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Dirty unisolated mains battery charger, viable?

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LAN_403

Storken

Sun Jun 10 2012, 08:25AM

Registered Member #1960 Joined: Thu Feb 05 2009, 11:57PM
Location:
Posts: 48

Hello

I've played with this circuit, but now I want to add a mcu and automatic cutoff to it when voltage wanted is reached.

The AC cap decides the current that will be delivered. I've got 40uF there now, and it delivers some 2,5A - I'm planning on having more to turn up currens soon. I'm charging a 75v16ah pack. Disregard the mosfet breakdown rating note, it is wrong. It will only need to block rectified mains - 370VDC. A bleed off resistor to bring it to ground is also needed, and missing in the schematic.

Other than that, does this look like a ok way to do it?

Sulaiman

Sun Jun 10 2012, 09:28AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I am never in favour of 'live' electronics,
if you are going that way it would be better to draw the -ve line as a wire rather than use the 'earth' symbol,
or use some other symbol.
because you cannot 'earth' any part of the circuit - every part is 'live'.

Your voltage sense divider is not measuring the voltage across the battery,
it is measuring the voltage output of the bridge rectifier
which is the voltage across the series combination of the battery, plus the mosfet drain-source, which will not be zero.

When the mosfet is turned off the full rectified mains voltage will be across the voltage divider,
the full voltage will also be across the 2k + 15V zener.

Pinky's Brain

Sun Jun 10 2012, 09:54AM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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Posts: 837

Sulaiman wrote ...

because you cannot 'earth' any part of the circuit - every part is 'live'.

Won't the capacitor on the mains side (and it's leakage resistance) allow you to ground one of the outputs of the rectifier? of course you have to make assumptions about the voltage difference between neutral and ground ... but it's not like you don't do that for transformers as well.

Dr. Dark Current

Sun Jun 10 2012, 10:06AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

At least here, the appliances must be able to operate normally when the L and N is switched over in the socket. In this case, it would probably mean a pretty decent "pop"...

Storken

Sun Jun 10 2012, 12:39PM

Registered Member #1960 Joined: Thu Feb 05 2009, 11:57PM
Location:
Posts: 48

Sulaiman wrote ...

I am never in favour of 'live' electronics,
if you are going that way it would be better to draw the -ve line as a wire rather than use the 'earth' symbol,
or use some other symbol.
because you cannot 'earth' any part of the circuit - every part is 'live'.

I'm not a huge fan of live electronics myself, but I cant be arsed to carry a huge iron lump to provide isolation. I need a lightweight portable and high current charger. Noted the point of the ground symbol, will use it where it's meant to be ;)

Sulaiman wrote ...

Your voltage sense divider is not measuring the voltage across the battery,
it is measuring the voltage output of the bridge rectifier
which is the voltage across the series combination of the battery, plus the mosfet drain-source, which will not be zero.

True. The only way to accurately check the battery voltage would be to shut down the mosfet and then sense the voltage around the ZVS. Or I could install a SSR on the other side of the bridge rectifier.

Sulaiman wrote ...

When the mosfet is turned off the full rectified mains voltage will be across the voltage divider,
the full voltage will also be across the 2k + 15V zener.

Hmm, yeah - I was having some trouble determiting the resistor to be used here. The current requirement is low, so adding a bigger resistor and a capacitor between zener and -ve to speed up transistion will probably do the trick.

I've only 'hand operated' my current revision of it, the caps get pretty warm - but overall it works fine.

plazmatron

Sun Jun 10 2012, 01:17PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

Well, I have to agree with others on this, why charge with no isolation from the mains?

I see two safer routes here, either build an SMPS, with a very small lump of ferrite instead of iron in there.

Or buy an SMPS and hack it!

I charged batteries off the mains, back in the day using chemical rectifiers and lamps for ballast, and to be honest, that contraption scared the hell of me, as well it should have!

Les

Thomas W

Sun Jun 10 2012, 01:18PM

Registered Member #3324 Joined: Sun Oct 17 2010, 06:57PM
Location:
Posts: 1276

Dr. Dark Current wrote ...

At least here, the appliances must be able to operate normally when the L and N is switched over in the socket. In this case, it would probably mean a pretty decent "pop"...

thats what happened to the PSU that i got off ben when it arrived ;~;
i had just finished fixing it up to a box :(

Storken

Sun Jun 10 2012, 07:11PM

Registered Member #1960 Joined: Thu Feb 05 2009, 11:57PM
Location:
Posts: 48

I appreciate the feedback :) I've built some motor controllers, but no charger - this should be easy in comparison, but my inexperience shines through :S

Anything resembling a buck topology, with adjustable duty cycle based on AC voltage or current feedback - would it be a viable solution?
I sketched out this, It's missing some resistors and capacitors and power supply connections - but how does this look for starters?

Would the duty cycle looks something like this?

I picture programming the avr to turn the duty cycle down when wanted current is met, then increasing it when current isn't met. A simple P-regulator algorithm would probably do?

And, how do I sense voltage while keeping isolation? optoisolator?

The battery act as a giant capacitor, I don't think it will need any decoupling caps.

plazmatron

Sun Jun 10 2012, 08:01PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

Storken wrote ...

And, how do I sense voltage while keeping isolation? optoisolator?

The battery act as a giant capacitor, I don't think it will need any decoupling caps.

That looks a lot safer!

I see no reason why you cant sense the output of the ferrite TX with a separate small secondary winding. This is commonly done on SMPS' , the Flyback transformers in computer monitors etc, and DRSSTC's!

Uzzors did quite a bit of work on home made SMPS', and a little SMPS hacking a while back. See, if you can get him to chime in, or check out uzzors2k.4hv.org

Les

Les

Dr. Dark Current

Sun Jun 10 2012, 10:33PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

I think you can hack most single-switch PSUs, which use the popular UC384x chip, into a charger. Just set the source resistor of the FET for the desired output current.

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