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Newbie- I want to pulse a low voltage high amp source.

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ukmxer

New Member
I only have old dormant NE555 experience, is this enough for a timed pulse or are there new devices these days that supersede this chip.

with the power being high amps there will be heat generated. So do I use a relay or some kind of isolator.

the power source is 240v and a welder.

Please help - I will attempt any useful suggestions offered.

thx
 

Klaus

New Member
You need to supply more information than just low voltage, high current. Some figures would help.
You say its for a welder, I am assuming you want to switch the welders secondary current. If so, you are better off switching the much lower current primary, which in turn controls the secondary output.
For that I would recommend a solid state switch. Pick one that can handle the expected AC current - you find that info on the welder.
These switches use low voltage DC for control, your 555 via a transistor driver should handle that.
Klaus
 

ukmxer

New Member
To Klaus

Thanks for your help.

Please accept I am a total novice and lacking confidence.

What I am trying to achieve is the switching of the output power to stop too much heat going into thin metal.

I dont know what power is drawn but the machine has a max output of 150 amps but that is for the thicker materials.

I am frightened to mess with the primary output transformer cant I just disconnect the final power and connenct that to a relay or something which would then Be controlled with a relay or opto isolator.

thx
 

bmcculla

New Member
Hi,

150A is a lot to switch. The leads of most packages cant handle that current let alone the silicon inside. I think Klaus is right about switching the primary. For a 150A output the primary probably draws 10 - 15 A at 110V (Half that for 220V) Which is not hard to switch. A triac like in a standard light dimmer (Get one rated for the current you need and heat sink it well) can handle those currents. You should use an opto isolator with zero crossing detection to drive the Triac. The Zero crossing detector only turns on when the AC voltage is zero, preventing large voltage transients in the transformer. Digikey should have these parts.

Hope this helps.

Brent
 

Klaus

New Member
And, all the parts that Brent mentioned you can find, packaged and ready to use, inside a solid state switch. It has 4 terminals, two large ones for the switching and two smaller, optical isolated ones for the control.

If you are talking about a simple stick welder you could try the following, if yours is a MIG welder, please disregard as I have not tinkered with these so far. For welding thin metal TIG is the way to go I'm told - wish I had one of these.

Now that you are telling us what you are trying to do, I think fast switching is perhaps not the best way to reduce the current with a welding transformer. It may cause complications to the welding process.
I take it there are some current settings on your transformer (most welding trannies have them) but none are low enough for your requirement. Some transformers regulate the current by moving part of the core, perhaps there is room for extracting it further if yours is that type.

To get a lower secondary current you can also lower the primary voltage to the transformer. It would, however, also lower the 'striking' voltage somewhat.
You need to connect something in series with the primary that causes a voltage drop. Have a look inside and see if there are several taps for different primary voltages. If so, just connect it to the next *higher* tap which, ideally, is no more than 20V higher than your supply voltage.

For example, I did the reverse on my welding transformer which had a 125A output that was not quite enough. So I reconnected the primary from the set 240V to the 220V tab and this solved the problem. Yes, feeding 240V into a 220V winding does make things hotter in there but since my welding was only for short bursts (stick) there was no heat problem.

Another thing you could try, connect the transformer via a long extension lead (uncoiled!) which has a low ampere rating. The voltage drop from this might be enough to lower your secondary current sufficiently.

I would not mess with the high current secondary output, switching that kind of current is just too troublesome. Mechanical relays would just weld the contacts together and solid state could be expensive.

IF you do try the above tricks, keep in mind you are dealing with lethal mains voltages and take the appropriate care.
Klaus
 

ukmxer

New Member
More Info, I hope this helps.

Thanks for your help guys.

The welder is a mig on UK 240V.
It has a four position output switch giving fixed power outputs of increasing power.

What I was wanting to do was take the power from any of the selected positions and then manipulate this power by pulsing it.

My idea was to control the time the pulse is present and also the time between the next pulse. So I am not playing with the size of the output power. I am controlling on/off as if triggering the switch on the gun.( This is what gave me the idea, I was welding 0.5mm material an I did not blow through by triggering at different mark/space ratios.)

Would it be possible to sketch me a circuit diagram.
My email is [email protected]

Thanks for your attention

Neil
 
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