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Why are you being so stubborn? The load current is too high and the voltage drop on R5 doesn't leave enough voltage for the zener to breakdown.
Why is voltage this way in this circuit and how to resolve it?
- Thread starter hhsting
- Start date
Why does V1 vary from 3V to 12V if it comes from a transformer and is rectified to DC? Can you show that part of the circuit, with values for ALL parts?
You should go back to square one.
Describe completely the project.
What are the different power sources?
You have now told us that the load is a 200mA @ 3VDC LED.
What ways do you want to control the LED?
This is a classic example of someone wanting to solve this specific problem but the problem actually lies some place else. That is why you need to give us the whole picture in the first place.
Describe completely the project.
What are the different power sources?
You have now told us that the load is a 200mA @ 3VDC LED.
What ways do you want to control the LED?
This is a classic example of someone wanting to solve this specific problem but the problem actually lies some place else. That is why you need to give us the whole picture in the first place.
its a plug all in one like this
ZOZO Universal AC/DC Adapter 30W Switching Power Supply 3V to 12V W/USB Port & 8 Selectable 8 Adapter Plugs for Household Electronics LED Strips Tablets Webcams Speakers USB Hubs and More https://a.co/d/2m0pQFk
So why don't you just set it to the the next voltage up from 3V (is that 4.5V, I can't read it from the photo!), pick a suitable series resistor for your LED (~7.5 Ohms 0.5W), and use that!
No. You want the highest voltage the adapter can supply. This requires a higher value series resistance and that gives better current regulation.
I could do that but in future i will have loads that are 7vdc, 5vdc etc then what? I would have to again switch the resistor to znere diodes?
Make a 200mA current source/sink.
One thing at a time. We are solving for this particular problem.
What do you mean by loads at 5VDC and 7VDC?
What kind of loads are these? Are we still talking about LEDs as loads?
As said before, you do not design for LED voltage. You design for LED current. An ideal constant 200mA current source should supply 200mA independent of the LED voltage.
Yes but different voltage. Its for landscape you know. Your telling me voltage can be 1.4V but if current is 200ma constant led works? Spec's for led says 200ma at 3vdc.
I dont follow how can i implement constant current with post #1 circuit second one that has all the transistor in it?
No. 1.4V cannot force 200mA through an LED that requires 3V at that current. I don’t think anyone said that.
It sounds to me like you are trying to make a circuit that does (something) given:
1. A control voltage between 0 and 3V.
2. A supply voltage anywhere from 3 to 12 V.
3. Some load that requires some other voltage and some unknown current.
This is a completely different thing than simply designing a circuit that works fore one specific supply voltage and load.
You have given us one soecific example:
12V supply
Load is an LED that draws 200mA at 3V
This does not help in designing a general purpose circuit.
That will dim the leds lights.
How does one build stable voltage regulator at constant 3vdc then in my scenario? What is good circuit in my scenario? I am open for Ics but i dont have 3bdc voltage reference
You don't want a voltage regulator You want a current sink/source.
With a logic level MOSFET, such as IRLZ44N, the circuit on the left would operate down to around 5V. With the values shown, it would sink up to 4A.
We have told you over and over again. You do not design a circuit to provide constant 3VDC for an LED.
You design a constant current source to deliver the amount of current required by the LED.
If you want a constant current of 200mA we have already given you a circuit. It will work for all LEDS that need 3V and greater, up to the limit of the power source voltage.
If you want a circuit without using ICs that is also possible. But first, you have to establish the operating conditions of all loads now and in the future that you want to drive. You cannot give us one condition now and then come back later and say that it does not work with a different load. Of course it does not work because it was not designed for a different condition.
You design a constant current source to deliver the amount of current required by the LED.
If you want a constant current of 200mA we have already given you a circuit. It will work for all LEDS that need 3V and greater, up to the limit of the power source voltage.
If you want a circuit without using ICs that is also possible. But first, you have to establish the operating conditions of all loads now and in the future that you want to drive. You cannot give us one condition now and then come back later and say that it does not work with a different load. Of course it does not work because it was not designed for a different condition.
I dont get it how this can help? I know i need constant current source
You set the current sink for 200mA and connect the LED between the power supply and the drain of the MOSFET. As long as the supply voltage is high enough, current in the LED will be independent of supply voltage.
I don't recommend the circuit with BJTs because you'd need a power transistor to be able to dissipate the required power and power transistors have low beta, so you'd need the split Darlington configuration.
MisterBill2
- Joined Jan 23, 2018
- 28,062
