Forums » General Electronics Discussion

Hi, I have made my first circuit and I need .5-2.0 mA at the anode based on what resistors are on... I am new to this and would like some help please!!! How do I check if this circuit even works and if it does work how do i check how many mA are coming out at the anode. Thanks for your help!

Hi @ahmads01,

Welcome to CL.

Please make your circuit public or unlisted.

No circuit: no help.

:)

OK, you made your circuit public but you did'n't post the link to it ...

First, please read:

https://www.circuitlab.com/docs/

If SW1 is closed then the current through the LED would be set by (battery voltage - LED forward drop) across 4k.

None of the rest of the circuit has any effect other than to momentarily very slightly dim the led as the battery voltage drops due to the current drain charging up C1 the instant SW1 is closed.

I say would because your LED is reverse biased so will not illuminate anyway.

SW4, SW9, SW10 & SW11 do nothing because they are shorted out.

If SW1 is open then there's no current flow anywhere because the battery is disconnected and the lED is reverse baised (although in practice 12V reverse bias across a LED would probably damage it).

With the LED the right way round then the LED will light up quickly when SW1 is closed. and when SW1 is opened, dim more slowly as C1 discharges through it and the 4k resistor.

If the short around the other switches were removed then the time the LED takes to dim would be changed by those switch settings. The effect would be small because the resistors in series with those switches are very much smaller than the 4k resistor already in series with the LED.

What do your 'Cathode -' and 'Anode +' connections go to? Is there some sort of external load you need to drive?

You need a ground node:

https://www.circuitlab.com/docs/the-basics/#ground

To look at the current through the LED read:

https://www.circuitlab.com/docs/the-basics/#volt_and_ammeter

and

https://www.circuitlab.com/docs/the-basics/#plotting_outputs

Hi, Thank you so much for the detailed reply. I made a few changes. So in order for the LED to not burn out, I had placed the 4kohm resistor. I dont really know where to place the ground... I want to know the current at the anode so I placed a ammeter there. So basically what I am trying to do is control the current that is coming at the anode...I would ideally like for a 2mA current to pass through the anode but would like it to be adjustible from .5,1.0,1.5, 2.0 mA. thats why I have the different resistors at R2-R5. This is for a TDCS device I am trying to make. I got this circuit from another person who had built one... The anode attaches to the head while the cathode attaches to a shoulder or anywhere else... I tried to simulate this modified version but still couldnt get a reading in the ammeter. Again thanks for your reply and sorry if I am asking for too much info. It has been years since I took physics :(

andd The led is just telling me that its on... The capacitor is used to create a "softer" start and stop... i checked out your webiste. you are a professional that is awesome! Thanks again!

Ah, you've edited the circuit and saved the edits under the same name. Doesn't really matter but it means anyone else reading this post won't understand the comments because they apply to a schematic that they can no longer see.

TCDS eh?

Hmmmmm.

You need to think very carefully about the possible implications of what you are doing and then read this:

https://www.circuitlab.com/forums/support/topic/hg55u3tb/adding-a-soft-start-to-my-circuit-lm334z-current-regulator/#comment_1911

There are several designs for TCDS sources in CL. I haven't looked at any in detail except this one:

https://www.circuitlab.com/forums/support/topic/hg55u3tb/adding-a-soft-start-to-my-circuit-lm334z-current-regulator/

The whole of that thread is about a pretty comprehensive design for a TCDS current source.

I can't comment on any of the others.

BTW: by anode, you mean the Anode + terminal and not the anode of the LED?

Some random comments about your first attempts ...

470uF cap directly across someone's cranium?

Absolute no-no.

Suppose the cap is accidentally charged to a high voltage?

Suppose it touched across someone's metal spectacle frame or a necklace.

No, no and did I mention it? No.

Your ground placement is fine. It's usually at the negative most point of a single supply circuit (i.e. the point at which all other supplies in the circut are referred to and are more positive than).

If it's a split or dual supply circuit such as with opamps and a +/-15V supply then the ground is usually placed at the mid point so the rails are equally + and - w.r.t. ground.

Not alway though. In a simulator sometimes it helps make the plots more sensible if the ground is at some other point in the circuit.

Sometimes if a simulation is running slowly or won't run at allit helps if you move the ground to a different and often arbitrary point in the circuit. Just remember that all single ended voltage plots (i.e. not measurements of voltage differences differences) are referred to the ground point.

Build some simple circuits and play to get the hang of that idea.

:)

This is about the simplest reasonably robust current source you can make.

Hey, thanks for all the information. It will take some time for me to digest and understand all of that lol.

Yeah by anode i meant the Anode+ terminal (not of the LED).

I am looking at a few circuit lessons on khanacademy.com to get started. I know I kind jumped right in to some complex stuff. I was a psychology major in school so have played with TDCS but I am not planning to put anything electrical near my head anytime soon. I just wanted to start it as a hobby... Thanks again !

And I saved the older "Circuito" and the modified version based on your suggestions as "Circuito Modified".

"Circuito"

"Circuito Modified"

Deep joy.

:)