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Old 03-06-2010, 07:38 AM #33
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Lightbulb Re: Why NOT to test your diodes on batteries!

Anyway, it's cheap enough to just made a constant current driver, to keep around for test your leds ..... just use the old, good, abused LM317 circuit ..... with only a pair of modifications .....

Place only a small capacitor on the output, like, 100nF ceramic in parallel with 1uF tantalium, and in parallel to them, place a 100 ohm 1W resistor, so you're sure that the capacitors don't have any residual charge, when you connect your diode ..... also, between the output of the LM317 and the capacitors, place a 3,3 ohm or similar, 1W, resistor, for safety ..... and, ofcourse, no on-off switches, just a pushbutton, so you cannot forgot it turned on (you can also place a led with its resistor, on the output pin of the LM317, just as indicator)

You can power up this circuit from your bench PSU, from a battery pack, from a 12V wall adaptor, or just incorporate it in the box, whatever you want ..... and for the ones that want and find good spring contacts, other than the wires with the alligator clips, you can place on it also "spring contact" sockets (can be helpful for test new, long pins LDs)

After all, you don't need a long time driver, here, but just a safe way for check your LDs, so this type of circuit is a good solution, is safe, is cheap ..... and if someone want to spend some more money and hook on it two lcd meters modules, one as voltmeter and one as ampmeter, you also end with a tester that not just test your diodes, but also gives you VF and current .....


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Old 03-06-2010, 08:11 AM #34
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Default Re: Why NOT to test your diodes on batteries!

excellent post HIMNL9
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Old 03-06-2010, 12:06 PM #35
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Default Re: Why NOT to test your diodes on batteries!

I've build my own laser diode drive, but it doesn't work wel with blueray diodes, the voltage drop of them is too large. But for testing I use this: I pick a large resistor and use that in series with the diode.

The voltage over the diode only varies a few tens of mv even if the current doubles, so the voltage drop will be approximately constant. But the resistor in series will make the load look nearly like a normal resistor (linear response between voltage and current), I pick a resistor that has about 5~6V across them at the operating current. This way, a stable variable voltage source supplies a stable variable current to the diode. Any voltage spike will be mostly across the resistor and only create a small current peak.
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Old 03-06-2010, 02:43 PM #36
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Default Re: Why NOT to test your diodes on batteries!

Lasersbee is of course correct. Not that anyone doubts that . I am no electrical engineer but I deal with the RC helicopter hobby and we must know the capacity and the current capabilities of the batteries.

LiPo and LiIon are pretty close the same behavior as far as ratings. Manufacturers of LiPo cells rate their battery by capacity and by C rating. Think of this as a water storage tank with a water hose and a constant speed of water coming out(in general current flows about the same speed). Capacity is how much total water is in the storage tank (mah). The C rating is how wide of a water hose you can use to pull out the water without abusing the battery. The wider the hose the more water comes out and the more current. Of course the manufactures of some LiIon batteries do not state the C rating. They might just give you the max amperage instead of a C rating or as most... nothing at all.

The C rating for LiPos are usually given as a continuous and burst rating(for some number of seconds) but for simplicity we will just use it as a max rating. The C rating or max amperage is mainly to let you know how wide of a hose you can use to pull the water out(current) before you start to adversely affect the capacity and voltage holding of the cell(s). You can use a wider hose to pull more water than the current rating but the battery will degrade rather quickly and you will not get as many charges form it and voltage will sag more when under loads. Or if you go too far over it could explode/melt, etc. There are no manufacturers that state their actual shorted current rating that I have ever seen. Go over the max ratings at your own risk.

Example of a battery rated for 1000mah (1ah) and a max C rating of 1 (or max current of 1amp):

You could safely pull 1 amp safely (1000 x 1). Keep in mind that LiIon batteries are not meant to be pulled all the way down to 0v or they will be damaged so you could not get a full hour from these batteries without some reduced capacity from the battery.

Example of a battery rated for 1000mah and a max C rating of 2 (or max current of 2 amps):

You could safely pull 2000 mah (2ah) but it would be drained in 30 minutes if you ran it all the way down(which you shouldn't do if you want it to last).

The reason LiPo battery current capability is rated as C ratings is mainly useful when you start buying LiPo packs in series or parallel.

My RC helicopter battery is a 6s 6500mah (6.5ah) battery with a C rating of 35C continuous and 60C burst. The 6s represents 6 cells in series.

This means I can pull 6.5 X 35 = 227.5 amps continuous out of the battery safely without hardly any degradation to the battery over the normal degradation expected with LiPo cells. I usually pull only 40 or so amps average and 105 amp bursts though (I have an in-flight data logger that records amps, volts, RPM, etc that I analyze after each flight).

Keep in mind that battery manufacturers over exaggerate their ratings for PR purposes so I suspect that around 150 amps continuous or so before I start degrading the battery capacity of the battery. We stress these batteries pretty good in the RC Helicopter hobby .

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Old 03-07-2010, 08:38 AM #37
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Default Re: Why NOT to test your diodes on batteries!

Although this thread is based on alkalines, Lithium batteries are even worse! These things will supply much more peak current than a standard alkaline battery, expect, you don't really want to test that, since a lithium battery isnt exactly something you want overheating.

Standard batteries do not have a discharge rating, as the manufacturer sells most of the batteries to regular consumers that would just get confused with all that extra information. If they can put the batteries into their TV remote, or whatever else you may be thinking of (), that's all they care about.

The analogy of the water tank is a good one.
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