My first MOSFET Blinking light - Page 1
Forum section: electronics / beginners

author: watchmaker
author_rank: Unknown
timestamp: November 02, 2023, 11:05:16 pm
content: Following the Analog Ckt Analysis course and labs. I decided not to skip any so I get used to the Discovery and breadboarding.Material management is not a small trick. I am starting with header pins and a set of premade jumpers. I do have the male to male jumpers, but that just looks wrong.I did get my initial hookups wrong. Columns look too much alike to my eyes unaided. Gotta use the 3x magnifier lamp at all times. But it is not like making a mistake with a 200V B+ and 150 uF capacitors (which we used to throw at each other in shop). The worst that happens is the Discovery shuts down or you smoke a semiconductor.Anyway, one of the very early labs is using a MOSFET to get practice using an ammeter, voltmeter and voltage sources (including WaveGen). Good practice.But, they call for a Mosfet I did not have. I did have 2N7000s. Looked it up and made the ckt with an LED and the Wavegen set to square wave at various voltages. I now get that Mosfets are very different from signal/switching transistors in that there is a gate voltage at which they do not conduct.BTW, I bit the bullet and bought one of the left over kits (apparently Analog stopped production) used in that course sequence. Cheaper than going to college! (So I tell my wife after a month or two of biweekly boxes).This made me read up on Mosfets. I get the channel is the key. We flood it with electrons (or holes) which increases the size of the depletion zone reducing current flow and at some point blocking it completely. Now I need to read the difference between this and an SCR. (No spoilers please).Here is my question, and I think my reading answered it. It looks to me like the source and drain designations are arbitrary. That in fact they can be interchanged.Is this correct?THANKS!Dewey

author: Ian.M
author_rank: Super Contributor
timestamp: November 02, 2023, 11:16:45 pm
content: Yes and no. It all depends on the substrate connection. There is a diode junction between the substrate and the channel, which must be kept reverse biased. If the substrate is brought out to a separate pin, then yes, the device's source and drain can be used either way round as long as the substrate is biased correctly, but most MOSFETs have the substrate tied to the Source internally so that must be negative with respect to the Drain for N channel, and positive for P channel, to keep the substrate|channel junction reverse biased.OTOH, most JFETs are reversible.

author: janoc
author_rank: Super Contributor
timestamp: November 02, 2023, 11:22:08 pm
content: Quote from: watchmaker on November 02, 2023, 11:05:16 pmNo, it isn't. Two issues:- Most components aren't constructed perfectly symmetrical for various reasons, such as cooling (one electrode is tied to the large metal tab for heatsinking, the other is not).- Parasitic body diode that emerges due to the fabrication process.Why don't you try it out? You have the components and all the equipment in front of you - flip the FET around and measure it.

author: watchmaker
author_rank: Unknown
timestamp: November 02, 2023, 11:29:09 pm
content: Quote from: janoc on November 02, 2023, 11:22:08 pmI WILL! Thank you; I need empirical verification to understand things. This is why I gravitated toward watchmaking; directly observable and measurable.I am done for the evening but will do this tomorrow.Regards,Dewey

author: janoc
author_rank: Super Contributor
timestamp: November 03, 2023, 02:48:27 pm
content: Quote from: watchmaker on November 02, 2023, 11:29:09 pmWell, with electronics things are rarely directly observable (not counting the BANGS! and magic smoke releases ...) but pretty much always measurable as long as you have the necessary equipment.So measure measure and again measure and compare that with the expected behavior. If it suddenly does something different and unexpected - great! Something is wrong with either the circuit or the mental model and there is an opportunity to learn something new.

author: watchmaker
author_rank: Unknown
timestamp: November 03, 2023, 05:21:57 pm
content: Quote from: janoc on November 03, 2023, 02:48:27 pmI made money thia MA to pay for this endeavor and will spend the rest of day measuring.Measured resistance and got results I do not yet kow how to interpret. A lot of infinities.I am now playing with he ckt and will spend the PM measuring. In the meantime I have seen the impact of stray voltage at the gate. I am using the LED as a visual indicator and above a VSS of 2.25Vdc and no gate input, the lamp lights. Once I apply the sq wave to the gate it turns the ckt off.I am using a known good InsteknGDM 8245 for measurements. Got infinite resistance across all terminals both ways except when positive is at the drain to source. Then recorded 47K. I want to puzzle this out for a while while I measure currents and voltages.Thank you for encouraging me to look at this more closely. Learning stuff.BTW, in this ckt Source Vs Drain makes no observable impact on the LED.Measurements are:LED Off OnVg -2vDC +2vDCId 0 .14uAIs 1.02uA 470uAIg 0.0uA 0.0uASo this confirms that it is gate voltage (w/o current flow) controling the current drain; I think.Also, I noticed significant loading from the ammeter when I measured the Is as witnessed by the dim LED.Thanks. Glad I followed that rabbit!Regards,DeweyRegards,Dewey

author: janoc
author_rank: Super Contributor
timestamp: November 03, 2023, 09:17:25 pm
content: Quote from: watchmaker on November 03, 2023, 05:21:57 pmDon't measure resistances, that is not very meaningful. FET is not a linear device. E.g. those 47k could have easily been either you measuring the body diode in the conductive direction or you have touched the gate and transferred static charge to it, making the FET start to partially conduct (linear regime).Measure drain current vs Vgs, for example - and plot them to obtain the characteristic. I.e. don't measure just a single voltage and single current. Then compare what you have measured with the characteristic that is in the datasheet.BTW, 2N7000 is very easy to destroy because it has no protection on the gate. So do make sure you observe proper ESD precautions (earthing strap, etc.) or you can easily zap it by merely touching the wires and then all bets are off. You could spend hours measuring and wondering why are you getting so weird values ...

author: julliesoner
author_rank: Newbie
timestamp: November 04, 2023, 08:34:04 pm
content: Yes, you are correct. In a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor), the source and drain designations are interchangeable. MOSFETs are symmetrical devices, and their operation does not depend on the specific source or drain terminal.You can swap the connections of the source and drain terminals, and the MOSFET will still function the same way. This property makes MOSFETs versatile for various applications where the orientation of the source and drain may need to be flexible or may change depending on the circuit's requirements.

author: Peabody
author_rank: Super Contributor
timestamp: November 04, 2023, 08:58:07 pm
content: If a mosfet is on, current can flow in either direction. However, it's specifically the relationship of the gate voltage to the source voltage that determines whether the mosfet is on. Also, the structure of the mosfet creates a "body diode" between source and drain, so the orientation of the mosfet does make a difference. So in practice, the drain and source are not interchangeable.

author: janoc
author_rank: Super Contributor
timestamp: November 04, 2023, 09:08:50 pm
content: Quote from: julliesoner on November 04, 2023, 08:34:04 pmSorry but that is completely false and has been explained above.What you are saying applies only to JFETs and some very rare MOSFET parts that have the substrate connected to a separate electrode so that the parasitic body diode can be kept reverse biased and thus non-conducting.So no, you really can't swap the source and drain terminals on most MOSFETs (even ignoring issues such as heat dissipation, etc.).OTOH, when I see your account description ("SEO specialist") and that this is your first post, I wonder whether this is a spam bot.

author: RJSV
author_rank: Super Contributor
timestamp: November 04, 2023, 11:05:20 pm
content: With your question about MOSFET terminal interchangeability I started thinking about the 'N' and 'P' doping done in manufacture, as that is different in the bipolar transistor regions, for emitter and collector.Apparently doesn't apply to question here, am I right ?

author: watchmaker
author_rank: Unknown
timestamp: November 05, 2023, 02:07:41 pm
content: When I looked at the pinout, I thought you were spot on. But in my ckt it did not matter where the drain and source were. Remeber though, I am using very low voltages and currents.Regards,dewey