A Good Idea When Using Adjustable Voltage Regulators

[For The Popcorn]

This article from EDN points out a potential problem when using a pot and adjustable voltage regulator like an LM317 – if the pot gets intermittent, the load will be subject to an "Oh crap" full-supply-voltage moment. I had never considered that possibility before but this could make for a bad day depending on what the load is.

The fix is pretty simple, requiring a single transistor to clamp the voltage if the pot opens.

 

[schmitt trigger]

Thanks, I have a home-brewed dual power supply which I have used, almost daily, for the past 15 or so years.
Although I employed high quality multiturn pots which have proved reliable over the years, they could become AWOL anytime.

 

[For The Popcorn]

The paper shows a similar arrangement for negative regulators as well.

 

[Lax Luthier]

Thanks for the tip. This is a very good point, especially when working with breadboards where components and/or connecting wires can accidentally become disconnected from the circuits. An ounce of prevention here can save a pound of chips. The clamp is important, I will be adding one to each of the three terminal regulator stages in the supplies used for prototyping and testing. Plus, now I can put to use some of the (hopefully) pre-counterfeit era 2N3906 and similar BJT's I stocked long ago which have remained unused.