RC Delay, Beginner 15 Year old EE,

AI Thread Summary
The discussion centers on a beginner's struggle to light an LED using an RC circuit with a 15V power supply, 100μF capacitance, and 100KΩ resistance, aiming for a voltage of 2-2.2V at 20mA. Participants highlight that the LED cannot be driven through a 100KΩ resistor and suggest using a 555 timer or transistors to create the desired time delay for the LED activation. The original poster is encouraged to provide a schematic for better assistance, but issues arise with uploading images. Additionally, it is clarified that a series capacitor blocks DC current, making it impossible to power the LED in this manner. The conversation emphasizes the need for proper circuit design to achieve the intended results.
hacker15
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I've been experimenting for hours, and my LED still won't light correctly when this method is used, and the fluke 87 reads voltage on the circuit in a non-logical way. Can you help? Here's what I have set up when using an online calculator:
Time delay: 2000 milliseconds
Power supply: 15v
Capacitance: 100μF
Resistance: 100KΩ
I want to achieve a result voltage of 2-2.2V, 20ma
Is this possible via this method?
 
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Could you please draw out your circuit and post it? Also measuring from the negative terminal of the supply (black lead) could you post the voltages (red lead) at various nodes in the circuit?
That will help a lot.
 
hacker15 said:
I've been experimenting for hours, and my LED still won't light correctly when this method is used, and the fluke 87 reads voltage on the circuit in a non-logical way. Can you help? Here's what I have set up when using an online calculator:
Time delay: 2000 milliseconds
Power supply: 15v
Capacitance: 100μF
Resistance: 100KΩ
I want to achieve a result voltage of 2-2.2V, 20ma
Is this possible via this method?

Welcome to the PF.

You have a 15V power supply feeding the RC to get a time constant of a second or two (right?). Where does the LED come into play? You can't drive the LED through the 100k resistor...
 
skeptic2 said:
Could you please draw out your circuit and post it? Also measuring from the negative terminal of the supply (black lead) could you post the voltages (red lead) at various nodes in the circuit?
That will help a lot.

Here is my circuit:
http://caltechhelp.net84.net/ledrc.jpg
Electricity flows from the orange (-) to the positive (+)
Sorry for the late reply, I'm new here and had to upload the image to one of my servers (I'm a web developer)
 
hacker15 said:
Here is my circuit:
http://caltechhelp.net84.net/ledrc.jpg
Electricity flows from the orange (-) to the positive (+)
Sorry for the late reply, I'm new here and had to upload the image to one of my servers (I'm a web developer)

Your image is getting blocked as a high-risk website. Can you just attach the JPG image to your post? Look for the paper clip icon in the edit window to upload an attachment.
 
I completely missed that, thank you! And that's because of my hosting service..
 

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Can you upload the schematic?
 
I know the schematic is horrid, but please bear with. Additionally i noticed my cap's were to smallin size, so i changed them. The led lit very dimly, then blew.
 
Never mind, it just quit lighting. I just tested it on another circuit. I apologize for any confusion!
 
  • #10
hacker15 said:
I know the schematic is horrid, but please bear with. Additionally i noticed my cap's were to smallin size, so i changed them. The led lit very dimly, then blew.

The schematic is unvisible! :-p
 
  • #11
berkeman said:
The schematic is unvisible! :-p

Lol sorry XD
 

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  • #13
Yeah, you can't drive an LED through a 100k Ohm resistor.

You need to do something different to drive the LED. Are you wanting to have a couple second delay after throwing a switch before the LED lights up? You would need to add something separate to sense when the RC waveform rises above some level, and then use a transistor or something to drive the LED.

If you drive the LED from your 15V supply, you will need to choose a series resistor to drop about 13V (most LEDs have about 2V across them when lit), and set the current at 10-20mA (whatever the LED current spec is). For example, if you wanted 20mA through the LED with 13V across the resistor, the resistor in series with the LED would be 13V/0.02A = 650 Ohms.
 
  • #14
berkeman said:
Yeah, you can't drive an LED through a 100k Ohm resistor.

You need to do something different to drive the LED. Are you wanting to have a couple second delay after throwing a switch before the LED lights up? You would need to add something separate to sense when the RC waveform rises above some level, and then use a transistor or something to drive the LED.

If you drive the LED from your 15V supply, you will need to choose a series resistor to drop about 13V (most LEDs have about 2V across them when lit), and set the current at 10-20mA (whatever the LED current spec is). For example, if you wanted 20mA through the LED with 13V across the resistor, the resistor in series with the LED would be 13V/0.02A = 650 Ohms.

I have run the led's previously, but I want to create a time delay (preferably make two different led's flash at different time intervals).
My supplies to achieve this are as follows:
(2x) https://www.amazon.com/BB400-Solderless-P…&tag=pfamazon01-20
https://www.amazon.com/Joe-Knows-Electron…&tag=pfamazon01-20
https://www.amazon.com/Joe-Knows-Electron…&tag=pfamazon01-20
https://www.amazon.com/Solderless-Flexibl…&tag=pfamazon01-20
https://www.amazon.com/Joe-Knows-Electron…&tag=pfamazon01-20
http://shop.vetcosurplus.com/catalog/pro…
 
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  • #15
hacker15 said:
I have run the led's previously, but I want to create a time delay (preferably make two different led's flash at different time intervals).
My supplies to achieve this are as follows:
(2x) https://www.amazon.com/BB400-Solderless-P…&tag=pfamazon01-20
https://www.amazon.com/Joe-Knows-Electron…&tag=pfamazon01-20
https://www.amazon.com/Joe-Knows-Electron…&tag=pfamazon01-20
https://www.amazon.com/Solderless-Flexibl…&tag=pfamazon01-20
https://www.amazon.com/Joe-Knows-Electron…&tag=pfamazon01-20
http://shop.vetcosurplus.com/catalog/pro…

Look at using 555 timer chips or similar to generate the delay functions you want, and then drive NPN transistors as switches to turn on your LEDs. Just Google 555 timer LED driver, or similar to see various circuits that you can use.
 
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  • #16
berkeman said:
Look at using 555 timer chips or similar to generate the delay functions you want, and then drive NPN transistors as switches to turn on your LEDs. Just Google 555 timer LED driver, or similar to see various circuits that you can use.

I've come across those alot.. I guess I'll have to tough it out and start buying diods and such? Lol, can you provide me with links to good components for beginners, with a quantity high enough to blow a few or more of each? XD Thank you!(:
 
  • #17
hacker15 said:
Additionally, here are my supplies:
(2x) https://www.amazon.com/BB400-Solderless-P…&tag=pfamazon01-20
https://www.amazon.com/Joe-Knows-Electron…&tag=pfamazon01-20
https://www.amazon.com/Joe-Knows-Electron…&tag=pfamazon01-20
https://www.amazon.com/Solderless-Flexibl…&tag=pfamazon01-20
https://www.amazon.com/Joe-Knows-Electron…&tag=pfamazon01-20
http://shop.vetcosurplus.com/catalog/pro…
None of those links work. Would it be possible to repost them?
berkeman said:
Yeah, you can't drive an LED through a 100k Ohm resistor.
True, especially when there are capacitors in series with that resistor :confused:
hacker15 said:
attachment.php?attachmentid=48756&d=1341009067.jpg
 
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  • #18
Google search on reading schematics

My thought is, if you read through or watch some of sites on that list you will then be able to draw us a schematic diagram. We need it to help you.

Also it may help you with getting your circuits working.
 
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  • #19
That circuit looks an awful lot like a circuit for driving an LED using alternating current. The LED would rectify the small alternating current and the resistor would limit the current so as not to burn out the LED. The capacitor would couple the AC to allow enough current to flow to allow the led to light.

If you use DC then you would not get any current through the capacitors. I supposed if you had a very unfiltered DC power supply the ripple might cause the LED to light.
 
  • #20
I am using a 15v dc 30mA power supply, which i found laying around and soldered/shrink wrapped breadboard jumpers on, for easy experimentation. Is it not possible to power though a capacitor this way?
 
  • #21
A series capacitor blocks DC current. So no you cannot power anything through a cap in a DC circuit.
 
  • #22
Darn. So I guess it's time to make another shopping list!
 

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