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Old 10th October 2011, 01:51 PM   #481
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I have 15R 5W on hand
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Old 10th October 2011, 02:04 PM   #482
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Quote:
Originally Posted by AndrewT View Post
if you plan to use a shunt regulator then the CCS feeding the shunt must deliver more current than the worst case maximum demand of the client circuit (FM4).

If 30mApk is the worst case demand then the CCS should be set for 30+X mA.
X can be any value >=1mA. I would suggest you aim for setting the CCS in the range 60mA to 100mA.

Note that the FM4 has a compartment housing the 3pin regulator. One does not have much room to fit in a BiB.
Thanks Andrew,
In fact I don't think it does draw 30mA. I measured the voltage across another power supply resistor on a Pi filter and it was 0.05V for a 0.27ohm resistor. I think it is drawing more like 0.185A. I have wired up the Salas regulator and it sound very good but I now can't get more than 9.7V output. Earlier whilst testing on a 390ohm resistor for the load I got bang on 12V but of course that had assumed 30mA draw.
I have 8.2ohm 3W resistor on the input side with the voltage reg configuration of 1 diode, 1k resistor and 5K pot (only one I had available).
Any suggestions would be gratefully received.
Chris

Last edited by Backbones; 10th October 2011 at 02:07 PM.
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Old 10th October 2011, 04:31 PM   #483
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Just carried out a few experiments with the voltage adjustment area and it seems that at these currents, the voltage is largely unresponsive to alterations in the potentiometer position. So I assume I'll have to increase the current via R301. Could I get away with 7W, 4.7ohm resistor here (Maplin don't have the 3.9ohm)?
Chris
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Old 10th October 2011, 05:13 PM   #484
AndrewT is offline AndrewT  Scotland
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First, one must set the CCS current !

If there is insufficient current, at any time no matter how short a time period that might be, then the shunt has no current to regulate with and the output voltage falls, while all the capacitors release their stored energy to try to meet that current demand.
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Old 10th October 2011, 06:12 PM   #485
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What is the CCS?
I'm running the reg from one of two secondaries, each rated at 16V/1A (external supply as you're right you haven't got room to mount anything inside the FM4 chassis).
Could I parallel the other secondary and double the current?
Chris
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Old 10th October 2011, 07:56 PM   #486
Salas is online now Salas  Greece
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The main constant current source is around the IRF9610 first Mosfet in line. There you set its steady current available with R101 etc. There is a 4th LED position to aid you if you don't have small resistors, read the last page of the guide rev.2 available for download at post#1 this thread.
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Old 10th October 2011, 08:00 PM   #487
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Quote:
Originally Posted by AndrewT View Post
X can be any value >=1mA.
X can be any value >=20mA to allow enough for the SSLV1.1 internal needs and some decent Zo.
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Old 10th October 2011, 08:36 PM   #488
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OK if I understand this correctly. For the green LEDs there is a 2.1v drop per diode which if we use four equals 8.4V. Looking at the ID/VGS chart for a projected value of CCS (150mA above peak current for FM4 which is 200mA + 150mA= 350mA) we get for 350mA a Vgs of 4.8. Subtract from 8.4V=3.6V. Current resistor is 8.2ohms. V/R=3.6/8.2 equals 0.43A, which exceeds my requirements. So if I add another green diode with the current resistor I should be alright.
Hard work these threads all of a sudden.
Chris
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Old 10th October 2011, 08:51 PM   #489
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Looks 5ish to me, better calculate based on Vgs=5V. Also your LEDS maybe ~1.9V at the small current they will run, their spec voltage drop Vf is given at 20mA nominal usually.
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Old 10th October 2011, 08:57 PM   #490
Salas is online now Salas  Greece
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P.S. Sticking the 8R2 you have available along with 4 green LEDS should get you comfortably above your load needs, around 340mA give or take. Just solder it in, and by measuring the drop across it, you will be able to figure out the current by dividing that voltage drop with 8.2.
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