Electronics assistance – reduction of output DC voltage

[duncan webster 1Participant @duncanwebster1]

This is getting well off topic, but that never stopped me! Some years ago I was involved in a project to heat a plasma to very high temperature using an RF heating coil. The very high voltages involved kept on causing flashovers. We were advised to fit two brass golf balls , one either side of the coil but physically close together, so that if there was a voltage spike the arc would be between the two balls which would be cheap to replace. There was then to be a light detector which would spot the arc and trigger a circuit to turn off the power. Part way through fitting it all, before the trigger circuit was in place, the boffins decided to have a run. Sure enough they got an arc between the balls, but once it had started the resistance of the air in between dropped and it just kept going. The balls vapourised and drifted off down the rig hall, quite spectacular I imagine, but I wasn't there at the time.

back on topic, I can't see what the issue is with winding a low voltage coil on a toroidal transformer, just don't damage the high tension windings. I've done it twice and I'm no great electrical engineer.

[John HaineParticipant @johnhaine32865]

Indeed, Duncan has it right – A toroidal transformer gives you a big hole to wind your secondary through without affecting the mains primary.

[choochoo_balooParticipant @choochoo_baloo]

Having successfully built and tested my PSU powered resistance soldering unit (RSU), I want to report my findings.

The premise of this DC powered RSU was to make use of a beefy DC PSU I already own.

Given this, here are my reasons for preferring DC over AC:

1. No home transformer secondary winding. Having consulted an electrical engineer, he talked me out of home transformer winding, danger of electrocution/fire. Especially since I had a PSU, I wasn't prepared to take the risk.
2. My PSU is 8-15V adjustable output. Such continuous power adjustment is a boon for etched kit building (especially for tackling whitemetal castings that are commonplace in 7mm scale locomotive kits). I gather you could squeeze 2 secondary windings out of a microwave transformer if you're careful.
3. DC can exploit a flyback diode to protect the foot switch contacts from premature arc damage arising from the inductor. This flyback diode protection isn't possible with AC

Therefore if you too have a PSU, I strongly suggest build your RSU for DC operation. Based on the previous circuit diagram, I experimented with resistances to settle on a 0.1 ohm, 50 W, ballast resistor. In an aluminium enclosure mounted with thermal paste, it only gets mildly warm to touch.

Edited By choochoo_baloo on 20/07/2018 01:57:35

[SillyOldDufferModerator @sillyoldduffer]

Posted by choochoo_baloo on 20/07/2018 01:56:55:

Having successfully built and tested my PSU powered resistance soldering unit (RSU), I want to report my findings.

The premise of this DC powered RSU was to make use of a beefy DC PSU I already own.

Given this, here are my reasons for preferring DC over AC:

1. No home transformer secondary winding. Having consulted an electrical engineer, he talked me out of home transformer winding, danger of electrocution/fire. Especially since I had a PSU, I wasn't prepared to take the risk.
2. My PSU is 8-15V adjustable output. Such continuous power adjustment is a boon for etched kit building (especially for tackling whitemetal castings that are commonplace in 7mm scale locomotive kits). I gather you could squeeze 2 secondary windings out of a microwave transformer if you're careful.
3. DC can exploit a flyback diode to protect the foot switch contacts from premature arc damage arising from the inductor. This flyback diode protection isn't possible with AC

Therefore if you too have a PSU, I strongly suggest build your RSU for DC operation. Based on the previous circuit diagram, I experimented with resistances to settle on a 0.1 ohm, 50 W, ballast resistor. In an aluminium enclosure mounted with thermal paste, it only gets mildly warm to touch.

Edited By choochoo_baloo on 20/07/2018 01:57:35

Glad it works but I'm not so happy with the reasons given:

1) When discussing the risks of rewinding a transformer with the electrical engineer was it clear that you were only rewinding the secondary? His concern makes more sense if he thought you were going to mess with the primary as well. Also, if you'd asked an electronics engineer rather than an electrical engineer, I suspect you would have got the other answer! He might have advised against a regulated PSU because of the risk of burning out the relatively delicate electronics and the possibility of other malfunctions. He would know that not all regulated PSUs are equal and that it's unwise to generalise about using them in non-standard ways. Not dangerous, just what worked for you may not work for others. (Good engineering is repeatable.)

2) 'I gather you could squeeze 2 secondary windings out of a microwave transformer if you're careful' is a misunderstanding. To get two or more voltages for an RSU, it's only necessary to tap a single secondary winding, which isn't difficult.

3) Flyback Diode protection is only necessary because DC is switching an inductor. Also a misunderstanding – a diode isn't needed when switching AC. The AC circuit is simpler.

That said, obtaining and rewinding a transformer to make an AC unit and boxing it up safely is a right faff. Although I disagree with Choochoo's reasons for recommending DC, in practice I say it's a fair compromise for a different good reason. Ready availability of the necessary components is sufficient justification. Chhochoo was able to build it, it does the job and it isn't dangerous.

As a matter of interest, can you strike arcs with your RSU choochoo? I'm sure it will arc if you deliberately manoeuvre the electrode, but does it ever arc accidentally?

Well done,

Dave

[choochoo_balooParticipant @choochoo_baloo]

Posted by SillyOldDuffer on 20/07/2018 10:01:14:

As a matter of interest, can you strike arcs with your RSU choochoo? I'm sure it will arc if you deliberately manoeuvre the electrode, but does it ever arc accidentally?

Well done,

Dave

Yes it does arc whenever I'm careless Dave – by removing the electrode before releasing the foot switch. No it's never arced accidently.

Edited By choochoo_baloo on 04/08/2018 01:05:45

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