I have built a circuit that uses very little current. To protect my circuit, I'd like to install a .100 Amp fuse or better yet an adjustable circuit breaker, that I could tweak until I get it to operate exactly the current level that suits me. May there already exists a little circuit that will do this for me. Does anyone have any simple cheap ideas about building such a circuit or point out one that already exists?

 

https://uk.rs-online.com/web/p/cartridge-fuses/6109579?gb=s
How about a PTC thermistor or a multifuse?

 

Thanks IanO, I will try some of those. I'm still going to be on the lookout for a little circuit that I could build that would be an adjustable circuit breaker.

 

You'll still need a fuse, to protect things in case your little circuit fails!

 

Here is an example of a 50 mA PTC resettable fuse that will trip at 100 mA.

https://www.digikey.com/en/products/detail/bel-fuse-inc/0ZRR0005FF2E/9468325

You can use LM317 adjustable voltage regulator as a current limiter.

 

If you need 'real' fuses, these work: https://www.littelfuse.com/products...fuses/pico-fuses-axial-radial-thru-hole-fuses

 

Search Digikey.
It lists 315 different types of 0.1 amp fuses.

 

I used some REALLY FAST 50 milliamp fuses to protect sensor power in a series of crash cars. The problem would be that 8 sensors development crash sensors were sharing one isolated regulated power supply, and that during a crash the cable to one sensor could be crushed and short circuit the supply for the other sensors. BUT those fuses are not cheap, over $2 each.With our supply and wiring a fuse would clear in less than two milliseconds, which would not cause a huge data loss. We were evaluating sensor locations, so we could live with that loss.
The bad news is that our management cancelled the project because the projected short term profit was not good enough. I won't name names, but the decision was made by MBAs, not engineers. And so crash sensing is done with accelerometers, just like always.

 

crash sensing is done with accelerometers, just like always.

The crash sensors I'm familiar with are part of the Air Bag Deployment System. It consists of a steel ball in a tube and two electrodes inside the tube. Should sufficient force move the ball to the contacts the air bags would deploy. Accelerometers are better suited for sensing the amount of force being exerted at the time of impact whereas the ball only needs sufficient force to close the contacts.

 

The crash sensors I'm familiar with are part of the Air Bag Deployment System. It consists of a steel ball in a tube and two electrodes inside the tube. Should sufficient force move the ball to the contacts the air bags would deploy. Accelerometers are better suited for sensing the amount of force being exerted at the time of impact whereas the ball only needs sufficient force to close the contacts.

Those sensors are crude accelerometers.
The sensors I was working with could signal an actual crash before the car started to change velocity. The video taken at one crash showed that the front had only pushed in about an inch and a half, and the sensor was signaling an impact. That violent deceleration had not even started. So the sensor would allow about 15 milliseconds more time for the airbag computer to decide what to do. That is a huge time benefit in an automotive crash. I don't know if any others were working on that sort of sensor, but somebody should.
But the bean-counters can only see the next quarter's profits. And it proves that greed is the primary qualification to be on the board of a publicly traded corporation. That product could, by itself, reduce serious accident injuries a whole lot!!

 

The airbag crash sensors that Tony described were used in the early 1990s. They were sufficient for vehicles with large hoods and crude airbags where timing errors of a few milliseconds were of no consequence.

Nowadays with shorter hoods (thanks to transversal engines) and significantly less powerful airbag deployment (to prevent injuries) all of the airbag sensors employ silicon MEMS sensors.
But as MB2 mentions, they were initially more expensive, and the bean counters initially fought them furiously.
I was closely involved with the Ford Aerostar system. This vehicle had a very short engine hood and the airbag timing was extremely critical.
A long story for another day.

 

OK, "S-T", I am wondering about the timing of those MEMS sensors, as that is an entirely different technology than the ones I was working with. AND, I was working with a group at the same company , but on a different line of vehicles. It would be interesting to compare the concepts. I suspect that MEM sensing would have solved the issues that were found with our sensors, which all of the issues were production related, not function related. I became friends with I.P. at the crash barrier as the tests were running. He may recall working with me, if you are still in a position to visit the barrier for tests.