三极管问题

cxwj2007

各位大师，小第莱鸟，想将48V直流电转换成12V，复杂的电路小弟不会，我想用一个三极管串在负载上分压，如图调节R3电位器使三极管BE间电压变化，从而使三极管CE间电阻变化，分压大小也变化，使负载上只有12V电压。不知这样是否可行，另小弟还有一个问题不明，三极管一般工作在开关截止饱和导通状态，若上述电路可行的话，三极管的CE间阻值可调，是工作在什么状态下的呢，多谢大家指点，不要笑话小弟，小第在此不甚感激！！！
(原文件名:~3U8(1@}IY(W2(0K_K2~H2J.jpg)

millwood0

it works if your load is constant.

otherwise, you need negative feedback.

jade1988

多大电流啊;电流小更简单用稳压管啊

Sullivan

回复【1楼】millwood0
it works if your load is constant.
otherwise, you need negative feedback.
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负载可变的就不能工作吗？还是要看作基准用的电阻和负载之间的阻抗比例。
这里不是已经有负反馈了吗？射随，电流负反馈。

dingtao213

42伏直接转12时电压差太高管子会发热.可以把42用电阻分压后再用LM2576负载电流大，或LM317负载电流小，转12伏

yeutay

这里不是已经有负反馈了吗？------沒有,除非射极不与R2相接通。

huayuliang

“三极管一般工作在开关截止饱和导通状态”，还有一个呢。。

dingtao213

截止，饱和，放大

millwood0

here is a quick sim, with the original design.

it is a dc sweep (x = V1 from 0 - 36v), and the y are the output voltage on R4, as R4 goes from 20ohm - 100ohm in 20ohm steps.

as you can see, as the load (R4)changes, the output voltage changes; and as the input voltage (V1) changes, the output voltage changes.

not a good thing for a power source.



(原文件名:transistor output.PNG)

millwood0

now, it is the same circuit, with Q2 being brought on line to add a degree of negative feedback: if the output goes up, it opens up Q2 more, which brings down Q1's base (thus its emitter voltage) -> lowers the output voltage.



(原文件名:transistor output 2.PNG)

we continue to have trouble with the case where R4 = 20ohm, but otherwise, once the input voltage goes above 16v, the output voltage starts to bend towards 14v (which is about the Q2's output voltage).

Q2 is called "Vbe multiplier".

forsuccess1

三级管一般工作在饱和导通截止状态，楼主的这个电路是要求C、E工作在导通状态，这是一个典型电路的变形，理论上可行！