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Old 1st August 2009, 10:31 AM   #931
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There is no universal design for all, too many variables. For proper design, you have to know VLED and Idss of JFET bellow and desired shunt current, then can you individually restrict adjustment range with resistors.
If you use single turn 500 Ohm trimmer, small movent of wiper & your shunt current will jump a lot.

You should use what you want to reach your goal, this is DIY
I like to use simplistic approach with minimum parts and will use multiturn for fine adjustment, then replace it with only one resistor.
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Old 1st August 2009, 12:47 PM   #932
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Quote:
Originally posted by stormsonic
[snip]If you use single turn 500 Ohm trimmer, small movent of wiper & your shunt current will jump a lot.
[snip]

But I think Chris' point was that you won't use it that way. You use it with resistors in series with both end terminals to set/restrict the range to what you need.
Seems like a sensible approach to me.

Of course you can use a multiturn during development, maybe that is the use you refer to?

jd
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Old 1st August 2009, 01:28 PM   #933
Salas is offline Salas  Greece
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Quote:
Originally posted by stormsonic
There is no universal design for all, too many variables. For proper design, you have to know VLED and Idss of JFET bellow and desired shunt current, then can you individually restrict adjustment range with resistors.
There is. If you use a transistor for Vbe CCS Vref (as I have shown in enough examples) then your current is more or less Vbe/R1.
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Old 1st August 2009, 09:47 PM   #934
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hi Janneman, I understand Chris' point.
My 5V shunt reg = shematic post#745, replaced D4 with trimmer.

With Rset 36.5 Ohm, shunt current is 250 mA. With Rset 105 Ohm, current is over 1A. This is very narrow range for adjustment.
If you use 2 resistors on each side of trimmer, as Chris sugested, adjustment range is even narrower.
To use 2 resistors, each resistor with value of 10 Ohm & set trimmer value to 0 Ohm, then Rset is 20 Ohm. With Rset 20 Ohm, shunt current is approx. 165 mA. For powering digital circuit with 20 or 30 mA consumption, this is too much, we are only producing excess heat.

Yes, I am using trimmer only during development & suggesting to use multiturn. It is easier to make fine adjustment

Hope you understand my point & my bad English
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Old 2nd August 2009, 03:32 AM   #935
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Hi Jan,
Thank you, you understand perfectly!

Hi stormsonic,
By using external resistors on each end, you are expanding the range that you want to control. This makes adjusting the current (in this case) very easy and repeatable with a single turn control. This is the proper way to execute this design. That way you can rotate the control full one way or the other and the extra resistors will restrict you current range to some safe level that you have defined.

An example. Anyone who has worked on older audio amplifiers from the 70s to mid 80s (approximately) will remember that setting the bias on a Marantz was pretty easy to get bang on where you wanted it. By the same token, Yamaha amplifiers tended to have a very sensitive bias control. It was difficult enough to put the bias even in a range where you wanted it. Marantz used external resistors to make most of the adjustment range usable, whereas Yamaha did not. The result was that in a Yamaha, you only had a small usable range on the trimmer. This ended up compressing the usable part to a small section and the ends would either shut the bias off, or allow it to run to destructive levels.

Using a multi-turn control for development is not what I would to. It is usually easy to figure out what the range of resistance will (should) be. If you miss the target, change padding values a little. This is, after all, DIY where you should use good design habits. Yes, you can do things any way you want, but I think we should only pass on good habits to other members.

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Old 2nd August 2009, 09:32 PM   #936
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Quote:
Originally posted by anatech

Hi stormsonic,
By using external resistors on each end, you are expanding the range that you want to control. This makes adjusting the current (in this case) very easy and repeatable with a single turn control. This is the proper way to execute this design. That way you can rotate the control full one way or the other and the extra resistors will restrict you current range to some safe level that you have defined.

@anatech
Maybe I am "slow minded" or maybe we are talking two different things. I do not full understand your proposal, can you please post shematic for me to understand. One pic is worth more than a 1000 words, hand drawn will be OK.
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Old 4th August 2009, 04:07 AM   #937
Tham is offline Tham  Malaysia
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Salas, will put a v1 together quickly for T amp . Can I use a 12V 3A transformer?
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Old 4th August 2009, 04:26 AM   #938
Salas is offline Salas  Greece
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If it will hold its 12V secondary at 2-2.5A constant current...then it will do OK. Try the transformer and see results.
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Old 4th August 2009, 04:36 AM   #939
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Thanks Salas. Will try that.
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Old 4th August 2009, 01:53 PM   #940
Tham is offline Tham  Malaysia
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Salas, builds a v1 for my T amp and ready to test soon...would a heatsink the size of Pentium 4 heatsink big enough?
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