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Old 17th October 2018, 01:27 PM   #1461
Dennis Hui is offline Dennis Hui  Canada
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Thank you PRR and Eric!
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Old 17th October 2018, 01:28 PM   #1462
Eric is offline Eric  United States
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I agree with good practice, but this also makes certain assumptions that I cannot always guarantee will always be met as I move things around to play/test/etc or give amps away to friends as gifts. My goal was to find something that prevented me from inadvertently screwing things up.

In some domains, this means not letting me touch things at all .

I'll have to grab a few IEC cords with a switch next time I place an Amazon order!
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Old 17th October 2018, 01:45 PM   #1463
Michael Rothacher is offline Michael Rothacher  United States
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Yes, I understood that. I wasn't debating. I was sharing.
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Old 17th October 2018, 01:48 PM   #1464
Eric is offline Eric  United States
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I understand completely - just trying to save my things from myself It's the computer scientist in me - always trying to keep the dummy user (often me) from doing things to screw it up.
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Old 17th October 2018, 02:29 PM   #1465
MEPER is offline MEPER  Denmark
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Now I hope I am better protected than before…...after installing a GA 1N5357 zener. Hopefully I have installed it correct……

Soon I am ready to power up the MoFo…...
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Old 17th October 2018, 02:34 PM   #1466
e_fortier is offline e_fortier  Canada
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You installation of Z1 is correct
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Old 17th October 2018, 03:13 PM   #1467
MEPER is offline MEPER  Denmark
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Thank you!
Now I have made some measurements on the choke so I can convert mV to a bias current.


1.0A gives 73 mV
2.0A gives 146 mV
3.0A gives 219 mV


….seems to be consistent…...and shows my choke has RDC = 73 mOhm.
Will be interesting to see if a RDC that low can give a stable bias......if not there is probably a solution for that…...and I have learned something……...again…...
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Old 17th October 2018, 07:26 PM   #1468
Eric is offline Eric  United States
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MEPER: I suspect you won't run into any specific challenge with maintaining stable bias. I would imagine that a choke that large will also have a pretty serious kickback, far larger than what I am experiencing with my choke. I am curious to see if your larger diode is able to tame the transformer on power down...
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Old 17th October 2018, 07:44 PM   #1469
MEPER is offline MEPER  Denmark
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We will see…….the problem with stable bias is that my choke has about same RDC as the mosfet and during warm up the mosfet will change its impedance (to smaller value as I understand)…..then current will rise....and more heat is generated….etc....so there could be a "thermal runaway". Think this is the main problem. But that is also part of the fun of DIY to experiment a bit.....
I start testing with a lap supply and using that I slowly turn up and down and I have current limiter activated. The final PSU will be LCLC based so there is a capacitor. I switch off at the AC line level. But it would be nice if the MoFo can handle e.g. if the fuse at the DC level breaks. The idea to have a capacitor on board is not bad...…
I have very little DC across the choke (max. 0.3 V) so maybe the kick back is not that large.....
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Old 17th October 2018, 07:52 PM   #1470
Eric is offline Eric  United States
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Since much of this is relatively new to me, I'm not sure what to expect for kick back. My guess is that it is directly related to the size of the magnetic field, but I am not sure how much physical size of the choke contributes to this vs the current flow or the voltage potential across the choke... If I had this whole "education thing" to do over again, I think I'd choose EE instead of CompSci

For comparison, my choke has a DCR of about 0R54, so getting 2.5A bias current means I can measure a 1.35v drop across the choke. With the added cap at DC input, my meter now registers a negative voltage - just briefly - as I pull the plug on the DC power supply. I wasn't able to see this before I added the cap.
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