[chbot] spark eroder power supply

Mark Atherton markaren1 at xtra.co.nz
Sat Nov 28 21:34:13 GMT 2020

Hi Graeme,

Not sure where Warren came up with the 30V figure from; magnetrons used 
in microwave ovens use transformers on the 2,000V range. I will talk to 
Warren Monday about this; most odd.

** so please don't buy any microwave transformers **

Andy's suggestion of 70V transformers from a large UPS is interesting. 
Given their power level, just wondering if small enough units would be 
available. 24V and above usually(?) for UPSs above 2kW(?) and you only 
need 750VA of iron.

In terms of wiring transformers together, obviously make sure that only 
the primary is attached to mains, the connect the secondaries in series. 
In phase will give you a multiple output, out of phase will give you a 
partial output. No harm should occur if you have the secondaries in 
series, but out of phase.

Just to re-iterate, please do not go with the microwave oven transformer 
option unless you intend removing the secondary and rewinding it by hand 
using (much) thicker wire.

Just out of interest, what is your budget for 70VAC, 10A of transformer?


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On 29/11/2020 9:43 AM, Graeme Absalom wrote:
> Hi Mark
> Thank you so much for your advice. The current is controlled by a bank 
> of 25 Ohm 50 Watt resistors that can be switched to allow more current. 
> The switching is managed by four power FETs (IRFB4127).
> I would not risk any switchmode power supply.
> I spoke to Warren and he suggested transformers from old microwaves. 
> They are 30 Volt and a high current. I could live with two of these in 
> series and it still meets the alternative options in the book that I 
> made the spark eroder from.
> I do have a question. Obviously, the output wires on the transformers 
> must be connected so the phases don't work against each other. Can this 
> be tested by reading the voltage and changing the orientation if the 
> voltage is wrong? Would an error like this damage the transformers? If 
> so, what checks could I do?
> I'm off to the recycling centre tomorrow for Microwaves 😊
> Thanks
> Graeme
> Message: 1
> Date: Sat, 28 Nov 2020 16:32:16 +1300
> From: Mark Atherton <markaren1 at xtra.co.nz>
> To: Christchurch Robotics <chchrobotics at lists.linuxnut.co.nz>
> Subject: [chbot] spark eroder power supply
> Message-ID: <0677a25b-eba2-6011-09ae-6e854d9587e8 at xtra.co.nz>
> Content-Type: text/plain; charset=utf-8; format=flowed
> Hi Graeme,
> How are you controlling the current ?
> I have some vague recollection that you were using an SCR or a TRIAC, in
> which case you need zero-crossing, so an SMPSU will not work.
> -Mark

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