iONic

I'm not 100% sure I understand the nomenclature with respect to the power supply in the image attached. To me it appears to be 9V AC, but what does the 2.5 VA mean, 2.5 Amps?

Also: How might one step down a 15VAC transformer to a 9VAC transformer?

Attached Images

ALKALINE CHARGER3.gif (4.7 KB, 46 views)

Nigel Goodwin

No, like it says '2.5 VOLT AMPS' - effectively 2.5 watts, but not exactly the same.

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kchriste

As Nigel alluded to, your 2.5VA transformer should be capable of outputting a maximum of 278ma @ 9Vac into a resistive load. See Wiki if you want to know more about power in reactive circuits:
http://en.wikipedia.org/wiki/Volt-ampere
You could remove turns from the secondary of the transformer until you got 9Vac out of it. An easier thing to do would be to change the 47hm: rheostats to 150 or 200hm: units. They would dissipate a bit more power, but this would be the simplest solution.
EDIT:
I don't know if I'd recommend the circuit that you posted above. It looks a little hokey; make sure the rheostats are set to their max resistance when powering up the circuit for the first time. The transistor are being used in an unconventional manner. Their emitter/base junctions will be in breakdown if wired as shown. I really don't know the designers intent as far as the transistors go.

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audioguru

I hope you are not trying to blow up 1.5V alkaline battery cells by charging them.

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ericgibbs

hi kchriste,
Is a form of a 'relaxation oscillator', the transistors do 'breakdown' and they drive the output to the battery with a 'charging pulse'
The large value cap charges up via the rheostat until it reaches the transistors breakdown voltage, the transistor effectively avalaches into a low resistance state and dumps the cap charge into the battery.
When the current falls to a low enough value the transistor recovers and this cycle continues.

EDIT: current link.
http://www.electro-tech-online.com/g...atic-work.html

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audioguru

The transistors are being used as zener diodes. I don't know why proper zener diodes weren't used instead.

What a stupid circuit to blow up 1.5V alkaline non-rechargeable battery cells.

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blueroomelectronics

I should post it on Electronics Lab

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audioguru

I think Aaron Cake has more projects on his website that don't work.
Electronics-Lab copied them and I fixed some.

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iONic

There are three kind of people on this site. 1) Those that have an optomistic
view, those that are neutral, but do what they can to help, and those that are, in polliticaly correct terms, likely to play the "devils advocate" with respect to ciruits and circuit ideas.

No one said the circuit was going to be used to blow up any alkaline batteries. My question was respect to the power supply, granted the interest in the unconventional us of the transistor was interesting as well.

Also, since BVceo across the BC337 or similar NPN transistors is 25 - 45V, it is more likely one will find the transistor lying around as apposed to the 25V or 45V Zener Diode.

ericgibbs

hi ionic,
If you check the circuit you will see the transistors are reversed biassed, so the conventional BVceo dosn't apply, the reverse voltage causes the transistor to 'breakdown' at a much lower voltage.

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iONic

Yes, you are right.

audioguru

The reverse breakdown voltage (VEBO) of the emitter-base junction of a BC337 is 5V or a little higher. The circuit has the transistor connected backwards so the emitter-base is reverse biased and the collector-base is forward biased.

Then the emitter-base will have avalanche breakdown at about 5.0V or a little higher plus the 0.7V of the collector-base diode equals a voltage at about 5.7V or a little higher.

It might even oscillate like a relaxation oscillator.

The circuit shows a 1.5V battery cell. Like a non-rechargeable alkaline cell, not like a 1.2V Ni-Cad or Ni-MH chargeable cell.

If an alkaline cell is charged too long from that circuit then it will explode.
Even a 1.2V rechargeable cell would be very over-charged if it charges too long from that circuit.

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mneary

Has anyone built this circuit? How long did the BC337's last?

kchriste

That makes sense. I now remember the LED flasher circuit that was posted earlier.
Don't know how good of a burp charger it will make due to component variations. Might be interesting to experiment with, but I wouldn't leave it unattended until it has been throughly tested.

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blueroomelectronics

Hard to beat Aaron's pimpin ride.

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