Relay switching off when under load (spot welder)

Question

I'm building a spot welder. Schematic is under this link here:

It is supposed to give impulse on relay for time determined by 555. On the other side of relay, I attach on one terminal + of 12V car battery (which I also use to supply the pcb), on the other terminal - ground. I use it to spot-weld nickel tape. It is a car relay, so it is a capable of driving high currents.

Everything is working fine when no load is applied to contact side of relay, For example relay contact closes for 0.5s. But as soon as I apply load (a very low resistance one), contact does not close for 0.5s, but it goes quickly on-off. Current does not flow through nickel tape for long enough and welding does not take place.

What might be the possible cause of this?

Because I connect power supply for welding pin by winding the copper cable around relay's contact terminal (on load side, those are also made of copper), I figured out that when high current goes through those copper wires and contact, it induces strong magnetic field (made by high current) that pushes back the contact inside of relay. Does it make any sense?

Answer 1

First of all, the RESET pin in your schematic is not connected to the supply voltage Vcc, which may cause spurious resets.

Secondly, the supply voltage is likely to drop during welding, which will lower the reset voltage threshold (in your schematic it's always 2/3 of Vcc); if the reset voltage threshold drops below the voltage on your capacitor, the 555 will turn off. I suggest that you power the 555 through a voltage regulator such as an LM3805/кр142ен5/кр1158ен5 and substitute a BJT for your MOSFET, as BJTs only need 0.6 V to turn on.

Note that if things happen quickly, you cannot measure the true voltage drop using a multimeter, as it always gives you the average voltage over a certain period.