I bought a cheap welder from hardware store, it has a large transformer inside and I found out it runs off A/C voltage. So I read some stuff where people convert it to D/C voltage. I bought a big fat bridge diode for it, mounted up. I also bought 100,000 uf capacitor. The capacitor says "50v. When I bought it.
Since then I read tutorials where people normal design their stuff use capacitors 2x the designed voltage (for reasons to me unknown - who knows if they know more than me).
I do not want to just waste a nice new capacitor just to see what happens. What I want to do is hook it back up again just using the bridge diode (without the capacitor) and test the voltage. Can I just put my multi-meter set on DC with leads on the gator clamp + wire & press the trigger to get an accurate reading? Is that safe ? This sounds like an obvious question, but since this monster makes such a wild reaction, I wanted to ask here first to be confident with my testing.
I'm assuming your multimeter is battery operated so you don't have to worry about what is grounded.
If so, you can:
- Set the meter on DC and read the output of the bridge rectifier. It will read about 63.6% of the peak value.
- Set you meter on AC and read before the bridge rectifier, It will read about 70.1% of the peak value
You should google "rectified sine wave" to see what the voltage vs time is for the output.
You also need to limit the current into the capacitor during power up. 90 Amp into an aluminum electrolytic capacitor will significantly shorten its life. There are several ways to do this, however we would need more about how you plan non using the output to help.
Hope this helps.
Yes I will be using a hand-held cheap digital multi-meter. Is there a cheap data-logging gizmo that can capture peak voltages? OH, stupid me - that probably what oscillioscopes are for.... I do have a scope, but I just started learning how to use it. It's a cheap student learning GW-INSTEK GDS-2102.
The only way I know how to limit current is by resistors, and the biggest one I can find is like 10 or 20 watts I think.
They also hacked up a microwave transformer and used 2nd coil to wrap their own cable to use as a choke. Does that device limit the current? Or is that for filtering ripple or something else ? It seemed like too much extra work for me to do, since the welder itself is already a huge coil.
A typical crude ac welder has an open circuit voltage of about 70vac to help the strike , and relies on transformer leakage inductance to bring that down to 20-ish volts at a fairly constant current when the arc is established.
I'd buy some higher rated capacitors before you have an interesting event...
Allan
Review this web site for a proper conversion of an AC welder to DC:Converting an inexpensive AC Welder to DC Service… | Mad Science
Don't use a capacitor. It will give you a high starting voltage and then quickly drop the voltage so your arc will start, and then blink out.
Paul
Hi DocStein99,
If the goal is to create a DC welder then I'm out of my league.
However before you connect the scope, be sure the secondary of the transformer is not connected to earth ground. The issue is if the transformer is connected to ground (internally) and the scope is connected to ground (internally) you will damage the scope. Especially if you measure on the output of the bridge rectifier.
If you want to use the scope I would:
- Put a small 100 ohm resistor in the ground lead of the scope. This will be a sacrificial item.
- Before you connect the scope, measure with the multimeter from where you plan on connecting the scope ground to the actual scope ground. I would expect a volt or two, more that that you should not connect the scope.
You can connect your scope to earth ground and probe each of the transformer / rectifier outputs. If this is not clear let me know. Wouldn't want you to fry your scope.
Good luck
John
Ok, so the tutorial explains to do the microwave hack and make a choke. I was trying to avoid that, since other tutorials actually use the microwave transformer to do the actual welding with.
As far as the oscilloscope goes, I run mine without earth ground. I got lost reading 100 pages from 50 different people, 25 explain why use the ground and 25 explain they do NOT use the earth ground. I don't hear noise through my computer speakers when my scope is NOT connected to earth ground, so that's the only tool I have to know something is interfering with something else. I do not even know if the ground in my a/c wall outlet for this whole room is actually grounded - I do not live in a certified science laboratory. I would be lucky to find out if the dryer, refrigerator, toaster, microwave, air conditioner were NOT all attached to the same circuit, and the single night-light lamp in the other room has it's own dedicated circuit.
I understand, I've floated my share of scopes. If you do, don't ground your self and touch the scope.
For clarification, the ground I was referring to is the ground on the scope probe. A 100 ohm resistor in the scope probe ground will absolutely not affect your readings and act as a current limit.
Good luck
According to the Harbor Freight web page for the welder, the open circuit output voltage is 27 VAC.
So a full wave bridge would give about 37V peak DC.
jremington:
According to the Harbor Freight web page for the welder, the open circuit output voltage is 27 VAC.
So a full wave bridge would give about 37V peak DC.
Thanks for the information. If that's true I guess the 50 volt capacitor should be strong enough - someone else I read parted out their lincoln welder it had a few capacitors added up to 43,000 uf.
I've never welded and I don't know anything about welders...
I assume your bridge rectifier is rated for more than 90 Amps?
If the rectifier blows, it's probably going to short. That could put AC on the capacitor and blow it, or other things could burn-up. (When I was a 'kid' we used to explode small electrolytic capacitors by connecting 120 VAC... Very stupid! Little bits of molten metal flying around out at supersonic speed... A larger capacitor will send-out larger pieces.)
They also hacked up a microwave transformer and used 2nd coil to wrap their own cable to use as a choke.
If you add a choke, add a reversed diode (rated for 90A or more) across the choke to kill the [u]Back-EMF[/u]. When the circuit is broken (a normal part of arc welding). Without the diode, the coil will generate thousands of volts with (obviously) enough current to kill you! (Normally, the low voltage along with your body's high resistance keeps you safe while allowing you to get 90A through the nearly zero-resistance of the weld.)