Mixing AC and DC

[ArmandTanzarian]

Possibly a stupid question but how to scoots that have a mix of AC and DC work? Surely the AC part is constantly switching from negative to positive earth. Does the -ve DC part of the circuit not connect to the frame?

[Grumpy225]

All voltages go to ground (frame) regardless of being AC or DC. The rectifier that converts the AC to DC is not grounded internally (floating ground) and that keeps all the voltages happy.

It would be easier to explain with a drawing.

I'm not sure what you mean by this "Surely the AC part is constantly switching from negative to positive earth." Is yours a + ground set up?

[ArmandTanzarian]

What I can't get my head round is that in an AC system, the current flow is constantly changing direction. How does that work in conjunction with DC which is all going in one direction. To my simple mind it would seem that one moment you have the AC current flowing in the same direction as the DC and the next moment its swapped direction and is flowing against the DC. If the AC voltage is a sine wave, why is it that when you add in the DC component, does the voltage not swap between zero (as they cancel each other out) and 24 as they add to each other.

Or, if the regulator has a floating ground does it just shift the waveform up so that it isn't alternating around zero volts but twelve and the potential difference between the positive and negative is still 12V.

I appreciate that I've misunderstood something here (these systems do work after all) I just wondered what.

[Grumpy225]

The AC feed and DC feed are not passed through the same wire until they get to ground. The rectifer converts the AC from the stator to DC (this is where the floating ground comes in to play and the end result is really a pulse DC, all positive voltage). Also the current flow doesn't change in AC, just the voltage potential

A little light reading.

http://www.allaboutcircuits.com/vol_2/chpt_1/3.html

Ohms law

Throw in some calculus

Have a pint and just accept the fact that it works

[ArmandTanzarian]

This would be why I failed my HND in electronics twice and gave up with the whole ghastly business 30 years ago!

[firekdp]

Possibly a stupid question but how to scoots that have a mix of AC and DC work? Surely the AC part is constantly switching from negative to positive earth. Does the -ve DC part of the circuit not connect to the frame?

It's not a problem using a common earth for "separate" AC and DC circuits, original battery models all do this.

What I can't get my head round is that in an AC system, the current flow is constantly changing direction. How does that work in conjunction with DC which is all going in one direction. To my simple mind it would seem that one moment you have the AC current flowing in the same direction as the DC and the next moment its swapped direction and is flowing against the DC. If the AC voltage is a sine wave, why is it that when you add in the DC component, does the voltage not swap between zero (as they cancel each other out) and 24 as they add to each other.

Take a standard series III Lambretta. Imagine the frame to have a reference voltage of 0V, obviously, the battery positive can only ever be +6v above it. The live AC feed wire from the stator (assuming correct load) will either be +6v(rms) above it or -6v(rms) below it. As long as the live sides of each circuit remain separate they will not affect each other.

Also the current flow doesn't change in AC, just the voltage potential .

Unsure what you're saying here, when the voltage changes polarity, electron flow must change direction, hence AC = "Alternating Current".

[Ian Hepworth]

I lost all ability to comprehend electrical stuff when I studied Physics A level and it all got turned round the other way as it's all about the flow of electrons.

[Adam_Winstone]

What I start scratching my head at is how to wire a system so that I can determine what the earth will be, positive or negative. Whilst the original 6V DC systems use a negative earth, the 12V systems use a positive earth. This then differs to the the original 6V systems combination of AC + DC with common negative earth.

With a little more understanding of wiring/electrics I could start to design my own stator schemes, rather than just copying the examples already presented. I have got as far as doing increased wattage 12V electronic AC (with and without final earth being remote for DC usage if wanted), original 6V electronic DC (starting with points stators), 12V AC increased wattage points ignition stators (with and without final earth being remote for DC usage if wanted) and 12V points DC (as per 60s conversion) but I'd love to be designing these to allow the mix of AC and DC on a common polarity earth, so as to switch between the 2 if required. I've read various posts about such systems but have never looked further into them. DC is great for running extras (tacho feed, etc) but my DC systems are an all or nothing affair, where battery problems can leave me without lights!

Oh well, something to keep me busy!

Adam

[Grumpy225]

Also the current flow doesn't change in AC, just the voltage potential .

Unsure what you're saying here, when the voltage changes polarity, electron flow must change direction, hence AC = "Alternating Current".

You are right. I did not think that all the way through. My bad.

[firekdp]

Whilst the original 6V DC systems use a negative earth, the 12V systems use a positive earth. This then differs to the the original 6V systems combination of AC + DC with common negative earth.Adam

I don't think you've quite got the concept of it. AC may share the earth with DC but can never have a constant pos or neg earth, if it did it wouldn't be AC.
DC systems can obviously be whatever polarity you like, the 60's 12v conversion was only pos earth because Lucas only made the zener and rec in that format. Nowadays copies come in both neg and pos versions, so you can choose.