Using PTCs to protect BJT output stages - Page 1
Forum section: electronics / projects

author: ComfyChair
author_rank: Contributor
timestamp: June 03, 2025, 10:29:43 am
content: Say I have a Sziklai output stage with BJTs (the same applies to Darlingtons as well, I just happened to have an image handy).Let's also suppose, to put some numbers on the table, that Q4 and Q5 are BD139/BD140 transistors (1.5A MAX collector current).What would happen if I replaced R8 and R9 with a PTC fuse? Again for the sake of numbers, a suitable part number could be belfuse 0ZCJ0075AF2E.What are the advantages/disadvantages? Has anyone tried this approach?My goal is to protect the output stage in a quick and easy way. I know there are better solutions like the ones presented inhttps://www.seventransistorlabs.com/Articles/ProtectedSwitch.htmlfor example.I just want to discuss the pros and cons of putting PTCs in series with BJT emitters and also know if you tried it and what happened.Thanks

author: oPossum
author_rank: Super Contributor
timestamp: June 03, 2025, 10:59:54 am
content: The usual way to protect the output stage is with a SOA circuit.https://hifisonix.com/wp-content/uploads/2013/05/soaprot_12-03-2012.pdfFuses are generally too slow to protect semiconductors. Mostly good for helping preventing fires.

author: srb1954
author_rank: Super Contributor
timestamp: June 03, 2025, 12:35:06 pm
content: Quote from: oPossum on June 03, 2025, 10:59:54 amPTC resistors are even slower than a fuse as they usually have significantly greater thermal mass than a typical fuse element.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 03, 2025, 01:10:40 pm
content: PPTC's are very slow, and also have an unreliable tripping point.Also, even if R8 is open, and (worst case) the output shorted to -42V, that would still be 2*42/150 = 560mA though R6 and it will dissipate 47W. If Q2 does not have enough base drive, it will also get quite hot and let out it's smoke too.I don't even want to think about what the variable resistance of the PPTC does to (presumably) audio quality and changes in setpoints when it gets hot. It changes the current ratio between Q2 and Q4.Also, How does that Vbe multiplier work without emitter resistors.If you want an inherent current limited output, then a BJT over a shunt resistor that starves the power transistors of base current is the most common circuit. It's simple and effective.

author: ComfyChair
author_rank: Contributor
timestamp: June 03, 2025, 01:20:02 pm
content: Quote from: oPossum on June 03, 2025, 10:59:54 amThis document is very interesting. I'm gonna steal some designs from that

author: LaserSteve
author_rank: Super Contributor
timestamp: June 03, 2025, 01:35:34 pm
content: One PTC at the CLASS AB output is a common fuse replacement on some DC to Audio frequency servo drives I work on. The LM3886 is the output driver on those amps, and the load drive coil in the limited rotation servo motor is about 3.2 Ohms on average. The PTC does a decent job, better than the fuse, of saving the system board when a coil shorts or the users wire the system wrong. May not be ideal, but for small loads the PTC saves the board about 75% of the time. As far as the servo loop is concerned, the PTC is a non-entity in normal operation. PTC saves the system until the LM3886 goes into shutdown and limits the current through the coil.In my case the system is DC to 2.4 Khz in normal operation and has to kill shaft resonances out to 19 Khz. Feedback is a current sensing resistor in the ground return and a optical position sensor. Just for a joke one day I did put one in an audio circuit, nothing noticeable by EAR at 30 or so watts.Your Milage May Vary,Just a thought.Steve

author: Gyro
author_rank: Super Contributor
timestamp: June 03, 2025, 06:16:27 pm
content: Quote from: LaserSteve on June 03, 2025, 01:35:34 pmRemember that the LM3886 hasa lotof internal output stage protection, instantaneous SOA and thermal (it will protect itself, driving into a short circuit, with no external protection). It isn't really representative of a discrete output stage, where a PTC would certainly be much too slow.