Rate this "simple" mosfet AC PWM control - Page 1
Forum section: electronics / projects

author: DavidAlfa
author_rank: Super Contributor
timestamp: March 07, 2024, 08:44:22 pm
content: We have some old desoldering stations at work, mine burned out after 2 years of heavy use, after replacing the heater I noticed it now almost melted steel!Before,the tip lasted 3-4 days before, now it's already corroding after 10 minutes!After a short talk with the maintainance guy - shrunk shoulders and made a defeated face -."Yeah, it's been like this for years, the old heaters were 35W, the new ones are 52W, so they get too damn hot,they aren't buying the correct one or it's EOL".Tried addign a diode in series to cut half of the wave, but 27W is too low, plus the transformer made angry noises.I may try with a triac, skipping entire AC cycles.Then I thought on doing this with fets, controlling AC directly, came to this idea.I won't be making this, was just to enjoy my afternoon tea.The idea is to run the AC cycle always at 99% duty, performing a small cut of ~200us in the positive Vpeak to get some power for the control circuitry.What do you think?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: March 07, 2024, 09:07:47 pm
content: Quote from: DavidAlfa on March 07, 2024, 08:44:22 pmI'm not sure zero cross will be true 0, because the AC is switched/clamped ?If you want to turn 52W into 35W, that's about 110' conduction angle, for 67% powerhttps://www.physicsforums.com/threads/how-to-calculate-rms-voltage-from-triac-phase-angle.572668/post-3737599

author: DavidAlfa
author_rank: Super Contributor
timestamp: March 07, 2024, 09:17:10 pm
content: But zero cross is taken from AC source, before the load. See the waveforms (It's 100% load, worst-case scenario), it's working nicely.

author: dietert1
author_rank: Super Contributor
timestamp: March 07, 2024, 09:56:28 pm
content: Basically the parasitic diodes of the mosfets and two 1N4007 could form a bridge rectifier.The rectified voltage will be close to the max input voltage spec of a LM7812.From what you reported about the application you might use a 555 timer to generate the fixed PWM ratio. MCU control would be desirable if you wanted to make a thermostat, e.g. by precision measurement of the heater resistance while the mosfets are off.Regards, Dieter

author: Benta
author_rank: Super Contributor
timestamp: March 07, 2024, 10:00:31 pm
content: Just make a triac phase-angle control. Were talking less than 10 parts for that.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: March 07, 2024, 10:43:38 pm
content: Quote from: DavidAlfa on March 07, 2024, 09:17:10 pmThe base is not driven by a clean sinewave, but here you get away with it, because the transistor has a small positive offset.

author: thm_w
author_rank: Super Contributor
timestamp: March 07, 2024, 11:04:55 pm
content: I don't get it, it has control circuitry but no form of temperature control?If it has no control circuitry you could try to power it from a fan or lamp dimmer.

author: DavidAlfa
author_rank: Super Contributor
timestamp: March 08, 2024, 04:54:28 am
content: This just was about the MOSFET switching!

author: dietert1
author_rank: Super Contributor
timestamp: March 08, 2024, 09:23:17 am
content: A simple solution is a power resistor of about 2 Ohm in series with the heater.Regards, Dieter

author: DavidAlfa
author_rank: Super Contributor
timestamp: March 08, 2024, 09:40:35 am
content: I know... It was about the idea, not fixing others's work!

author: dietert1
author_rank: Super Contributor
timestamp: March 09, 2024, 08:36:46 am
content: Yesterday i tested a Weller antistatic soldering iron, heating its tip with a WTCP-S with a 380 °C tip. It has two elements: a heater with about 11.4 Ohm and a temperature sensor with about 22.4 Ohm (both measurements when cold, including cable resistance). While heating the tip with the other soldering iron, resistances went up to 12.3 and 36.6 Ohm. This is an increase of 7.8 and 63.4 %, both pretty easy to measure.I think the temperature of a simple soldering iron without temperature control can be controlled to some degree measuring the heater resistance and using some power stearing.Regards, Dieter