[chbot] Chchrobotics Digest, Vol 158, Issue 11

Sat Nov 28 20:43:22 GMT 2020

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
________________________________
From: Chchrobotics <chchrobotics-bounces at lists.ourshack.com> on behalf of chchrobotics-request at lists.ourshack.com <chchrobotics-request at lists.ourshack.com>
Sent: Sunday, 29 November 2020 1:00 AM
To: chchrobotics at lists.ourshack.com <chchrobotics at lists.ourshack.com>
Subject: Chchrobotics Digest, Vol 158, Issue 11

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Today's Topics:

   1. spark eroder power supply (Mark Atherton)
   2. Re: Chchrobotics Digest, Vol 158, Issue 9 (Andy Gardner)

----------------------------------------------------------------------

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

On 28/11/2020 3:13 PM, Robert W. Walker,  wrote:
> Hi... There are a range of heavy duty transformers but, particularly,
> heavy duty switch mode and regulated supplies that are commonly used and
> are available for E-bikes, E-scooters, Mobility chairs etc. They range
> from 12volts up to 70 volts or slightly more. Check out Google or
> Alibaba, Aliexpress or Ebay....
> Nga mihi...
> Bob, ZL2ROB (QTHR)
>
> Mark Atherton wrote on 28/11/2020 12:46 pm:
>> Hi Graeme,
>>
>> A 75V/10A transformer is a pretty specialist voltage, and while you
>> might get lucky finding one, the idea of 3 x 24V/10A with secondaries
>> in series sounds a more likely/lower cost option.
>>
>> You might be able to do something clever with beefy ex-UPS
>> transformers, but they are not going to have their windings marked so
>> there is a possibility of injury if you wire them up wrongly.
>>
>> Obvious places to scour include Echotech, Trademe, and Molten Media.
>>
>> Regards,
>>
>> Mark
>>
>> PS Warren may also have some ideas, or something squirrelled away :)
>>
>>
>>
>>
>> On 28/11/2020 11:51 AM, Graeme Absalom wrote:
>>> Hi All
>>> I need some help. Recently I've been pondering on why my spark eroder
>>> only delivers 3 Amps on the onboard ammeter. It hadn't worried me
>>> before because it does work. It is rated for 10 Amp. I decided that
>>> the transformer is far too small to deliver 10 Amps. I tried putting
>>> in a 120 volt 12 Amp and proved that the installed transformer is not
>>> capable but blew a power resister. Reading the book / specs again
>>> what I need is a wire wound, iron core, 230 volts to 75 volts AC and
>>> 10 Amp. The book uses 3 x 24volts AC 10Amp transformers in series.
>>>
>>> Does anyone know where I could get a approx 75 volt, 10 transformer?
>>>
>>> Thanks
>>>
>>> Graeme Absalom

------------------------------

Message: 2
Date: Sat, 28 Nov 2020 23:06:13 +1300
From: Andy Gardner <ceo at andygardner.com>
To: chchrobotics at lists.ourshack.com
Subject: Re: [chbot] Chchrobotics Digest, Vol 158, Issue 9
Message-ID: <e5a3975a-6cc9-e3ca-cc35-6078423a74a3 at andygardner.com>
Content-Type: text/plain; charset=utf-8; format=flowed

Many of the bigger UPS's run off 72 volts on the DC side, so there's a good chance a transformer can be found with the amperage required.

On 28/11/20 12:46 pm, Mark Atherton wrote:
> Hi Graeme,
>
> A 75V/10A transformer is a pretty specialist voltage, and while you might get lucky finding one, the idea of 3 x 24V/10A with secondaries in series sounds a more likely/lower cost option.
>
> You might be able to do something clever with beefy ex-UPS transformers, but they are not going to have their windings marked so there is a possibility of injury if you wire them up wrongly.
>
> Obvious places to scour include Echotech, Trademe, and Molten Media.
>
> Regards,
>
> Mark
>

------------------------------

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