# Help needed to fix circuit

Discussion in 'The Projects Forum' started by RedHunter, Aug 2, 2012.

1. ### RedHunter Thread Starter New Member

Aug 2, 2012
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I'm a mechanical engineer not electrical so please excuse my ignorance. I am trying to get a circuit working which seems pretty simple. It is effectively a level switch, so if the probes are in water the bulb is out, if there is no water the bulb is on.
If I substitute the bulb with an LED and 680 resitor the circuit works fine, but if I put a bulb in it does not light.
The bulb current accross 12v is 23mA and the tramsistors in the darlington pair can handle 1A.
I thought I could simulate it in LTspice, but Ive failed again, I can draw the circuit but the simulation doesn't run.

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2. ### praondevou AAC Fanatic!

Jul 9, 2011
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Did you try to decrease the 2k2 base resistor value? Worst case DC gain of the BD138 seems to be 25. For a 5W bulb at 12V there would not be enough base current, meaning the BD is not fully turned on.

When the bulb is supposed to be ON did you measure the voltage drop over collecter emitter of the BD138?

3. ### RedHunter Thread Starter New Member

Aug 2, 2012
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Yes I reduced the 2k2 first to 470 then 120 with same result.
with LED onlt Vec=8.79
with 5w lamp Vec=12.4 which is pretty much Vcc

4. ### praondevou AAC Fanatic!

Jul 9, 2011
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ok, step by step. If you connect the base of the last BD138 via a 470R reistor to ground, does the lamp light up?

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5. ### SgtWookie Expert

Jul 17, 2007
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5 Watts with a 12v supply means 5/12 = 0.416A current flow; the transistor will need about 1/10th that much current, so 11.2/0.0416 ~= a 270 Ohm resistor on the base of the transistor.

However, it'll take a LOT more than 416mA to get that lamp started, as the filament when cold will look like nearly a dead short. The BD138 is rated for 1.5A absolute maximum collector current, so less than half that is OK - but you'll need lots more than 416mA to get the bulb lit.

You may have burned up the transistor when current was flowing through it, but there was not enough base current to saturate the transistor with the load that the bulb was presenting.

I suggest that you could really use a MOSFET there instead of a transistor; a Darlington would probably work too.

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6. ### RedHunter Thread Starter New Member

Aug 2, 2012
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Thanks guys, for pointing me in the right direction.
With 470 ohms base to 0v the lamp did not light, so I dropped a Tip 30 in and all is good.
the 5 watts was a mistake the current is only 23ma so its nearer 1/4 watt
but still mystified as to why it was fine with LED but not bulb.
However as circuit now works fine I'm not going to spend too much time on it when I could be watching the olympics