Astable 555-timer not switching - Page 1
Forum section: electronics / beginners

author: Moriambar
author_rank: Unknown
timestamp: September 23, 2023, 02:46:09 pm
content: Hi guys.There's something wrong with my 555 astable configuration.The thing is: the timer does not switch.The "load" is the gate of a mosfet.I'm utterly clueless. I'm attaching the schematic for anyone whishing to help.Thanks!

author: Kim Christensen
author_rank: Super Contributor
timestamp: September 23, 2023, 03:47:11 pm
content: Show the entire diagram. I assume the other end of R1 (CMD) goes to the gate of the mosfet? If so, R1 should be a much lower value. More like 100 ohms or so.What's the part number of the mosfet?What does pin 2 (TR) of the 555 connect to?If you want the 555 in astable (oscillator) mode, then pin 2 should connect to the same point as pin 6.

author: pqass
author_rank: Super Contributor
timestamp: September 23, 2023, 04:24:29 pm
content: The 555 should work. See attached. Simulatorhere.Is it a CMOS 555? They do have a minimum supply of 3V but an ordinary NE555 starts at 4.5V.Add an LED and resistor to the output to confirm it blink (1.5sec on/1.5sec off).100K (from pin 3) to drive a MOSFET is pretty weak; should be 100R.Also, 3.3V on a MOSFET gate is low for most. You need to confirm the specific part can be turned on fully with 3.3V.

author: Moriambar
author_rank: Unknown
timestamp: September 23, 2023, 06:52:19 pm
content: Quote from: Kim Christensen on September 23, 2023, 03:47:11 pmthis was it.How/why?Thanks

author: Moriambar
author_rank: Unknown
timestamp: September 23, 2023, 06:53:14 pm
content: Quote from: pqass on September 23, 2023, 04:24:29 pmyep I confirmed it all on the DS. everything works now, the problem was the large output resistance, which I have to figure why was too large (as per my other message)thanks

author: Kim Christensen
author_rank: Super Contributor
timestamp: September 23, 2023, 08:02:14 pm
content: Quote from: Moriambar on September 23, 2023, 06:52:19 pm100K is too high of a resistance to charge/discharge the gate capacitance of the mosfet quickly enough. Though with the sub 1Hz switching speed, it probably should have half assed worked. If you gave us a part number for the mosfet, we could give you a more accurate explanation. (could also be due to gate/source leakage current)

author: Moriambar
author_rank: Unknown
timestamp: September 23, 2023, 08:18:48 pm
content: Quote from: Kim Christensen on September 23, 2023, 08:02:14 pm2N7002.Thanks, can't wait for more!

author: Kim Christensen
author_rank: Super Contributor
timestamp: September 23, 2023, 10:02:57 pm
content: The maxspec'd VGS(th) (Gate Threshold Voltage) of the 2N7002 is 2.5V @ 25C... That means it'sbarely turning onat that gate voltage. So the 100K series resistance combined with a bit of gate and board leakage could be enough that it doesn't get enough gate voltage to turn on at all. With 100 ohms, it's a bit higher, but still not great. (The 7555 isn't going to output exactly 3.3V on pin 3 when it goes high either) So if you try and switch higher drain currents with it you'll find that the 2N7002 drops too much voltage causing it to heat up, and the load will not get the full voltage across it. RDS(ON) for the 2N7002 is spec'd at 5V gate voltage or higher. Below that, it's not guaranteed.The better solution is a mosfet that has it's RDS(ON) spec'd much lower. Something like 2.8V or less.

author: Moriambar
author_rank: Unknown
timestamp: September 23, 2023, 10:32:35 pm
content: Quote from: Kim Christensen on September 23, 2023, 10:02:57 pmthank you. The current I'm switching is around 2-6mA so I think I'm fine (with 100R) but I'm glad I understand this aspect too, for future projects (or if this crashes and burns I know where to look).