Resistance-No Resistance wire welder

[bapgood]

No, LM339 is a comparitor and the inputs are swapped. (That's why my first schematic was wrong - I was wiring as I would a LM339) A diode would probably solve the 12V problem - I take it you mean 12V on the cap.

Sorry yes the cap.....I hooked it up just like your schematic (i think) with 100k resistors and a 10k pot and it still drains the cap pretty fast, even without the led, plus the led only stays on for like 1 volt drop.

I tried swapping the inputs on the 339 and nothing. I can't get the 339 to output poo.

 

[dsy5]

Sorry yes the cap.....I hooked it up just like your schematic (i think) with 100k resistors and a 10k pot and it still drains the cap pretty fast, even without the led, plus the led only stays on for like 1 volt drop.

I tried swapping the inputs on the 339 and nothing. I can't get the 339 to output poo.

You need a 100kΩ pot. All values should be the same. May need even higher values, such as meg ohm. The LED will go out since you're using a small 10kΩ pot - the reference voltage is only at about 3.6 V. A 100kΩ would make it about 18V reference; more room to adjust.

You can set up whatever ratio you would like with the voltage dividers - this is just an example. The formula for a resistive divider is similar to the one I gave you for the capacitive divider - V_R1 = V * (R1 / (R1 +R2))

 

[bapgood]

You need a 100kΩ pot. All values should be the same. May need even higher values, such as meg ohm. The LED will go out since you're using a small 10kΩ pot - the reference voltage is only at about 3.6 V. A 100kΩ would make it about 18V reference; more room to adjust.

You can set up whatever ratio you would like with the voltage dividers - this is just an example. The formula for a resistive divider is similar to the one I gave you for the capacitive divider - V * _R1 = V (R1 / (R1 +R2))

Ok....I was using a 100k resistor + the 10k pot....plus I just realized I had hooked up my voltage divider resistors incorrectly....so I'm fixing that.

Any suggestions on the 339? Even without all the voltage dividers I can't get it to output, I have tried two different ones.

 

[dsy5]

Ok....I was using a 100k resistor + the 10k pot....plus I just realized I had hooked up my voltage divider resistors incorrectly....so I'm fixing that.

Any suggestions on the 339? Even without all the voltage dividers I can't get it to output, I have tried two different ones.

Yeah, I forgot that you will need a pull-up resistor from the chip positive supply to the output pin of whatever comparator output you are using - try about a 1kΩ - 2.2kΩ. The comparitors have a open collector output.

 

[bapgood]

Yeah, I forgot that you will need a pull-up resistor from the chip positive supply to the output pin of whatever comparator output you are using - try about a 1kΩ - 2.2kΩ. The comparitors have a open collector output.

thanks now onto the 339

I got the 741 figured out....kinda.... I don't have a 100k pot here so I have been swapping out resistors. I got it to where it drops 2 volts in 8 seconds and starts slowing off after that. I can probably dial it in further with different resistors. The 741 has the 12v input voltage on the cap voltage pin....I can't explain why (just a feeling) but I think that has something to do with to sucking the cap down.

 

[dsy5]

I just finished breadboarding an LM339 and it is working as expected with the pull-up.

 

[dsy5]

thanks now onto the 339

I got the 741 figured out....kinda.... I don't have a 100k pot here so I have been swapping out resistors. I got it to where it drops 2 volts in 8 seconds and starts slowing off after that. I can probably dial it in further with different resistors. The 741 has the 12v input voltage on the cap voltage pin....I can't explain why (just a feeling) but I think that has something to do with to sucking the cap down.

You may vave better luck with the LM339... BTW, mail came and I got my LED voltage meters!

 

[dsy5]

This is what I would suggest - In the cap divider use a couple of 1Meg resistors. On the other divider, since we dont really care how much current it draws, use a 10kΩ and the 10kΩ pot. At the input pins you'll get around 18v each at max charge. Adjust your pot to get around 15V at the input and see how it goes.

 

[dsy5]

This is for bap:

[


The values selected for R1 and R2 must be chosen so that the voltage at positive input does not exceed the chip supply voltage (12V). Maximum supply voltage for the LM339 is 36V. The LM339 is very sensitive and will respond to millivolt differences, so it can be very accurate.

Just charge the cap fully and adjust the pot to a volt or two difference in the two inputs (use your meter to read the two inputs - minus input is the lower of the two voltages).

 

[bapgood]

This is for bap:

View attachment 174800


The LM339 is very sensitive and will respond to millivolt differences, so it can be very accurate. With the pot above the neg input in the voltage divider leg, you will be able to set the reference much closer to the LM2577's output. Just charge the cap fully and adjust the pot to a volt or two difference in the two inputs (use your meter to read the two inputs - minus input is the lower of the two voltages).

Thanks dsy5 for all your help and patience!!! The LM339 is far superior and only a $1 more at radio shack. I have a s similar setup going and it's working great but I will revise with the latest updates.

Dsy5 what do think about setting the threshold voltage to somewhere like 5v-12v? Reason being is it would be nice to know I'm not going to shocked handling it (or someone else). With a resistor in the main on/off switch I guess it would be just as easy to turn it off and drain the charge.


Sorry to anyone wondering WTeff....this has been a little off topic...basically dsy5 has been helping me turn on an LED when the cap is charged when using the LM2577, and to do it without draining the cap charge. I can say with current lm339 op-amp setup I let the cap set charged with led on for 6 minutes and lost ~1.4 volts of charge from the cap, likely less with the latest schematic.

 

[dsy5]

Thanks dsy5 for all your help and patience!!! The LM339 is far superior and only a $1 more at radio shack. I have a s similar setup going and it's working great but I will revise with the latest updates.

Dsy5 what do think about setting the threshold voltage to somewhere like 5v-12v? Reason being is it would be nice to know I'm not going to shocked handling it (or someone else). With a resistor in the main on/off switch I guess it would be just as easy to turn it off and drain the charge.


Sorry to anyone wondering WTeff....this has been a little off topic...basically dsy5 has been helping me turn on an LED when the cap is charged when using the LM2577, and to do it without draining the cap charge. I can say with current lm339 op-amp setup I let the cap set charged with led on for 6 minutes and lost ~1.4 volts of charge from the cap, likely less with the latest schematic.

Well, you could always use one of the other 3 comparators in the chip to light an amber warning LED...

 

[bapgood]

Well, you could always use one of the other 3 comparators in the chip to light an amber warning LED...

Stop it...I need to finish this gd thing....but I will have an extra hole now that I'm taking out the dual pole momentary switch

 

[lowboy]

I have subscribed to this thread and been reading it up to now.
I am so confused. I am ellectronically challenged.
I do good to just solder anything and get it right.
I would love to have one of these as it would be pretty handy.
I did acquire a old disposable camera but that is about as far as I got.
If someone decides they want to get rid of a prototype I will be glad to help you out in getting some new parts for your future projects.

 

[TBinAZ]

You have the basic equation right there lowboy. Crack the camera case open, get yourself a few alligator clips to make two thick wire leads (be sure to insulate the ones you hold), Clip one on each of the leads coming from the end of the capacitor, make sure the battery is good, and then tap the flash charge button a few times. Obviously, the basic rig needs a bit of tweaking to get a flawless weld on the Kanthal, but it will give you an idea of how easy it really is to put something like this together.

Look back to post #34 where TomCatt quotes a couple of key posts.

I have subscribed to this thread and been reading it up to now.
I am so confused. I am ellectronically challenged.
I do good to just solder anything and get it right.
I would love to have one of these as it would be pretty handy.
I did acquire a old disposable camera but that is about as far as I got.
If someone decides they want to get rid of a prototype I will be glad to help you out in getting some new parts for your future projects.

 

[dsy5]

Dsy5 what do think about setting the threshold voltage to somewhere like 5v-12v? Reason being is it would be nice to know I'm not going to shocked handling it (or someone else). With a resistor in the main on/off switch I guess it would be just as easy to turn it off and drain the charge.

To be honest, at a voltage of around the 36V that the LM2577 puts out, you won't be shocked. I've handled the camera box leads with 100V on them and didn't get a shock. 'Course not everyones skin conductivity is the same...

The biggest shock potential of the camera board comes from the high voltage step-up transformer that actually powers the flash - the charged cap of around 300V is stepped up to around 4000V to trigger the flash. I have removed my step-up transformer to avoid the potential for HV shocks.

It is probably best though to err on safety.

 

[Jellyfish]

With the camera boards the voltage is increased by increasing the charging time via a charge button. With the LM2577 the pot is maxed out to supply around 36 V. If a charge button is used in the LM2577 circuit, can the capacitor voltage be set to a lower value with a charge button or do you need to trim the onboard pot back? I'm basically just wondering whether I need easy access to the trim pot.Thanks!

 

[dsy5]

With the camera boards the voltage is increased by increasing the charging time via a charge button. With the LM2577 the pot is maxed out to supply around 36 V. If a charge button is used in the LM2577 circuit, can the capacitor voltage be set to a lower value with a charge button or do you need to trim the onboard pot back? I'm basically just wondering whether I need easy access to the trim pot.Thanks!

The capacitor actually needs to be higher; since the voltage is lower, you will not receive the same amount of charge unless the cap is bigger. Most have been using caps in the 1000µF - 2200µF range.

Edit: And the charge can be regulated by tapping on the charge button quickly, so you don't necessarily need to back off on the pot.

 

[bapgood]

Used the LM2577 output voltage for the supply voltage and used one of the other outputs to feed the feed the cap. It doesn't get much more stable than this.

 

[comptechltd]

Bap,

I absolutely love your pictures but every time I see one I think 'rube goldberg'......lol

Steve

 

[BJ43]

Ok I got all the stuff. Now what?