Drive Mosfet Gate From Tractor's Altenator Voltage - Page 1
Forum section: electronics / projects

author: 2X
author_rank: Frequent Contributor
timestamp: November 19, 2024, 10:02:08 pm
content: Hello,I want to enable lights from an ald tractor with a solid "state relay" ( I don't want to use an electromechanical relay) but the mosfet's gate after some time of use is burned out. I have placed a zener diode on the Gate of the mosfet but can't sink the voltages spikes. Also, I tried the zener diode with 100ohm in series instead of 10Kohm but the same result... mosfet's gate burned.Do you know if there is any buffer that could accept voltage spikes or something else that could protect the Gate of the mosfet? I thought to use a 10V voltage regulator in order to drive the gate of the mosfet but I don't know if this is the best way.Any recommendations are welcomed.

author: Manul
author_rank: Super Contributor
timestamp: November 19, 2024, 10:17:27 pm
content: Well, put zener between gate and source, where it belongs, not between gate and ground. And it would be better to use 10-12V zener not 17V. And then you need to add some current limiting series resistor to your NPN transistor's collector, otherwise if zener starts conducting, it's a straight path through NPN transitor to ground. But beware that you will be making a voltage divider with that 10k GS resistor, so maybe it needs to be increased.

author: SuzyC
author_rank: Frequent Contributor
timestamp: November 19, 2024, 10:48:16 pm
content: This is all you need.If the switch leads are far from the MOSFET then add a 47k resistor in parallel with a .1uF cap from the gate to source to prevent noise from flickering the tractor lights.As Benta replies below, adding back the BJT transistor instead of a switch directly to ground will simplify using the old +24V switch wiring to the lamps.

author: Benta
author_rank: Super Contributor
timestamp: November 19, 2024, 11:01:54 pm
content: Quote from: SuzyC on November 19, 2024, 10:48:16 pmPerhaps.But I suspect the switch is already wired to plus in an old tractor. Changing the wiring harness is hell, and prolly not what the OP wants.I think moving the Zener to G-S is all that's needed.

author: SuzyC
author_rank: Frequent Contributor
timestamp: November 19, 2024, 11:11:08 pm
content: Benta forgets that the original BJT transistor circuit with moved zener G-S, without a current limiting resistor to the BC547 collector will fry the zener and the BJT transistor.

author: Benta
author_rank: Super Contributor
timestamp: November 19, 2024, 11:14:13 pm
content: Why?Nominal voltage is 12 V, when running it's 14 V. Seems you read it as 24 V (damn European "1"s).But I agree that a gate resistor should be added for fault protection.

author: SuzyC
author_rank: Frequent Contributor
timestamp: November 19, 2024, 11:17:28 pm
content: Benta, yes I mistakenly read 14V as 24V.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 19, 2024, 11:22:54 pm
content: Quote from: 2X on November 19, 2024, 10:02:08 pmWhat is the load inrush and operating current.The MOSFET here is only modest, and they do not like high temperatures. It may simply be underrated too.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: November 19, 2024, 11:23:54 pm
content: Quote from: SuzyC on November 19, 2024, 10:48:16 pmThat is missing a G-S resistor, to properly turn off the FET.

author: SuzyC
author_rank: Frequent Contributor
timestamp: November 19, 2024, 11:53:54 pm
content: Ok, and the MOSFET could need a heatsink.How many Watt bulb does does an old tractor light use?

author: 2X
author_rank: Frequent Contributor
timestamp: November 20, 2024, 09:40:24 pm
content: Thanks all of you for the replies.The the total watt of the lamps are: 4bulbs x 23W = 92W -> I = P/V =92W/12V = 7.66WI find this Mosfet: SQM100P10-19L_GE3 (I selected for his low RDSon because I don't want to use heatsink)For the tranzistor I am going to use: NSVMMBT6429LT1GFor zener I am going to use: BZX384C12-HE3-18I have one question... It would be better if I used TVS diode instead of zener diode because TVS is faster than zener diode or it is too much?

author: Benta
author_rank: Super Contributor
timestamp: November 20, 2024, 10:13:47 pm
content: Quote from: 2X on November 20, 2024, 09:40:24 pmWhy do you think a TVS is faster?And I'd suggest a 15 V Zener instead of 12 V.Your new choice of MOSFET is good. Cold resistance of bulbs can be 6...7 times lower than hot resistance.

author: 2X
author_rank: Frequent Contributor
timestamp: November 20, 2024, 10:17:45 pm
content: Quote from: Benta on November 20, 2024, 10:13:47 pmFrom reading on the internet, but maybe is not true in practical use?https://www.google.com/search?q=tvs+vs+zener+diode+time&sca_esv=dfd2af4a0faad991&sxsrf=ADLYWIJaEZmgA4gfmALgkVzSZVToSxTyCA%3A1732140903847&ei=Z18-Z-OlM5eN7NYPi-uKsAw&ved=0ahUKEwij9Zja9-uJAxWXBtsEHYu1AsYQ4dUDCA8&uact=5&oq=tvs+vs+zener+diode+time&gs_lp=Egxnd3Mtd2l6LXNlcnAiF3R2cyB2cyB6ZW5lciBkaW9kZSB0aW1lMgUQIRigATIFECEYoAEyBRAhGKABMgUQIRigATIFECEYnwVIoxFQvQhY_A9wAXgAkAEAmAHEAaABzweqAQMwLja4AQPIAQD4AQGYAgagAtkGwgIKEAAYsAMY1gQYR8ICBhAAGBYYHsICCBAAGIAEGKIEmAMAiAYBkAYIkgcDMS41oAf5Gw&sclient=gws-wiz-serp

author: Benta
author_rank: Super Contributor
timestamp: November 20, 2024, 10:25:33 pm
content: It's irrelevant in your case. The gate capacitance of the MOSFET will slow everything down, anyway.

author: David Hess
author_rank: Super Contributor
timestamp: November 21, 2024, 05:29:03 pm
content: The zener diode needs be across the gate and source rather than the gate and drain. Then a resistor in series with the collector will be needed to limit the current. I would also use a lower voltage zener diode.Alternatively use a photovoltaic optocoupler to drive the MOSFET between its gate and source. Optical isolation is not strictly necessary for your application, but it will make the circuit more rugged.

author: 2X
author_rank: Frequent Contributor
timestamp: November 21, 2024, 06:54:50 pm
content: Quote from: David Hess on November 21, 2024, 05:29:03 pmThanks for your reply. Do you have any part number of photovoltaic optocoupler? What voltage zener diode would you use?

author: Benta
author_rank: Super Contributor
timestamp: November 21, 2024, 07:23:18 pm
content: When I saw your OP, I considered a PV optocoupler as well, but found it overkill.It's a 3-Euro device as opposed to the few cents that otherwise is suggested.But OK, Vishay VO1263 could be an option.

author: DavidAlfa
author_rank: Super Contributor
timestamp: November 21, 2024, 07:27:25 pm
content: There's nothing more simple and robust like a relay, why all this overcomplicated mosfet mess to turn some lights on?It's a tractor, not a Mars Rover...

author: 2X
author_rank: Frequent Contributor
timestamp: November 21, 2024, 09:32:00 pm
content: Quote from: Benta on November 21, 2024, 07:23:18 pmAlso If any guy in future will need... I found with one channel but the Voc is 8.4V typical so the mosfet will not be fully enabled in order to reach the minimum RDSon.VOM1271T

author: David Hess
author_rank: Super Contributor
timestamp: November 21, 2024, 10:06:15 pm
content: Quote from: 2X on November 21, 2024, 06:54:50 pmI would use a 10 or 12 volt zener diode.I used to use the ToshibaTLP591Bphotovoltaic optocoupler in a 6-pin DIP package, however it has been discontinued and replaced with the ToshibaTLP3906which has a 6-pin SOIC package. If you want a DIP devices, then the more expensiveAPV1122might be suitable if you can figure out how to get the datasheet from Panasonic.Quote from: Benta on November 21, 2024, 07:23:18 pmA photovoltaic optocoupler is overkill, but is a simple way to get a more robust design in an automotive environment.

author: Someone
author_rank: Super Contributor
timestamp: November 21, 2024, 10:14:50 pm
content: Quote from: DavidAlfa on November 21, 2024, 07:27:25 pmand if low power consumption is really important, an Automotive solid state relay is still cheaper than engineering one from scratch.

author: 2X
author_rank: Frequent Contributor
timestamp: November 22, 2024, 05:55:52 am
content: Quote from: Someone on November 21, 2024, 10:14:50 pmDo you have any trusted automotive solid state relay to suggest me?I found the below (made in China) but I am not sure about the quality.https://www.hongfa.com/product/solid-state-relay/HFS60-C-12

author: David Hess
author_rank: Super Contributor
timestamp: November 22, 2024, 06:14:50 am
content: Quote from: Someone on November 21, 2024, 10:14:50 pmI designed and built an automotive solid state relay last year because nothing available at a reasonable price was suitable.https://www.eevblog.com/forum/projects/homemade-solid-state-relay-for-12-volt-automotive-application/msg4955389/#msg4955389

author: Someone
author_rank: Super Contributor
timestamp: November 22, 2024, 07:56:34 am
content: Quote from: 2X on November 22, 2024, 05:55:52 amI walk into a local Automotive stockist and ask them (since they are the first place to go for a warranty if it goes wrong) choices between $40 and $70 here.Quote from: David Hess on November 22, 2024, 06:14:50 amI see you also spend your time adding spacing to peoples quotes, so your labour must be cheap. Engineering says otherwise for me:Conventional Relay, $5, burns around 1W of excess power in the OPs application.Solid State Relay, $50, burns around 0.7W of excess power in the OPs application.asking a forum to design something for them... hours of peoples time, for a solution that will still burn something in the order of 0.3W of excess power and cost at least as much as an off the shelf conventional relay. Where is the benefit? Labour costs need to approach zero for that to make any sense.Having replaced a conventional relay in one of the households vehicles that welded its contacts, I still stuck with a conventional relay.

author: elektryk
author_rank: Regular Contributor
timestamp: November 22, 2024, 12:10:15 pm
content: Quote from: 2X on November 22, 2024, 05:55:52 amWhat's wrong with mechanical relay to you?They are still used in vehicle lighting today without any problems.

author: 2X
author_rank: Frequent Contributor
timestamp: November 22, 2024, 05:04:07 pm
content: Quote from: elektryk on November 22, 2024, 12:10:15 pmBecause, I have made a try and I burned the mosfet the simple solution would be to buy a mecjanical relay, but the simple way didn't give you new knoeledge... I learnead new things (like the photovoltaic optocoupler etc.) where could be useful for me in the future. Personally I prefer the hard way because this way pays off in very well indirectly and not well directly like the easy way.Thanks all fo you for the help.

author: elektryk
author_rank: Regular Contributor
timestamp: November 22, 2024, 08:19:39 pm
content: Its always good to try something new, but sometimes it ends up with off shelf solution.In this case mechanical relay should be more rugged than plain mosfet. One short circuit and mosfet will burn faster than fuse.Some automotive modules use transistor outputs, but usually together with short circuit detection, so wiring failure isn't that dangerous.