Nope, the red circle really does go to Vcc as shown, the same Vcc noted everywhere else. The drain is switching the low side - completing the path to ground - for the load.

I am sorry, now I am really baffled. The Mosfet that I have only has 3 connections Drain, Source, and Gate as shown below. Looking at the schematic, I have shown how I perceive to connect it. The question is, where does Vcc connect to?

 

Might be helpful if you tell us what exactly you're trying to build.

 

R8 and Q1 do NOT connect into the drain. The drain sees the low side of your load, and only that. The high side of the load is connected to Vcc. Bill's drawing in Fig 10.4 is quite explicit.

 

Might be helpful if you tell us what exactly you're trying to build.

I am trying to use the schematic that Wayne provided to build a high speed flasher to power a smaller coil for automotive application. I do know that this is the circuit that I need, however I am having some issues with how to connect the MOSFET. I have assembled circuits in the past, but have never had to use a MOSFET. The problem I am having is that I have never ran across a schematic that ties a run inside of a component before. When I saw Q1 and R8 tie into the MOSFET at the top like this with the additional Vcc tie at the corner I became a little confused. At this point, I understood it as I described as above. I am feeling a bit small as an electrician right now. I have worked with aircraft electronics and wiring for over ten years and I am stumbling pretty bad on how to connect 1 simple component. I appreciate all the help on this one. Take care.

 

Can you post the full schematic?

 

...for automotive application.

Gaaa!!!!

You'll need to explain how this is not a (forbidden topic) automotive application or it will be shut down by the mods.

 

Gaaa!!!!

You'll need to explain how this is not a (forbidden topic) automotive application or it will be shut down by the mods.

Sorry, poor choice of words. The coil is simular to that of a coil for automotive use, it is for a experiment that I am conductiong. It will not be used on a vehicle. Is there any way to show an image of the MOSFET showing where the connections go? I am a visual learner. I think once I see this I will understand how the final output of this component works. Thank you again.

 

I'm a visual guy too, and that's why I'm baffled about what is not clear in Fig. 10.4. I'm not taking a dig at you, just saying that I cannot see the ambiguity that seems to be troubling you. Maybe if you zoom in and post another diagram I'll finally get it.

The 3 pins of the MOSFET are labelled there. Check the pinout for your particular MOSFET (it's always good to double check, and then check again) and hook it up as shown there.

 

I'm a visual guy too, and that's why I'm baffled about what is not clear in Fig. 10.4. I'm not taking a dig at you, just saying that I cannot see the ambiguity that seems to be troubling you. Maybe if you zoom in and post another diagram I'll finally get it.

The 3 pins of the MOSFET are labelled there. Check the pinout for your particular MOSFET (it's always good to double check, and then check again) and hook it up as shown there.

Ok Wayne, I feel like a complete idiot. What I have circled in RED below is what I thought was part of the MOSFET; it is not, it is the LED load shown in the demonstration isn't it. Essentially, R10-R14 and D1-D15 is the LED load. Essentially, now how I understand it R8 and Q1 ties into Vcc at the entry point of my coil and the negative side of my coil goes back into the circuit tying into the Drain of the MOSFET, hence the N type MOSFET. I am so sorry for racking your head with confusion like that. The second schematic is how I believe that my circuit needs to be connected. Thank you again so much for your help.

Donny

 

Haha, slapping forehead now.

Oh hey, that flyback diode needs to go from drain to Vcc - antiparallel to the coil - so that a high voltage on the drain side of the coil can dissipate to Vcc.

 

Haha, slapping forehead now.

Oh hey, that flyback diode needs to go from drain to Vcc - antiparallel to the coil - so that a high voltage on the drain side of the coil can dissipate to Vcc.

Wayne yes it was a slapping the forehead moment, lol. And about the flyback diode, that was another slapping a forehead moment. I know better than that and do not know why I drew it like that, I did mean to draw it from the Drain to Vcc, guess I was caught up in the moment. I think the reason why I was so stumped with the whole thing was because I was use to seeing LED drawn the old fashion way on schematics, not like the was it was on this schematic. Again, I greatly appreciate all your help on this and will let you know the outcome of my project. Until then, take care and thank you.

Donny

 

If you are driving a coil you will need a coil similar to CR1 to absorb the back voltage spike generated by the coil when the transistor stops conducting.

You can use this for other things, as I show in my article here...

CMOS 555 Long Duration LED Flyback Flasher

BTW, the original article you referred to was also one of my articles.

 

Ok, so I built the circuit today and put it to the test. It appears to flash correctly, but when I put a meter across the output it it only reading 4vdc when I have 12vdc input. Is this the norm for this circuit or am I suppose to have the same out put voltage as input. I am definatly needing the 12vdc output. Thank you again.

 

If you are driving a coil you will need a coil similar to CR1 to absorb the back voltage spike generated by the coil when the transistor stops conducting.

You can use this for other things, as I show in my article here...

CMOS 555 Long Duration LED Flyback Flasher

BTW, the original article you referred to was also one of my articles.

Bill, sorry I just noticed your post. Thank you for your hard work on this schematic, secondly thank you for chilling in on it. Thus far Wayne has been a great help on my project. Earlier on Wayne confirmed what I was thinking about with needing a flyback diode (CR1), and you confirmed it again. I do have a question about this circuit you designed. As you have probably notice in my other post and the one above this one, I replaced the LED load with a coil. I finished building it today and put it to the test. I am not sure how fast it is flashing, but I do know that I have 12vdc comming into Vcc but only have 4vdc leaving it. No I tested the voltage by removing the load and placing the meter in its place. I know that works in most other electrical applications, but I wanted to make sure that I am testing it right to begin with. I was thinking about testing the voltage by place the red lead on the voltage filming out with the load connected and placing the black lead on the same ground feeding into the circuit. I was concerned if I did this I could potentially fry something within the circuit. All this said, I am wanting to get this circuit to where I can adjust flashes between 5 to 200 flashes per second with a 12vdc output and 3amp maximum. Thank you Wayne and Bill very much for you help. Take care.

 

What are you using to measure the output voltage? Since this is not pure DC, but oscillating, you shouldn't use a vom to measure the voltage. Depending on the cycle time (ie if the on time to total period is 1/3) your results are expected. Do you have an oscilloscope?

 

What are you using to measure the output voltage? Since this is not pure DC, but oscillating, you shouldn't use a vom to measure the voltage. Depending on the cycle time (ie if the on time to total period is 1/3) your results are expected. Do you have an oscilloscope?

Thank you for the reply, I am using a Fluke Multimeter to take my readings., unfortunately I do not have a oscilloscope or Fluke scopemeter. I did suspect that this is one reason why I was getting the reading I was.

 

Good afternoon, I am now having a new issue with my circuit and hope that someone can help. Today, I was tweaking my circuit with the load attached and 12vdc on and noticed that as I was turning on r5 my MOSFET began to smoke. I immediately secured power and waited for it to cool. Once it was cool I installed a large heatsink with Artic Silver 5 compound and fired the circuit back up. The MOSFET is no longer smoking, but it is getting extremely hot. The problem is I am now about at my peak efficiency for what I need this circuit to do. At this point, do I need to upgrade my MOSFET or another component within my circuit? I am currently using a IRF540 MOSFET. Many thanks for any help. Take care.

Donny

 

Are we still talking about switching 3A at 200Hz with Bill's circuit?

The gate voltage when "on" could be a bit low depending on your 555, causing too high a resistance in the MOSFET. You might need to use a transistor to drive the gate.

 

Hi Wayne, yes I am still on with driving Bill's circuit with 3amps. Thank you for the insight, I am assuming that a power transistor behind the resistor feeding the gate should do the trick?

 

You don't need "power", as no current flows into the gate, just something to ensure higher voltage on the gate. That may not be the problem, just something to consider. Actually a pull-up resistor on the gate might be all you need to address the issue.