Will this circuit work AND THEN, is it safe ?  - Page 1
Forum section: electronics / projects

author: CountParadox
author_rank: Contributor
timestamp: March 01, 2015, 07:21:24 am
content: Hey guys, making an LED strip dimmer thing with an arduino and some mosfets, basically,please take a look at these photos and let me know ifA) this circuit should work ? (im gonna test with 5v first before 12v)B) is it safe???do you any problems with how i layed it out (i know its ugly/dodgy but its ehhhh)The LED strips will be 12v LED strips, with 12v 6amp power supplies, Ill mark on an image where everything will be hooked up!!Thank you!my references:http://www.jaycar.com.au/products_uploaded/zt-2468.pdfhttp://www.drinking-water.com/pictures/mosfet510.bmphttp://iot-playground.com/2-uncategorised/28-esp8266-wifi-led-dimmer

author: picandmix
author_rank: Frequent Contributor
timestamp: March 01, 2015, 09:40:33 am
content: Your 'references' show two different types of Mosfet, but for use with the Arduino you must use the one with the Logic Level gate.

author: CountParadox
author_rank: Contributor
timestamp: March 01, 2015, 10:08:47 am
content: Quote from: picandmix on March 01, 2015, 09:40:33 amthe pinout is the same for them from what i understand.im new to this sort of thing,im using the one i linked the datasheet to, the IRF1405I see in its datasheet it mentiones the gate threshold minimum at 2 volts, ill be using 3.3v logic..

author: Niklas
author_rank: Frequent Contributor
timestamp: March 01, 2015, 02:22:23 pm
content: There are two things about the MOSFET you selected that might cause some problems. Scroll down to page 2 in the datasheet and check:1. Vgs(th) which is stated as Min 2.0 V and Max 4.0 V. In case you have the Min or something Nominal it can work with 3.3 V logic, but what about Max at 4.0V? The Vgs(th) is the point where the MOSFET will start to conduct, similar to when the water starts dripping from the tap. As the Vgs voltage is increased, the MOSFET conducts better, similar to a higher rate of water flow from the tap.2. The capacitive parameters (Ciss, Coss etc) a little bit further down are hinting about possible issues with switching frequency for the PWM. Let's assume that the capacitor is a bucket that you want to fill with water from a hose. The current that the Arduino board can source is equal to the water flow in that hose and that is quite limited (a few milliamps). With a low rate of water flow it will take some time to fill up that bucket and each PWM period must be able to completely fill the bucket.I spotted another thing that will inhibit this circuit from working. You have diodes from the Arduino PWM to the gate pins on the MOSFETs. You will only be able to turn the MOSFETs on, but the diodes will block any attempt to discharge the gates. The diodes also have a forward voltage drop that will remove at least 0.5 V of the 3.3 V.Replace the diodes from the Arduino PWM to the MOSFETs with resistors of 100-200 ohms (100, 120, 150, 180, 220...) something. Also add a 10k resistor from the Arduino PWM down to GND to make sure that the gates are discharged if the Arduino will reset and have a high impedance output.

author: Corporate666
author_rank: Unknown
timestamp: March 01, 2015, 04:42:00 pm
content: In addition to what's already been said about the FET's, you are running all the power for your strips through the two diodes.That's fine - but keep in mind the diodes have a max amp rating and will dissipate power, so your strips can't be too long.

author: CountParadox
author_rank: Contributor
timestamp: March 02, 2015, 10:49:54 am
content: Quote from: Niklas on March 01, 2015, 02:22:23 pmSo does this mean only use it at 3.3v, do NOT use it with 5v logic ?Quote from: Niklas on March 01, 2015, 02:22:23 pmwhat capacitor is this? also:Quote from: blueskull on March 01, 2015, 07:29:55 pmim using the sketch detailed in the 3rd "reference" link,can you give me some pointers about the PWM frequency ? im not sure about thisQuote from: blueskull on March 01, 2015, 07:29:55 pmwould I be able to use one of the ones im using here?Ive just spent $20 on 4 of them at expensive-car err, jaycar so i would prefer to use them, i think if im reading the datasheet correctly they handle 100A + ??as far as i know the MAX I will be using is 12.Quote from: Niklas on March 01, 2015, 02:22:23 pmooh thank you, this completely skipped my mindits small like 1A isnt it!?shoot,this is gonna sound stupid, but do resistors have a current limit...?(I presume they do for obvious reasons, but like, would cheap jaycar [radioshack?] ones be fine in this case...?Quote from: Corporate666 on March 01, 2015, 04:42:00 pmRight! as mentioned above I didn't even think of this...ill be rid of the diodes there, I only used them to get over the other wire without as much risk of shorting hahaaThanks for helping me guysPretty new to this stuff but keen to learn!also, sorry how messy this reply is!also also,Its perfectly fine to hook up the two PSUs in parallel right?>http://www.ebay.com.au/itm/261740183173