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MrHeckles

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Everything posted by MrHeckles

  1. When I connected pin 3 via the red line it fired up no worries. Of course, no way to vary the output. When I connect pin 3 at the blue circle, it doesnt fire up at all (unless I put a resistor in series with the lamp) even with R7 being just a piece of wire. I figure the IC works fine, but I dont seem to be able to get a low enough resistance between the blue circle connection point and ground. Is there a better place for the pin 3 connection and sense resistor or some way of making the whole system a little less sensitive to such small changes in the sense resistor?
  2. I tried using a piece of wire. It still wouldnt get the lamp (or the motor) to start up. I am also getting a reading of 0v at pin 3, and the mosfet starts heating up. If there was still too much resistance for the piece of wire (sense resistor) why is there no voltage on pin 3. If it was limiting, wouldnt there be 1v or more on pin 3? If I detach pin 3, then it sits there with 1.8 - 2.3 v depending on the error amp output and nothing goes. Is there any way to make this thing start up and limit properly (like a buffer start for high current things)?
  3. I did see this when I was figureing out how to do the variable duty cycle. Hence you can see the input to the error amp is the same on my circuit. But doesnt this mean that there wont actually be any current sensing of the load? I thought that I could choose the series current sense resistor to provide 1v to pin 3 at the maximum amperage I wanted to allow and then adjust the output from the error amplifyer to determine the clamp level in the comparator. I tried bypassing the sense resistor all together on my circuit (i.e 0 volts at pin 3) and it did not work. What am I supposed to be after here in order to get the circuit to run, a high voltage on pin 3 or a low voltage?
  4. This circuit was a test to see if I could get the UC3843 to do a variable duty cycle. It was going to drive a mosfet halfbridge driver for a motor. I hooked up a 12v motor (with a diode) instead of a lamp, but the output from the chip was still low. I do not understand why the low resistance would prematurely limit the output due to the sensing at pin 3. Could you please explain why. How can I get around these issues and get the thing to output the correct voltage to turn on the mosfet?
  5. Right, I have moved the lamp so it is between the +ve rail and the drain of the mosfet. However, I am still only getting an output of about 0.09 volts from PIN 6 of the UC3843 IC. I dont understand why the output is so low. It should be around 12v.
  6. Here we go... I know there are easier ways, but isnt the idea to try and learn things you dont know? Anways - ideas about why the output is so low?
  7. Hi guys. I have made a circuit that uses a UC3843 in order to run a 12v lamp and be able to dim the lamp. I can get an the voltage on PIN 1 of the chip (the ouput of its error amplifier) to vary between 0.8 and 7.26 volts (which I believe should be enough to vary the output duty cycle between 0 and about 96%). However the output on PIN 6 is only varying between 0 and 0.13 volts (not the so called 12v for a high output). My question is are there any common sort of reasons I can look for as to why this output is so low?
  8. Could someone let me know if the following setup on the UC3843 chip is actually going to produce a user variable PWM pulse to the MOSFET driver chip? Specifically, if the variable resistor doesnt work like this, how could I set it up? Thanks
  9. Now that I think about it. If someone could explain to me how to setup a good current limit system, the PWM generator based on the dual opamp in another thread could probably be used. The PWM would be used to drive a half bridge driver at this stage. Later I may go to a full bridge, but not until I know a bit more about whether I can build it.
  10. I was going to use the UC3843BN from ON. It's cheap and easily available for me. How about the above questions? Is there a better chip for providing a variable duty time PWM signal with current limiting around (that I can actually get in NZ)?
  11. I am tossing around a few ideas as to making a motor controller for an electric wheel chair type thing (it'll need to handle a fair current from a 24v motor). I was thinking that pehaps the UC3843 would be a good PWM generator for this. My reasons for thinking this (which may be incorrect) are that it allows variation of the PWM duty cycle between 0 and (close to) 100%, and it has built in current limitation, and undervoltage lockout. Could anyone tell me if I am correct in these assumptions? Also, am I right in thinking that to make it variable duty cycle, I can just make the Rt a variable resistor?
  12. Can a soft-start being implemented on a 555 timer through the control voltage pin (number 5). What I am after is to start the 555 up at a low frequency and have it ramp up to a higher frequency slowly. This is what I have in mind so far My question is - will this circuit addition mean that the boost converter slowly ramps up (due to the slowed frequency increase) or do I need another method?
  13. Okay, I made some changes to the circuit. The mosfet driver is now powered from the 12v supply. I also added a schottky diode to ground between the driver and the fet, and a small value resistor. This solved the heating problems. I also upped the bleeder resistors to 100k and a 5k pot. I added a 1k resistor after the LED, and dropped the cap from the 5k pot to the transistor down to 0.01uF. Hey hey, and now it all works swimmingly. On startup minimum charge level is about 30v, and I could wind it up to 200v with no problems. I couldnt go higher since I had no capacitors better than 200v. There was still a bit of movement left in the pot so I guess maybe 350v with this frequency setting. It also charges very quickly, with no overheating issues. I'll take a photo of the finished board and post it soon. Thanks for all your help guys.
  14. Just to clarify, my circuit is a little different from the one the anotherforummember posted. Here is the latest schematic of mine. I understand the mosfet wont be heating during the off period (hence the advantage to a larger cap from the transistors base), however I am curious as to what happens during the turning on and turning off times of the mosfet. Does it generate alot of heat at this time, or is heat generation only due to the current passed through it while on? The mosfet I am using now is the SPP20N60S5. The extreme heat up occurs at any voltage preset between 35v and 100v (I dont go higher because of the main capacitors I have are only rated to 100v). Below 35v there seems to be enough time between charge-up bursts to keep heat under control. The transistor is the right way round. I checked. Last important question, in regards to a 2N3904 transistor. How much voltage and current is needed to turn it on?
  15. Okay, now I am confused. I got a new mosfet of much lower resistance. This one has an Rds (on) of 0.19 ohms. It has a 20 amp rating. However. Once the capacitor reaches the preset charge level, the mosfet heats up even quicker than the last one. It reaches the preset charge very quickly (faster than the old one). Could this overheating be due to the preset voltage cutoff system? At the lower levels of charge (up to 25v) you can see the voltage peak, and then drop a bit (about 2v or so) and then peak again. This I think is due to the effect of the 1uF cap between the transistor (not the mos, the other one, Q2 in my schematic) and the pot. But at the higher voltage levels, there is none of this drop off. So would I be right in assuming that the mosfet is being switched on and off very quickly because the 555 timer is being reset all the time. Would this cause the super heating effect? If this is the case. How can I overcome this? Do I need a much bigger(or smaller) capacitor, or perhaps a different type? It is a 1uF electrolytic at the moment.
  16. Actually, now that I think about it, could it just be that there is too much current passing through the mosfet. After all, it says for the IRF830 that the RDS(on) is 1.5 ohms. Am I right in assuming this means that when it is on, it is going to be passing 12/1.5 = 8 amps? Could I reduce this by placing small resistor just infront of the mosfet (between the inductor and it), say a 5 ohm or something. Would that reduce the current through the mosfet? What effect would this have on the rest of the circuit?
  17. Righto, I changed some bits around. It is now running at 4.8 kHz with a 66% duty cycle. That seems to have fixed the limited voltage issue. I should merrily be able to blow up capacitors now :-) As far as that diode is concerned, I am using an MR856, which according to its datasheet should have a reverse recovery time of at worst 300 ns. Why would it be discharging the capacitor bank through the diode if this recovery time seems to be short enough? What sort of diode should I be using to stop this from happening? Is there another way to stop it from heating up the mosfet once it has reached its preset voltage level?
  18. Well, I built it. It all seems to work (after some intial diffuculties). However I have a few questions as to its performance. Firstly, with the pot wound right up, the best charge I can get out of the unit is 75 volts. I presume this has something to do with the frequency or duty cycle I am running the 555 timer at. I am running it at 64 kHz. With a 55% duty cycle. I find that the circuit draws around 0.1 amps steady when set for the highest level (75 volts as above). If I wind it down somewhat (to say between 35v and 55v) then the mosfet gets real hot (and yes, that is with a big fat heat sink) and the current draw ups to around 3.8 amps. If it has a lower setting (up to about 30v) then it will charge to a level, then wait until the voltage drops by a few volts then kick in to charge back up again. It does not heat up the mosfet significantly doing this. Why would it only heat the mosfet up in the middle ranges? Thanks for any feedback guys
  19. Hmm, the forum seems to be "resizing" it. Try this link, should be alot clearer
  20. Hi guys. I have come up with a schematic for a coil gun capacitor charger circuit. It should run off a car battery (12v). It is currently set up to charge up a 50v capacitor but I designed it to go up to close to 500v (the max rating for the diode), by just changing R4. What do you think?
  21. Hi everyone. I have been trying to design a ciruit for a boost converter in order to charge up a big capacitor. I have come up with a schematic. I would welcome feedback on this circuit, specifically will it work, and what adjustments/additions would be needed to make it work (or work better). The idea of the circuit is to use the 555 timer to proivde the oscillation (at about 48kHz) for the Mosfet driver. The LED between the IC's will be on to indicate charging. The transistor on the right side should activate when the voltage entering the large capacitor hits around 48v. In this case the right LED turns on the the 555 timer is switched off by the reset pin (number 4). The thing I am most unsure of is the values of the components and whether I got them correct. Thanks for your help.
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