simple question re: current and welding - Page 1
Forum section: electronics / beginners

author: chconnor
author_rank: Contributor
timestamp: March 26, 2023, 03:29:48 pm
content: There are a number of 120V 20A appropriate MIG welders out there that are e.g. "80A" devices and I'm trying to understand how that works.I understand that the power draw is balanced by the fact that the transformer is changing to some lower voltage, thus the theoretical 2400W of the supply is not exceeded, at least not long enough to trip a breaker.But isn't there also a 20A limit on the current draw? If the circuit can supply 20A sustained, and current is a measure of charge per second, how does the device draw 80A, regardless of the voltage? Where would the "extra" 60A charge per second come from?Or is this a case of "80A" meaning "80A for brief periods of time only", and it's smoothed out by a capacitor or something?Thanks!

author: mikerj
author_rank: Super Contributor
timestamp: March 26, 2023, 04:00:09 pm
content: A transformer allows you to trade current for voltage and vice versa. The 80 Amps comes from the fact that the secondary winding on the transformer has fewer turns of much thicker wire than the primary, so voltage is lower but available current is higher.

author: jpanhalt
author_rank: Super Contributor
timestamp: March 26, 2023, 04:31:49 pm
content: Your Open Circuit Voltage (OCV) output is probably DC. So instead of 120V RMS, use 165V DC +/- (i.e., 2^1/2 x 120V) . "80 A" therefore equates to about 41 OVC, which might be a little low. Check what the OCV is specified as. Also, as you point out, that is not 80A continuous. There is a duty cycle rating too.

author: Stray Electron
author_rank: Super Contributor
timestamp: March 26, 2023, 05:41:21 pm
content: Quote from: chconnor on March 26, 2023, 03:29:48 pmA: The 80 volt output is at a much lower voltage level than the 120 V 20 Amp output. In theoryat 100% efficiency, 120 V 20 A input is the same amount of power as 30 VAC 80 Amp output.FYI I just bought one of those cheapish 120/220 VAC welders and after I got it I found that it can draw up to 33 amps on 120 VAC! But even 33 Amps input isn't enough for it to deliver a full 80 Amp output. IIRC according to the manual it only delivers a maximum of about 56 Amps when operated on 120 VAC. I haven't used it yet so I can't comment on how well it works.https://www.amazon.com/dp/B07W7P943W?ref=ppx_yo2ov_dt_b_product_details&th=1SERIOUS, if you want to weld;FORGET 120 VACand go with a 220 VAC circuit with at least 50 Amps capacityminimum.I have several bigger welders including a Miller Synchrowave 350. I only bought THE TOY because it was cheap and very portable.Somebody forgot to tell the Chinese that 15 Amp or 20 Amp are the usual 120 VAC outlet maximum in the US.

author: james_s
author_rank: Super Contributor
timestamp: March 26, 2023, 06:03:01 pm
content: Think of a transformer like a gearbox, it lets you exchange voltage for current of vice versa, the same way a gearbox exchanges torque for speed. Power is volts * amps, the power is the same, but if the voltage is lower the current can be higher.I concur with others that 120V welders are pretty worthless though, there just isn't enough power available to do proper welding.

author: Stray Electron
author_rank: Super Contributor
timestamp: March 27, 2023, 02:12:09 pm
content: Quote from: james_s on March 26, 2023, 06:03:01 pmX2. It's not that the 120 VAC welders are bad designs or anything, it's the fact that 120 VAC at 15 amps (or 20 amps If you lucky) circuits simply can't supply enough power to realistically weld. The welder that I bought is only rated to weld with a maximum size of 3/32" rod, even if you have a 33 Amp 120 V supply. IIRC at 20 amps it's doubtful that it could weld with 1/16" rod.The wired-in 120 VAC power cord on my welder (33 Amp input) will plug into a NEMA 5-15 socket which is only rated for 15 Amps and by NEC code that outlet is not supposed to be connected any larger than a 15 Amp circuit breaker!But in the interest of full disclosure, it also comes with an adapter that will plug into a 30 Amp 240VAC socket and it is rated at 23 Amps input at 240 VAC. So hopefully, that will be enough power for small jobs.

author: chconnor
author_rank: Contributor
timestamp: March 27, 2023, 02:29:35 pm
content: Thanks everyone - the part i forgot was that the secondary is isolated from the primary so you can just push as much charge around in that circuit as you want (given whatever the ratio is). Thanks for getting my head back on straight.And I'll avoid the 120V devices. :-)