:: current limiting (circuit simulations)

simulations of a NPN based current limiting setup (high HFE into a power trans, darlington paired)
my conclusion/findings :
1) larger zener, allow main trans to handle larger current ranges. 3.3v seems to max out at 25A,
2) larger current ranges require more sink, and larger power handling of darlington feeder 
3) at larger than 20A, VR units may require larger power handling envelop. ie : zener 5.1v, 2.53ma into VR = ixixR = 62mW. some smaller VR pots only handle 80mW, power spikes when conductor lead in becomes resistive element may cause VR to burn out at lead in region and cause the VR to fail. the bulk of the current goes thru wiper into feed trans (higher HFE = lesser current)

problem will arise when lead out fails, the feed transistor will fail to operate properly and will only apply 9/10 of limiting ability.

@ 5.1v zener, the wiper will feed 12ma of current (for total of 50A loading). the VR will need to handle about 1.5watts of power.

3.9v zener, FZT690 (HFE > 500)

further 2 more simulations suggests that if more than 20A of current control is required. the zener should be upsized slightly to 3.9v AND a change of feed NPN to high HFE versions (to reduce power loading on VR wiper, power load is shifted onto the feed trans instead.

3.9v zener, FZT653 (HFE <400)
but however, SMD based FZT690B is only rated at 2watts. this only allows for VCEsat of below 2 volts (which is clearly not going to happen)

as we can see using a lower HFE feed trans, we are risking a shorter life span of VR

3.9v zener, 2N2222 (current overload)

conventional small signal 2N2222 is useless here. it will not handle the collector current of over 1A

finally this is the version, limit just below 10A, minimum current limit adjustable to 10mA, power dissipation safety observed esp on VR.

 using zener 10v

R2 -- 5%
R2 -- 100%

using zener 5.1v

R2 -- 5%

R2 -- 90%

using zener 3.3v

R2 -- 5%
R2 -- 100%

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