Large capacitor on welder

[TheBrick]

Hi all,


I've picked up a mig welder for chep that doesn't work and I'm working my way though the components checking them. I've two question related to this.

#1 After the rectifier is a bank of 5 20,000 micro farad caps, one of which has obviously blown. The cap is rates as


20,00 mico farad, 40 v DC

Look online is seems cap this big are harder to find. I understand that it doesn't matter if I get a cap rated to a higher voltage but dose it matter if I get a cap that has a slightly higher capacitance? If it was a lone cap I would not worry but as it is part of a bank I worry about the different impedance or is this not really a worry in this application?

This looks like a good choice if the capacitate value is not circuital to match.
http://cpc.farnell.com/vishay-bc-components/mal210117223e3/capacitor-40v-22000uf/dp/CA05316?in_merch=Products%20From%20This%20Range


#2. The welder is rated at 180 Amp. If I need to replace the diodes being as each one will be only operating on 50% duty cycle are  some 150 Amp 1N5162. If I'm reading the data sheet correctly the 150 Amps is the average over the full cycle.

[Paul Moir]

In that application, the specific capacitor value will not make any real difference.   Anyway you're well inside the tolerance.  Electrolytic capacitors are not ever made very precisely; they're not made for that.  They're made for bulk.  That one you linked for example is 20% tolerance.  It could be 18000uF or 26000uF, and it would be considered acceptable.  (Most cheapy MIGs won't have any capacitors at all.) 

As for the diode, looking at the datasheet they define "Average" as being over a 180deg sinusoid wave, so they've already taken the off time on you.  After all, what are you going to do with a 150A diode but rectify AC?  They do define it at a 150degC junction temperature, which is achievable.  (Note:  look at that when you're looking at power semiconductors.  Sometimes max power dissipation can only be achieved with an infinite heat sink that magically keeps the case temperature at 25degC.)

Don't know if these are any help.  I'm presuming you're in the UK:
http://www.amazon.co.uk/1200V-Recovery-Rectifier-Diode-ZP200A/dp/B008DETLS2
Bit of a gamble since you don't have complete information, but pretty cheap.  He also has some 300A ones for not much more, and some with heat sinks.

[G7PSK]

The mig welder must be a single phase unit then as capacitors are just there to smoothe the the power output a bit. The welder should work without those cap's but will spit a whole lot more, if there are several capacitors (there usually are) you could try taking the blown one out of circuit, I would suspect that if the contactor is closing when you pull the trigger on the torch that one or more diode's are blown. In my experience with mig welders (35 years) the most common fault is in the lead from the trigger switch down through the torch, the wires tend to break just short of the handle, depending on the torch configuration try shorting the contacts for the trigger switch this is much easier if the torch connection is of the euro type as the torch just unscrews as a unit a lot of the really cheap units the torch just goes into the case where the various leads diverge to their relevant connections, there is a US system where the torch is terminated in a series of brass rings of diminishing size to form a taper that then pushes into a socket and is held there with a clamp its so long since I have seen one of those that I cannot remember the name of the torch manufacturer. The open circuit voltage of most mig units is around 24 volts big units may go to 31 volts or so the voltage will droop a bit on load but not as much as you get from a mma welder (stick) as they are a so called constant voltage system the current is controlled by the wire feed speed which alters the arc length.

[TheBrick]

@Paul good point on the tolerance front. I also realised today while at work that it would not matter as all the caps are in parallel so just creating one big cap for smoothing. Good fine on the diode. That's good price on them. To be honest I'd probably be ok with 150a diode as the mig would be used more for car work as I have access to a three phase arc for anything thick. I'm only back yard tinkerer.

The mig welder must be a single phase unit then as capacitors are just there to smoothe the the power output a bit. The welder should work without those cap's but will spit a whole lot more, if there are several capacitors (there usually are) you could try taking the blown one out of circuit

Yep, there are 5 in total. Yep to single phase too.(migatronic automig 180 )

 

I would suspect that if the contactor is closing when you pull the trigger on the torch that one or more diode's are blown.

Contractor is not closing at the moment.

In my experience with mig welders (35 years) the most common fault is in the lead from the trigger switch down through the torch, the wires tend to break just short of the handle, depending on the torch configuration try shorting the contacts for the trigger switch this is much easier if the torch connection is of the euro type as the torch just unscrews as a unit a lot of the really cheap units the torch just goes into the case where the various leads diverge to their relevant connections,

Yep its euro style connection.

there is a US system where the torch is terminated in a series of brass rings of diminishing size to form a taper that then pushes into a socket and is held there with a clamp its so long since I have seen one of those that I cannot remember the name of the torch manufacturer. The open circuit voltage of most mig units is around 24 volts big units may go to 31 volts or so the voltage will droop a bit on load but not as much as you get from a mma welder (stick) as they are a so called constant voltage system the current is controlled by the wire feed speed which alters the arc length.

Top information thanks.