Simple Resistor Question

[Sir TophamHatt]

I have a bag of 1k resistors, 5.6k ohms, and 560ohms.

 

I presume:

5.6k (5600?)

1000k

560k

 

Is the order of resistance? 

 

Would the 560ohm resistors be safe to use with LEDs? I've been using the 1k ones before which are fine.

 

Thanks

[melmerby]

Possibly, it depends on what voltage you are using. On 12v the current would be somewhat higher than you would typically use these days.

(5.6k ,{5600 ohm} is a bit high for series resistance with a LED on 5v or 12v.)

 

Using the 560 ohms with 12v:

For a rough guesstimate a LED drops about 2v - 3v. With 12v than means between 9v & 10v across the resistor.

Using ohm's law Current (I) = Voltage (V) divided by Resistance (R) = 10V/560R = 18mA

[Steven B]

5.6k = 5600

1k = 1000

 

560 Ohm resistors would be OK for 5-6V supplies. I'd be warry about using them for 12V unless you used two in series giving 1120 Ohms.

 

Steven B.

[sharris]

18 minutes ago, Sir TophamHatt said:

I have a bag of 1k resistors, 5.6k ohms, and 560ohms.

 

I presume:

5.6k (5600?)

1000k

560k

 

Is the order of resistance? 

 

Would the 560ohm resistors be safe to use with LEDs? I've been using the 1k ones before which are fine.

 

Thanks

 

In descending order...

5.6k          = 5600ohms

1k             = 1000ohms

560ohms = 560 ohms

 

(just to clarify as you have a few errant 'k's in your list). 

 

Whether they are safe to use depends on the supply voltage and the forward voltage and current rating of the LEDs. 

Your LEDs will probably have a voltage drop of around 1.8V-3.5V depending on the colour and chemistry used to obtain that colour (red generally has the lowest forward voltage, blue and white the highest).

 

Subtract the forward voltage for your LEDs from the supply voltage, divide what's left by the current the LEDs need, and that will tell you the resistor to use.

 

If the 1k resistors were fine before, why not use them again? 

 

 

 

 

[Sir TophamHatt]

Thanks.

I'll leave the 560ohms ones at the bottom of the tub then!

[kevinlms]

18 minutes ago, Sir TophamHatt said:

Thanks.

I'll leave the 560ohms ones at the bottom of the tub then!

You could try using them in a series pair 1.2k ohms. You probably won't notice any difference in brightness, but if you don't like it, put them back in your tub. It won't cost you anything to try & no risk of blowing the LEDs up.

[Sir TophamHatt]

3 minutes ago, kevinlms said:

You could try using them in a series pair 1.2k ohms. You probably won't notice any difference in brightness, but if you don't like it, put them back in your tub. It won't cost you anything to try & no risk of blowing the LEDs up.

True.

So I'd go Decoder > Resistor > Resistor > LED > Decoder ?

 

Could I go:

 

Decoder > Resistor > LED (1) > LED (2) > Decoder ? (in series)

 

I guess it's just a case of being able to use them up.

[kevinlms]

7 minutes ago, Sir TophamHatt said:

True.

So I'd go Decoder > Resistor > Resistor > LED > Decoder ?

 

Could I go:

 

Decoder > Resistor > LED (1) > LED (2) > Decoder ? (in series)

 

I guess it's just a case of being able to use them up.

It shouldn't make any difference. A question of what actually fits best. Watchout for shorts and use heat shrink, once you know it works.

[Sir TophamHatt]

12 minutes ago, kevinlms said:

A question of what actually fits best.

 

True.

Although the current project is seeing three white LEDs and two reds at one end of the loco (same on the other) anyway - all have their own resistors.

 

Might be a case of leaving it for a loco with loads of room.

 

Cheers!

[kevinlms]

29 minutes ago, Sir TophamHatt said:

True.

So I'd go Decoder > Resistor > Resistor > LED > Decoder ?

 

Could I go:

 

Decoder > Resistor > LED (1) > LED (2) > Decoder ? (in series)

 

I guess it's just a case of being able to use them up.

I see that you're now talking DCC in the non-DCC section. You could end up with misleading/unsuitable answers.

[Junctionmad]

for 3mm leds , on 5V I regularly use 1.8KOhms , as I find modern LEDs only need about 1-2mA these days 

[Nigelcliffe]

Don't worry about reusing resistors, or daisy chaining them together. Just buy the right size needed - both resistance (ohms) and physical size.

 

From the first electronics supplier I looked up, UK based, including VAT:

A standard carbon film resistor is less than 1p each in packs of 100.

A metal film resistor (smaller than carbon film) is under 2p for pack of 100.

SMD resistors in "805" size (medium as SMD stuff goes) are under 0.5p each in pack of 100.

 

Some years ago, I bought a mixed (labelled bags) pack of metal film resistors for a few pounds.  That gives me a wide range to select from for a very low outlay.  The bags are all clearly labelled, just a matter of storing them sensibly so finding the right bag doesn't take ages.  

 

- Nigel

 

 

[sharris]

3 hours ago, kevinlms said:

It shouldn't make any difference. A question of what actually fits best. Watchout for shorts and use heat shrink, once you know it works.

 

It will make a difference if the LEDs are different types with different current requirements (e.g. a high efficiency LED with forward current of 3mA won't match well in series with an ye olde-worlde LED with a forward current of 20mA - you'll either blow one, or have the other hardly lit). It's usually best to run the LEDs in parallel, each with its own resistor, unless you know they're all well matched, in which case you can run them in series with a smaller resistor. One thing never to do is run them in parallel with them all sharing the same resistor - a slight variation in forward voltage between them can have one hogging all the current. 

 

 

[kevinlms]

1 hour ago, sharris said:

 

It will make a difference if the LEDs are different types with different current requirements (e.g. a high efficiency LED with forward current of 3mA won't match well in series with an ye olde-worlde LED with a forward current of 20mA - you'll either blow one, or have the other hardly lit). It's usually best to run the LEDs in parallel, each with its own resistor, unless you know they're all well matched, in which case you can run them in series with a smaller resistor. One thing never to do is run them in parallel with them all sharing the same resistor - a slight variation in forward voltage between them can have one hogging all the current. 

 

 

The answer was given for using two resistors in series, as basically the equivalent of a 1k resistor. I made no mention of using different type of LED's.

 

Agreed about never using parallel LED's with a common resistor.

[melmerby]

5 hours ago, Sir TophamHatt said:

True.

So I'd go Decoder > Resistor > Resistor > LED > Decoder ?

 

Could I go:

 

Decoder > Resistor > LED (1) > LED (2) > Decoder ? (in series)

 

I guess it's just a case of being able to use them up.

If you are talking DCC decoders the lighting ouputs are typically around 12v - 15v depending on track voltage (Lenz quote 16v as a guide)

That means you need higher value resistors than if using 5V

Using the function output of a decoder I would be looking to use at least 2.2k ohms with modern LEDs

Those 5600 (5.6k) resistors might be usable if the LEDs are low enough current, depending how bright you want them

Have you got a DC supply of 12v-15v that you could use to set the right value (before you commit to installing them)?

 

You should only use LEDs in series if they are either the same type or have been current matched.

Best practice is a series resistor for each LED
 

[sharris]

1 hour ago, kevinlms said:

The answer was given for using two resistors in series, as basically the equivalent of a 1k resistor. I made no mention of using different type of LED's.

 

Agreed about never using parallel LED's with a common resistor.

 

It wasn't clear as the question was in two parts - 1 with two resistors, one with 2 LEDs

[DCB]

Usual problem with sizing resistors is not checking what the actual input voltage is not the nominal.   Quite often model railway 12 volts is 12 VA with over 20 volts off load but 12 volts at 1 amp,  which makes sizing a resistor for 12 volts a bit difficult as the more LEDs you add the dimmer they get.  I start with a 3 volt supply and  I drive people mad by not using resistors on my orange and yellow building lighting old school LEDs only on the greens and especially the reds which are much brighter for any given voltage than the green.

I bought a Poundland string of LEDs with 2AA batteries and no resistors which work well.  Don't ever use rechargable batteries with LEDs, they can make wires glow orange and set the insulation on thin wires on fire if shorted, 

Edited June 2, 2019 by DavidCBroad

[melmerby]

5 hours ago, DavidCBroad said:

 I drive people mad by not using resistors on my orange and yellow building lighting old school LEDs only on the greens and especially the reds which are much brighter for any given voltage than the green.

I bought a Poundland string of LEDs with 2AA batteries and no resistors which work well. 

And that sort of reply is not helpful, you can do whatever you like but don't suggest it to other people because it is just wrong and can cause problems.

 

[Dave John]

I'd always have a resistor for each LED. The 560 ohm ones will protect the LED from damage, but they will be very bright. 

 

I purchased a couple of dozen cheap pwm motor controls. 

 

https://www.ebay.co.uk/i/332412126587?chn=ps

 

Built in switch and they are excellent for dimming LEDS and mini incandescent lamps. 

 

Thing is you want LEDS to be a bit brighter if you are lighting the layout strongly for a photo , but dimmer for normal viewing. These sort the problem for not much more than the price of a switch on its own. 

 

 

 

 

 

[DCB]

5 minutes ago, melmerby said:

And that sort of reply is not helpful, you can do whatever you like but don't suggest it to other people because it is just wrong and can cause problems.

 

Your reply is also unhelpful.    If the Vf of the LED is less than the supply voltage and the resulting brightness its not too bright for your taste  you don't need resistors.  Don't perpetuate the myth that you must use resistors with LEDs per se.  The world has moved on.

[kevinlms]

3 minutes ago, DavidCBroad said:

Your reply is also unhelpful.    If the Vf of the LED is less than the supply voltage and the resulting brightness its not too bright for your taste  you don't need resistors.  Don't perpetuate the myth that you must use resistors with LEDs per se.  The world has moved on.

How is it 'moving on' to not use resisters?

 

I can see that in special circumstances (a battery powered device, where extra voltage CANNOT be supplied), it may not be entirely essential, but for most cases it is recommended.

[melmerby]

2 hours ago, DavidCBroad said:

Your reply is also unhelpful.    If the Vf of the LED is less than the supply voltage and the resulting brightness its not too bright for your taste  you don't need resistors.  Don't perpetuate the myth that you must use resistors with LEDs per se.  The world has moved on.

If the Vf is less than the supply voltage you need a resistor. No ifs no buts.

[AndyID]

2 hours ago, melmerby said:

If the Vf is less than the supply voltage you need a resistor. No ifs no buts.

 

All diodes have a resistive component. The forward voltage increases with current so you can get away with omitting a resistor if you match the source voltage very carefully and/or use a high impedance voltage source.

 

But it's still not a good idea to do that.

[Tricky Dicky]

6 hours ago, DavidCBroad said:

Your reply is also unhelpful.    If the Vf of the LED is less than the supply voltage and the resulting brightness its not too bright for your taste  you don't need resistors.  Don't perpetuate the myth that you must use resistors with LEDs per se.  The world has moved on.

 

Here we go again! I am surprised we have got so far into 2019 before this old chestnut has surfaced. I cannot believe we are arguing over a component that costs 1p requires a few seconds soldering and ensures a circuit is correct!

 

Richard

 

[melmerby]

1 hour ago, AndyID said:

 

All diodes have a resistive component. The forward voltage increases with current so you can get away with omitting a resistor if you match the source voltage very carefully and/or use a high impedance voltage source.

 

But it's still not a good idea to do that.

A LED has a voltage/current curve in the usable area similar to (but less well defined than) a zener diode.

For most purposes it can be assumed to be essentally flat so the curent flow (and hence brightness) is defined by the series resistor whilst the voltage across the LED only varies slightly.

 

If you are using cheapo non linear LEDs and/or pound shop batteries (with chemically challenged filling) you can no doubt under certain circumstances just run them off a battery. The series resistor is the internal resistance of the battery.

I had one of these cheapo led strings and a 2xAA battery case but there was a resistor hidden in the wiring.

Edited June 2, 2019 by melmerby