44
votes
Accepted
What's the intuition for reading transistor/resistor circuits?
What's a step by step way of thinking about the flow of electrons in such a circuit? How can we tell it would provide constant current for instance?
The first step is to forget about flow of ...
- 160k
33
votes
'Leftover' voltage when using constant current
This circuit is particularly useful for a variety of reasons. So I wanted to do it a little more justice than just two sentences' worth.
OVERVIEW
In the following circuit, I've intentionally boxed up ...
- 70.1k
27
votes
Accepted
How do constant current constant voltage devices work?
The devices work by not being constant current and constant voltage at the same time.
You set a voltage (V) for the output. You set a current (A)for the output.
It will supply constant V for any ...
- 58.2k
24
votes
Accepted
What is the dual of a diode?
Yes, they are called current regulator diodes. They are essentially a JFET with gate joined internally to source so that you get approximately IDSS for voltages above the pinch-off voltage (and below ...
- 331k
21
votes
Accepted
How to calculate the tolerance of this constant current circuit?
A couple of notes may help clear the air.
Early Effect
One of the problems of BJTs is something called the Early Effect. This is where the collector current depends on the collector to emitter voltage ...
- 70.1k
20
votes
LED current driver
Lose the series resistors. It's not clear what you think they do for you, but they won't do anything useful. You have the LEDs connected to constant current drivers. Series resistors only get in ...
- 306k
19
votes
Accepted
What are some practical examples of a constant current source?
Photovoltaic cells and betavoltaic cells are examples of devices that produce more-or-less constant current up to some maximum open-circuit voltage, just as a battery produces more-or-less constant ...
- 331k
16
votes
Accepted
How does this constant current sink actually work?
The circuit employs negative feedback and utilizes the very high gain of the op amp. The op amp will try to keep its non-inverting and inverting inputs at the same voltage \$V_{\text{set}}\$ due to ...
- 6,971
16
votes
can leds take varying voltage > listed voltage, with constant current?
An LED is a very simple device. It behaves according to:
$$I_{LED}=I_{SAT}\cdot\left(e^\frac{V_{LED}}{n \cdot V_T}-1\right)$$
Or, alternately,
$$V_{LED}=n\cdot V_T\cdot \operatorname{ln}\left(\frac{...
- 70.1k
14
votes
How to limit current to LEDs
No, that's the right way to do it. Each LED having it's own ballast/series current limiting resistor, sized in Ohms and Wattage to fit the desired forward current of the LED and the Voltage source.
...
- 71k
14
votes
Why can't electronic systems use constant current power source and vary the voltage as power consumption varies?
While there's no reason why you couldn't design electronics with constant current sources, there's a few good reasons why we don't. Batteries and mains power are supplied more as constant voltage ...
- 12.4k
13
votes
What's the intuition for reading transistor/resistor circuits?
What's the intuition for reading transistor/resistor circuits?
More precisely speaking, "reading" means "understanding" here. So, the question is, "How do we intuitively ...
- 9,379
13
votes
How do constant current constant voltage devices work?
It might be better to think of these supplies as "maximum voltage, maximum current" supplies. The potentiometers adjust the maximum current and voltage that the supply will deliver.
With no ...
- 51.6k
12
votes
LM317 µA Constant Current source possibility?
Yes, that is clever, I think it would 'work', however the problem is that Iadj is 50-100uA so that you will not be able to get an accurate load current. First, it is large, so your 200uA current might ...
- 331k
12
votes
What is the dual of a diode?
I think you mean a constant current diode. They will attempt to drop any voltage past a fixed current. From Wiki:
Constant-current diode is an electronic device that limits current to a maximum ...
- 71k
12
votes
Accepted
Need for Temperature Compensation of Current Mirror
The three main steps are
a) Use as much emitter degeneration as you can
b) Match the temperatures of Q1 and Q2
c) Match the dissipation of Q1 and Q2
For (b), at the very least, glue Q1 and Q2 ...
- 141k
11
votes
Accepted
Why is PWM used to control LEDs even when they're only used On or Off?
Contrary to initial intuition, it's actually to increase the brightness.
LEDs can be driven at a constant current, or they can be driven with a pulsed current.
With constant current you have to limit ...
- 55.5k
11
votes
Accepted
Explanation of LED driver circuit
It's a buck-boost configuration. If it was connected in boost then the input voltage could not exceed the LED voltage or they would burn out. There's more voltage across the inductor and the switch ...
- 331k
11
votes
Accepted
LM317T with weird pinout?
Neither of your circuits is correct. The correct circuit looks like this:
simulate this circuit – Schematic created using CircuitLab
If you connect it incorrectly (as in the first circuit) it ...
- 331k
11
votes
Accepted
'Leftover' voltage when using constant current
The remaining voltage will be dropped across Q1. Or, the voltage across Q1 will vary to ensure that 20 mA flows through the LED and R2.
- 51.6k
11
votes
Constant current source drop
Your load requires 15V to maintain 10mA (10mA x 1.5k).
Below about 16V there is no voltage available across Q2 for it to control the current so the current has to drop.
As @mkeith points out the ...
- 29.6k
10
votes
Will a current mirror prevent thermal runaway of these LEDs in parallel
A current mirror can only servo the output current if it has the voltage headroom (compliance) to do so.
If D2 is lower voltage drop and tries to hog the current, then Q2 will reduce its current by ...
- 141k
9
votes
Low Overhead Constant Current LED Driver
You could use something like the below, which requires less than 60mV of headroom. Sense voltage is only 50mV so the offset voltage of the op-amp will tend to greatly affect the current. The LED could ...
- 331k
9
votes
Accepted
can leds take varying voltage > listed voltage, with constant current?
The driver is inappropriate for the LED because the minimum voltage from the driver (18V) is greater than the minimum LED voltage at 600mA (3V). The driver is likely designed for LED arrays that have ...
- 331k
9
votes
Accepted
LM317 intermittent current source
To drive 50 mA through 330 ohms, you need 16.5 V.
For 50 mA through 1330 ohms you need 66.5 V.
In the constant current mode, the maximum output voltage of the LM317 circuit is \$V_{in} - 3\ {\rm V} ...
- 121k
8
votes
Accepted
Why can't electronic systems use constant current power source and vary the voltage as power consumption varies?
Most power sources are constant voltage, and not constant current. If you take the two main sources of electrical energy, which are batteries and rotating generators(regardless of size), the one thing ...
8
votes
Accepted
Precision circuit that is CC and CV circuit or power supply
If you want precision, then your CC source doesn't cut it, what with transistor alpha an' all.
The classic way to do this is with two loops
simulate this circuit – Schematic created using ...
- 141k
8
votes
Op-Amp output not stabilizing in precision current sink
Do this to isolate the heavy load of the gate from the AC feedback path of the op-amp. The DC feedback path remains accurate.
simulate this circuit – Schematic created using CircuitLab
- 331k
8
votes
Accepted
Constant voltage charging phase of Li-ion batteries
I have not yet been able to find a good visual representation of the
saturation stage of the charging and how it is different from the
constant current stage of charging. Does anyone have/know of a ...
- 51.4k
7
votes
How does this constant current sink actually work?
The opamp is acting as a unity-gain buffer, though it may not be obvious:
The rule for opamps is that the output does whatever it has to to keep the two inputs equal, provided that it doesn't clip of ...
- 5,356
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