MOSFET or RELAY and Resistive v Inductive Question - Page 1
Forum section: electronics / beginners

author: FearTec
author_rank: Regular Contributor
timestamp: October 29, 2020, 02:39:01 am
content: Hello, Newbie here.I have a 12V 8A DC Solenoid that I want to power from a ESP32 (3.3v) with a MOSEFT or Relay.This is the project (basically, a clock that that rings a bell every hour):https://fearby.com/article/adafruit-huzzah32-esp32-feather-rtc-alpha-numeric-led-solenoid-zen-clock/I chose an Adafruit ESP32 Hazzah (https://www.adafruit.com/product/3591) and originally, I planned to use an N Channel MOSFET to switch on an off the 12V Power. I made a simple circuit to test the MOSFET with a 5V 0.5A DC motor.12V 8A Solenoid:https://www.amazon.com/uxcell-JF-1578B-Linear-Solenoid-Electromagnet/dp/B07FKSX779Two MOSFETS’s I could use?STP36NF06L N-Channel MOSFET 60V 30A (Part: STP36NF06L)Threshold Voltage Vgs Typ: 2.5VSTP36NF06L Datasheet:http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00003405.pdforN-Channel MOSFET 60V 30A MOSFET Part (FQP30N06L).FQP30N06L Datasheet:https://dlnmh9ip6v2uc.cloudfront.net/datasheets/Components/General/FQP30N06L.pdfRelayI ended up ordering a Adafruit Power Relay FeatherWing as I was not sure if the MOSFETS were ok?Snip “This Wing has a non-latching type relay. You can switch up to 10A of resistive-load current at 120VAC, 5A at 240VAC. With inductive loads, about half that.”Adafruit Power Relay FeatherWing Datasheet:https://cdn-shop.adafruit.com/product-files/3191/G5LE-14-DC3-Omron-datasheet-10841140.pdfI will use a a1N4004 1A 400V Diode to protect the feathers when the solenoids collapsing magnetic field is turned off.Q1) Would you use a MOSFET (and what one) or the relay?Q2) I assume I will have an inductive load o the Solenoid and the relay would not be the best choice?Thanks In Advance, Data sheets scare me.

author: BlackICE
author_rank: Regular Contributor
timestamp: October 29, 2020, 10:25:33 am
content: I'm no expert but in general if you're switching it on and off quite often I would lean towards mosfet. So I was leaving it on for long periods of time I would lean towards the relay. Look up flyback diode to protect the mosfet.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 29, 2020, 11:05:39 am
content: If a N-Channel MOSfet fits the task then I'd prefer it over a relay.Relays are mostly useful for switching AC, or when you need the floating contacts, or high voltage isolation barrier.A 12V 8A solenoid is quite a beast and it can also destroy relay contacts over time, so adding a freewheel diode, and maybe also a snubber is advisable.For a MOSfet, almost any that can handle the voltage and amps will do.Some of them have an avalanche rating, and if this avalanche rating is (significantly) higher then the energy in your solenoid, you can even do without the freewheel diode (though I still recommend putting it in).Most MOSfets are not compatible with 3V3 Gate voltages, but you already mentioned that Ugs of your STP36NF06L is 2V5 so you probably are already aware of that.But watch out for the power dissipation of the FET. Rds increases at low Ugs. You can already see it increasing to 50mOhm @ Ugs = 5V.P = I^2 * R => 64*.05 = 3.2W and this is already too much to use it without heatsink, and with 3V3 it would be even worse. Too small a Gate voltage dramatically increases the power dissipation in a MOSfet, and the heat can easily kill it. Even if a 3V3 compatible MOSfet "should work", I'd still put in a better gate driver which preferably can deliver more then 8V to the Gate, and this mostly makes the use of a "logic compatible" MOSfet superfluous.Do you have a higher voltage available nearby?If not, you can use a PWM output with a DC of 50% and a voltage tripler from some diodes and resistors.Hmmm,Maybe a good old fashioned relay ain't so bad after all...

author: FearTec
author_rank: Regular Contributor
timestamp: October 30, 2020, 04:05:07 am
content: Thanks for the replies, I am switching to a 12V 2A Solenoid with 3KG force (not 5KG) and I will switch back to a MOSFETI irdered20x SCHOTTKY DIODE, 1A, 20V, DO-41 made by MULTICOMP for protectionI might switch back to MOSFETS even though the relay is working

author: Zero999
author_rank: Super Contributor
timestamp: October 30, 2020, 10:37:38 am
content: Quote from: FearTec on October 29, 2020, 02:39:01 amWhat MOSFET driver do you intend to use?Are you intending to drive them directly from the ESP32 output, which is only 3.3V? If so, I wouldn't recommend either MOSFET. The threshold voltage of 2.5V is specified at a tiny current of 250 μA. The FQP30N06L for example will drop around 1V, with a drain current of 8A and a gate-source voltage of 3V, resulting in a power dissipation of around 8W. The STP36NF06L isn't specified at VGS< 4V, but will probably be similar.I'd recommend a MOSFET driver IC, e.g. the MCP1416 or FAN3111, but if you must directly drive it from 3.3V, it seems the best way is to go overkill. Try the IRLB3034.https://www.infineon.com/dgdl/irlb3034pbf.pdf?fileId=5546d462533600a40153566027b22585

author: Doctorandus_P
author_rank: Super Contributor
timestamp: October 30, 2020, 10:49:41 pm
content: A very good reason for using a MOSfet may be if you want to activate the coil for longer time periods.Then you can start with a high current, and once it has been activated you can PWM to reduce the power consumption. With PWM, the coil itself and the freewheel diode form a buck converter, and the coil current will depend on the duty cycle.Using a scottky diode in this application has no real benefit over a "normal" silicon diode (Unless maybe if you use PWM) A diode rated for 1A is also a bit smallish (Though it's likely specified for higher peak currents) But if you start using PWM, your 1A diode may get overloaded easily.