Help verifying a power circuit - Page 1
Forum section: electronics / projects

author: Zinger
author_rank: Contributor
timestamp: April 11, 2017, 11:32:02 pm
content: I'm designing a power circuit to control a 12V 4.67A strip of LEDs. I've come up with something I think will work but I'm a novice at this. I'd really appreciate it if someone who knows what they're doing would take a look at it.I've attached the schematic and simulation I did on SystemVision. Big thanks to everyone who replied to this thread.https://www.eevblog.com/forum/beginners/easy-to-learn-circuit-simulator/Your input was invaluable.The power supply on the left is 12V. In the real circuit I will use a reed switch to drive the MOSFET's gate. For the simulation, I used a voltage controlled switch and hooked it up to a pulsed 5V. I couldn't find a simple switch in the parts list.The resistor on the left is to limit the current going into the reed switch. There seem to be a million reed switches that can handle 12V and 80mA.I modeled the MOSFET on a BUZ11 because several sites said it's a popular NPN and it looks cheap on eBay.I modeled the lamp load on my LED strip. 12V, 4.67A, 56W. I think the current spike when I first turn on the transistor is because the lamp model has a fairly large inrush current. The LED strip I'm using is a high quality one from HitLights. It's protected so I don't think that will happen in the real circuit. BTW, Hit has fantastic customer service, as I found out this morning.https://www.hitlights.com/Will this thing work? Is it overkill? Will it kill someone? What am I missing?Thanks

author: ebclr
author_rank: Super Contributor
timestamp: April 11, 2017, 11:38:53 pm
content: You can even buy similar circuit readyhttp://www.ebay.com/itm/New-MOSFET-Button-IRF520-MOSFET-Driver-Module-for-Raspberry-pi-Arduino-ARM-/221890885367?hash=item33a9ba36f7:g:AeEAAOSw9r1V~fW2

author: Zinger
author_rank: Contributor
timestamp: April 12, 2017, 12:06:59 am
content: I want to use my 12V to drive the MOSFET. This part is logic level. Could I just use a resistor between the 12V and SIG to drop the voltage to logic level?Then wire the battery to VCC and GND and my switched power comes out the screw terminals? All for 99¢ plus shipping?

author: RJFreeman
author_rank: Regular Contributor
timestamp: April 12, 2017, 12:40:04 am
content: As you are a novice, lets run through some of the things you need to consider, referring to the BUZ911 data sheet at:https://www.fairchildsemi.com/products/discretes/fets/mosfets/BUZ11.htmlhave you exceeded the Breakdown Voltage of the mosfet?No, all good, the BUZ11 can handle 50VHave you exceeded the Gate Voltage of the mosfet?No, all good at 12V, the BUZ11 can handle a VGS of 20Vhave you exceeded the current handling of the Mosfet?No, all good at 12V - a BUZ11 can handle 50 Ampsare you driving the Gate hard enough to turn the Mosfet fully on?Yes, VGS of over 5V should have the Mosfet in saturation at around 5 Amps, and minimum rDS(on) is given as 0.03 ohms at 10V VGS, so 12V is good.Are you switching the Mosfet on fast enough?- this is not a major concern in this application, as you are not pushing a BUZ11 anywhere near its limits.Having said that though, R2, the 100 ohm resistor is going to slow the turn on slightly, due to the 1500 to 2000 pF input capacitance.Likewise it will be slow to turn off, as the Gate discharges via R1This would be a concern if/when you are pushing a Mosfet to its limits as you want the Mosfet to switch on and off as quickly as possible, so it does not spend too much time 'partly on' which is where it gets hot....Have you exceeded the safe operating area of the Mosfet?your specified current is 4.67 Ampsworst case VDS should be about 0.04 x 4.67 = 0.187 Volts (where VDS=rDS(on) x Current )- That is well in the Safe operating area of the BUZ11 shown in Figure 4 of the Data sheet.

author: RJFreeman
author_rank: Regular Contributor
timestamp: April 12, 2017, 12:46:51 am
content: Quote from: Zinger on April 12, 2017, 12:06:59 amProbably not needed, as most Mosfets will handle at least 12V VGS, but just for the sake of the exercise, the picture of that module shows the Mosfet as an IRF520N.We can find the Data sheet for the IRF520N here:http://www.infineon.com/cms/en/product/power/mosfet/20v-300v-n-channel-power-mosfet/80v-100v-n-channel-power-mosfet/IRF520N/productType.html?productType=5546d462533600a401533d2b9de87385So, does 12V exceed the maximum VGS for an IRF520N?

author: Zinger
author_rank: Contributor
timestamp: April 12, 2017, 12:51:04 am
content: Datasheet says ±20V so I'd be good at 12V.Thanks!

author: RJFreeman
author_rank: Regular Contributor
timestamp: April 12, 2017, 12:54:42 am
content: Quote from: Zinger on April 12, 2017, 12:51:04 amYep, should all be good.Quoteno worries

author: Zinger
author_rank: Contributor
timestamp: April 12, 2017, 12:58:22 am
content: Thanks again. This is a great reply. Exactly the level of detail I need at this point in my "career".

author: RJFreeman
author_rank: Regular Contributor
timestamp: April 13, 2017, 01:17:47 am
content: This is a good starter to understanding Mosfet Parameters:http://www.newelectronics.co.uk/electronics-technology/making-sense-of-mosfet-datasheets/48782/although the pictures/diagrams do not load from the web-page for me, but if you scroll down to near the bottom, you can download a PDF version which does include all the diagrams.Look out for :Related Downloads48782\P23-24.pdf*Comment on this articleWhile this document looks like (I haven't finished reading through it) it goes into a bit more detail:http://www.infineon.com/dgdl/mosfet.pdf?fileId=5546d462533600a4015357444e913f4f

author: Audioguru
author_rank: Super Contributor
timestamp: April 13, 2017, 02:40:17 am
content: Why do you think the IRF520 Mosfet is logic level? It is not. An IRL520 Mosfet is logic level. See the "L"?When you buy cheap stuff from ebay then the Mosfet might not meet its specs, be fake or be defective. It also might be used and near the end of its life.

author: exoticelectron
author_rank: Regular Contributor
timestamp: April 13, 2017, 08:46:09 pm
content: Quote from: Audioguru on April 13, 2017, 02:40:17 amSeriously..there needs to be a sticky telling beginners where to get parts. Its not 1993 anymore. Order your parts where everybody else gets them for prototyping, digikey and mouser. USPS shipping for small stuff is $4 and saturday delivery is free. Stop wasting time getting unknown stuff from ebay.Unless you arent in USA and in that case good luck

author: Zero999
author_rank: Super Contributor
timestamp: April 13, 2017, 10:35:32 pm
content: Quote from: exoticelectron on April 13, 2017, 08:46:09 pmWhat the heck? For those not in the U$ there's Farnel, RS Components, Rapid Electronics, etc.

author: exoticelectron
author_rank: Regular Contributor
timestamp: April 13, 2017, 10:37:30 pm
content: Quote from: Hero999 on April 13, 2017, 10:35:32 pmI thought with Brexit that all British business stopped and its like stone age over there with people eating each other....no?

author: janoc
author_rank: Super Contributor
timestamp: April 13, 2017, 11:06:13 pm
content: Quote from: Hero999 on April 13, 2017, 10:35:32 pmWell, here in France RS charges 10€ s&h, Farnell 20€, Digikey the same + VAT + customs handling thanks to UPS doing the customs for them, Mouser also charges 20€ s&h unless you buy for at least 50-60€.That MOSFET gets quite a bit expensive in such caseStill wondering why are people buying crap from AliExpress, eBay and similar sources? Consider yourself lucky if you don't have to deal with the above in UK.RS here is actually the cheapest and has free s&h if you manage to time your order on a weekend. On the other hand, their website is probably by far the most unusable and they don't stock many things that Farnell or Mouser haveAnd it isn't that long time ago where the likes of RS, Farnell or Mouser didn't even sell to private persons at all, because they didn't want to bother with charging VAT and dealing with small orders here.

author: Zinger
author_rank: Contributor
timestamp: April 21, 2017, 09:53:40 pm
content: Quote from: exoticelectron on April 13, 2017, 08:46:09 pmEE - Thanks for the pointer to Digi-key. Great web site. Much better than scrounging around on eBay. I got my components and am ready to wire the circuit.Of course, today is the day the lead decided to fall off my 20 year old DMM's negative probe. And, of course, today I was already at the hobby store, a Home Depot and one of the last Radio Shacks in Virginia. Could have got a new one cheap, if I'd known it was going to happen.

author: Zinger
author_rank: Contributor
timestamp: April 22, 2017, 02:00:09 pm
content: Thanks everyone for your pointers and advice. I breadboarded the circuit this morning and it works perfectly.I'll solder and shrinkwrap it this afternoon and then start the next part of this project.Thanks again.