Without it SEEMING to be damaged ? ? ? Just because something works for a five minute video doesn't mean it's a long term solution.

Right, that's why I'm asking. Even in this thread, at least one person has said it should theoretically be fine.

Just guessing here but it seems like you're trying to control the heat output of a heat gun. A thermostat switch should do the same thing without having to build all that stuff and hope it lasts. OK, I did see where you said you wanted to do this with parts in hand. I get it. I do the same thing. Once used a MOSFET rated for 600V on a 12 volt circuit. Not always the brightest idea, but it was what I had in hand, like you.

I want to control the temperature of the heat gun, not just on and off periodically. A think a thermostat switch won't work for that.

You mentioned a soldering iron in series with your heat gun. What about a light bulb? Easier to see when something is on or off.

That was really just a joke, haha.

 

Typical gate cap is 1350pF. That isn't insignificant and is going to affect edge speeds.

Either I don't understand, or it doesn't matter for my purposes. I don't need anything precise, I just need it to function reasonably without overheating in a 30-60 second time span.

BJTs don't have channels.

This has been established already in this thread.

 

Either I don't understand, or it doesn't matter for my purposes. I don't need anything precise, I just need it to function reasonably without overheating in a 30-60 second time span.

You haven't provided sufficient information for anyone to determine whether rise/fall time of the control signal will matter. With slow edge transitions, the MOSFET won't switch quickly between on and off states. That will mean increased power dissipation. Since we don't know the voltages or currents involved, we can only state the potential issues.

 

You haven't provided sufficient information for anyone to determine whether rise/fall time of the control signal will matter. With slow edge transitions, the MOSFET won't switch quickly between on and off states. That will mean increased power dissipation. Since we don't know the voltages or currents involved, we can only state the potential issues.

Yes, I understand all that. I'm saying that I'm not concerned, I think it will be fine with the heatsinks I have. Thank you for your help.

 

at least one person has said it should theoretically be fine.

Four out of five dentists recommend "it". Wonder what the one dentist knows that the rest don't?! At least you've found one person to agree with you despite all the others suggesting it might not be a good idea. But there's no greater teacher than true life experiences. That being the case - build it. Let us know if it works. Let us know if it keeps working long term. Stranger things have happened, so there's no saying what you want to do will end up in failure.

Modulating the temperature - not the fan - should be the aim. That'd mean sensing the output temperature somehow. I know that's not an easy thing to do. Well, actually it is. But controlling just the heating elements - that's where you want to focus your attention, not on slowing the fan and lowering the temperature.

At this point I don't think I can offer anything further, so I wish you success. Just be sure to let us know how things turn out.

 

Four out of five dentists recommend "it". Wonder what the one dentist knows that the rest don't?! At least you've found one person to agree with you despite all the others suggesting it might not be a good idea. But there's no greater teacher than true life experiences. That being the case - build it. Let us know if it works. Let us know if it keeps working long term. Stranger things have happened, so there's no saying what you want to do will end up in failure.

Modulating the temperature - not the fan - should be the aim. That'd mean sensing the output temperature somehow. I know that's not an easy thing to do. Well, actually it is. But controlling just the heating elements - that's where you want to focus your attention, not on slowing the fan and lowering the temperature.

At this point I don't think I can offer anything further, so I wish you success. Just be sure to let us know how things turn out.

*Sigh* you completely misunderstood my point. My instinct is and has been to use resistors to limit the current, but I wasn't sure if it's needed or not. No one has "agreed with me" or "disagreed with me" since all I did in the first place was ask questions, and in the post you're quoting I was only making the point that me not knowing whether it was necessary is understandable as there's not a consensus of information available. Some of the information may be bad (all those videos and such with people connecting them straight to the MCUs), but it's still out there. I also didn't know what effect would necessarily be had by using the resistor, which apparently is increased heat in the MOSFET. It seems like you're criticising me while completely misunderstanding what I've been saying and what my position is.

 

*Sigh* you completely misunderstood my point. My instinct is and has been to use resistors to limit the current, but I wasn't sure if it's needed or not. No one has "agreed with me" or "disagreed with me" since all I did in the first place was ask questions, and in the post you're quoting I was only making the point that me not knowing whether it was necessary is understandable as there's not a consensus of information available. Some of the information may be bad (all those videos and such with people connecting them straight to the MCUs), but it's still out there. I also didn't know what effect would necessarily be had by using the resistor, which apparently is increased heat in the MOSFET. It seems like you're criticising me while completely misunderstanding what I've been saying and what my position is.

I wouldn't worry too much about harsh replies.. sometimes people don't understand and sometimes we don't explain ourselves effectively to be understood.

I'm no expert on your topic but I can say if you measure voltages, current and view each node on the scope you can learn a lot about if you need resistors. I put my faith in the datasheets tolerance and when in doubt I limit current.

 

You should not be getting harsh replies here. If you are getting such replies you can use the Report button to bring it to the moderation team's attention.

 

A 10 Ohm resistor is not going to limit the current of a micro GPIO by any meaningful amount. That is not the purpose of the resistor when it is used. It is to damp out ringing, and would be much more important if your source had very low impedance, like a MOSFET gate driver, which often recommend using a series resistor.

Bob

 

It the MCU outputs can tolerate momentary short circuits on their outputs, then there is no need for a series resistor to limit the current to the MOSFET gate.

As noted, a small series resistor (e.g. 10 ohms) is sometimes added to prevent oscillations, but that likely is not be a problem with your circuit.

 

I also didn't know what effect would necessarily be had by using the resistor

The only result I know of is that the turn on and turn off times would be slowed, depending on how much resistance you use. The gate of a MOSFET acts like a capacitor. The faster it charges (or discharges) the faster the MOSFET turns on (or off). The higher the resistance the slower it turns on. Keep in mind that "Slow" doesn't mean it takes several seconds to switch on and off. Slow is a relative term meaning it may take a few microseconds to possibly several mS's turn on or off.

Sorry - I didn't mean to come off sounding harsh. It sounded to me like you asked a question and everyone was saying one thing but you found the one person who said what you wanted to hear. I'm certainly no expert on electronics. Neither am I an expert in human relations, so if I made you feel small - my biggest apologies. I only wanted to point out what I saw as being someone who only wants to hear what they want to hear. So on that vein if you want to build without a resistor then the choice is yours. Just let us know how it turns out for you.

Many years ago I built a circuit to keep the car running even after I turned the key off. That way I could start the car and have it warm up without having the key in the ignition. No one could unlock the wheel or shift out of gear. And if they did - as soon as they stepped on the brake pedal the engine would shut off. A fail safe. The circuit worked fantastic. For about a week. It then failed because I didn't take into account of switching loads and transient voltage spikes that occurs within automotive electrical systems. It didn't take long to build the circuit but it took hours to install. In the end I went with relays and diodes to steer the voltages. It accomplished the same thing and was far more robust. And back then we didn't have this forum to ask. In fact, back then it was long before home computers became a popular item.

The car circuit wasn't the only thing I've built not understanding all the nuances one needs to know. After many failed circuits - circuits that once worked beautifully - I've learned a little more. One thing I've learned is there's wisdom in the crowd. If most are saying one thing then that's the way I'm going to lean. Even if I feel very strongly that my circuit will work the other way. There are those who have many years - even decades of experience engineering electronic circuits, whereas I probably haven't built more than 20 projects over my entire lifetime. So I defer to their expertise.

Again, if I made you feel small or insulted - it wasn't my intention. Just an awkward way of saying listen to the consensus.

 

I also didn't know what effect would necessarily be had by using the resistor, which apparently is increased heat in the MOSFET.

I don't think that's the case. Of course I'm assuming you're talking about putting a resistor on the gate. Since there is no conduction between the gate and either the source or drain, there shouldn't be any current flowing at all. Only the time it takes to charge the gate capacitance. Resistance - or "More" resistance will only serve to slow down the current, thus, reducing the amount of heat - if any.

 

Slow switching combined with high frequency can keep the MOSFET partway on for a significant fraction if the time causing high dissipation.

It should not be an issue at switching speeds one would use to PWM a heating element.

Bob

 

Slow switching combined with high frequency can keep the MOSFET partway on for a significant fraction if the time causing high dissipation.

It should not be an issue at switching speeds one would use to PWM a heating element.

Bob

Heating element and fan most likely (although I could just make the fan constant power instead).

 

Is it a PWM fan? (4 wire)

Because they typically need about 25k hz on the control input @ 5 volts, if it's a 2 or 3 wire you can just control it with the power connections.

To run a PWM fan at full speed, just leave the control connection open.

 

Is it a PWM fan? (4 wire)

Because they typically need about 25k hz on the control input @ 5 volts, if it's a 2 or 3 wire you can just control it with the power connections.

Nope, two wire fan. PWM on the power lines, not as a control signal.

 

The kind of fan motor is important. If it's an induction motor then chances of controlling its speed is questionable by PWM.

Been thinking about a heat gun I saw a long time ago - it had louvers on the intake to control the amount of air flow into the blower. Less air flow meant hotter air at the outlet.

 

I can't tell on this motor, but it has some sort of small IC that seems to make it able to take AC or DC current and it only spins one direction. Would you like pictures?

 

Pictures would help