What determines a low leakage capacitor?

I am trying to find info about what a low leakage capacitor is.

I read that aluminum electrolytic caps have a higher leakage rate than most other types.

How do you determine if a capacitor is low leakage?

I was looking at this one.

Specifications

Voltage rating: 250 VDC

Size: 14mm x 8.5mm x 4.5mm (W x H x D) Axial leads

Reply to
AK
Loading thread data ...

How fast does it lose it's charge just sitting there?

Metal film caps are all low leakage compared to electrolytics. There's something else call "dissipation factor" that only comes into play at some AC frequencies (how much power the dielectric wastes as heat).

What you are doing is low frequency and the capacitors aren't critical.

Reply to
default

ost other types.

Ok.

Thanks. I field tested my circuit on my patio.

Without the laser, ambient light is enough to keep buzzer off.

I passed my hand over the LDR from about a foot away and it sounded.

But it won't work from about 6 feet away which is about the range I need.

So it looks like even with a laser shining and someone breaks the beam, it may not sound if the light level is high enough.

Reply to
AK

Dump the LDR and use a photodiode instead.

--
This message may be freely reproduced without limit or charge only via  
the Usenet protocol. Reproduction in whole or part through other  
 Click to see the full signature
Reply to
Cursitor Doom

Would this work?

Silicon Photodiode with Daylight Filter PIN Photodiode Chip, BPW34 Features

Reverse voltage: 32V Forward voltage @ current: 1.3V @ 80uA Total power dissipation: 150mW Spectral sensitivity: 0.62 A/W Wavelength of max. sensitivity: 850nm

Short switching time: 20ns typ.

Reply to
AK

For a 'laser alarm', i.e. detecting a light beam, the presence of anbient light (generally white-ish and not modulated) is a guaranteed interfering signal.

If you use a photodiode (instead of LDR) your receiver can pick up any frequency from zero (direct current output) to 1 MHz, as opposed to zero to 1 kHz (for CdS, which has a storage time).

If you start with a laser, you can use a long black-painted tube to aim the receiver in one and only one direction. Alignment will be fiddly, though.

Laser or LED light also has a characteristic color, not common in nature, so some kinds of filtering will reject interference while passing the whole intended signal.

The best ( easiest) way to make it all work, might be to make the laser flicker at some frequency, and use a LM567 'tone decoder' that can pick up on one-and-only-one frequency of light-brightness-oscillation. Because both the laser and receiver require power, it could be good to package them together, and use retroreflective tape or other backscatter optics at the 'target'.

Reply to
whit3rd

Put the LDR in a 1 inch plastic tube,20 inch long and paint the tube inside with black flat paint(used for school boards). That will keep the ambient light out, and does not inhibit the laser beam.

Reply to
Sjouke Burry

LDR is too slow. He wants something that *briefly* interrupts the beam to produce a 'meaningful' output from the detector. The 'burglar' is not going to stand in front of the beam until the LDR has time to change resistance.

--
This message may be freely reproduced without limit or charge only via  
the Usenet protocol. Reproduction in whole or part through other  
 Click to see the full signature
Reply to
Cursitor Doom

You might try adding a potentiometer so you can adjust the sensitivity, then take some time and effort and install the detector in a light-tight tube with a non-reflective black inside coating, and position the detector so the only light it sees is the emitter (laser in this case).

6' should be doable even with your simple system.

You are using a laser for a light source. It should work the way it is, but if you think about it, wouldn't it make more sense to use a narrow very bright laser beam on a small area like a photodiode or photo-transistor? Concentrate the light on a small target then any extraneous light will be much weaker by comparison.

Reply to
default

LDRs are used at audio frequencies and work very well. They aren't fast if you are talking about data transmission rates, but in human terms they are very very fast.

His circuit just needs to drop below a reference voltage to trigger, speed isn't important.

Reply to
default

am,

I put it in a 3 inch tube.

It worked but then it was overcast.

I will try again when I have full sun.

The photo diode might work as I can adjust the laser beam to be wider.

Andy

Reply to
AK

I'm suggesting you keep the beam width narrow - unless it becomes impossible to aim. The sensitive area for a photo transistor or diode is less than a millimeter square. So keeping the beam narrow and bright will help.

But go ahead with the photocell I believe it can work too. The photo alarm I built for my motorcycle parking area was easily 6 feet. Across the whole carport ~8 feet and into a small laundry alcove. (down in Puerto Rico where pipes never freeze the laundry can be done outdoors) The carport provided shade from the sun so I didn't need a baffle to keep sunlight out of the detector.

Reply to
default

Cap leakage is sometimes defined as a time, the self-discharge time constant. That's how long a charged cap, connected to nothing, takes to discharge itself to 37% of its original voltage. It can range from minutes to years. Good film caps can be many years. Electrolytics vary a lot, minutes to weeks maybe.

Sometimes caps are rated for megaohm-microfarads, which is exactly this same time constant. Namely Rl * C, where Rl is the equivalent leakage resistance.

Film caps are usually way, way better than the circuit they are used in.

--
John Larkin         Highland Technology, Inc 

lunatic fringe electronics
 Click to see the full signature
Reply to
John Larkin

Thank god for bleeder resistors.

Reply to
Cursitor Doom

Big oil or film metal-can energy-storage caps are usually shipped with a bunch of bare wire shorting the screw terminals. This prevents killing customers.

--
John Larkin         Highland Technology, Inc 

lunatic fringe electronics
 Click to see the full signature
Reply to
John Larkin

Those are typically quite low F value caps, though. Not much energy can be stored in them. In any case, why not just send them out uncharged? Seems the obvious thing. :-/

--
This message may be freely reproduced without limit or charge only via  
the Usenet protocol. Reproduction in whole or part through other  
 Click to see the full signature
Reply to
Cursitor Doom

That would take too long. Dielectric absorbtion.

--
  When I tried casting out nines I made a hash of it.
Reply to
Jasen Betts

Basically a parasistic parallel resistance that drive some current inside the cap. For electrolytics, there are ionic recombination too. Also thermal shocks can affect the auto-discharge rate. For electrolytics,, those parameters merely affect the nominal value. The tolerance (+/10...20%) has no relation with auto-decharge.

The more you put caps in //, the more you put these resistor in // and

With caps in series, it is the opposite.

I think tantalum ones are the worst. And as already written above, mica and plastic films the best, and of course, air ones for low values. Oil and glass caps are rather good too but uncommon (UHT voltages). The auto-healing once "holed" lower their // resistance.

electrons and the // resistance is a matrimonial agency !

The ideal cap, though unreal, is the vacuum cap.

Reply to
Look165

A serious energy storage cap can store kilojoules, or 10s of kJ. 10 joules is considered to be leathally dangerous in some places.

Discharging 100 joules is an impressive event.

--
John Larkin         Highland Technology, Inc 

lunatic fringe electronics
 Click to see the full signature
Reply to
John Larkin

There are some effects that cause capacitors to develop bothersome charge even after having been discharged. One is dielectric absorption, sort of like charge hiding away in nooks and crannies for a while, before coming out again when you least want it. Another is the slow accumulation of stray free charge. That's probably self- limiting.

Anyway, it's safer to ship big capacitors shorted. I've had some nasty surprises in cases where this wasn't done.

Jeroen Belleman

Reply to
Jeroen Belleman

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.