Resistance-No Resistance wire welder

[yo han]

I don't need the momentary switch, the LM 2587 will be online all the time which is why I upped the power rating and resistance of the surge resistor and the discharge resistor.

I understand but the discharge resistor path is only needed when you want to lower the voltage. Keeping the caps online will keep them fully charged all the time without using much energy. Keeping R2 connected all the time will unnecessarily use up power. I guess it'll use around 10 to 15W all the time?

 

[awsum140]

At 60 volts, full output, the load on the LM2587 will be about 2.6 watts, quite negligible and that assumes full output. At 30 volts it will be a little over a watt. The total load is R1 plus R2, 1360 ohms, unless welding and that's only a momentary load of R1, 680 ohms or about five watts plus the quick surge of charging the capacitors. I think it'll work out OK, I can't think of why it wouldn't, but will have to wait until everything comes in and it's put together and I can try welding with it. I did oversize those resistors, just to be safe. A 10 watt rating would also work out, but 25 watts gives a very large margin of extra heat dissipation.

 

[yo han]

Thanks! That's a lot less than I assumed.

 

[awsum140]

P=E^2/R

10char

 

[yo han]

I still have to memorize all the basic formulas

 

[awsum140]

I always end up using Google, LOL. The mind is a terrible thing when it turns to mush.

 

[yo han]

Don't worry, I'm also past 45 and need Google to calculate my age. So lucky we're living in the digital age

 

[Peter_C]

I was born in 1962. I used to go, "1962 to 1992=30, then add however many years to that". So then I got used to, 1962 to 2002 = 40, but for some reason my mind gets stuck on "1962 to 2012=50" <<-- that part stresses my head for some reason.

/sigh

 

[yo han]

I was born in 1962. I used to go, "1962 to 1992=30, then add however many years to that". So then I got used to, 1962 to 2002 = 40, but for some reason my mind gets stuck on "1962 to 2012=50" <<-- that part stresses my head for some reason. /sigh

Brilliant! I just thought up a calculation: 1968 (my birth year) minus 1962 (your birth year) = 6
That would mean we're 6 years apart. My God, it's all starting to make sense now!

 

[Peter_C]

Brilliant! I just thought up a calculation: 1968 (my birth year) minus 1962 (your birth year) = 6
That would mean we're 6 years apart. My God, it's all starting to make sense now!

OK Smarty - tell me quick, how old are you if you are 6yrs younger than me

 

[awsum140]

You youngsters! Born in the 60's. Heck, I was driving then.

 

[yo han]

OK Smarty - tell me quick, how old are you if you are 6yrs younger than me

That's a tough one...
Let me see, you were 40 in 2002.
I'm 6 years younger so I was 34 back then.
Now it's 2013 so 2013-2002=11
Add this to my age in 2002 and presto! I must be 45

 

[tobarger]

You kids behave now

 

[Peter_C]

The sad part is that it probably took both of us as long to think it out, as it did to type it out...

 

[dezyner]

If you want to splurge (lol) and spend $1.64, here's one with extremely low input bias current in a dip package. .

Lol, ok, so I did splurge. got that "spendy" OP AMP LMC662 in today, wired it into the circuit between the first CAP and the LED, and now my voltage reading on the LED maxs out at 9.2v? take it out of the circuit, rewire, and back to golden on the rest of the circuit. anybody got any idea of what went wrong?

 

[fogMann]

Lol, ok, so I did splurge. got that "spendy" OP AMP LMC662 in today, wired it into the circuit between the first CAP and the LED, and now my voltage reading on the LED maxs out at 9.2v? take it out of the circuit, rewire, and back to golden on the rest of the circuit. anybody got any idea of what went wrong?

I'm coming in late to this thread. I see a few schematics but haven't seen the source supply to the circuit. What is the voltage going to pin 8 on the op-amp? That is probably the key to your problem.
If it from a Li battery, then it is in the range of 2.8-4.2V. If so, that presents two problems. The first one isn’t so bad – 4.2V is below the Vsupply spec for this part so it won’t work right. However, the second problem isn’t so good. It appears that you are applying the voltage from the caps to the LMC662 input which could be as high as 35V. If so, that’s a huge problem. The max rating for input voltage is Vsupply + 0.3V. If Vsupply really is a Li cell and 3-4v nominal, then you have probably fried (destroyed) the op-amp.
If however the Vsupply to the LMC662 (pin 8) is from the LM2577 and is 35V, then there is no problem with input voltage being higher than Vsupply. Unfortunately, 35V is way above the Vupply spec for the opamp and the part is probably fried.
You need to provide a voltage supply to pin 8 which is equal or higher than the input voltage. One way to do that is to reduce the input voltage by scaling it down with a voltage divider. However, that defeats the whole purpose of the buffer. The other way is to use an opamp which can accept a Vsupply >= the input voltage. I see the label “35V” on the schematic so you need an opamp with Vsupply of 35V or more. Ideally, use a part rated at 38-40V to provide some margin. Unfortunately, opamps with Vsupply much above 36V gets pretty specialized but they are available. If you can lower the output of the LM3577 to around 32-33V you can use an opamp with Vsupply of 36 which is a more common spec.
Also, the LMC662 is overkill for this circuit. It isn't just the input current to the opamp that bleeds the voltage off the capacitor(s). The capacitors themselves have a leakage current so they discharge themselves over time and I expect for these caps that is in the range of 20-100nA. Therefore, picking an opamp with Ibias lower than 1nA gets you diminishing returns.
Finally, you should always connect unused input pins 5 and 6 to ground. Leaving them floating might cause oscillations and/or saturations. NOTE – ground INPUT pins only. Don’t connect unused outputs (like pin 7) to ground!
As always, I reserve the right to make stupid mistakes and mis-understandings. The missing piece of the puzzle for me is “what is the value (voltage) going to pin 8? Is it a Li cell or something else? A complete schematic would be helpful.

 

[dsy5]

^^^^^He is right — you always will get slightly less output from an Op Amp than the voltage used to supply it...

 

[dezyner]

thanks guys, appreciate the info. yep, all of the above except was not using Li bat, was using 9 volt on it's own circuit. yes, I did use both sides of the OP AMP, all pins(not #8 tho), eventually, for what that was worth. nothing was fried, no magic smoke, no fireworks, nobody was harmed during filming....thought that the OP AMP might work on the CAPS bleed. for $1.60, I rolled the dice. lol. no joy. and yeah, scaling it defeats the purpose. I didn't read up to any extent on specs, just thought that it might work. Sorry about the diagram, yes, I know, it isn't exactly a circuit diagram, or a wiring diagram for that matter, but it was close enough, didn't want to spend a couple more hours to make it look better, and you got the drift. ...On the bright side, I learned way more about OP AMPS than I ever needed or wanted to. lol. I did find out about the cost of an OP AMP rated for that high of voltage, was way cheaper and easier to get a new voltmeter LED with high enough impedance to keep it from draining the caps while it read them, I think/hope/maybe. lol, I like tinkering, so was no big deal, just a huge time suck this weekend. but hey, i did get to read and google for hours.

 

[awsum140]

I found it easier to make a modified 1/4" shaft and bring the trimpot outside the enclosure. That lets me keep the LM board online all the time which prevents any drop in voltage on the caps. All it took was drilling a small hole into one end of a 1/4" shaft to fit the trimpot screw. Then I cut a slit into that end with a Dremel so that it clamps onto the trimpot adjustment screw. Simple, fast and very stable with no added electronics.

 

[asidrave]

Hi guys im having an issue with my zapper. I built it following the parts and instructions from the instructables post. The issue im having is that i get little to no spark when joining wires and wires won't stick or when they do it breaks apart easily. Also the highest volts im able to get is 32.2v and then will drop fairly quickly when i release the charge button. here is a pic of what i've done. Please dont mind the colors of the wires, i had to use a black wire to attach to the positive of the wire lead clip. any help would be greatly appreciated.