I'm currently working on making a volt switch attached to a welding earth cable. Feedback voltages when the welder are in use range from 60mv-150mv DC. I've created a simple NPN transistor volt switch however how would i increase the voltage from the earth so that its sufficient enough to switch the state of the NPN releasing a 12v feedback voltage to start a secondary machine. I was thinking an op amp circuit, however it needs to be that the switches only activate when the welder is in use.

 

Use a comparator.

 

Really, I need a more detailed description of both the source of the feedback signal and the actual requirements for switching on a second machine. So there is a lot more explanation required about the system.

 

Use a current shunt amplifier (such as INA180, AD8418) etc. They are meant for sensing voltage drops across small resistances and are protected from the transients that are likely to occur.

 

when the npn switches and closes the latch the 12 volt dc signal is then fed back to the secondary machine this in turn activates a 12v relay initiating the machine. its shown in the circuit i posted. The issue I'm having is that I'm taking dc voltage readings from a welding earth in mv ranges (60-150mv) depending on setting this is too low to activate the NPN. anything over 0.8v will activate the npn transistors. I just need to either increase the DC voltage gain or use a comparator/ amplifier. any help would be greatly appreciated.

 

when the npn switches and closes the latch the 12 volt dc signal is then fed back to the secondary machine this in turn activates a 12v relay initiating the machine. its shown in the circuit i posted. The issue I'm having is that I'm taking dc voltage readings from a welding earth in mv ranges (60-150mv) depending on setting this is too low to activate the NPN. anything over 0.8v will activate the npn transistors. I just need to either increase the DC voltage gain or use a comparator/ amplifier. any help would be greatly appreciated.

See post #4

 

See post #4

okay thanks I've never used them before, would it be possible to see a sample circuit. I have the AD8418 on ltspice

 

okay thanks I've never used them before, would it be possible to see a sample circuit. I have the AD8418 on ltspice

Have a look at the INA180 datasheet
https://www.ti.com/lit/ds/symlink/i...87521&ref_url=https%3A%2F%2Fwww.google.com%2F
There are plenty of application circuits towards the end. In your case your cable is Rsense.
Another possibility you might want to consider is the Hall effect sensor:
https://www.micro-transformer.com/Hall-Current-Sensor-pl3337725.html
Much more difficult to blow up!

 

So far,
it would appear that You want to use 2 Welders at the same time.
This is NOT going to work.
Purchase a Welder that is adequate, or over-rated, for the job.
.
.
.

 

So far,
it would appear that You want to use 2 Welders at the same time.
This is NOT going to work.
Purchase a Welder that is adequate, or over-rated, for the job.
.
.
.

Indeed, it does not make sense to consider operation of two welder machines in parallel, at least not for arc welding.
From my welding experience it is entirely reasonable to know the required working current for the rod size or wire size selected. In fact, that knowledge is mandatory for producing anything close to a "good weld."
If the application is other than arc welding then the success of paralleled welding supplies is still not certain by any means.

So a great deal of additional information as to the actual application is required to enable any useful suggestions.