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> Multiple Power Supplies Wired In Sereies, Will it work?
u4edot
Posted: August 06, 2010 10:42 pm
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I went and bought a NTE270 (Fry's didn't have a FDP26N40. I'll order one online if the NTE doesn't work), because it is rated for 10A. It looks like it takes 500mA to power trigger the gate. And, it looks like the mosfet (got tired of typing with the caps lock on) can take 10A, but I'm really not sure. Here is the pdf nte270.pdf

So I think that if I use 3 TL494 chips I can provide enough amperage to trigger the gate. I am using the TL494 chips for now because I have them on-hand, but am looking a getting a few TL598 that where suggested above. They have the same current output so I would still need to use three. Speaking of the TL598, Tim and johansen mentioned using a totem pole to drive the mosfet. Did you mean that I should wire one part to the mosfet gate an another to a leg instead of going to ground?

One other concern I have is that the nte270 is rated for 100v, I thought that that was a good thing when I bought it. However, I intend to run 36v, and I don't know if this mosfet has a higher resistance. And if it does, I wonder if that will dampen the output of the speaker.

Again thank you all for your help
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Sch3mat1c
Posted: August 06, 2010 11:30 pm
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Gate? MOSFET? NTE270 is a darlington bipolar transistor. It has a base.


NTE270 looks to be the replacement for TIP142. Beware, NTE parts are custom replacements. Most I'm sure are just relabeled, like this TIP142, but they get to charge a premium nonetheless.

Darlingtons are very slow (page 2, t_s, t_f = 2.5us) and are a poor choice for high speed switching applications. The saturation voltage is also higher (3V at 10A, 40mA Ib with grunting, whereas FDP26N40 is 0.2 ohms or so, or 3V at 15A, with headroom to spare!).

Tim


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u4edot
Posted: August 06, 2010 11:54 pm
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QUOTE
  Gate? MOSFET? NTE270 is a darlington bipolar transistor... Darlingtons are very slow (page 2, t_s, t_f = 2.5us) and are a poor choice for high speed switching applications.

Well OK then, a FDP26N40 it is

Also you mentioned using a TL598 or UC3525 for the totem pole outputs. I'm guessing that one lead of the totem pole output is supposed to go to the gate (it is a gate right?), and the other is supposed to go to one of the legs, instead of going to ground.
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u4edot
Posted: August 07, 2010 09:37 pm
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I'm now trying to build the RCD snubber that Tim recommended. This is how it would work right? Sorry for the bad texted based drawing (I had to use an * instead of a space)
__________________ +27v
|*****|**********|
>****__*********|
> 1k *__ *.1UF ***|
>**** | *********|
|_____| *********|
|*************** |
__**************|
/\ *UF4004*******|
|_______________|
|
[][][]
[][][] FDP26N40
[][][]
|
0v

If this is right, what wattage rating should the resistor be?
Thank you for your time

This post has been edited by u4edot on August 07, 2010 09:49 pm
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u4edot
Posted: August 08, 2010 03:02 am
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Well I finished the circuit, without the Snubber, so I decided to fire it up. I have 9 more mosfets, so I wasn't worried about destroying one (and I'm talking about the IRF540 that I bought when I first stared this project). I have three TL494 chips driving the mosfet, and 27v with a switch so that the batteries are not engaged until I'm ready to turn the circuit on. It was much louder than I expected, so much so that it startled me. And Tim, I must say you where right. It didn't take 2 seconds, before I saw a puff of smoke billowing from my circuit board, and then there was silence...

I'll let you guys know when I test my circuit again with the right components this time.
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Sch3mat1c
Posted: August 08, 2010 11:51 am
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Three chips at once..???

Do you have a complete circuit diagram yet?

For the snubber, I would guess 100 ohm 10W as a starting point, maybe lower resistance, maybe higher.

Tim


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u4edot
Posted: August 08, 2010 03:29 pm
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I'm working on it, but the only thing I changed was really the number of chips. I bussed the audio signal (witch I am afraid will make it weaker), the output of the potentiometers, and the output signal from the chipsuser posted image
I'll upload a circuit diagram here in a few minutes
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u4edot
Posted: August 08, 2010 03:31 pm
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user posted image
Let's try that image again
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u4edot
Posted: August 08, 2010 04:19 pm
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Here is the circuit I tested on
user posted image

And here is what I want to build
user posted image
Is this right, the snuber I mean
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johansen
Posted: August 08, 2010 04:27 pm
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you still don't have a pull down on the gate as far as i can tell, 10 ohms would be about right.

edit...

your snubber is a separate circuit, connected to the 12 votl rail. as it is now you have it shorted out, in line with the drain.


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u4edot
Posted: August 08, 2010 09:04 pm
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Ok, how about this? I still am not sure I'm wiring the snuber right.
user posted image
One question though, if I have a 10ohm pull down resistor across the 27v circuit won't it have to be rated for at least 73W

Thank you for your time
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MacFromOK
Posted: August 08, 2010 09:13 pm
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QUOTE (u4edot @ August 08, 2010 03:04 pm)
One question though, if I have a 10ohm pull down resistor across the 27v circuit won't it have to be rated for at least 73W

I'm thinking he probably meant a 10k pulldown (possibly 10M, but that seems a bit high)... blink.gif


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Sch3mat1c
Posted: August 08, 2010 09:21 pm
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Why do you have three?


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johansen
Posted: August 08, 2010 10:23 pm
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oh, my bad.

100 ohms would be a better choice for the pull down resistor, it only sees 12 volts at most 48% of the time btw.


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u4edot
Posted: August 09, 2010 12:49 am
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I thought someone mentioned that the TL494 didn't provide enough amperage to open and close the gate quick enough. So I put three TL494 chips so that to that I they could provide 600mA instead of 200mA

For the 100ohm resistor, should I buy one that is rated for 10W, or do you think that since it isn't going to be a full 27v all the time, I could get away with using something smaller.

So is the snuber in the right place, and wired correctly?

Thanks for your help
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johansen
Posted: August 09, 2010 01:29 am
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yah know, i've never tried actually measuring the maximum output current of those chips.
it might be a lot more than 200ma for the first 200 nanoseconds.
200ma is enough to switch the fet fairly quickly, the problem is at least half of the current is dumped into the gate pull down resistor, so the most you can get into the fet, or out of it, is only 100ma. Which don't get me wrong is still plenty.

and no the snubber is not wired right
take a look at the schematic Tim posted.

edit: just noticed where you put the pull down resistor... its supposed to go from the gate to the source.

the snubber prevents the drain from going too much higher than the supply rail, 27 volts.


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u4edot
Posted: August 09, 2010 02:53 am
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I'm a bit confused. If the snuber goes on the 12v circuit, how can it prevent voltage spikes on the 27v circuit? The way Tim's diagram showed it, it was between the drain and the +18v lead. Unless it goes to the 6 turns of wire on the transformer.

I moved the snuber back, and repositioned the pulldown between the gate and source
user posted image

Thanks for your help



This post has been edited by u4edot on August 09, 2010 02:55 am
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Sch3mat1c
Posted: August 09, 2010 01:18 pm
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QUOTE (u4edot @ August 08, 2010 06:49 pm)
I thought someone mentioned that the TL494 didn't provide enough amperage to open and close the gate quick enough. So I put three TL494 chips so that to that I they could provide 600mA instead of 200mA

You can't just parallel them! You didn't even parallel them as such, anyway! (RTFM, the application note clearly states how to synchronize a master-slave series of chips.)

If it doesn't supply enough current (a dubious claim for small transistors), then add a driver. They make drivers. TC4420 is quite popular. Or you can use a variety of discrete circuits, one method of which I posted above, which has gone unused.

All your attempts with the snubber have been "not even wrong".

Use this circuit.
user posted image


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u4edot
Posted: August 10, 2010 12:58 am
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Sorry I've been such a long running case
I'll build that and let you know what happens

Thanks for all your help
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u4edot
Posted: August 11, 2010 12:10 am
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Hay Tim, I'm building the circuit your uploaded, and I have one question. the 2N4403 is a PNP transistor, so the lead connected to the diode is the emitter, the lead between the diode and resistor is the base, and the lead connected to the ground is the collector, right?
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CWB
Posted: August 11, 2010 12:35 am
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u4edot
Posted: August 11, 2010 12:41 am
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CWB, that website shows that the arrow indicates the emitter for PNP transistors. I looked up something similar, but I wasn't sure if I was reading it right.
OK, Thanks

This post has been edited by u4edot on August 11, 2010 12:42 am
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CWB
Posted: August 11, 2010 01:03 am
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a little saying to help you remember :
if the arrowhead on the emitter points towards the base this is a PiN Point
if it points away fron the base this Not a PiN point .


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u4edot
Posted: August 11, 2010 01:08 am
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Oh, OK, yeah that will help allot, thanks
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u4edot
Posted: August 11, 2010 10:11 pm
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The reason I insisted on using a TL494 was because I tried to find a TL598 on ebay and they where about $11 a peace. This is also why I didn't build the driver that Tim mentioned, I mean because I didn't know where to go to buy a bunch of individual electronic components. Well I recently found Mouser Electronics, and bought a TL598. They where much cheaper there.
Anyway I was looking at the datasheets for both the TL494 and the TL598, and I think I have made the correct changes to the diagram that Tim uploaded.
pins 1-6 & 12-16 are the same
* * No changes needed
Pin 7 changed from a common ground to a signal ground
* *I assume this means that I tie the negative (neutral, ground, or whatever) lead from my audio jack
Pin 10 changed from E2 (emitter 2 I assume) to power ground
* * So I tie pin 10 to the common ground
Pin 11 changed from C2 (collector 2 I assume) to Vc (it looks like the collector for both ouputs)
* * So that should remain tied high
And finnaly pins 8 & 9 have changed to output 1 and output 2
* * 8 is low when 9 is high, and 8 is high when 9 is low (that's what you guys meant my totem pole outputs). I don't know if it matters witch one I use, so I have made pin 9 the output for my circuit.
user posted image

This post has been edited by u4edot on August 11, 2010 10:14 pm
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