<div dir="ltr">As microwaves normally run around 800-1100W, and you need 75V or less at 10A which is 750W, you should get away with a single microwave transformer. However you will probably need enamelled copper wire rather than conventional insulated wire otherwise the insulation will take up too much room and not leave enough for the copper. You may also need to wind two secondary windings and connect them in parallel to achieve the required current rating without having the wire too thick (which adds to eddy current losses). I expect your secondary will take more than one layer of wire, so you might need some insulating tape to separate the layers. Both the tape and insulation need to have a decent temperature rating - not normal PVC tape! (You might be able to salvage the tape from the transformer and re-use it.<div>Beware - I have never rewound a transformer so the above advice is what I have gathered second or third hand, etc.</div><div>Good luck.</div><div>Trevor<br><div> </div></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Mon, Nov 30, 2020 at 8:15 PM Mark Atherton <<a href="mailto:markaren1@xtra.co.nz">markaren1@xtra.co.nz</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><u></u>
<div><p>Hi Graeme,</p><p>You need to remove all signs of the old secondary, and you may have to drift out a magnetic-short in the transformer, but in principle, you should be OK.</p><p><br></p><p>Have a look at youtube for videos about making welders from microwave oven transformers, the only difference is you need many turns of thinner insulated wire.</p><p>Regards,</p><p>Mark</p><blockquote type="cite">On 30 November 2020 at 16:40 Graeme Absalom <<a href="mailto:clock_man_OK@hotmail.com" target="_blank">clock_man_OK@hotmail.com</a>> wrote: <br> <br><div style="font-family:Calibri,Helvetica,sans-serif;font-size:12pt;color:rgb(0,0,0)">Hi</div><div style="font-family:Calibri,Helvetica,sans-serif;font-size:12pt;color:rgb(0,0,0)"><br></div><div style="font-family:Calibri,Helvetica,sans-serif;font-size:12pt;color:rgb(0,0,0)">Thank you for your concern for my safety, it is appreciated.</div><div style="font-family:Calibri,Helvetica,sans-serif;font-size:12pt;color:rgb(0,0,0)"><br></div><div style="font-family:Calibri,Helvetica,sans-serif;font-size:12pt;color:rgb(0,0,0)">I have another option that William has mentioned, if I understand correctly. It is to take out the high voltage wires from the microwave transformer and replace these. I have seen You tubes with this being done. I estimate from one clip that 60 turns would produce about 35 volts. Then using two in series would give me 70 volts (hopefully 10 Amp) which would suit me nicely. I was planning to use 10 Amp domestic plastic covered wire;</div><div style="font-family:Calibri,Helvetica,sans-serif;font-size:12pt;color:rgb(0,0,0)">would this be OK?</div><div style="font-family:Calibri,Helvetica,sans-serif;font-size:12pt;color:rgb(0,0,0)"><br></div><div style="font-family:Calibri,Helvetica,sans-serif;font-size:12pt;color:rgb(0,0,0)">Thank you to all those who posted advice.</div><div style="font-family:Calibri,Helvetica,sans-serif;font-size:12pt;color:rgb(0,0,0)"><br></div><div style="font-family:Calibri,Helvetica,sans-serif;font-size:12pt;color:rgb(0,0,0)">Graeme Absalom <br></div> <br><hr style="display:inline-block;width:98%"><div id="gmail-m_2698505006172808833ox-db7eb75991-divRplyFwdMsg" dir="ltr"><span style="font-size:11pt;color:rgb(0,0,0);font-family:Calibri,sans-serif"><strong>From:</strong> Chchrobotics <<a href="mailto:chchrobotics-bounces@lists.ourshack.com" target="_blank">chchrobotics-bounces@lists.ourshack.com</a>> on behalf of <a href="mailto:chchrobotics-request@lists.ourshack.com" target="_blank">chchrobotics-request@lists.ourshack.com</a> <<a href="mailto:chchrobotics-request@lists.ourshack.com" target="_blank">chchrobotics-request@lists.ourshack.com</a>><br> <strong>Sent:</strong> Monday, 30 November 2020 1:00 AM<br> <strong>To:</strong> <a href="mailto:chchrobotics@lists.ourshack.com" target="_blank">chchrobotics@lists.ourshack.com</a> <<a href="mailto:chchrobotics@lists.ourshack.com" target="_blank">chchrobotics@lists.ourshack.com</a>><br> <strong>Subject:</strong> Chchrobotics Digest, Vol 158, Issue 13</span><div> </div></div><div><div>Send Chchrobotics mailing list submissions to <br> <a href="mailto:chchrobotics@lists.ourshack.com" target="_blank">chchrobotics@lists.ourshack.com</a> <br> <br> To subscribe or unsubscribe via the World Wide Web, visit <br> <a href="https://lists.ourshack.com/mailman/listinfo/chchrobotics" target="_blank">https://lists.ourshack.com/mailman/listinfo/chchrobotics</a> <br> or, via email, send a message with subject or body 'help' to <br> <a href="mailto:chchrobotics-request@lists.ourshack.com" target="_blank">chchrobotics-request@lists.ourshack.com</a> <br> <br> You can reach the person managing the list at <br> <a href="mailto:chchrobotics-owner@lists.ourshack.com" target="_blank">chchrobotics-owner@lists.ourshack.com</a> <br> <br> When replying, please edit your Subject line so it is more specific <br> than "Re: Contents of Chchrobotics digest..." <br> <br> <br> Today's Topics: <br> <br> 1. Re: Chchrobotics Digest, Vol 158, Issue 11 (william fleete) <br> 2. Re: Chchrobotics Digest, Vol 158, Issue 11 (Volker Kuhlmann) <br> 3. Re: Chchrobotics Digest, Vol 158, Issue 11 (Mark Atherton) <br> <br> <br> ---------------------------------------------------------------------- <br> <br> Message: 1 <br> Date: Sun, 29 Nov 2020 12:40:31 +1300 <br> From: william fleete <<a href="mailto:fleetwil@gmail.com" target="_blank">fleetwil@gmail.com</a>> <br> To: Christchurch Robotics <<a href="mailto:chchrobotics@lists.ourshack.com" target="_blank">chchrobotics@lists.ourshack.com</a>> <br> Subject: Re: [chbot] Chchrobotics Digest, Vol 158, Issue 11 <br> Message-ID: <br> <<a href="mailto:CAFNRJjwCjim3SxYu0K2vH6UP3Y9_L4D92kKv7a3Lch_XYK_31g@mail.gmail.com" target="_blank">CAFNRJjwCjim3SxYu0K2vH6UP3Y9_L4D92kKv7a3Lch_XYK_31g@mail.gmail.com</a>> <br> Content-Type: text/plain; charset="utf-8" <br> <br> I think the heater supply of a magnatron is only a few volts (2-3volts) at <br> a handful of amps, I might be wrong though. you could certainly re-wind a <br> MOT though if you are willing to punch out the HT winding. <br> <br> On Sun, Nov 29, 2020 at 9:43 AM Graeme Absalom <<a href="mailto:clock_man_ok@hotmail.com" target="_blank">clock_man_ok@hotmail.com</a>> <br> wrote: <br> <br> > Hi Mark <br> > <br> > Thank you so much for your advice. The current is controlled by a bank of <br> > 25 Ohm 50 Watt resistors that can be switched to allow more current. The <br> > switching is managed by four power FETs (IRFB4127). <br> > I would not risk any switchmode power supply. <br> > I spoke to Warren and he suggested transformers from old microwaves. They <br> > are 30 Volt and a high current. I could live with two of these in series <br> > and it still meets the alternative options in the book that I made the <br> > spark eroder from. <br> > <br> > I do have a question. Obviously, the output wires on the transformers must <br> > be connected so the phases don't work against each other. Can this be <br> > tested by reading the voltage and changing the orientation if the voltage <br> > is wrong? Would an error like this damage the transformers? If so, what <br> > checks could I do? <br> > <br> > I'm off to the recycling centre tomorrow for Microwaves 😊 <br> > <br> > Thanks <br> > Graeme <br> > ------------------------------ <br> > *From:* Chchrobotics <<a href="mailto:chchrobotics-bounces@lists.ourshack.com" target="_blank">chchrobotics-bounces@lists.ourshack.com</a>> on behalf <br> > of <a href="mailto:chchrobotics-request@lists.ourshack.com" target="_blank">chchrobotics-request@lists.ourshack.com</a> < <br> > <a href="mailto:chchrobotics-request@lists.ourshack.com" target="_blank">chchrobotics-request@lists.ourshack.com</a>> <br> > *Sent:* Sunday, 29 November 2020 1:00 AM <br> > *To:* <a href="mailto:chchrobotics@lists.ourshack.com" target="_blank">chchrobotics@lists.ourshack.com</a> <<a href="mailto:chchrobotics@lists.ourshack.com" target="_blank">chchrobotics@lists.ourshack.com</a>> <br> > *Subject:* Chchrobotics Digest, Vol 158, Issue 11 <br> > <br> > Send Chchrobotics mailing list submissions to <br> > <a href="mailto:chchrobotics@lists.ourshack.com" target="_blank">chchrobotics@lists.ourshack.com</a> <br> > <br> > To subscribe or unsubscribe via the World Wide Web, visit <br> > <a href="https://lists.ourshack.com/mailman/listinfo/chchrobotics" target="_blank">https://lists.ourshack.com/mailman/listinfo/chchrobotics</a> <br> > or, via email, send a message with subject or body 'help' to <br> > <a href="mailto:chchrobotics-request@lists.ourshack.com" target="_blank">chchrobotics-request@lists.ourshack.com</a> <br> > <br> > You can reach the person managing the list at <br> > <a href="mailto:chchrobotics-owner@lists.ourshack.com" target="_blank">chchrobotics-owner@lists.ourshack.com</a> <br> > <br> > When replying, please edit your Subject line so it is more specific <br> > than "Re: Contents of Chchrobotics digest..." <br> > <br> > <br> > Today's Topics: <br> > <br> > 1. spark eroder power supply (Mark Atherton) <br> > 2. Re: Chchrobotics Digest, Vol 158, Issue 9 (Andy Gardner) <br> > <br> > <br> > ---------------------------------------------------------------------- <br> > <br> > Message: 1 <br> > Date: Sat, 28 Nov 2020 16:32:16 +1300 <br> > From: Mark Atherton <<a href="mailto:markaren1@xtra.co.nz" target="_blank">markaren1@xtra.co.nz</a>> <br> > To: Christchurch Robotics <<a href="mailto:chchrobotics@lists.linuxnut.co.nz" target="_blank">chchrobotics@lists.linuxnut.co.nz</a>> <br> > Subject: [chbot] spark eroder power supply <br> > Message-ID: <<a href="mailto:0677a25b-eba2-6011-09ae-6e854d9587e8@xtra.co.nz" target="_blank">0677a25b-eba2-6011-09ae-6e854d9587e8@xtra.co.nz</a>> <br> > Content-Type: text/plain; charset=utf-8; format=flowed <br> > <br> > Hi Graeme, <br> > <br> > How are you controlling the current ? <br> > <br> > I have some vague recollection that you were using an SCR or a TRIAC, in <br> > which case you need zero-crossing, so an SMPSU will not work. <br> > <br> > -Mark <br> > <br> > <br> > On 28/11/2020 3:13 PM, Robert W. Walker, wrote: <br> > > Hi... There are a range of heavy duty transformers but, particularly, <br> > > heavy duty switch mode and regulated supplies that are commonly used and <br> > > are available for E-bikes, E-scooters, Mobility chairs etc. They range <br> > > from 12volts up to 70 volts or slightly more. Check out Google or <br> > > Alibaba, Aliexpress or Ebay.... <br> > > Nga mihi... <br> > > Bob, ZL2ROB (QTHR) <br> > > <br> > > Mark Atherton wrote on 28/11/2020 12:46 pm: <br> > >> Hi Graeme, <br> > >> <br> > >> A 75V/10A transformer is a pretty specialist voltage, and while you <br> > >> might get lucky finding one, the idea of 3 x 24V/10A with secondaries <br> > >> in series sounds a more likely/lower cost option. <br> > >> <br> > >> You might be able to do something clever with beefy ex-UPS <br> > >> transformers, but they are not going to have their windings marked so <br> > >> there is a possibility of injury if you wire them up wrongly. <br> > >> <br> > >> Obvious places to scour include Echotech, Trademe, and Molten Media. <br> > >> <br> > >> Regards, <br> > >> <br> > >> Mark <br> > >> <br> > >> PS Warren may also have some ideas, or something squirrelled away :) <br> > >> <br> > >> <br> > >> <br> > >> <br> > >> On 28/11/2020 11:51 AM, Graeme Absalom wrote: <br> > >>> Hi All <br> > >>> I need some help. Recently I've been pondering on why my spark eroder <br> > >>> only delivers 3 Amps on the onboard ammeter. It hadn't worried me <br> > >>> before because it does work. It is rated for 10 Amp. I decided that <br> > >>> the transformer is far too small to deliver 10 Amps. I tried putting <br> > >>> in a 120 volt 12 Amp and proved that the installed transformer is not <br> > >>> capable but blew a power resister. Reading the book / specs again <br> > >>> what I need is a wire wound, iron core, 230 volts to 75 volts AC and <br> > >>> 10 Amp. The book uses 3 x 24volts AC 10Amp transformers in series. <br> > >>> <br> > >>> Does anyone know where I could get a approx 75 volt, 10 transformer? <br> > >>> <br> > >>> Thanks <br> > >>> <br> > >>> Graeme Absalom <br> > <br> > <br> > <br> > ------------------------------ <br> > <br> > Message: 2 <br> > Date: Sat, 28 Nov 2020 23:06:13 +1300 <br> > From: Andy Gardner <<a href="mailto:ceo@andygardner.com" target="_blank">ceo@andygardner.com</a>> <br> > To: <a href="mailto:chchrobotics@lists.ourshack.com" target="_blank">chchrobotics@lists.ourshack.com</a> <br> > Subject: Re: [chbot] Chchrobotics Digest, Vol 158, Issue 9 <br> > Message-ID: <<a href="mailto:e5a3975a-6cc9-e3ca-cc35-6078423a74a3@andygardner.com" target="_blank">e5a3975a-6cc9-e3ca-cc35-6078423a74a3@andygardner.com</a>> <br> > Content-Type: text/plain; charset=utf-8; format=flowed <br> > <br> > <br> > Many of the bigger UPS's run off 72 volts on the DC side, so there's a <br> > good chance a transformer can be found with the amperage required. <br> > <br> > On 28/11/20 12:46 pm, Mark Atherton wrote: <br> > > Hi Graeme, <br> > > <br> > > A 75V/10A transformer is a pretty specialist voltage, and while you <br> > might get lucky finding one, the idea of 3 x 24V/10A with secondaries in <br> > series sounds a more likely/lower cost option. <br> > > <br> > > You might be able to do something clever with beefy ex-UPS transformers, <br> > but they are not going to have their windings marked so there is a <br> > possibility of injury if you wire them up wrongly. <br> > > <br> > > Obvious places to scour include Echotech, Trademe, and Molten Media. <br> > > <br> > > Regards, <br> > > <br> > > Mark <br> > > <br> > <br> > <br> > <br> > ------------------------------ <br> > <br> > Subject: Digest Footer <br> > <br> > _______________________________________________ <br> > Chchrobotics mailing list <br> > <a href="mailto:Chchrobotics@lists.ourshack.com" target="_blank">Chchrobotics@lists.ourshack.com</a> <br> > <a href="https://lists.ourshack.com/mailman/listinfo/chchrobotics" target="_blank">https://lists.ourshack.com/mailman/listinfo/chchrobotics</a> <br> > Mail Archives: <a href="http://lists.ourshack.com/pipermail/chchrobotics/" target="_blank">http://lists.ourshack.com/pipermail/chchrobotics/</a> <br> > Meetings usually 3rd Monday each month. See <a href="http://kiwibots.org" target="_blank">http://kiwibots.org</a> for <br> > venue, directions and dates. <br> > Please edit your Subject line to reflect new subjects. <br> > <br> > ------------------------------ <br> > <br> > End of Chchrobotics Digest, Vol 158, Issue 11 <br> > ********************************************* <br> > _______________________________________________ <br> > Chchrobotics mailing list <a href="mailto:Chchrobotics@lists.ourshack.com" target="_blank">Chchrobotics@lists.ourshack.com</a> <br> > <a href="https://lists.ourshack.com/mailman/listinfo/chchrobotics" target="_blank">https://lists.ourshack.com/mailman/listinfo/chchrobotics</a> <br> > Mail Archives: <a href="http://lists.ourshack.com/pipermail/chchrobotics/" target="_blank">http://lists.ourshack.com/pipermail/chchrobotics/</a> <br> > Meetings usually 3rd Monday each month. See <a href="http://kiwibots.org" target="_blank">http://kiwibots.org</a> for <br> > venue, directions and dates. <br> > When replying, please edit your Subject line to reflect new subjects. <br> -------------- next part -------------- <br> An HTML attachment was scrubbed... <br> URL: < <a href="http://lists.ourshack.com/pipermail/chchrobotics/attachments/20201129/abc928d6/attachment-0001.html" target="_blank">http://lists.ourshack.com/pipermail/chchrobotics/attachments/20201129/abc928d6/attachment-0001.html</a>> <br> <br> ------------------------------ <br> <br> Message: 2 <br> Date: Sun, 29 Nov 2020 13:23:21 +1300 <br> From: Volker Kuhlmann <<a href="mailto:list57@top.geek.nz" target="_blank">list57@top.geek.nz</a>> <br> To: Christchurch Robotics <<a href="mailto:chchrobotics@lists.ourshack.com" target="_blank">chchrobotics@lists.ourshack.com</a>> <br> Subject: Re: [chbot] Chchrobotics Digest, Vol 158, Issue 11 <br> Message-ID: <<a href="mailto:20201129002321.GA22482@top.geek.nz" target="_blank">20201129002321.GA22482@top.geek.nz</a>> <br> Content-Type: text/plain; charset=utf-8 <br> <br> > I'm off to the recycling centre tomorrow for Microwaves 😊 <br> <br> Do not, under any circumstances, plug in the microwaves to see if they <br> still go, then attempt to take transformers out, unless maybe your life <br> insurance is current or you've thought about how to deal with big <br> capacitors holding kilovolts. <br> <br> In this case one wouldn't care about damaging anything else, I might be <br> happy with safety glasses and a crowbar with a 30kV isolated handle. <br> I'm sure Mark will also advise a change of undies... <br> <br> There are video instructions on youtube showing how to safely discharge <br> those things, it's a multi-step process. <br> <br> Volker <br> <br> -- <br> Volker Kuhlmann <br> <a href="http://volker.top.geek.nz/" target="_blank">http://volker.top.geek.nz/</a> Please do not CC list postings to me. <br> <br> <br> <br> ------------------------------ <br> <br> Message: 3 <br> Date: Sun, 29 Nov 2020 13:38:42 +1300 <br> From: Mark Atherton <<a href="mailto:markaren1@xtra.co.nz" target="_blank">markaren1@xtra.co.nz</a>> <br> To: <a href="mailto:chchrobotics@lists.ourshack.com" target="_blank">chchrobotics@lists.ourshack.com</a> <br> Subject: Re: [chbot] Chchrobotics Digest, Vol 158, Issue 11 <br> Message-ID: <<a href="mailto:803475ae-048e-29cc-33c5-6152229d0d50@xtra.co.nz" target="_blank">803475ae-048e-29cc-33c5-6152229d0d50@xtra.co.nz</a>> <br> Content-Type: text/plain; charset=utf-8; format=flowed <br> <br> Thank you Volker. <br> <br> Standard kit for handling high energy kit: <br> <br> - Rubber gloves <br> - Fire extinguisher <br> - Change of underwear <br> <br> -Mark <br> <br> <br> On 29/11/2020 1:23 PM, Volker Kuhlmann wrote: <br> >> I'm off to the recycling centre tomorrow for Microwaves 😊 <br> > <br> > Do not, under any circumstances, plug in the microwaves to see if they <br> > still go, then attempt to take transformers out, unless maybe your life <br> > insurance is current or you've thought about how to deal with big <br> > capacitors holding kilovolts. <br> > <br> > In this case one wouldn't care about damaging anything else, I might be <br> > happy with safety glasses and a crowbar with a 30kV isolated handle. <br> > I'm sure Mark will also advise a change of undies... <br> > <br> > There are video instructions on youtube showing how to safely discharge <br> > those things, it's a multi-step process. <br> > <br> > Volker <br> > <br> <br> <br> <br> <br> ------------------------------ <br> <br> Subject: Digest Footer <br> <br> _______________________________________________ <br> Chchrobotics mailing list <br> <a href="mailto:Chchrobotics@lists.ourshack.com" target="_blank">Chchrobotics@lists.ourshack.com</a> <br> <a href="https://lists.ourshack.com/mailman/listinfo/chchrobotics" target="_blank">https://lists.ourshack.com/mailman/listinfo/chchrobotics</a> <br> Mail Archives: <a href="http://lists.ourshack.com/pipermail/chchrobotics/" target="_blank">http://lists.ourshack.com/pipermail/chchrobotics/</a> <br> Meetings usually 3rd Monday each month. See <a href="http://kiwibots.org" target="_blank">http://kiwibots.org</a> for venue, directions and dates. <br> Please edit your Subject line to reflect new subjects. <br> <br> ------------------------------ <br> <br> End of Chchrobotics Digest, Vol 158, Issue 13 <br> ********************************************* <br></div></div></blockquote><p><br> </p><blockquote type="cite">_______________________________________________ <br>Chchrobotics mailing list <a href="mailto:Chchrobotics@lists.ourshack.com" target="_blank">Chchrobotics@lists.ourshack.com</a> <br><a href="https://lists.ourshack.com/mailman/listinfo/chchrobotics" target="_blank">https://lists.ourshack.com/mailman/listinfo/chchrobotics</a> <br>Mail Archives: <a href="http://lists.ourshack.com/pipermail/chchrobotics/" target="_blank">http://lists.ourshack.com/pipermail/chchrobotics/</a> <br>Meetings usually 3rd Monday each month. 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