555 Transformer driver issues - Page 1
Forum section: electronics / projects

author: howlingmadpanda
author_rank: Newbie
timestamp: November 03, 2017, 02:19:57 am
content: Hello, this is my first post, so please feel free to advise on etiquette. As a disclaimer, I've got a little bit of EE experience under my belt from some of my simple past projects, but the bulk of my knowledge is still in theory. I started working on a 555 driver for another project but got kind of backlogged, I've been experimenting with it again and found that I am getting acceptable output up to the pins of the MOSFET, but after that nothing on the secondary. For reference, the transformer I'm using was a double tap 12.6 - 0 -12.6 to 120 V, the MOSFET is an IRL7833, which appears to be suitable for logic level applications. I've pretty thoroughly checked my solder joints and replaced some components, perhaps somebody more experienced can give assist me.

author: Audioguru
author_rank: Super Contributor
timestamp: November 03, 2017, 02:59:27 am
content: I guess you did not look at the datasheet for a 555. The supply voltage for the 555 in your circuit jumps up and down too much. Remove the 220 ohms resistor between the 555 and the battery and connect the supply pins of the 555 and the transformer directly to +9V. The datasheet says a supply bypass capacitor must be used, use 1uF or 10uF connected from +9V to ground.Your timing resistor values are much too low and the timing capacitor value is much too high. Use 22k, 22k and 0.47uF.Maybe you have the pins of the Mosfet mixed up, they are shown in its datasheet.

author: howlingmadpanda
author_rank: Newbie
timestamp: November 03, 2017, 03:28:31 am
content: Never really thought to look at 555 datasheet, past few times I've used them I've gotten by without a bypass cap, though I've seen it in other people's designs, I guess it's good practice. I can try that and modify the resistor and cap values tomorrow, pretty sure the MOSFET pinouts are correct as I've checked that numerous times. Thanks for the help and guidance, if that doesn't work any other ideas? Edit: Come to think of it, the absence of bypass caps in many of the microcontrollers I've worked with might explain the odd results I've gotten. Like I said, I'm still pretty inexperienced as far as IC's go.

author: Mjolinor
author_rank: Frequent Contributor
timestamp: November 03, 2017, 03:52:55 am
content: Way back in the 70s I used the 555 when it came out. Very quickly I learned that it crowbars the power supply when it switches and for that reason I haven't used one since.I suppose if they do CMOS ones now they won't have that problem but if they are still BJT based then they must have. Power supply smoothing capacitors are a must for devices with that habit.

author: pe6o991
author_rank: Contributor
timestamp: November 21, 2017, 09:56:55 pm
content: I've had bad times with the 555 timer chip when i first started playing with it.It's frequency changes with voltage and because of the weird power issue it was hard to get it to start oscillating.So now i always use a 7805 regulator with the necessary in and output caps,the 10nf cap on pin 5 is a must,using 2 trimmers(1k and above) instead of fixed resistors and a non-electrolytic oscillator cap.You can squeeze the components on a 5x3cm prototyping board.