# Current capacity of capacitors?

1. May 30, 2015

### Integral

Staff Emeritus
Recently I have obtained and have been maintain several pinball machines. On one of them, the 6v winding on the transformer is producing 7.3V. I am also having dificlty keeping lamps in this machine, they burn out faster (a week or so) then I think they should. My idea is that the 20% over voltage is the root cause. I have inspected the transformer hoping the someone had moved a connection to increase the brightness of the bulbs. I would like to turn down the intensity a bit. My first concept is to put a 2500mF (if I did the arithmetic right that should drop about 1V at 60hz. My uncertainty is how much current a cap will carry? I am not going to blow it up am I? There are many bulbs being fed by this so may be carring more then 1A . Basic question is what is the current carrying capacity of a capacitor?

2. May 30, 2015

### nsaspook

I would investigate the possible problem of too high input voltage to the transformer first because if the lamp voltage is too high so is probably everything else adding to stress and heat. If there are no tap adjustments, a simple variac or auto-transformer with correction taps on the input should work.

3. May 30, 2015

### Integral

Staff Emeritus
Good point, I have not checked other output voltages. Seems to me I looked at the input wiring to xformer and it was as expected. Thanks for the response.

4. May 30, 2015

### davenn

Capacitors have a voltage rating because their dielectric ( for a given capacitor) has a breakdown voltage
There is no current rating as current doesn't flow through a capacitor
Charge is added to or depleted from the capacitor plates.

Dave

5. May 30, 2015

### jim hardy

well, actually there is an AC rating but it's in the fine print and not always explicitly stated. Usually it's called "Ripple Current Rating"
Capacitors have a parameter called "Equivalent Series Resistance which is a combination of actual resistance of the capacitor plates and connections, and the work done by vibrating the polar molecules in the dielectric. Both effects produce heat inside the capacitor which must flow to the case and be dissipated.

Cornell Dubelier is an old line manufacturer who still has tutorials available. Page 10 of this one speaks to ripple current rating for aluminum electrolytics.

http://www.cde.com/resources/catalogs/AEappGUIDE.pdf

Sometimes they just give Equivalent Series Resistance and maximum power for the case size. I like to use parts from quality suppliers who give thorough information........

here's a Cornell-Dubelier datasheet that gives unambiguous ratings for "ripple current", which is the amount of AC that one shouldn't exceed.

http://www.cde.com/resources/catalogs/MLP.pdf

I think you should investigate why you have 6 volt bulbs in a 7 volt circuit. Bulb life is inversely proportional to the twelfth power of applied voltage.

(6/7)^12 = 0.157 . You'll get 15.7% rated life from a 6 volt bulb operated at 7 volts. Surely the guys who built it knew that.

have fun - dlgoff has a featured thread on antique electrical gizmos.

.

6. May 30, 2015

### nsaspook

I've seen this exact voltage increase when a 100vac (we have tons of OEM Japanese equipment with 120-> 100 step-down transformers) mains transformer in equipment is used on a 120vac circuit.

https://www.amazon.com/s/?ie=UTF8&k...vptwo=&hvqmt=b&hvdev=c&ref=pd_sl_5u6a81qumv_b

Last edited by a moderator: May 7, 2017
7. May 31, 2015

### jim hardy

Interesting !

Hopefully the transformer primary has taps for 100 and 120 volt that were wired at the factory depending on where the machine was going. I have some old jukebox transformers made that way. I always wondered what was the 100 volt tap for...

Feel that transformer and look it over carefully - if it runs hot it probably really needs to have either lower primary voltage or primary wired to a 120 volt tap.

8. May 31, 2015

### jim hardy

BTW here's the basis for lamp life in proportion to (12th power of voltage)-1
www.allegromicro.com/~/media/Files/Technical-Documents/an295012-A-Primer-On-Driving-Incandescent-Lamps.ashx

9. May 31, 2015

### Integral

Staff Emeritus
Thanks all for your input. The way the xformer is mounted I cannot easily see the primary winding connections. However there is a jumper block used to set up the inputs to the xformer. It is set for 110V. I need to get to the bottom side to verify that the wiring is factory original.

10. May 31, 2015

### jim hardy

hmm you get 7.3 volts from the lamp winding

assume they wanted 6.3

7.3 / 6.3 X 110 = 127.5 volts on a 110 volt primary would give you that.

In US 120 volts is only nominal so check your incoming. I think the utility is allowed 6% high which would be 127.2
If they set your service transformer tap expecting a big household heat or airconditioning load , you'll find voltage a bit high when those aren't running.

If you find it higher than that call the electric company and ask what's their limit. My friend was blowing light bulbs, we measured 132 volts , and our local utility changed the tap in his service feed for him.

see http://www.pge.com/includes/docs/pd...ergystatus/powerquality/voltage_tolerance.pdf

11. Jun 1, 2015

### dlgoff

While restoring some of the stuff shown in the my Beauty of old electrical and measuring things etc. thread, I've measured 6 volt tube filament transformer supply voltages at 7.3 volts. This is because, like Jim says, our present day ac supply voltage is ~120 volts, while back then 110 volt was the standard. Also, the 6 volt tube filament transformers windings supplied 6.3 volts as the standard. Check out this old Philco tube tester supply transformer (primary = 110 volts, filament secondary = 6.3 volts):

I've also noticed that some "old stuff transformer" voltages were specified while fully loaded.

So your transformer is good and you should have a bunch of lamps on hand ....

Edit:
BTW: Welcome to the beauty of the old.

Last edited: Jun 1, 2015