Big current draw in simple timer circuit. - Page 1
Forum section: electronics / beginners

author: MarkusAnd
author_rank: Contributor
timestamp: March 30, 2019, 05:24:37 pm
content: 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

author: JackJones
author_rank: Regular Contributor
timestamp: March 30, 2019, 05:35:44 pm
content: 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.

author: MarkusAnd
author_rank: Contributor
timestamp: March 30, 2019, 05:59:02 pm
content: 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?

author: jlmoon
author_rank: Unknown
timestamp: March 30, 2019, 06:16:46 pm
content: 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

author: MarkusAnd
author_rank: Contributor
timestamp: March 30, 2019, 06:38:14 pm
content: 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

author: jlmoon
author_rank: Unknown
timestamp: March 30, 2019, 07:10:13 pm
content: The graph is only a generalized representation of what you need to make the transistor behave correctly. They call this the Quiescent point (operating point) on a load line.The first thing you need to establish is the required "I" (not max) current that your led will require. Take a look at a general led and figure most only need 15 - 20 milli amps to light up with a voltage drop around 2.5 Vdc. Once you have this you can establish the required current through the Collector to Emitter junction. Keep in mind the voltage drop your going to experience with that led using 20ma. I would suggest you take a led (selected resistor for 20ma based on voltage drop across led) plus your power supply. Explore this and determine the voltage drop your experiencing across that "diode" then you can explore the concept of using a transistor as a amplifier / switch to make the led respond to the bias voltages on the base of your transistor. A TIP120 might be a bit much for just a simple switch, but if that is what you only have then so be it. I have not looked at the hfe of a 120. Can't remember if that one is a darlington or not. If so, then yes hfe (gain) could be around 1000 or even higher. I do see one that is rated at hfe=2500 at Ic of 4A. So you're close. Basically if you're setting the Collector current around 20mA then you should require 1/2000th that current to make the Tip120 turn on. Don't forget you have (I think) around a 1.2 Volt drop across the Base - Emitter junction when that TIP 120 is in conduction or turned on. Do the current limiting on the Emitter leg and see what you get. If your supply is 12 Volts, try a 200 - 400 ohm resistor and see what you get. Just don't apply 12V and ground directly to that led. It will let the smoke out fast!Hope all this makes sense to youHave fun!JM

author: cvanc
author_rank: Frequent Contributor
timestamp: March 30, 2019, 07:11:39 pm
content: The current spike when you first push the button is the charging current of the capacitor. An 'empty' (discharged) capacitor, when DC is first applied to it, acts briefly as a short circuit and this apparent short eases as the capacitor charges up.Add a resistor to limit the charging 'inrush current' of the cap.

author: MarkusAnd
author_rank: Contributor
timestamp: March 30, 2019, 07:46:22 pm
content: I have used all of your tips and everything seems to work fine now, and all the values are acceptable. Thank you for all the replies! As a new member, I'm glad to see that people are this helpful!//Markus

author: Zero999
author_rank: Super Contributor
timestamp: March 30, 2019, 07:59:45 pm
content: We don't know what sort of LED the original poster is using, as they've not posted a part number, description or schematic. 320mA would be perfectly reasonable for a high powered LED and many have a built-in resistor or driver, so no series resistor is required.The reason for the massive initial current draw is the large capacitor charging up, when the switch is pressed. In reality, the peak current could be much higher, than that indicated by the meter: possibly hundreds of amps. It can shortern the life of the switch contacts. Add a 10Ohm resistor in series with the switch, to limit the current to a safe level.

author: MarkusAnd
author_rank: Contributor
timestamp: March 31, 2019, 12:59:31 pm
content: The LEDs I use are the "normal" low powered LEDs you can buy as an assortment. According to the product description, they should be fed 20mA.

author: Zero999
author_rank: Super Contributor
timestamp: March 31, 2019, 07:47:35 pm
content: The transistor's gain is probably much higher than specified on the data sheet. Never rely on the hfe of a transistor to limit the current to an LED. You need a series resistor.The current surge due to the capacitor is unavoidable. There are better ways of implementating a timer, such as the classic NE555.