CYCLON battery charging circuit, how does it work ?

[Buzby123]

This is the circuit from a portable oscilloscope. It charges three 2.5Ah 'D' CYCLON cells.

Can anyone explain the operation of this circuit to me ?

Also, it needs a low voltage AC supply from the external transformer, but what voltage would you suggest ?

Thanks,

Buzby

[HamishBoxer]

Possibly 6V AC ?

[KeithsTV]

The TL431 is a "programmable zener". The voltage at the slider of the pot is 2.5V which gives the voltage at the base of the transistor a range of 3.56V to 4.28V with the voltage at the emitter approximately 0.65V lower (2.9V to 3.6V) which seems a bit too low for charging 3 Cyclon cells.

Keith

[trsomian]

The 431 and its associated resistors form a voltage reference, and the transistor is an emitter follower from that. Note that there is no smoothing capacitor, so the output will be a series of voltage regulated pulses.

I think cyclons are lead acid technology, so around 2V3 or 2V4 per cell on charge, so around 7 volts across the cells, a few volts of loss in the regulator, so need 10V or so (plus a bit maybe) out of the rectifier, call it 12V peak into the rectifier gives around 8 to 9 V RMS out of the transformer. Note that there is no current limiting other than the fuse, which may not be ideal

Disclaimer - I haven't done any calculations on the circuit so I don't know what voltage it is likely to actually produce

[Herald1360]

The battery charger regulator is a simple emitter follower pass transistor with its base voltage controlled by the programmable "zener" VR1. It's doubly "odd" in that it looks upside down and it's fed with unsmoothed full wave rectified dc so the battery will see a series of voltage pulses each peak limited at the level set by the regulator minus the Vbe of the pass transistor. There may be unseen "tricks" to doing it this way, perhaps wrt charging voltage temperature compensation or it may just be the quick'n'dirty design that it appears to be. It does save the cost of an (unreliable) electrolyticsmoothing capacitor though?....

[Herald1360]

Originally Posted by KeithsTV

The TL431 is a "programmable zener". The voltage at the slider of the pot is 2.5V which gives the voltage at the base of the transistor a range of 3.56V to 4.28V with the voltage at the emitter approximately 0.65V lower (2.9V to 3.6V) which seems a bit too low for charging 3 Cyclon cells.

Keith

I reckon the adjustment range for the TL431 is from 8.38V to 6.02V nominal which minus the Vbe of the transistor sounds about right for a 3-cell lead acid which would be float charged at around 6.9V or so.

You get the odd figures above if you transpose the values for the programming resistors in the the app note equation Vout = Vref*(1+R1/R2).

[MotorBikeLes]

To trickle charge three cells in series, you need 2.203v per cell, a little more for faster charging. Obviously a Tek circuit. As already pointed out, no capacitors, but the cells are in effect one large capacitor. Taking a 4.8v secondary, Xs 1.414, we get 6.9v, so no more than 5v secondary surely?
les.

[Herald1360]

I would expect a somewhat higher ac than 5V- the regulator effectively clamps the peak voltage at the charging voltage for the battery so the higher the input ac, the more of each half cycle is useful to charge the battery. An upper limit would be set by the dissipation limit for the pass transistor but a guesstimate might be that ac input that makes perhaps 25-50% of the half cycle "useful"

[KeithsTV]

Originally Posted by Herald1360

I reckon the adjustment range for the TL431 is from 8.38V to 6.02V nominal which minus the Vbe of the transistor sounds about right for a 3-cell lead acid which would be float charged at around 6.9V or so.

You get the odd figures above if you transpose the values for the programming resistors in the the app note equation Vout = Vref*(1+R1/R2).

Yes you are correct. Must be because the circuit is "upside down". Wonder why they did it that way?

Keith

[John_BS]

1. Must work when mains is 10% low
2. double diode loss in bridge rectifier


That would suggest transformer nominal output closer to 6.5V min.

[Radio Wrangler]

Using 'dirty DC' is an old trick considered to prolong battery life over more charge/discharge cycles. Sometimes used with all sorts of cell chemistry

David

[Herald1360]

Ahh- I'd come across dirty DC as a good idea for NiCads but not encountered it (except incidentally in old car battery chargers) as being a benefit for PbSO4.

[snowman_al]

Here is a snip from the CYCLON data sheet. Re constant voltage charging.
It suggests a range of 2.3 - 2.5 volts per cell depending on trickle or fast charge.

I see the circuit as a simple voltage regulator. And likely set to a suitable charge voltage as long as the pot (R112) has not been twiddled?

You need a transformer that exceeds the double diode drop and pass transistor drop, so about 9 volts.

Data sheet here: https://docs.rs-online.com/c7b6/0900766b800278eb.pdf

[Buzby123]

For a bit more info, here is the PSU circuit.

Regarding the batteries, I took the pack out and put it on a 7.3v Constant voltage supply. Left it running all night. The current drawn varied started at 1.25A, and finished at 0.8A.

Battery voltage tests now at 6.25v, but it won't drive the 'scope.

Tried a 7.3v supply in place of the battery pack, and the 'scope works fine.

It works fine down to about 6.6v, so I think the cyclons are dead !.

I can't test the charging side of things until I find a suitable AC PSU.

( I did have one, it came with the 'scope, but that was twenty years ago ! )

Cheers,

Buzby

[snowman_al]

Just an observation, normal lead acid cells give about 2.1 volts off load and charged, so best possible output from 3 cells will be 6.3 volts. Less as soon as a load is applied.

Before drawing any conclusion about the Cyclons, you need to establish if the scope works from a solid 6 volt (or even 5.5 volt) supply.

[Buzby123]

Originally Posted by snowman_al

Before drawing any conclusion about the Cyclons, you need to establish if the scope works from a solid 6 volt (or even 5.5 volt) supply.

Good point, and one I'm investigating now !.

The manual describes how to set the PSU shutdown voltage, and it seems to require shutdown when the supply from the battery is below 5.75v, which is 1.92v per cell

My quick experiment earlier showed shutdown at around 6.6v, so that doesn't look right.

I'll be back to investigate further after mowing the lawn !.

[Buzby123]

After mowing the lawn I did a bit more testing.

It definitely looks like the shutdown circuit was set too high.

The scope is now running off the cyclons, at 6.07v at the moment.

Spec says it should run for three hours on battery.

I'll check that tomorrow.

Cheers,

Buzby