Post by thread:Signal Diodes

Hi I wish to know if anyone knows exactly what the characteristics are of the 1N4148 signal diode (a.k.a. 1N914). I have seen a circuit where the signal diode was connected in series with a LED to limit the LED's current. Is this possible? How well does it work? Thanks Werner Soekoe IS Manager Free State Legislature spam_OUTwsoekoeTakeThisOuTglobal.co.za Tel. +27 (51) 407 1109 Fax. +27 (51) 407 1137 Cell. +27 82 376 8383

In message <000f01bf89c5$b7592aa0$0a01a8c0@werner>, Werner Soekoe <.....wsoekoeKILLspam@spam@GLOBAL.CO.ZA> writes >Hi > >I wish to know if anyone knows exactly what the characteristics are of the >1N4148 signal diode (a.k.a. 1N914). I have seen a circuit where the signal >diode was connected in series with a LED to limit the LED's current. Is this >possible? How well does it work? I don't see as it's possible, the only reasons I can think of for having a diode in series is to feed it from an AC supply, protect it against reverse DC connection, or to drop the voltage fed to it by 0.7 volts or so (but why is a different matter!). -- Nigel. /--------------------------------------------------------------\ | Nigel Goodwin | Internet : nigelgKILLspamlpilsley.demon.co.uk | | Lower Pilsley | Web Page : http://www.lpilsley.demon.co.uk | | Chesterfield | Official site for Shin Ki and New Spirit | | England | Ju Jitsu | \--------------------------------------------------------------/

By dropping V you limit current (ohms law). Although I don't think this is a practical application of a diode, I have used them to drop V for various reasons, but not exactly for current limiting, that is just a bi-product. Anyway the 1n914 is obviously not the only diode that can do this, just a very common diode in any project. My 2 cents, Joe {Original Message removed}

In message <004c01bf8a81$6fb1c9c0$9341fc9e@oemcomputer>, Joe Hamilton <.....joeh100KILLspam.....CROSSWINDS.NET> writes >By dropping V you limit current (ohms law). Although I don't think this is >a practical application of a diode, I have used them to drop V for various >reasons, but not exactly for current limiting, that is just a bi-product. >Anyway the 1n914 is obviously not the only diode that can do this, just a >very common diode in any project. Sorry, I disagree!. Applying Ohms law shows that by dropping V you reduce current, you don't limit it, a 5 volt supply feeding through a silicon diode to an LED is going to pass a serious amount of current and destroy the LED. -- Nigel. /--------------------------------------------------------------\ | Nigel Goodwin | Internet : EraseMEnigelgspam_OUTTakeThisOuTlpilsley.demon.co.uk | | Lower Pilsley | Web Page : http://www.lpilsley.demon.co.uk | | Chesterfield | Official site for Shin Ki and New Spirit | | England | Ju Jitsu | \--------------------------------------------------------------/

At 06:50 PM 3/10/00 +0000, you wrote: >Sorry, I disagree!. Applying Ohms law shows that by dropping V you >reduce current, you don't limit it, a 5 volt supply feeding through a >silicon diode to an LED is going to pass a serious amount of current and >destroy the LED. > Yes, in general. It may be possible to operate an LED from a supply of the right voltage with no dropping resistor (they do have significant internal resistance, meaning the I/V curve does not go straight up when you get to the usual Vf, it has a significant slope). The original LED calculator from Clive Sinclair used this method to save power (and money and space). Personally, I don't use this method and I don't recommend it. It's too sensitive to the LED characteristics (which are not guaranteed closely enough) and the exact supply voltage and the temperature. =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-= Spehro Pefhany "The Journey is the reward" speffspam_OUTinterlog.com Fax:(905) 271-9838 (small micro system devt hw/sw + mfg) =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=

> In message <004c01bf8a81$6fb1c9c0$9341fc9e@oemcomputer>, Joe Hamilton > <@spam@joeh100KILLspamCROSSWINDS.NET> writes > >By dropping V you limit current (ohms law). Although I don't think this is > >a practical application of a diode, I have used them to drop V for various > >reasons, but not exactly for current limiting, that is just a bi-product. > >Anyway the 1n914 is obviously not the only diode that can do this, just a > >very common diode in any project. > > Sorry, I disagree!. Applying Ohms law shows that by dropping V you > reduce current, you don't limit it, a 5 volt supply feeding through a > silicon diode to an LED is going to pass a serious amount of current and > destroy the LED. "REDUCE, LIMIT, Geee, I'm Sorry" Hey I didn't say to go and burn up your led. I meant that by dropping, say .7 volts you "limit" current by decreasing the V to the load. 5 - .7 = 4.3. --- Now 4.3 - 1.9 (led V) = 2.4 --- 2.4 / .008 (led I) = 300. So for the led I just described you would use a 300 ohm resistor in series with the LED and diode. I never said to throw out ohm's law. {Quote hidden}> -- > > Nigel. > > /--------------------------------------------------------------\ > | Nigel Goodwin | Internet : KILLspamnigelgKILLspamlpilsley.demon.co.uk | > | Lower Pilsley | Web Page : http://www.lpilsley.demon.co.uk | > | Chesterfield | Official site for Shin Ki and New Spirit | > | England | Ju Jitsu | > \--------------------------------------------------------------/ >

In message <003601bf8b51$bce50240$9a41fc9e@oemcomputer>, Joe Hamilton <RemoveMEjoeh100TakeThisOuTCROSSWINDS.NET> writes >"REDUCE, LIMIT, Geee, I'm Sorry" Two very different words, with entirely different meanings :-). If a diode could limit the current, that would be ideal, but they don't - obviously one of those FET constant current devices would work even better than a series resistor. >Hey I didn't say to go and burn up your led. I meant that by dropping, say >.7 volts you "limit" current by decreasing the V to the load. 5 - .7 = >4.3. --- Now 4.3 - 1.9 (led V) = 2.4 --- >2.4 / .008 (led I) = 300. So for the led I just described you would use a >300 ohm resistor in series with the LED and diode. I never said to throw >out ohm's law. The thread was about using JUST! a diode to feed an LED, no resistor at all, which is why I said it wouldn't be a workable solution. -- Nigel. /--------------------------------------------------------------\ | Nigel Goodwin | Internet : spamBeGonenigelgspamBeGonelpilsley.demon.co.uk | | Lower Pilsley | Web Page : http://www.lpilsley.demon.co.uk | | Chesterfield | Official site for Shin Ki and New Spirit | | England | Ju Jitsu | \--------------------------------------------------------------/