AC welder to DC welder by adding a bridge in the primary...possible?

Hello guys,

This is my first post Here. Also I am not a native English speaker, so I apologize in advance for my sometimes funny english.

Well, I was so happy with my AC stick welder until I read good critics for stick DC welders, which I think are also called inverter wleders. they say welding is much easier, but they are pricy too:they would hurt my wallet.

I have read that some smart guys have converted their AC welders into DC ones by adding a fullwave bridge rectifier (and some other stuffs to smooth the output) in the secondary, which means that you need electronics that can handle high currents (70-150A in most cases). This is not a cheap solution for me, since these things are really expensive here. Buying the inverter seems to be, sadly, the only option.

However, it came to my mind that it could be possible to add the rectifier in the primary. This approach has the large adavantage that a rectifier bridge that can support up to 35A costs about 2 bucks, and I would need only one!. On the other hand, I dunno how dangerous this circuit could be (and what the final output will look like). I ran some simulations in a cheap software and I found this approach very promising, actually showing a somewaht smooth DC current/voltage. I am not an expert in elctronics and these stuffs, so any comment is welcome before I proceed.

DanSan
Mexico

Hi,

A transformer works on AC in order to convert the higher voltage lower current primary to higher current lower voltage secondary.
The rectifier diodes will have to be placed in the secondary circuit. You may be able to locate some cheap surplus rectifiers online.

Regards,
scanman

Thanks for your help scanman, I appreciate it. I know that the rectifier diodes normally are placed in the secondary coil, it is the usual way, plus some capacitors and some other stuffs if required. I was just thinking the other way around...I mean, it is still cheaper and easier to get diodes that support high voltages-low currents than low voltages-high currents. So the question is if I get the same results with both approaches (rectifier in the primary vs rectifier in the secondary.

Cheers,
Dansan

If you had a transformer and put a diode in the primary, you would get pulsed DC into the primary. This pulsed DC would, very inefficiently, generate some AC voltage in the secondary. There would be no DC level in the secondary.

So, the desired effect--a DC output to weld with, is not achieved.

What you are suggesting is similar to the way an inverter welding unit works. THey use FETs or IGBTs to switch, without a transformer involved, the AC into DC current.

biff44 said:

If you had a transformer and put a diode in the primary, you would get pulsed DC into the primary. This pulsed DC would, very inefficiently, generate some AC voltage in the secondary. There would be no DC level in the secondary.

So, the desired effect--a DC output to weld with, is not achieved.

What you are suggesting is similar to the way an inverter welding unit works. THey use FETs or IGBTs to switch, without a transformer involved, the AC into DC current.

Click to expand...

Right, thanks for the explanation, now its clear. Saved me time. SO now I will take a look to the dc welders and buy one.

cheers,

dansan

Hi,
I was reading this post and wondering if recified but unfiltered AC voltage could be used to excite a transformer? After all transformers are based on changing magnetic field , one can see that a rectified DC using a bridge rectifier(full wave rectifier) will have an average DC component which will get wasted in heating the primary. Also considering the transformer to be an inductor it will try to smoothen the fullwave rectified DC. But what if we use a half wave rectifier. In that case the ripple may be too much for the inductance to filter out and it might work, the obvious drawback is that the average output voltage will be half as that of a full wave rectifier but In this this should work.


May be i will try out something like this with audio transformer and report the results!!
Asimov

Hi asimov,

That sounds interesting. Actually I had not considered the heating part when thinking in placing a rectifier for the primary. I dunno much of electronics, I admit.

however, for your idea I think you could double the voltage, rectify and send it right to the primary, this way you could compensate the loss of voltage.

By the way, thanks to the help of scanman I found that using a rectifier, a 555 timer and mosfets one can get a stick welder close to an inverter welder, which is still affordable for me. I will research a few days more and will let you know.

dansan