# Series Transistor Regulator

Discussion in 'General Electronics Chat' started by Roam, May 5, 2011.

1. ### Roam Thread Starter New Member

May 5, 2011
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I'm new to these forums so just saying hi before I make my post. The following is the diagram of a series transistor regulator. Could anyone explain, what factors here would limit the maximum current that could be supplied by this transistor-regulated supply? Any explanation is greatly appreciated.

$R_L$ is the resistive load and $V_o$ is the output.

2. ### R!f@@ AAC Fanatic!

Apr 2, 2009
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Welcome dear.

MAX current is always limited by the source.
In other words, the tx supplies the current demand and the series element regulates the current.The smoothing cap comes into play when the rectified AC dips..which is known as ripple.

Last edited: May 5, 2011
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Dec 26, 2010
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The transistor will also have limitations due to its own current rating, and the amount of heat it can safely dissipate. If either of these results in a lower limit than other factors, it may define the limit overall.

There is also a more basic limit in this simple circuit. Rz, 100Ω, supplies the Zener current and thr transistor base current. If we know the rectifier output voltage, the Zener current and voltage, and the transistor current gain we can estimate a maximum output current.

Example: Rectifier output =10V, Zener voltage = 6.8V Zener current =10mA, transistor current gain = 50 times.

Current in Rz = (10V - 6.8V)/100Ω = 32mA. If the Zener needs 10mA, the current available for the transistor base is 32mA - 10mA = 22mA.

Thus the maximum output current in this made-up example would be 22mA * 50 = 1100mA or 1.1A

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4. ### THE_RB AAC Fanatic!

Feb 11, 2008
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About the only useful information in your schematic is that the main filter cap is 1000uF so that would *suggest* maximum amps at about 1 amp, possibly 0.5 amp.

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5. ### BillO Distinguished Member

Nov 24, 2008
993
137
There is no current limiting designed into this circuit. Shorting the output will likely take the transformer, the pass transistor, the bridge or any combination thereof. If you were to build this design around a specific current capability, Id suggest a fuses on both input and output.

Edit: That being said...the transformer, the bridge and the pass transistor all have to meet the current requirements with a little head room to keep everything cool and happy. For instance, it you required a 12v output @ 3A I would specify a 12.7v 4A transformer, a 12.7V 1W zener, Rz at 1/2W, a 6A bridge, a 6,800uf 25v Cs, and a TIP35 pass transistor with a decent heat sink enough to dissipate 10W or more. I'd also put a 4A fast blow fuse on the output and a 1/2A sow blow on the transformer primary.

Yes, I over design a bit for my own purposes. I hate stuff failing. If this is for a commercial product, you might want to tighten up on the capacitor, rectifier and transistor, but only if it saves considerable money.

Last edited: May 5, 2011
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