Old 09-09-2009, 03:01 PM #1
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Default Tantalum capacitor in DDL

Will it make any difference if I use tantalum capacitor instead of normal electrolitic

and also will 47mF 16v be fine or I need 10mF

Thanks


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Old 09-09-2009, 03:29 PM #2
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Default Re: Tantalum capacitor in DDL

Tantalum caps in general are more expensive but are higher grade
than electrolytics.... You can easily substitute the electrolytic gor
the Tantalum...

Since the cap is used for despiking... the bigger (to a point) the
better.. Just remember... a larger cap will hold a charge longer.


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Old 09-09-2009, 05:34 PM #3
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Default Re: Tantalum capacitor in DDL

Only a pair of advices about tantalium capacitors (just cause i've already fried some of them )

They are much more sensitive to normal ones to inversion of polarity and overvoltage.

A polarity inversion put them on fire in few seconds (literally, i ignited one in 3 seconds, soldering it reversed)

And if you plan to use them, just double the voltage ..... i mean, with normal capacitors, if you use it on 5V, a 6,5V can be ok, for tantalium ones, better use 12 or 16 V ones ..... for 12V, usually are used 16V common capacitors, tantalium ones must be at least 25 V, and so on (this if you want that your capacitors have a long and happy life )

For the rest, they are better than common ones, in results
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Old 09-09-2009, 06:35 PM #4
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Default Re: Tantalum capacitor in DDL

^LOL, fire hazard. I think i will stick with my tried and tested electrolytics
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Old 09-09-2009, 06:39 PM #5
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Default Re: Tantalum capacitor in DDL

I haven't actually been using a capacitor on any circuit that I've soldered together in quite some time. That output capacitor has killed at least 2 of my diodes, and I've yet to have a single diode die when the capacitor was not there. So until a diode unexplainably dies while running on one of these drivers that doesn't have an output cap, I'll just leave it out.
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Old 09-09-2009, 06:41 PM #6
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Default Re: Tantalum capacitor in DDL

Why did the cap kill your diodes?
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Old 09-09-2009, 08:02 PM #7
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Default Re: Tantalum capacitor in DDL

I'm in agreement with PBD... as long as it is a Battery powered Voltage
or Current Driver...
If using external (non Battery) power to the V or C driver I would use a
small output cap and a larger input cap... just in case the supply introduces
some transients (spikes)...

If it is a boost Driver you have no choice but to use an output filter capacitor..

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Old 09-09-2009, 09:31 PM #8
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Default Re: Tantalum capacitor in DDL

What pullbangdead says has been my advice all along - leave off the capacitor. First, electrolytic caps are simply useless if you want them for spike protection - their parasitic inductance is way too high. Also, any "despiking" capacitance has to be very close to the regulator IC, for the same reason, so soldering it across the diode pins is again useless. The CC or CV drivers are made for ohmic loads. With a capacitive load, you may get a phase shift which in turn may start the whole setup oscillating, thus supplying current peaks that are higher than what you'll be measuring with a DMM. And possibly more problems. If you want to know whether (and how!) to use a capacitance, read the datasheet for the regulator IC.

You can use large elcos in an AC power supply, after the rectifyer bridge; but that's for smoothing, not for spike rejection.
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Old 09-10-2009, 03:47 AM #9
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Default Re: Tantalum capacitor in DDL

^^ Yeah, I stopped using caps a while ago because IMO they always seemed to do more harm than good.

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Old 09-10-2009, 04:34 AM #10
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Default Re: Tantalum capacitor in DDL

I've used "bleeder" resistors on power supplies and drivers. I calculate them to draw ~ 1 or 2 mA so the regulator always has some load.
This is an old school trick I learned 45 years ago !!!

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Old 09-10-2009, 06:48 AM #11
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Default Re: Tantalum capacitor in DDL

Good idea, however it doesn't work with "constant current" drivers obviously... here, you'd need a Zener diode with a voltage threshold above the normal load (laser diode or LED) and below the max rated output voltage of the driver as an open circuit protection.
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Old 09-10-2009, 07:39 AM #12
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Default Re: Tantalum capacitor in DDL

Quote:
Originally Posted by Gryphon View Post
Why did the cap kill your diodes?
If a connection between the diode and cap is broken (as in diode disconnected from the driver, cap still connected), it'll charge up the cap to the full max voltage of your driver. If that connection is then reformed to your diode, the capacitor will dump that full amount of energy through the diode at once, killing it. Hemlock_Mike's "bleeder" resistor could be a solution to this, but I've not had any trouble leaving the cap out.
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Old 09-10-2009, 08:36 AM #13
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Question Re: Tantalum capacitor in DDL

For LD's with battery powered drivers, i usually use a 100nf capacitor on the LD ..... most of the times, directly soldered on the LD, in parallel with a 10 Kohm resistor, and if the LD is wired far from the driver, also an 1N4148, for prevent any accidental inversion of the wires (i don't use just red and black, and sometimes this can cause confusions )

As far as i recall, i fried few diodes for my mistakes (overheating and overcurrents), but had not a single one fried for capacitors problems (still have around 12 or 13 PHR in aixiz, some 6X, some LCC and SOC reds, none of them never suffered anything from capacitors)

On my drivers that i solder directly on the LD, i putted a capacitor on the input and output of the LM1117, so there's already one (why have connections that detach from driver, anyway ? ..... i once ripped the pins of an old IR diode, bending bad the driver PCB, but never had a single soldering point "detached" ..... do you use right tools and alloy, for soldering ?)
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