# dumb cap question

Discussion in 'General Electronics Chat' started by skinner927, Feb 5, 2008.

1. ### skinner927 Thread Starter Active Member

Dec 31, 2007
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let's say you have a 100uF cap, it's rated for 35v, and you're giving it 12v, totally within spec. If You have that 12v running to that cap for.... forever, will it explode, or stop functioning after a while?

2. ### Papabravo Expert

Feb 24, 2006
10,135
1,786
It sounds like it might be an electrolytic capacitor. When the electrolyte dries out they tend to fail short.

3. ### skinner927 Thread Starter Active Member

Dec 31, 2007
36
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yes, it's an Electrolytic cap.
What's the ETA on something like that failing? Is there any way to do something different to prevent it from failing?

and when you say fail short, do you mean ground out?

4. ### mik3 Senior Member

Feb 4, 2008
4,846
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If you apply a DC voltage across it, the current which passes through it will decrease exponentially until it is fully charged. When it is fully charged no current will flow through the capacitor theoretically (practically a tiny amount of current flows).
If you apply an AC voltage across it the current through it will depend upon the frequency of the AC voltage. The current will be: i=(Vrms)*2*π*f*C

where Vrms is the rms value of the ac voltage in Volts
f is the frequency of the ac voltage in Hz
C is the capacitance in Farads

If u apply an AC voltage across the capacitor and the current through it goes over a limit (i think it will be noted on the data sheet) then it will get overheated and will fail or explode because i guess your capacitor is electrolitic.

5. ### skinner927 Thread Starter Active Member

Dec 31, 2007
36
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so the deal is, don't worry about it?

I'm mainly wondering what's the expected lifetime on it? I'm going to be running 12v DC to it for nearly 90% of it's life.

Can I get 3 years out of it before the cap dries up?

It's going in my car and it holds a charge when the car's off, but it's always getting power, so I was just wondering how bad it will really be.

6. ### SgtWookie Expert

Jul 17, 2007
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1,728
EVERYTHING electromechanical eventially will fail. It is not a question of if it will fail, but merely when it will fail. Unfortunately, components usually choose the most inconvenient times to fail, such as flights to the moon (remember Apollo 13?)

Mil-spec parts have a MTBF, or Mean Time Between Failure, or the average time a given item will take before it will break.

If you are using a component that is rated well above the expected operating environment, your odds of it lasting longer are greatly increased. However, automotive environments are notably brutal, particularly in regards to temperature.

Generally, components inside the passenger compartment have a relatively easy time of it, but the engine compartment is brutal. Wide temperature extremes, vibration, heinous chemicals, water, and corrosion all take their toll. If you're installing something in the engine compartment, it better be at least rated for the industrial temp range ( -55°C < temp < 150°C ) or it will not last long.

Be aware that there have been "bad" electrolytics being manufacured by certain overseas companies for a number of years, who I don't wish to name. If you want to be certain that your cap will have a long and healthy life:
1) Use one at least rated for the industrial temperature range.
2) Use a cap of recent manufacture, as electrolytics do degrade as a function of time.
3) Select a cap made by a U.S. manufacturer.
In order to avoid getting a cap that's been sitting around for a long time, order from a place that does a lot of business like Mouser.com or Digi-key.

7. ### skinner927 Thread Starter Active Member

Dec 31, 2007
36
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Thank you very much for all that information. I'll be using the cap inside the car, behind the radio and the ducts from the AC will unintentionally keep it cooler, and yes, it will be protected from condensation.

Does anyone have any experience with doing something like this? that is, keeping power on a cap. How long can I expect this to last and continue to work as "expected". I'm hoping to get at least 2 years out of this thing.

Any straight forward opinions/experiences? (yes/no)

Most companies have tested their caps under load for 1,000 hours, thats ~42 days, not much to stand behind or get an idea of what will happen.

8. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
OK, I'll ask the real question.

Why do you plan on keeping this cap supplied with voltage indefinitely?

Electrolytic capacitors do have leakage current.

Therefore, they will tend to drain your vehicle's battery, if left connected across it.

Once an automotive lead/acid battery discharges to 12.4 volts, it begins a process called sulfation, which coats the plates with a hard gook, making them less able to perform their designed function, thus shortening their service life.

Is this cap going to provide some kind of vital function?

9. ### thingmaker3 Retired Moderator

May 16, 2005
5,072
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If the DC is a steady level, expect the cap to last many years. If the DC is pulsating, then things get complicated quickly.

10. ### skinner927 Thread Starter Active Member

Dec 31, 2007
36
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Here's the circuit.

It's for my radio. It's from a different model year and cannot be turned on and off with the car. this one communicates with the ECU [Engine Control Unit] and the old one uses the regular 12v accessory line, my car is equiped with the old one and cannot support the ECU style. My option is to turn the radio manually on and off every time I get in the car or create a circuit that will "press" the button for me.

In the circuit, the LED on the far right is representing the switch on the radio.

I ask this question because one of the caps will always be connected to 12v, except when the car is on.

how bad could the drain possibly be? The car keeps the clock and the odometer on and I've never had a problem before, what's one more little cap?

And the cap gets "fully" charged then "fully" drained, it doesn't really pulsate at all.

11. ### arthur92710 Active Member

Jun 25, 2007
307
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If its just to turn it on and off a switch would be the simplest, cheapest, and will always work.

12. ### skinner927 Thread Starter Active Member

Dec 31, 2007
36
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yes, obviously. But it already works that way, i'm trying to remove the human element from it. We tend to make mistakes /forget the radio is on thus draining the battery.

13. ### nomurphy AAC Fanatic!

Aug 8, 2005
567
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You need to look at the datasheet for the capacitor you are using.

See attached for an example, and note the specs such as load life, ripple life, shelf life.

All capacitors have these types of ratings, which will vary with type and manufacturer. Note that this cap is designed to last 3,000 hours, minimum, if you stay within the given design parameters.

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