# Resistors?

Discussion in 'The Projects Forum' started by Gadersd, Jul 9, 2013.

Dec 8, 2012
98
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Things like diodes and transistors need resistors while things like motors and ics don't need resistors. How can I find out if something needs a resistor or not? Whenever I look at data sheets I see the max current. I don't know whether this is the max current it will draw without a resistor or if it is the max current that can sadly pass through it. Where in the data sheets can I find out if it needs a resistor or not?

2. ### MrChips Moderator

Oct 2, 2009
17,603
5,472
There is no rule that says diodes and transistors need resistors.
Show a circuit diagram and then we will say if it is correct or not.

Dec 8, 2012
98
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I want to connect a laser diode to a 2.5v power supply. The data sheet says that it needs a min of 45ma and a max of 60ma. It is a diode so it defiantly needs a resistor, but some electronic components don't need a resistor. They have enough eternal resistance to prevent a short circuit and keep the current from going too high. How would I be able to tell from the data sheet that it has enough external resistance to not need a resistor?

4. ### Ramussons Well-Known Member

May 3, 2013
753
132
Thumb rule:
If the specs define a Max or Min value for a parameter (voltage, current, wattage, temperature, ...), then this parameter is to be controlled externally.

In the present case, the " ...data sheet says that it needs a min of 45ma and a max of 60ma ..." means that the current needs to be externally limited normally by a resistor or current source.

Ramesh

5. ### wayneh Expert

Sep 9, 2010
15,253
5,582
To be honest, it's mostly experience. After a while, you come to "know" and hardly give it any thought. You'll get there.

In the meanwhile and for everyone, a useful way to look at things is to assume everything needs current limiting until you prove otherwise. Any path for current to flow is a potential problem for the components in that path, until you can identify something in the circuit that is restricting current flow to within the specifications of each component, especially the weakest link. ICs that need power are usually meant to be connected directly to the power rails without current limiting. (Their outputs may need protection, but not their power supply.) LEDs need a specific limit on current. Transistors, resistors, diodes and such all have limits on how much current they can take (usually it depends on the power they must dissipate) and you need to make the calculations to be sure you're OK. These components cannot on their own protect themselves.

6. ### MrChips Moderator

Oct 2, 2009
17,603
5,472
Basically, spec sheets alone do not tell you when you need resistors.
Know-how will.
Learn the fundamentals of electronics and you will know the answer.

Dec 8, 2012
98
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Data sheets don't specify the internal resistance do they? To calaculate the current that will pass through it I would need to know the internal resistance.

8. ### Markd77 Senior Member

Sep 7, 2009
2,803
596
For a laser diode you probably need something a bit better than a resistor, especially if you want it to run at the same power all the time. Search for:
laser diode driver circuit
Some of the links will probably explain why it is a good idea.

Last edited: Jul 10, 2013
9. ### crutschow Expert

Mar 14, 2008
20,554
5,825
ICs normally take the current they require without any external impedance required. The current they take is given in the data sheet, sometimes with min and max limits. The internal resistance may be quite non-linear, i.e. the IC will have an operating current that varies only slightly with applied voltage. That's typical of op amps, for example.

If an IC requires an external impedance it should be apparent from the data sheet.