side.

I have no values for L1 as its got no markings on it, less the transformer everything on the secondary side is wedged into an area about an inch square, it might be possible to tack on a few ceramic caps on the solder side though there is a limited amount of clearance between the board and the chassis.

Caps marked with ** are the ones that got replaced.

Am I missing something, or does D4 look backwards for a -V source... ?

I'd add a 0.1 uF (104k) Ceramic cap across C14 and C15, and definitely consider changing C11 and C14 to a Low-ESR electrolytic cap... Ex: »www.digikey.com/product- ··· /3134191 (yes, it's 330uF @ 25v, but it should still fit.)

No doubt the fault was mine charting it out (if its the only error I made I will be happy - made by digikey's free online software and I have to say its a nice tool, never made use of it till now).

Any reason to also upgrade the down side filter caps C15 and C12 to the same series or would it just be a fun exercise in soldering that wouldn't yield much of an improvement?

Everything is clustered around the linear voltage regulator and that little 7905 does get warm when its powering the KVM, no doubt it was cooking the caps to some degree.

It probably wouldn't hurt to change the output caps to Low-ESR types as well, as they should have better HF ripple filtering performance (and longer life)... But, it's also a matter of price/availability/size, as well...

Digikey has some 56uF Low-ESR caps in stock, but the lowest voltage ones they have are 35v (not really a problem in itself), but they are 6.3 mm x 12.5 mm... »www.digikey.com/product- ··· dor=1189 If they will fit, and you're buying the other ones from them anyway, then I'd go for it myself...