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Black & Decker 2xF4T5 battery lantern

Black & Decker 2xF4T5 battery lantern

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A schematic redrawn from here and here .
It is rather basic Royer oscillator ballast, each lamp have it's own ballasting capacitor.
Lamps are cold started by the transformer's secondary (I guessed about 300V there)
What is interesting, with the single lamp mode the second lamp is kept OFF by simply shorting it out. It is nice trick to keep all the stuff very simple and yet to not overdrive the single lamp...

T5_CAPAFB_EOLPTC.GIF PLC_BASIC.GIF BlackAndDecker2xF4T5.GIF SN57Sims.gif

Light Information

Light Information

Manufacturer:Black&Decker
Lamp
Lamp Type:F4T5
Base:G5
Fixture
Ballast Type:Royer oscillator instant start
Electrical
Wattage:2x4W

File information

File information

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Filename:BlackAndDecker2xF4T5.GIF
Album name:Medved / Reverse engineering
Keywords:Gear
File Size:60 KB
Date added:Jan 13, 2013
Dimensions:1600 x 1200 pixels
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Ash
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Jan 13, 2013 at 02:37 PM Author: Ash
This circuit is perfectly symmetric (unlike others where the resistor to the base is going only to 1 transistor etc). What guarantees that it wont be stuck (both transistors slightly conducting dc) when powered on
Medved
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Jan 13, 2013 at 02:46 PM Author: Medved
Given the resistance of the feedback winding is in mOhm range (it is about two turns), it does not matter at all, if there are used two resistors or only single one.
Two resistors are used only, when the power dissipation become too high for a single piece, what could be the case with rather low beta power transistors. Here the transistors apparently need 120mA as the base drive, what would mean ~0.7W power dissipation. So there are two 100Ohm resistor, what mean about 60mA, so 360mW in each of them, what become manageable in the 0207 size. And if two resistors are used, the circuit become neater looking when really fully symmetric, but it have no real influence on the operation.

No more selfballasted c***

rjluna2
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Robert


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Jan 13, 2013 at 03:15 PM Author: rjluna2
According the circuit I have shown here has 200 Ohm 1/2 Watts 5% tolerance resistor

By the way, thanks for the reverse engineering layout

Pretty, please no more Chinese failure.

Medved
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Jan 13, 2013 at 04:45 PM Author: Medved
You may be right, the first band look a bit different from the 3'rd one, it could be indeed red, so then the 200Ohm would be the correct value.
For these resistors, they have to pass sufficient base current for the transistors to be fully ON, but otherwise the exact value have no influence on the circuit (beside eating extra power from the input), the operating frequency is given by the LC resonance (L is the transformer's inductance, C is the sum of all capacitors transformed to the winding the inductance stands for) and voltage by the input voltage. And lamp current is given by the secondary voltage (that is proportional to the input) and the capacitor's impedance (constant at given resonance frequency). So no dependence on the transistor's gain, base current, temperature, actual lamp voltage variation (the lamp is nearly a short circuit, compare to the secondary voltage), only the output power is directly proportional on the input voltage...

This is the main difference from the blocking oscillator circuits (most common in the cheap ballasts; most commonly features one transistor only), where the moment (so collector current, so the energy stored in the magnetics), when the transistor switches OFF is steered by when the real primary current drive the transistor out of saturation. And that depend on the transistor gain and base current. And mainly the gain is strongly dependent on the transistor temperature. And the lamp power follow more the Vinput square rule (like resistors do), so quite a variation over the battery voltage range...
All that make the blocking circuits quite temperamental, by far not as stable as the Royer oscillator type. And consequently not as efficient.

No more selfballasted c***

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