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Onvinyl 18th September 2007 08:50 PM

Simple shunt reg
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Here is a simple shunt regulator with good isolation and low noise. One of the design goals was to use things waiting in the parts bin for too long. Another was low parts count because I plan to use some of those in a phono pre.
Not shown are output/input caps.
There nothing too special about it except it works really well.

One thing I'd like to know is: How can I test/verify for output impedance?


Werner 19th September 2007 06:31 AM

By simulation: attach a 1A current source as load and sweep it over the frequency range of interest. The voltage measured at the output node is 1:1 equivalent to the output impedance.

aparatusonitus 19th September 2007 07:03 AM

Re: Simple shunt reg

Originally posted by Onvinyl

One thing I'd like to know is: How can I test/verify for output impedance?


Since you are using LT SwCad do this: attach an 1A AC current source I1 at ouput, run AC analize, when card popoup, mark Vout, change Vout to Vout/I(I1) by presing Alt, OK, you will be able to see output impedance vs freq.

Onvinyl 19th September 2007 07:12 AM

2 Attachment(s)
Ok, I did.
How do the dB-numbers correlate to Zout? Is it mandatory to get the curve flat?

Werner 19th September 2007 07:20 AM

plot voltage (or impedance) on a linear scale, not a log/dB scale!

aparatusonitus 19th September 2007 07:23 AM

Place a cursor on the Y-axis, press left or right ( I can't remember right now), mark linear scale instead dB.

Impedance shuld be as low as posible and flat as well to 100kHz to 1Mhz region, after that you will see a rise up, play a little bit with a "real" capacitor at output (or without it).

Onvinyl 19th September 2007 07:48 AM

2 Attachment(s)
Thanks! yes, I forgot to place an output cap before sim :cannotbe:

this is with 100uF and assumed ESR of 0.1R, at the output

unclejed613 19th September 2007 09:02 PM

right click on the db scale, you can change it to lin or log voltage scale. you can do this with the FFT output as well to easier compute %THD (log is better for that). if you find that your results are "compressed" by using a lin scale, see if you get better results with a log scale. while in the plot scaling window, you can also set the high and low limits of your trace, and you can use this feature to magnify certain portions of the trace (you can also do this by selecting a portion of the output trace by holding down the left mouse button and drawing a box around the portion of interest).

sawreyrw 19th September 2007 10:16 PM


How does this circuit work? Where is the voltage reference? If it's dependent to the transistor parameters, I don't think it will be very dependable.


Onvinyl 19th September 2007 10:59 PM

I'll do my best:

Q2/M1 and R12 form the current source against the shunt reg will work.
The differential fet (2sj109) is the error amplifier. The leds are the voltage reference, held stiff by J3. A portion of the ouput voltage (both DC and AC) is compared to the voltage reference through R2/R6. I made part of R2 a trimpot, so I can vary between 20 - 26 Volts DC.
Q1/Q3 give further gain and provide current. The current through Q3 varies against load changes, so net current through R12 is always the same. In fact, you adjust the amount of the netcurrent through this resistor.
The point to see here is that Q3 shunts current to ground, the less is needed by the load, the more it eats.
Hope that's accurate,

edit: if a circuit would not depend on transistor parameters, one wouldn't need them, right?:goodbad:

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