Oscilloscope quad buffer/line driver blown - Page 1
Forum section: electronics / beginners

author: Dunk_c
author_rank: Contributor
timestamp: October 01, 2022, 06:24:40 am
content: HiI see me to always learn from my mistakes. Here is another.I was wanting to back probe the ABS active sensor on my car using a battery powered DSO Nano (Seed studios) to see if it was faulty. I connected the ground wire to the battery negative and the signal wire to the sensor cable - no response, not even 12v on either pin. Got frustrated and touched the signal lead to the positive battery terminal to see if 12v would register. I thought the maxi input voltage was 80v but the magic smoke was released !The quad buffer/line driver (74HC125d) was burnt. No other damaged components visible. What did I do wrong and is it likely to be fixable by replacing the chip?Thanks

author: jonpaul
author_rank: Super Contributor
timestamp: October 01, 2022, 06:58:56 am
content: check "alternator load dump transient"80 v max scope rating is not useable in vehicle or auto electronics.Use a classic analog scope like Tektronix 465Jon

author: Dunk_c
author_rank: Contributor
timestamp: October 01, 2022, 07:07:08 am
content: Car was not running. Please explain further Jon, thanksI thought the scope would have a high input impedance and not draw much current, but something smoked the chipSpec: Input impedance. >500K. Max input voltage. 80Vpp (by ×1 probe)

author: jonpaul
author_rank: Super Contributor
timestamp: October 01, 2022, 08:58:54 am
content: Engine off, still any inductive load switching, will induce jules and kilovolts of transients on the 12v bus.Scope Zin has little to do with transients protection or sustability.Suggest you get a scope rated for 400 v input and transients protected for kV.j

author: Dunk_c
author_rank: Contributor
timestamp: October 01, 2022, 09:50:42 pm
content: The 3.5 mm signal jack has the 510k resistors nearby but the chip that blew seems to be connected to the audio jack without any inout resistors and this chip tolerates <7v according to the spec sheet.I am probably reading the diagrams all wrong

author: james_s
author_rank: Super Contributor
timestamp: October 02, 2022, 01:28:47 am
content: A load dump, while technically possible, is not something that should happen in any reasonably modern car with a properly functioning electrical system. There can be large spikes but I wouldn't expect them to damage a device like this either assuming there is SOME sort of input protection.

author: Dunk_c
author_rank: Contributor
timestamp: October 02, 2022, 01:52:14 am
content: The car was not running.There cannot be any “hidden voltage sources” connected to the battery! The whole system is at battery voltage, surely. Why did the DSO get nuked when connected to the 12v car battery? Anyone?

author: james_s
author_rank: Super Contributor
timestamp: October 02, 2022, 02:17:55 am
content: It's possible to get HV spikes from inductive loads switching even without the car running, however I would not expect this to damage an instrument good for 80V.

author: Dunk_c
author_rank: Contributor
timestamp: October 02, 2022, 03:08:10 am
content: Actually, it just occurred to me that I had the ignition ON in order to power the active ABS sensor -sorry to confuse.