Post by thread:[EE] Logic Level Mosfet 2 PIC

Hello everyone, it must have been an about 4 or 5 years since I backed out of the piclist and the world of PICs. Now, a few weeks before Xmas I decided to do a new PIC project again. I have a rather simple question, however, some extensive search on the web and http://www.piclist.com did not delivered enough information for me. I plan to drive a few lightbulbs (12V) by a PIC and to minimize a loss of energy this should be done with a logic level mosfet. The IRLZ 34N seems a good chice for me (Ron=35mOhms, rather cheap and _available_ at my local dealer). Now: How to attach this device to a PIC port. An approach like this sould do the work: bulb | ------ D RBx |--------------*-----G ------ | S Z | 10k Z | Z | | | | | GND GND I am not very familliar with this sort of device (In fact I am only hobbyist) something is very confusing to me: In some schematics the authors use some low-valued resistors directly befre the gate. I assume this is to protect the driving device from some high current pulses when the gate has to be charged. Would it be necessary for the PIC as well like bulb | ------ 100Ohms D RBx |--------------*----|/\/\|--G ------ | S Z | 10k Z | Z | | | | | GND GND Any hints or web-pointers iwth similar schematics are welcome. If you think this generates too much noise over the list, please email me privately. Best Regards Markus

Markus The resistor in series with the gate is typically used to prevent oscillaton at RF frequencies. It may not be required or it may be essential. Layout is a major factor as well as the type of load and mosfet characterisics. For the cost, I'd add it. 100ohms is a reasonable figure but if your switching frequency is high you may want to drop it somewhat (10 ohms minimum ?) It does provide some protection to the PIC in the event of device failure, but that is more a benifit than a reason. Richard P On 27/11/06, Markus Rohe <spam_OUTpiclist.roheTakeThisOuTgmx.net> wrote: {Quote hidden}> Hello everyone, > > it must have been an about 4 or 5 years since I backed out of the > piclist and the world of PICs. Now, a few weeks before Xmas I decided to > do a new PIC project again. > > I have a rather simple question, however, some extensive search on the > web and http://www.piclist.com did not delivered enough information for me. I > plan to drive a few lightbulbs (12V) by a PIC and to minimize a loss of > energy this should be done with a logic level mosfet. The IRLZ 34N seems > a good chice for me (Ron=35mOhms, rather cheap and _available_ at my > local dealer). Now: How to attach this device to a PIC port. An approach > like this sould do the work: > > > bulb > | > ------ D > RBx |--------------*-----G > ------ | S > Z | > 10k Z | > Z | > | | > | | > GND GND > > > I am not very familliar with this sort of device (In fact I am only > hobbyist) something is very confusing to me: In some schematics the > authors use some low-valued resistors directly befre the gate. I assume > this is to protect the driving device from some high current pulses when > the gate has to be charged. Would it be necessary for the PIC as well like > > > bulb > | > ------ 100Ohms D > RBx |--------------*----|/\/\|--G > ------ | S > Z | > 10k Z | > Z | > | | > | | > GND GND > > Any hints or web-pointers iwth similar schematics are welcome. If you > think this generates too much noise over the list, please email me > privately. > > > Best Regards > Markus > -

> The resistor in series with the gate is typically used to prevent > oscillaton at RF frequencies. IIRC it is also used to prevent the high dI/dt in the load and hence in the FET, capacitivly coupled back to the gate, from harming the uC. Wouter van Ooijen -- ------------------------------------------- Van Ooijen Technische Informatica: http://www.voti.nl consultancy, development, PICmicro products docent Hogeschool van Utrecht: http://www.voti.nl/hvu

Hello everyone, thank you for the responses so far. I was a little bit confused because today I saw some schematics with and without a resistor in serial to the gate and I was wondering about this purpose. The resistor from the RAx or RBx to GND are used suppress voltage spikes caused by HF noise since the gate has a quite high impedance. The circuit will consist of 12 MOSFET drivers all independently driven in PWM mode such that a good noise reduction is essential. Unfortunatelly, I do not have an oscilloscope such that I simply have to "guess" some "good" value and to hope that everything works well enough. So, what are your suggestions? Is this resistor value critical. Do you have some "empirical values"? 10 Ohms, 50 Ohms, 100 Ohms? Best Markus

William Chops Westfield schrieb: > On Nov 27, 2006, at 10:49 AM, Markus Rohe wrote: > >> The resistor from the RAx or RBx to GND are used suppress voltage >> spikes >> caused by HF noise since the gate has a quite high impedance. >> > The PIC output, however, has low impedance. I wouldn't think the > resistor to ground would be necessary unless you planned to float > the PIC outputs for some reason (fail safe? Power on transient > protection?) > > BillW No, the port pins will constantly used as outputs for driving the gates and never be set as inputs. Ok, since designing such a mosfet driver seems to depend a lot on the given mechanical and electrical paramenters (board, wires, current of the loads, switching frequency) such that I dcided to start like this and see (and measure) what happens. o +12V | \/ bulb /\ 12V/10W | ------ 47 Ohms D RBx |---------|/\/\/|-----G ------ S | | | | GND Markus

At 07:41 AM 11/28/2006, you wrote: > >No, the port pins will constantly used as outputs > >for driving the gates and never be set as inputs. > >but don't forget that for a few microseconds while the unit boots up and >initialises the pots they are defaulting to inputs, which is why the >resistors to source are wise - to ensure you don't end up with something >accidentally turning on when it shouldn't. Or drifting to half-on (which can cause power devices to automatically unsolder themselves) if the micro is socketed and is removed from the socket. >Best regards, Spehro Pefhany --"it's the network..." "The Journey is the reward" .....speffKILLspam@spam@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com ->>Test equipment, parts OLED displys http://search.ebay.com/_W0QQsassZspeff

On 28/11/06, Alan B. Pearce <A.B.PearceKILLspamrl.ac.uk> wrote: > >No, the port pins will constantly used as outputs > >for driving the gates and never be set as inputs. > > but don't forget that for a few microseconds while the unit boots up and > initialises the pots they are defaulting to inputs, which is why the > resistors to source are wise - to ensure you don't end up with something > accidentally turning on when it shouldn't. > And during programming if you're using an ICP. RP