搵resistance問題



我要搵R1, R4, R5, opto 係4N25, Q1 係BC337-40

BC337 datasheet
[url=http://www.datasheetcatalog.org/datasheet/SGSThomsonMicroelectronics/mXytvuq.pdf ]http://www.datasheetcatalog.org/datasheet/SGSThomsonMicroelectronics/mXytvuq.pdf [/url]

a. What are the constraints you have in selecting R1?

According to the 4N25 and BC337 data sheet, the typical forward voltage of opto-coupler (Q2) is 1.15V at 25℃ and 0.7V in BC337-40 (Q1). So the range of voltage across is from 3.15V (5V - 1.15V - 0.7V). And the input current is 10mA. We can use ohm’s law (R = V/I) in order to find out the value of R1 which is 315Ω.

上面係我e 家既Answer, 有冇人可以指教下. 知唔知點解係用10mA, 唔係用input 既1A

P.S. 可能錯區, 但係我唔知可以貼去邊, 又見e 版既人quality 好高, 所以係度問

回覆 1# 的帖子

師兄!你可以Download番個4N25既Spec睇睇,呢粒Coupler的LED輸入電流 If (cont')最大都係頂得住60mA,而TYPICAL亦是只用10mA就可以推著粒LED,如果你想試下1A (記得接粒0.1ohm / 1W電阻),可以去試看看是否有煙出來!

[ 本帖最後由 edwardfcy 於 2009-3-25 20:11 編輯 ]

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原帖由 edwardfcy 於 2009-3-25 20:07 發表
師兄!你可以Download番個4N25既Spec睇睇,呢粒Coupler的LED輸入電流 If (cont')最大都係頂得住60mA,而TYPICAL亦是只用10mA就可以推著粒LED,如果你想試下1A (記得接粒0.1ohm / 1W電阻),可以去試看看是否有煙出來!

所以我奇怪點解佢specific current 係1A, 個source 應該係control current, DC5V.
anyway, 我R1 搵得arm 唔arm?

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The 5 V / 1 A power source is supposed supplying up to 1 A continuously. On the other hand, the opto-coupler 4N28 input LED requires only 10 mA typically to work. Therefore you need not care about the 1 A specification of the supply.

The 0.7 V saturation voltage Vce of Q1 (BC337-40) is the value when Ic = 500 mA.
For Ic = 10 mA, Vce,sat should be much smaller. Let it be 0.2 V or at most 0.3 V.

When choosing a resistor for non-critical use, you should always consider the preferred values,

e.g. E12 ( 10%): 10 12 15 18 22 27 33 39 47 56 68 82
http://en.wikipedia.org/wiki/Resistor#Preferred_values

So if your calculation result is a  315 Ω resistor, you have to choose between a 270 Ω or a 330 Ω resistor.

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