Simple soft starter for semi-automatic welding machine

[Nix2]

Hi
I want to assemble a (maximum) simple soft starter for a small semi-automatic welding machine, I saw that Electra Beckum (later Metabo) used one, but I don't know the capacitor values. Dashed frame in the diagram.

I know from experience that the starting resistors should be around 50-60ohm/10-20W, but what capacitors to get a switching time of around 1 second?
Regards!

 

[Seadog]

T=CR
Where T is seconds
C is in Farads
R is in Ohms

C=1/50 for a 50Ω resistor
Which gives a value of 0.02F or 20μF

 

[Nix2]

Ok, but which capacitor are we talking about? There are 2, I marked C1 and C2 for clarity. Or maybe someone has these values marked on the factory E.Beckum diagram?

 

[eddie49]

I think that the two soft-start resistors of 50 or 60 Ohms each and rated at 10 or 20 watts are in series, and they are intended to reduce the initial current spike when the main welding transformer is triggered on, then get bypassed by the relay contacts after a delay. This will happen every time the torch trigger is pressed, which may be annoying, especially if the power supply for the wirefeed motor is not delayed. Perhaps the soft-start should only be in series with the higher steps of the voltage range switch?

Neither of those two resistors is the RC timeconstant charging resistor. There isn't one. The time before the relay operates depends only on the internal resistance of the power source ( a bridge rectifier ), the value of the capacitor, and the minimum pull-in voltage of the relay.

Capacitor C1 is electrolytic, running on DC, and it is the one that delays ( "slugs" ) the relay. I guess it has to be very big, maybe 2,000 uF.
Capacitor C2 is non-polarised, and it is in the AC supply line to the rectifier. It will reduce the voltage. I am not sure what that voltage is - maybe 230v mains, or less, from the small transformer on the PCB.

Here is the same soft-start circuit in another Metabo welder:

Metabo 0021031900 10 Mig/mag 170/30 Xtc Eu 230v SPARE_0021031900_10_EU_230V from Spare Parts World

www.sparepartsworld.co.uk

Select Diagram 3.

BTW, a Farad is very very big, so .02F is 20,000 microFarads.

 

[Seadog]

I think that the two soft-start resistors of 50 or 60 Ohms each and rated at 10 or 20 watts are in series, and they are intended to reduce the initial current spike when the main welding transformer is triggered on, then get bypassed by the relay contacts after a delay.

Neither of those two resistors is the RC timeconstant charging resistor. There isn't one. The time before the relay operates depends only on the internal resistance of the power source ( a bridge rectifier ), the value of the capacitor, and the minimum pull-in voltage of the relay.

Capacitor C1 is electrolytic, running on DC, and it is the one that delays ( "slugs" ) the relay. I guess it has to be very big, maybe 2,000 uF.
Capacitor C2 is non-polarised, and it is in the AC supply line to the rectifier. It will reduce the voltage. I am not sure what that voltage is - maybe 230v mains, or less, from the small transformer on the PCB.

Here is the same soft-start circuit in another Metabo welder:

Metabo 0021031900 10 Mig/mag 170/30 Xtc Eu 230v SPARE_0021031900_10_EU_230V from Spare Parts World

www.sparepartsworld.co.uk

Select Diagram 3.

BTW, a Farad is very very big, so .02F is 20,000 microFarads.

Doh!

 

[Nix2]

Perhaps the soft-start should only be in series with the higher steps of the voltage range switch?

Softstarter operation only at higher power is not desirable, it should work on every range (it is like this for simplicity, but it is good)BTW. So what about capacitor values and obtained time? I had such a semi-automatic in my hands but unfortunately I did not take pictures...

 

[Nix2]

This is what the softstarter looks like.