Dear Dennis...
EN-Genius Network's Dennis Feucht answers your design
queries in his new Circuit Design Clinic!
November, 2009
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EN-Genius Network presents a new, interactive
analog design service to readers! Send us your design questions (with
relevant data; schematics in JPEG or GIF, please) for some free
engineering advice from EN-Genius Network's circuit
consultant, Dennis Feucht, on how you might solve a design problem or
improve circuit performance. Submissions may be edited for clarity or
brevity, and submitters and their email addresses will remain anonymous
(unless otherwise indicated). Please send your questions to Dennis here.
A User-Proof Connection to an Internal Supply: a Simple Current-Limited Output
Circuit 1: The Primitive One-BJT Current Limiter
by Dennis L Feucht
Ordinary power supply design includes current limiting as a protection feature. In product design, this is usually sufficient in that the engineer decides the loads on the supply by design. However, in some applications a supply must be brought outside the box where it enters the realm of the user. Examples include sensor bridge and solid-state relay (SSR) supplies for data acquisition systems and USB power of computer peripherals. There are multiple commercial products for protected USB supplies but these are not always optimal for non-USB applications.
This article presents three simple circuits that can drive a current-limited supply port intended for use by a potentially terminal-shorting user. The design goal is a circuit that has a maximum short-circuit current by design so that this amount of current can be budgeted in the design of its internal supply. They also use fewer than half a dozen parts and are very low in cost.
...download complete article here (154 KB PDF)
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