Resistance-No Resistance wire welder

[awsum140]

No offense, but the connection at the positive side of the capacitor doesn't look very solid. Another comment is to get some tape on all the bare wires, a short, anywhere, will keep it from working properly. How much capacitance are you using? At 32 volts you will need 2000mfd to get a solid weld. Personally, I'd move that resistor over to the output side of the LM board to limit charge surges, right now it seems to be dropping the supply voltage to the LM board.

 

[asidrave]

No offense, but the connection at the positive side of the capacitor doesn't look very solid. Another comment is to get some tape on all the bare wires, a short, anywhere, will keep it from working properly. How much capacitance are you using? At 32 volts you will need 2000mfd to get a solid weld. Personally, I'd move that resistor over to the output side of the LM board to limit charge surges, right now it seems to be dropping the supply voltage to the LM board.

No offense taken at all. This is my first time building such a device. Im using this capacitor.
Invalid Request
again...taken from the parts list on instructables. it is 1000uf @ 50v.

I will clean up and better solder the connection. The resistor is on the in+ of the board. Are you saying i should move it to the out- of the board?

thanks

 

[awsum140]

OK, 1000mfd probably won't give a good weld unless you can get up to about 50 volts, well past the limit of the board you're using, so I'd add a second capacitor first. Simply wire it in parallel with the existing one, positive to positive, negative to negative. As far as the resistor is concerned, not knowing its value could be misleading me. The LM board has a "+" input and "+" output. I've always used a resistor in the "+" output to limit the current surge when charging the capacitor(s) which protects the LM board.

 

[asidrave]

the resistor value is 3.6 OHM 10W 5% AXIAL..info from digikey part# i used. I will look for another capacitor at work on monday. So if i put the resistor in the +out of the board...does it still need the diode? or where would it go with the diode?

 

[awsum140]

The resistor could go either before or after the diode, it is not critical, but it does need to be ahead of the capacitors. At 3.6 ohms it really won't limit current all that much when charging the capacitors, and because it's such a low value it shouldn't effect voltage to the LM board very much at all. Leave it ... is and just try the second capacitor, that will probably cure your problem with weak or not real weld.

 

[dsy5]

Make sure the charging pushbutton is right before the cap...

 

[tobarger]

asidrave
By the time you are able to touch the two wires together the voltage is way under 30 volts
This is not enough voltage to weld the wires together
First you need to figure out why the voltage is dropping like a lead balloon
You really need the voltage to be steady so you can take your time to touch the wires just right (parallel)
Then you will be able to figure out whether you have enough voltage and capacitance by trial and error
A schematic diagram of what you have built would really help

 

[Mad Scientist]

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?

My fault. I was focused on the input impedance and not the device power supply voltage (being an old timer, the usual 36v rail to rail is stuck in my head). I'd either divide the input by 10 and connect to the meter (so no fancy math to read it, just the decimal point) or divide by 10 and run its output into another op amp with lower input impedance but higher voltage tolerance (the ubiquitous 741 should do -- the modern ones will go 44V rail to rail) and set the second op amp for appropriate gain and direct reading. Some work involved but might be a fun experiment.

 

[asidrave]

The resistor could go either before or after the diode, it is not critical, but it does need to be ahead of the capacitors. At 3.6 ohms it really won't limit current all that much when charging the capacitors, and because it's such a low value it shouldn't effect voltage to the LM board very much at all. Leave it ... is and just try the second capacitor, that will probably cure your problem with weak or not real weld.

I will add the second cap before i move around any electronics.

 

[asidrave]

Make sure the charging pushbutton is right before the cap...

hmmm.....does it need to be right after the out+ of the board and before the diode?

 

[asidrave]

asidrave
By the time you are able to touch the two wires together the voltage is way under 30 volts
This is not enough voltage to weld the wires together
First you need to figure out why the voltage is dropping like a lead balloon
You really need the voltage to be steady so you can take your time to touch the wires just right (parallel)
Then you will be able to figure out whether you have enough voltage and capacitance by trial and error
A schematic diagram of what you have built would really help

I followed these instructions and bought the exact electrical parts listed.
Small Welder for joining Nichrome and Nickel Wires
is there a revision to these instructions?

 

[tobarger]

According to that schematic the capacitor voltage can only be draining through the LM2577 board.
Which would indicate a problem with the wiring at the LM2577 board, a bad board or a crappy capacitor.
As I said earlier, FIRST you MUST solve the problem of the capacitor voltage dropping quickly BEFORE you can determine whether you have enough voltage and capacitance.

 

[dsy5]

hmmm.....does it need to be right after the out+ of the board and before the diode?

It's the last thing before the cap... you want the cap isolated from all the other parts after it is charged.

 

[awsum140]

Another comment regarding a potential source of fast discharge of the capacitor is the input impedance of the meter being used to monitor the voltage and the quality of the capacitor itself. If the input impedance is low, as in 20-100K ohms, it will drain the capacitor fairly rapidly. If the capacitor is of low quality, the leakage through the dielectric will cause it to drain more rapidly as well.

 

[dsy5]

I agree that the meter can drain the cap — I find that a meter is really not necessary. Once you get a feel for holding the charge button, you really don't need it.

If you must use one, get a push button for the charging button that has 2 poles — thereby disconnecting the meter from the circuit after it is released.

 

[tobarger]

asidrave
You provided a link to a schematic and claim that it is exactly the same as what you built
I am suspecting that what you built is not exactly like the schematic in the link
If you added ANYTHING than it is not the same and I am not going to be able to help you
Is the schematic diagram EXACTLY what you built?

 

[asidrave]

asidrave
You provided a link to a schematic and claim that it is exactly the same as what you built
I am suspecting that what you built is not exactly like the schematic in the link
If you added ANYTHING than it is not the same and I am not going to be able to help you
Is the schematic diagram EXACTLY what you built?

Its exact using the board, capacitor, resistor and diode and wired using the pictures as a reference. the only thing i added was an on/off switch wired between the + of the battery wire and the momentary switch.

BTW, i could not find a capacitor at work 2000uf @ 50v. My father may have some in his stash. Ill check tomorrow after work at his house

 

[awsum140]

If you want to increase the capacitance, a second 1000mfd capacitor wired in parallel with the first will work to give you 2000mfd.

 

[asidrave]

If you want to increase the capacitance, a second 1000mfd capacitor wired in parallel with the first will work to give you 2000mfd.

I was able to find 3 1000mfd capacitors @35v not 50v. would those work?

 

[awsum140]

I was able to find 3 1000mfd capacitors @35v not 50v. would those work?

Well, you're "pushing the envelope" a little. As a rule of thumb, you want the working voltage rating to be at least50% over the actual voltage, just for safety. Electrolytics can go off like a large firecracker when they fail catastrophically so being judicious with voltage ratings is good practice. To give you an idea, I saw an 80,000mfd, 150 volt, blow a hole right through a 14 gauge steel cabinet when it was wired backwards and everyone sat around wondering why the current draw on that power cabinet was so high. Nobody was hurt, but a lot of people ran like he77!

That said, you can probably get away with 35 volt rated capacitors, just be aware that you are working near the ragged edge and things could go wrong. No guarantee or warrantee is expressed or implied here, LOL.