Path: news.eternal-september.org!eternal-september.org!.POSTED!not-for-mail
From: JM <sunaecoNoChoppedPork@gmail.com>
Newsgroups: sci.electronics.design
Subject: Re: dumper circuit
Date: Mon, 10 Mar 2025 23:37:36 +0000
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On Sat, 08 Mar 2025 14:06:50 -0800, john larkin <jlArbor.com> wrote:

>On Sat, 8 Mar 2025 16:46:14 -0500, bitrex <user@example.net> wrote:
>
>>On 3/8/2025 3:03 PM, john larkin wrote:
>>> On Sat, 08 Mar 2025 18:57:11 +0000, JM
>>> <sunaecoNoChoppedPork@gmail.com> wrote:
>>> 
>>>> On Sat, 08 Mar 2025 08:57:43 -0800, john larkin <jl@650pot.com> wrote:
>>>>
>>>>> I'll have an isolated class-D amplifier powered by a dc/dc converter
>>>> >from my big 48 volt supply. Simple push-pull, transformer, rectifier,
>>>>> making maybe isolated 46.
>>>>>
>>>>> In one situation, the class-D amp can push current uphill into the
>>>>> isolated supply, and we need to dump it. This circuit acts sort of
>>>>> like a 25-watt zener diode.
>>>>>
>>>>> https://www.dropbox.com/scl/fi/98n07mnf3qokvbewuv1oy/Dumper_1.jpg?rlkey=poiep8hzhz33qecswdj50sdne&raw=1
>>>>>
>>>>> It didn't work in simulation. It was driving me to despair. No amount
>>>>> of chocolate would help.
>>>>>
>>>>> The problem was of course that the Spice initial condition simulation
>>>>> perfectly biased everything, balanced the pencil on its point.
>>>>>
>>>>> So how can the initial conditions sim ignore the positive feedback?
>>>>
>>>> Use .ic together with uic.
>>> 
>>> It sims fine if I click the box to skip the initial condition
>>> solution. That's faster too. Delaying the power supplies breaks the
>>> tie too.
>>> 
>>> What I don't understand is how negative feedbacks work during the ic
>>> solution, but positive feedbacks don't.
>>> 
>>
>>
>>Strongly non-linear ODEs and systems of ODEs are more likely to have 
>>singular solutions i.e. parts of the domain where the initial value 
>>problem fails to return a unique answer...I'm not an expert in how this 
>>translates to discretized/numerical solutions but I believe the same 
>>sort of thing applies, a system of finite difference equations 
>>describing a positive-feedback circuit is more likely to be strongly 
>>non-linear than one for negative feedback.
>>
>>Sometimes an initial value problem is only troublesome at a single point 
>>like t = 0 and if you move off that point the problem is well-posed.
>
>Version 4
>SHEET 1 880 680
>WIRE 272 -16 208 -16
>WIRE 336 -16 272 -16
>WIRE 336 0 336 -16
>WIRE 336 96 336 80
>WIRE 208 112 208 -16
>WIRE 80 128 32 128
>WIRE 176 128 80 128
>WIRE 272 144 240 144
>WIRE 288 144 272 144
>WIRE 304 144 288 144
>WIRE 176 160 128 160
>WIRE 208 208 208 176
>WIRE 128 240 128 160
>WIRE 272 240 272 144
>WIRE 272 240 128 240
>WIRE 32 288 32 128
>WIRE 208 288 32 288
>WIRE 272 288 208 288
>WIRE 32 320 32 288
>WIRE 272 320 272 288
>WIRE 32 432 32 400
>WIRE 224 448 208 448
>WIRE 272 448 272 400
>WIRE 272 448 224 448
>FLAG 208 208 0
>FLAG 336 96 0
>FLAG 32 432 0
>FLAG 272 -16 +10
>FLAG 224 448 +10
>FLAG 80 128 N
>FLAG 208 288 P
>FLAG 288 144 U
>SYMBOL OpAmps\\UniversalOpAmp2 208 144 R0
>WINDOW 0 -63 48 Left 2
>SYMATTR InstName U1
>SYMBOL voltage 336 -16 R0
>SYMATTR InstName V1
>SYMATTR Value 10
>SYMBOL res 48 416 R180
>WINDOW 0 -52 67 Left 2
>WINDOW 3 -58 38 Left 2
>SYMATTR InstName R2
>SYMATTR Value 10K
>SYMBOL res 288 416 R180
>WINDOW 0 -46 69 Left 2
>WINDOW 3 -54 39 Left 2
>SYMATTR InstName R4
>SYMATTR Value 10K
>TEXT -176 304 Left 2 !.tran 1
>TEXT -224 208 Left 2 ;Opamp Startup
>TEXT -224 248 Left 2 ;JL  Mar 8  2025
>
>
>Run that for 1 second and look at node U.
>
>Repeat but run for 10 seconds.
>
>Then run for 1000 seconds.
>
>

It's doing nothing unusual.  An output of about 5V is a valid state.
If you need the simulation to start in a different state use .nodeset
v(U)=0.  That's exactly what the directive is for.