Acceleration sensitive current switch - Page 1
Forum section: electronics / projects

author: Mandelbrot
author_rank: Regular Contributor
timestamp: March 31, 2015, 06:21:44 pm
content: Hello,I'm helping a friend with a rocket he is building and I'm stumped on something that doesn't seem like it should be too hard. The rocket contains a small microcontroller which receives signals from various sensors on board. After certain conditions are met, an igniter is lit to deploy a parachute (one for each stage). The igniter needs at least 1.25A to ignite and has a resistance of 1 ohm. We are hesitant to use mechanical relays because we are expecting a maximum of 24 g to be exerted on the rocket during the launch and we really don't want the parachute deploying prematurely. We tested a simple switch using a logic level gate FET but had trouble getting it to work correctly. We have also considered using a solid state relay which we will receive and test on Wednesday. Does anyone have any suggestions for an elegant solution? What is available on the rocket is a 9V battery and a 5V microcontroller for each stage. Space is also limited, but this probably won't be an issue for the electronics.Any suggestions or input would be greatly appreciated!

author: wraper
author_rank: Unknown
timestamp: March 31, 2015, 06:26:13 pm
content: Quote from: Mandelbrot on March 31, 2015, 06:21:44 pmAnd what is the issue? There is no obvious reason why it should not work.

author: ajb
author_rank: Super Contributor
timestamp: March 31, 2015, 06:48:10 pm
content: A FET is really the best choice here.Are you trying to run the igniter from a regular 9V battery? That may have been part of the problem, if the battery's internal resistance is too high to heat the igniter. Otherwise, take a close look at the specs on the FET you were using. Not all "logic level" FETs are created equal, and if you chose the wrong part with a Vgs of 5V the Rds may be too high for the igniter to work properly.

author: Mandelbrot
author_rank: Regular Contributor
timestamp: March 31, 2015, 08:01:50 pm
content: A 9 volt battery should be sufficient to light the igniter because only 1.25A is needed to light it. Unfortunately the FET wasn't picked out too carefully because it was chosen by my friend who is an aerospace person and has very little experience in electronics. It's an IRLZ14. In testing, the FET turns on fine and supplies enough current when on, but it leaks about 200mA when Vgs is zero. This keeps increasing as the FET heats up. I feel kind of stupid because I graduate in electrical engineering in December and I can't figure out a simple circuit like this.

author: ConnorGames
author_rank: Regular Contributor
timestamp: March 31, 2015, 08:27:20 pm
content: That fet is specified for a max of 250uA leakage at 150*C, 60v D-S. If you are getting 200mA of leakage, either your MOSFET is broken or the circuit is incorrect. It is n- channel, so you are using it as a low-side switch with a grounded source, right? Note the very low Vgs max of 10v, these would be easy to fry with ESD or a simple mistake putting together the circuit. Measure the resistance between gate and source with a meter. If it is not infinite, the MOSFET is bad. If you are at a university studying EE, I am sure they have some logic-level fets you could replace it with. It is most likely that you have the fet connected incorrectly, however(source not grounded). Also make sure the 9v battery and the MCU share a common ground.Good luck!

author: Mandelbrot
author_rank: Regular Contributor
timestamp: March 31, 2015, 08:43:53 pm
content: Quote from: ConnorGames on March 31, 2015, 08:27:20 pmYep, you nailed it. The FET was bad. It measured about 70 ohms between gate and source. Luckily my friend had the good sense to buy some extras. The circuit works fine now. I guess this goes to show how a faulty component can screw you over (although it was likely because our apartment isn't exactly ESD safe). I've been working almost exclusively on low power stuff with SPICE the last few months so I think I was in the wrong mindset to think about anything but the circuit topology. Well, guess I get to Widlarize this FET now.Thanks!