555 driving 50V DC motor with FET - Page 1
Forum section: electronics / beginners

author: Wojta
author_rank: Contributor
timestamp: March 06, 2017, 02:43:41 pm
content: Hey all,I need to do a "pretty easy" (hopefully) switching circuit. Basically a 555 driving a 50V DC motor 10A max.(grill motor) using pwm. Not any high frequencies are needed, at least I think so, you will correct me if Im wrong. The thing is I never used MOSFETS before and I guess thats the right component for the job. Will I be able to switch the fet correctly? I wanted to use the IRF640, is it enough?Thanks for any suggestions

author: Seekonk
author_rank: Super Contributor
timestamp: March 06, 2017, 08:52:44 pm
content: What are you roasting, a whole pig? 640 seems a little small if you have a 10A motor. Hope you buy a bunch.

author: Ammar
author_rank: Regular Contributor
timestamp: March 06, 2017, 09:05:24 pm
content: At a minimum you might be looking for a device like this:http://www.digikey.com.au/product-detail/en/microsemi-corporation/APT21M100J/APT21M100J-ND/1494383Seekonk is correct, an IRF640 will be the first thing to get grilled.

author: Zero999
author_rank: Super Contributor
timestamp: March 06, 2017, 09:10:44 pm
content: A 555 timer can drive a MOSFET directly.I wouldn't use the IRF640 though. It has too higher on resistance for 10A. Using a 200V MOSFET is overkill. A lower voltage rated MOSFET will have a much lower of the on resistance, everything else being equal.

author: Seekonk
author_rank: Super Contributor
timestamp: March 06, 2017, 09:41:37 pm
content: Where did that 10A come from? is that LRA locked rotor amps? Run current would be a half horsepower motor. A couple IRFB4110 in parallel would be a cheap alternative. Don't forget a large fast diode on the motor.

author: Zero999
author_rank: Super Contributor
timestamp: March 06, 2017, 10:20:05 pm
content: Quote from: Seekonk on March 06, 2017, 09:41:37 pmI'd go for the IRFB7537PBF or PSMN5R0. Much cheaper than the IRFB4110 at my local supplier and lower on resistance.

author: Seekonk
author_rank: Super Contributor
timestamp: March 06, 2017, 11:55:45 pm
content: There are a million FET out there. Hopefully you got the idea. 60V is playing it pretty close to the chest for an inductive load. From my sources that looks pretty expensive. Use a couple in parallel just to be safe. You know you will likely burn something up.

author: Wojta
author_rank: Contributor
timestamp: March 07, 2017, 11:43:41 am
content: So to sum it for myself. 555 is capable of driving the fet directly according to the schematic. It should be OK if I use the CMOS 555 right? My question is at what voltage do I have to run it? I dont really get what voltage will trigger the FET enough. For the fet I should look at something capable of doing more. So arround 100V and above 20A should be good or do I need more?Thanks for helping

author: Seekonk
author_rank: Super Contributor
timestamp: March 07, 2017, 01:38:49 pm
content: If you look at many FET spec, the high currents are at a gate voltage of 10V or over. You didn't specify what your motor current REALLY is. No reason to use a wimpy FET or two. A 100A FET doesn't cost any more. I normally won't even talk to someone with a 555 project so can't say anything about the drive capabilities.

author: Wojta
author_rank: Contributor
timestamp: March 07, 2017, 08:25:17 pm
content: Sorry, I had really not much info about the motor till now. The thing is Im doing it for a friend of mine and he didnt really specify the motor ratings so I just guessed 10A max. Today he sent me full info. The motor has label which says4380rpm150W24V8,1AThe voltage was one I didnt expect. He told me he used a 50V supply to run the motor before so I guessed it would be 50V. Still the voltage should be fixable with limiting the top pwm to less than 100% right? So there are the actual specs!

author: Wojta
author_rank: Contributor
timestamp: March 08, 2017, 07:05:35 am
content: Ok. Can someone please help me understand what voltage does it take to open the fet? Can I find that in datasheet? What does it depend on?Thats basicaly all I wanted to understand + the questions that were answered.Thanks

author: nugglix
author_rank: Regular Contributor
timestamp: March 08, 2017, 07:09:19 am
content: Just in casehttps://www.ixquick.com/do/dsearch?query=how+to+read+datasheet+fetdoesn't work:https://www.embeddedrelated.com/showarticle/809.phphttp://www.physics.unlv.edu/~bill/PHYS483/fet_datasheet.pdfhttps://training.ti.com/understanding-mosfet-datasheets

author: Wojta
author_rank: Contributor
timestamp: March 08, 2017, 07:53:22 am
content: Quote from: nugglix on March 08, 2017, 07:09:19 amThats what Ive been looking for.Thank you.

author: Zero999
author_rank: Super Contributor
timestamp: March 08, 2017, 10:54:13 am
content: Quote from: Wojta on March 07, 2017, 11:43:41 amOne thing to bear in mind with most 555s, the output is not symmetrical. With a bipolar 555 the output voltage loss is higher when the output is high than it is when it's low. In the case of a CMOS 555, such as the ICM7555, the output can provide more current when low, than when high. This means the MOSFET will turn off, faster than it will turn on. This may not be a problem, especially at low frequencies, but it's something to beware of.