EEVblog Electronics Community Forum

Electronics => Beginners => Topic started by: LarsT on July 05, 2015, 10:27:16 pm

Title: Latching circuit
Post by: LarsT on July 05, 2015, 10:27:16 pm

Ok, so I've tried making a latching circuit (seen in eevblog #262) using two complementary transistors and two switches for setting and releasing the latch (see attachment). When connecting power to the circuit it does (most of the time) turn itself on automatically unless I've closed the releasing switch right before. I've also tried adding a 10k pull up on the PNP as well as a 10k pull down on the NPN, as well as a capacitor (0.1uF) in parallell to the releasing switch in order to short this when I connect power. None of these measures have been 100% successful. What am I doing wrong here?

Title: Re: Latching circuit
Post by: T3sl4co1l on July 06, 2015, 01:40:40 am

Try 1k B-E resistors.

Tim

Title: Re: Latching circuit
Post by: LarsT on July 06, 2015, 09:05:26 pm

Quote from: T3sl4co1l on July 06, 2015, 01:40:40 am

Try 1k B-E resistors.

Tim

I tried, but with same result as before. I tried a 0.1uF cap on release switch in combination with a 10k pull down on the NPN and that seem to work reliably.

I have modified the curcuit to cut the power when supply voltage drops below a set voltage by comparing a defined fraction of the supply voltage to a fixed voltage reference of 3.30V. This circuit does seem to work, but is there anything I should be aware of?