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The simplistic Salas low voltage shunt regulator
The simplistic Salas low voltage shunt regulator
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Old 28th July 2009, 02:29 PM   #851
Salas is online now Salas  Greece
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The simplistic Salas low voltage shunt regulator
So a 100R or 220R trimmer will do. The only prediction variable will be individual Mosfet part Vgs if with fixed Jfet IDSS, I guess.
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Old 28th July 2009, 02:35 PM   #852
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The simplistic Salas low voltage shunt regulator
Quote:
Originally posted by Salas
10mA IDSS in a 100pcs BL bag are a dime a dozen and seem adequate for our purposes. A bit more complex for a DIYer to set, but hey, this is V2. Takes a bit of more involvement.
Come on, it's not so bad

Rset = (Vbias - VLED1 - VLED2) / Idss

For a desired bias voltage for the CCS mosfet of 4 volts, if the voltage drop across each LED is about 1.8 volts, and if Idss is 10mA

Rset = (4 - 1.8 - 1.8) / 0.01 = 40 ohm

Simple
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Old 28th July 2009, 02:37 PM   #853
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The simplistic Salas low voltage shunt regulator
Quote:
Originally posted by Salas
So a 100R or 220R trimmer will do. The only prediction variable will be individual Mosfet part Vgs if with fixed Jfet IDSS, I guess.
Yes, and perhaps people use slightly different LEDs, so that could be a variable too. But as long as the LEDs have a voltage drop of less than 2 volts, the procedure would be to set the trimmer in advance to a low value, such as 10 ohm, and then increase its value in the circuit, while measuring the current. Very easy, really.
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Old 28th July 2009, 02:38 PM   #854
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The simplistic Salas low voltage shunt regulator
I know, but just checking Vdrop across a CCS resistor for total current is more hands on for the DIYer. But lets not be so pampered.
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Old 28th July 2009, 02:44 PM   #855
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The simplistic Salas low voltage shunt regulator
JLH69 or any high bias class A like Zen etc. are the best customers for a high current shunt by the way. They already burn the juice on their outputs on big sinks all the time, and the shunt needs only a comparatively small sink for idling just a bit over that, and the bulk of its heat is only on the CCS Mosfet, which can be held at 5-6V higher only, if the input ripple is under 1Vpp by using a not so big filter cap, or two with 0.33R in between. Can make do with a common between IRFPs 1W/C sink in other words if the load is always connected.
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Old 28th July 2009, 02:55 PM   #856
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The simplistic Salas low voltage shunt regulator
I agree, that would be a good setup. For high current applications, the more I think about it, the more I would happily stay with v1. I think that's what I'll do for my chipamp.
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Old 28th July 2009, 04:07 PM   #857
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The simplistic Salas low voltage shunt regulator
Tham, sorry about the moving target here. If you really wanted to try something with a wider bandwidth than v1 for your t-amp, I have this suggestion:

Click the image to open in full size.

This is the type of application where the loop gain is best provided by high frequency transistors. Q2 is BFT92, nicely working up to 2GHz, and Q1 is its complement, BFR92P. They are also cheap, and most online places will carry them. Q1 can also be 2N2222 (not so good, but OK). Using these high freq transistors in simulation makes a big difference in terms of stability and step response. It might be a good idea to use them for the DAC shunt as well. Then it is proper broadband treatment.
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Old 28th July 2009, 04:45 PM   #858
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The simplistic Salas low voltage shunt regulator
Good results also with Q1 = 2N2222A and Q2 = 2N4403.
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Old 28th July 2009, 04:52 PM   #859
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Hi Iko,
use the schematic in post823 for all your future versions.
It shows the measuring bridge more like it should be implemented in a practical PCB.

I am not sure that Q3 & J4 should be included in the bridge. They pass a correction current (although near constant), the same as the output FET does.

Finally 823 would be even better if the top and bottom tie in points, together with the whole bridge, were brought over to the other side of the output FET and shown connected to the output terminals.
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Old 28th July 2009, 05:21 PM   #860
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The simplistic Salas low voltage shunt regulator
Quote:
Originally posted by AndrewT
Hi Iko,
use the schematic in post823 for all your future versions.
It shows the measuring bridge more like it should be implemented in a practical PCB.

I am not sure that Q3 & J4 should be included in the bridge. They pass a correction current (although near constant), the same as the output FET does.

Finally 823 would be even better if the top and bottom tie in points, together with the whole bridge, were brought over to the other side of the output FET and shown connected to the output terminals.
Hi Andrew, if you could please have another look at post 857. Is this more like it?

Honestly, caught in the design process, running many tests for each change, I completely ignored layout. But you're right, many people see the schematic and automatically think that also shows the layout/connection points, which may often not be the case.
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