| Electronics > Beginners |
| Big current draw in simple timer circuit. |
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| MarkusAnd:
Hi! Electronics beginner here who is trying out some simple circuits. This circuit consists of a switch, resistor, transistor, led and a capacitor. When the switch is closed the capacitor is charging and when the switch is open the cap discharges through the base of the transistor which lights up the led. When the voltage from the cap drops below 1.2V the led doesn't get any current. When I test this circuit I get a big current draw from my power supply immediately when I close the switch. I measured the current with my DMM and it has a peak of 2.1A. What is it that is causing this big current draw? Picture of the schematic as an attachment //Markus |
| JackJones:
You have forgotten the current limiting resistor for the led. When you close the switch you are basically shorting out the supply though the led and the transistor. |
| MarkusAnd:
I tried that but it only affects the current draw marginally. I changed the size on the resistor going to the transistor's base so that it only is fed around 0.32mA. If I'm not wrong the tip-122 has a current gain of 1000 so the led should only be able to draw around 320mA, which my DMM also shows. So maybe the resistor value on the base was too low? |
| jlmoon:
I hope your not running 320ma through that LED. As previously stated you need to limit the current through that led to a reasonable level such as .020 A or 20 ma. You also have your emitter directly tied to ground, you need a little negative feed back there. Transistors work much better when you move the Q point up to the middle of the IC / Vce (Ic on Y axis, Vce on X axis) load line. Jon |
| MarkusAnd:
So should I adjust the current and the voltage to match the line in the graph? And yeah 320mA isn't good for the led but it's an improvement from 2A I guess ;) |
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