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Avia
June 29th, 2017, 03:02 AM
Hi All,

I am building a small card for my TRS 80 Color Computer that will generate an 80 column display. While I can solder PTH components to a PCB, my technical electronics knowledge is very weak and just need a bit of guidance. A picture of the completed board (file photo), PCB layout and schematic is included here.

The board has a collection of radial capacitors, specifically
3x 10uF Elko
4x 220uF Elko
2x 22uF Elko

and 4 transistors
3x 2N5401
1x BC548

Regarding this particular application, I need to know a suitable voltage rating for the capacitors.

Thanks for any help.

Cary

393813938239383

KC9UDX
June 29th, 2017, 03:08 AM
It depends on the operating voltage. Take 150% of that and round up to the next nominal value.

Chuck(G)
June 29th, 2017, 07:31 AM
I don't see anything in the circuit diagram that isn't TTL level. 10WVDC caps should be fine--you can use 6.3WVDC as well, but the difference in dimensional size is very small, so probably not worth it.

Avia
June 29th, 2017, 08:40 AM
Thanks as always for the help guys. I have some 50V caps lying around, but will order some 10V - I am guessing 50 V would be excessive in this application?

Curious what the "W" stands for in your nomenclature Chuck?

Dwight Elvey
June 29th, 2017, 09:06 AM
W stands for 'W'orking DCV.
Surprisingly, using a 50V electrolytic for a 5V circuit is not a good idea.
Wet electrolytic are funny critters. While under voltage, the plates are
constantly changing. The voltage maintains the thickness of the oxide
on the aluminum electrolytic.
The problem is that if the voltage is too low, you tend to get small
areas that get thinner while the average thickness remains the same.
These areas tend to concentrate the electrical fields and eventually
the oxide ruptures.
Dwight

Chuck beat me to it.

Chuck(G)
June 29th, 2017, 09:15 AM
DCWV = DC working volts. Maybe it's an antique notation, but it signifies the difference between the continuous voltage and the peak instantaneous voltage that a capacitor is rated for. Also seen as "WVDC".

If the 50V caps fit, use them. There's no penalty in over-rating the working voltage.

Although probably not germane in your situation, there's another rating that comes into play when capacitors are subjected to "ripple" currents, such as in a power supply. That's ESR, or "equivalent series resistance". Most electronic passive devices are not simple single-dimensional objects. Resistors exhibit a bit of inductance and even capacitance. Inductors, likewise possess a similar amount of all three. It's useful to think of passives as being essentially the same device type, but with certain aspects emphasized.

So a capacitor, particularly an electrolytic one, can be viewed as a pure capacitance in series with a resistance. Clearly, when an AC-ish voltage is applied (i.e. high ripple current), that resistance is going to result in heating (I2R losses). As heat is the enemy of things electronic, a capacitor with a high ESR is going to enjoy a shorter lifetime in certain applications.

Electrolytics also exhibit a parallel resistance, called leakage. In many DC circuits, this can be disregarded, but in some, it's important to take into account.

Avia
June 29th, 2017, 09:27 AM
Excellent info...thanks Chuck!

GiGaBiTe
June 29th, 2017, 11:07 AM
DCWV = DC working volts. Maybe it's an antique notation.

"WV" is an archaic term in capacitor ratings which isn't really used anymore. The last time I saw it used was on Mallory capacitors from the late 70s/early 80s. Mallory still uses the archaic "MFD" instead of "uF" on their capacitors, but not WV.

eeguru
June 29th, 2017, 11:28 AM
Even the 150% (or 2x) rule isn't always best practice. Capacitors never have a static capacitance value. Capacitance is a function of voltage. The Farad value printed on the side is a nominal one. Check the data sheet for a C vs V curve. Capacitance may significantly de-rate as you approach the maximum rated voltage. eg. you can run a larger cap with lower max voltage and still get similar response curves.

Chuck(G)
June 29th, 2017, 04:39 PM
"WV" is an archaic term in capacitor ratings which isn't really used anymore. The last time I saw it used was on Mallory capacitors from the late 70s/early 80s. Mallory still uses the archaic "MFD" instead of "uF" on their capacitors, but not WV.

If you sell mil-spec stuff, WVDC or DCWV (take your pick) still has a definite and well-established meaning. Take this brochure from Evans (http://www.trendsetter.com/v/vspfiles/assets/datasheets/Evans%20Capacitor%20Company/THQA2-HT_Series.pdf).

But, unless you're in aerospace or the military, you don't see the term much. Consumer goods aren't designed for long service lives anymore, so any old junk will do. That doesn't render the term any less relevant. A good engineer knows the difference between surge voltage and working voltage--and knows how to derate according to environmental conditions.

But maybe it's like script kiddies calling themselves "computer scientists". I'm old and words used to have meanings. :(

KC9UDX
June 29th, 2017, 05:23 PM
Script kiddies as computer scientists; :) the one I see a lot recently is anyone who installs a popular Linux distru calling themselves a programmer!

(Saw a TV commercial the other day calling Trekies "Trekkers"...?)

GiGaBiTe
June 30th, 2017, 05:09 PM
Script kiddies as computer scientists; :) the one I see a lot recently is anyone who installs a popular Linux distru calling themselves a programmer!

(Saw a TV commercial the other day calling Trekies "Trekkers"...?)

I'm a certified CPU engineer! Trust me, I installed a gpu in one the other day..