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Old 17th August 2012, 01:35 AM   #831
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and then hasta la vista baby ......... to simplicity and elegance

having 2 caps is small price for elegance , especially considering that their influence is exactly opposite in character , in every moment

2 caps and few resistors , without need for bigger xformers , which eventually will be needed , considering fact of pushing twice 40++mA through tiny nickel cored bstrd

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Old 17th August 2012, 01:47 AM   #832
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Default CCS noise problem?

One aspect of constant current sources that has not been discussed is noise. The noise current of the ccs multiplied by the DC resistance of the winding will be an additional noise voltage on the FET gate. I am not sure how this might compare with other noise sources, but it is likely to be much higher than the noise injected from the circuits shown in methods A, A1, and A2 of this post post F6 Amplifier.
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Old 17th August 2012, 03:20 AM   #833
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noise will be transposed via impedance of winding , not resistance
it will be noisier than simpler solutions , but I'm not seeing noise as major issue with CCS option
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Old 17th August 2012, 03:22 AM   #834
lhquam is offline lhquam  United States
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Quote:
Originally Posted by Zen Mod View Post
and then hasta la vista baby ......... to simplicity and elegance

having 2 caps is small price for elegance , especially considering that their influence is exactly opposite in character , in every moment

2 caps and few resistors , without need for bigger xformers , which eventually will be needed , considering fact of pushing twice 40++mA through tiny nickel cored bstrd

At this point I am not advocating any particular circuit, but evaluating "out-of-the-box" alternative approaches. I thought that the ccs/dc-winding-resistance idea was worth looking at, nothing more.
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Old 17th August 2012, 03:24 AM   #835
lhquam is offline lhquam  United States
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Quote:
Originally Posted by Zen Mod View Post
noise will be transposed via impedance of winding , not resistance
it will be noisier than simpler solutions , but I'm not seeing noise as major issue with CCS option
You are right about that. Will it be the reflected impedence of the front end driver? If so, the impedance might be fairly low.
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Old 17th August 2012, 03:33 AM   #836
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nope


secondary inductance at any freq. covered with noise , so impedance
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Old 17th August 2012, 01:37 PM   #837
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Quote:
Originally Posted by lhquam View Post
Power dissipation is the problem here. About 50mA current is required thru the transformer winding with an average of over 20 volts across the ccs when using 24V rails. The means 1 watt average power dissipation. We need a high power regulator or a ccs circuit which uses a power transistor for the main current.
TL783 is a high power positive voltage regulator [TO-220AB]. It has a published free air dissipation of 2 W at 25 Celsius, and 20 W on an adequate heat sink. The maximum recommended current is 0.7A at 125 V. Plenty!

Clearly lhquam you are chasing an experiment which you thought has value. As Mr. Pass will say: put it together and see what happens!

ZM: many methods to tan a skin to make leather. Each leather has its unique balance of valuable properties; an outcome of experiments.
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Old 17th August 2012, 02:30 PM   #838
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The maximum recommended current is 0.7A at 125 V. Plenty!
My apology, the underlined is a mistake as written. [at] is the problem. The current [0.7A] times 125 V will violate the 20 W upper dissipation limit of TL783. Instead, operate the regulator at 0.7A such that the In-Out voltage differential is 28 VDC. Or operate the regualtor at an In-Out voltage differential of 120 V such that the output current is ~0.16A. Each of the above 2 situations do not violate the maximum dissipation constraint of 20W.
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Old 17th August 2012, 06:45 PM   #839
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My apology, the underlined is a mistake as written. [at] is the problem. The current [0.7A] times 125 V will violate the 20 W upper dissipation limit of TL783. Instead, operate the regulator at 0.7A such that the In-Out voltage differential is 28 VDC. Or operate the regualtor at an In-Out voltage differential of 120 V such that the output current is ~0.16A. Each of the above 2 situations do not violate the maximum dissipation constraint of 20W.
The TL783 appears to have a problem for this application. The datasheet shows a graph of output current vs. in-to-out voltage. The current rapidly falls off below 25v. A Spice simulation for the part shows that below 12V in-to-out the part will not maintain 50mA current.
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Old 17th August 2012, 06:46 PM   #840
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How bout biasing schema like in BA1 output?
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