Please advise or Spec me a circuit...

Audioguru

Joined Dec 20, 2007
11,248
Guys, the LEDs I have, have a Vf of 3.2-3.8. I have based my wiring schematic on 3.8. Is that the best thing to do?
Then when the Vf of the LEDs is the lowest value (3.2V) they might burn out.
Calculate the current when the Vf is the highest and the lowest. then select a resistance that does not overload the LEDs but allows enough brightness.

I also got a laptop power brick which is DC 19V 65W. If I divide 19v by 3.8 (Vf) it comes back with a straight 5. The LED wizards out there all say this is fine with a 1ohm resistor. Is the resistor even necessary?
The Wizards are VERY STUPID!
You need a fairly high value resistor because the "19V" supply might actually be 18V or it might actually be 21V. Also the "3.8V" LEDs might actually all be only 3.2V.

Example:
21V supply, five 3.2V LEDs in series, 1 ohm current-limiting resistor.
The total voltage needed by the LEDs is 5 x 3.2V= 16V.
The 1 ohm resistor has a voltage of 21V - 16V= 5V across it.
Then the current in the LEDs is 5V/1 ohm= 5A!
If the "19V" supply is actually only 18V then five 3.8V LEDs will barely light up.

Are you going to measure the voltage of all your LEDs then throw away the hundreds that are not 3.8V? Maybe none will be 3.8V.
Design a circuit that works with ANY spec.

The circuit I've drawn up has a series of 4 LEDs with a 150 ohm resistor. Is this better?
1) 21V supply, four 3.2V LEDs, 150 ohm current-limiting resistor.
The total voltage needed by the LEDs is 4 x 3.2V= 12.8V.
The 150 ohm resistor has a voltage of 21V - 12.8V= 8.2V across it.
Then the current in the LEDs is 8.2V/150 ohms= 55mA.

Aren't your LEDs supposed to use only 20mA and might burn out if the current is more than 30mA?

Then a resistor value of 8.2V/28mA= 300 ohms should be used. It heats with (8.2V squared)/150 ohms= 0.22W so use a 1/2W resistor.

Now what happens if the supply is actually 18V and the LEDs are actually 3.8V?
2) 18V supply, four 3.8V LEDs, 300 ohm current-limiting resistor.
The total voltage needed by the LEDs is 4 x 3.8V= 15.2V.
The 300 ohm resistor has a voltage of 18V - 15.2V= 2.8V across it.
Then the current in the LEDs is 2.8V/300 ohms= 9.3mA.
Will that be bright enough?
 

Thread Starter

dbov22

Joined Aug 28, 2012
24
aw no...
Design a circuit that works with ANY spec.
I actually for the first time in my life understand what you're saying BUT dont understand how to select the correct components to make it all work.

Here is what i have just ordered (resistors wise)

470 ohm 1/2 watt x 100
82 ohm 1/4 watt x100
150 ohm 1/4 watt x 100

I put a value of 28mA into wizard thinking that the LED's would be brighter than if I allowed just 20mA to them.

I am not too bothered about some LED's being slightly brighter than others, just more concerned with safety and lasting a decent amount of time.

Also, this is the power supply - is it OK in everyone's oppinion and this is my schematic. I plan on having one power supply split into 7 different/separate DC female connectors then I can just plug in different panels as and when i need them. Hope this theory works!


By squeekybean at 2012-09-01


By squeekybean at 2012-09-01

could I put 2x 150ohm resitors in series to = 300ohm?
 

KJ6EAD

Joined Apr 30, 2011
1,581
That 19V supply will not drift more than a couple of tenths of a Volt since it's a well-regulated switcher so you can use 19V in calculations. 20mA is a good target current for maximum life and brightness. If you use the average Vf (3.5) for calculation you get 250 for a string of 4 LEDs but the nearest higher value is 270Ω. If you calculate the range of potential LED currents with 19V, 270Ω and Vf from 3.2 to 3.8, you get 14mA to 23mA. The current will be 18mA in most cases.
 

elec_mech

Joined Nov 12, 2008
1,500
For numerous reasons Audioguru mentioned, it is never a good idea to not use a current-limiting resistor, regardless of what the math tells you. It is also not a good idea to use a tiny resistance value either. Always give yourself a buffer, in other words, if x LEDs in series equals the power supply voltage, use x-1 LEDs so you can figure in a good current-limiting resistor. Also note, most resistors have 1% to 5% tolerance, meaning their values will differ in reality, even from each other.

I can't say it better than KJ6EAD did. If you want to really nail it down, follow KJ6EAD's advice and then measure the current going through one of the series sets of LEDs. This will help you determine if the resistor is too big or small and help to safely determine the maximum current and thus brightness you can get out of your LEDs.

If you've never taken a current measurement before, post a picture of your meter or tell us the make and model. It isn't hard to take current measurements, but it is easy to damage your meter or circuit if you don't know what you are doing. With the make and model, I can show you how to do this.
 

Thread Starter

dbov22

Joined Aug 28, 2012
24
Good, well at least this is starting to sink in as I knew it wasn't good to use no resistors or a low value.

I have never measured the current across before so here is my multimeter.

Also, Should the resistors go before the +ve end of the LEDs or at the end of the series attached to the -ve leg. I've seen it done a few different ways so unsure whether it matters.

I have ordered lots of LEDs and resistors and some dc connectors so will be building soon!

Anyway, here is my meter....

 

elec_mech

Joined Nov 12, 2008
1,500
Here you go. Be sure power supply is off until meter is hooked up and set correctly. Let me know if you have any questions.

Also, Should the resistors go before the +ve end of the LEDs or at the end of the series attached to the -ve leg. I've seen it done a few different ways so unsure whether it matters.
It does not matter, you can place them on the anode side (+ve) or the cathode side (-ve).
 

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Thread Starter

dbov22

Joined Aug 28, 2012
24
Thanks a lot elec mech! You couldn't have made that easier if you tried! I appreciate it man! Most of the goodies have just arrived, just waiting for the new resistors to arrive and all my LEDs and I'll be ready to start soldering again!
 

KJ6EAD

Joined Apr 30, 2011
1,581
Another note on measuring current with your meter. If the value of the current is potentially above 200mA, start with the meter set up for 10A and then change to the 200mA set up only if the value shown is less than 0.2A (200mA).

Of course, with the present circuit you're expecting no more than 23mA but it's good practice.
 

MrChips

Joined Oct 2, 2009
30,706
Just one other detail. I do not know how the switch on a meter is designed so I avoid turning the switch while the meter is connected.

My recommendation would be:

STEP 1: Connect the probes on the meter to appropriate jack for 200mA or 10A input.
STEP 2: Turn the meter switch to desired 200mA current range or 10A range.
STEP 3: Connect probes in series with circuit
STEP 4: Turn ON power to circuit

To disconnect the meter, reverse the steps.

NEVER switch the meter to a different type of measurement Volts/Current/Resistance while the meter is connected to a live circuit.

When measuring unknown current less than 200mA it is best to start at the highest range (200mA). It is OK to switch to different current range while in circuit.

Edit: Follow what KJ6EAD says above. If you are uncertain if the current is below 0.2A start with the 10A jack and setting.
 
Last edited:

Thread Starter

dbov22

Joined Aug 28, 2012
24
Ah, lads...you're gonna hang your heads down and rub your foreheads here... When I was measuring the voltage in the old circuit I had the probes on the circuit And tried the multimeter on every setting, spinning whilst connected...

Will this have damaged the circuit or the meter or have I been lucky and got away with it? I can buy a new meter but this one seems to work fine...

Also guys questions from this post...

1. If the current measurement is say 20-25 ohms...what do I do with that? Put another resistor in series or replace the resistor with a new one of higher value?

2. Is 20mA to a LED the brightest it will go? And putting 21-30mA is ok but no brighter and only likely to decrease the life of the LED? Or does it make it brighter and decrease its life?

I love this forum!! Thanks also kj6ead and mr chips!
 

elec_mech

Joined Nov 12, 2008
1,500
Thank you KJ6EAD and MrChips for putting in more detail - I'm trying to multi-task, goodness help me, and didn't get into all the important details you highlighted.

Ah, lads...you're gonna hang your heads down and rub your foreheads here... When I was measuring the voltage in the old circuit I had the probes on the circuit And tried the multimeter on every setting, spinning whilst connected...

Will this have damaged the circuit or the meter or have I been lucky and got away with it? I can buy a new meter but this one seems to work fine...
Usually the worst that happens is you blow a fuse inside the meter - assuming the meter has one. Most have two fuses minimum - one for the 200mA range and one for the 10A range. Looking at the manual for your's, you don't have a fuse for the 10A range, but you do have one for the 200mA - a 250mA, 250V fast-blow fuse. You can check the fuse with another meter if you have it (check for continuity or low resistance) or just set the meter to 200mA and have a go at it. If your LEDs don't work with the meter connected in current mode but work with the original circuit, then the fuse probably needs replacement.

If the current measurement is say 20-25 ohms...what do I do with that? Put another resistor in series or replace the resistor with a new one of higher value?
Do you mean mA? Ohms is a resistance measurement. That would be ideal. You could increase the current and brightness by using a slightly smaller resistor value. Post your measurements and we can advise you further.

Is 20mA to a LED the brightest it will go? And putting 21-30mA is ok but no brighter and only likely to decrease the life of the LED? Or does it make it brighter and decrease its life?
Generally speaking, for standard LEDs 20mA continuous is the norm, 30mA is the max. Sometimes you notice a big difference in the light output, sometimes you don't, depends on the LED. I'm not an LED expert, but I would expect the life would drop a little with more current added. However, you're still talking years, so I don't think that is too much of a concern in the 20-30mA range. If you buy some quality LEDs, you can go crazy and add a PWM circuit to pulse a higher current in rapid spurts and get them even brighter. The pulses (on-off-on-off) allow you to safely apply more current without destroying the LED. I would imagine this would shorten the life further, but still, you'd get many years. I wouldn't worry about doing this now, just something to think about in the future if you need more light output.
 

wayneh

Joined Sep 9, 2010
17,496
Will this have damaged the circuit or the meter or have I been lucky and got away with it? I can buy a new meter but this one seems to work fine...
If it works fine, it probably is fine. What you did - if you in fact hit the ammeter setting - was similar to laying a short on your circuit. It may have also survived if the resulting currents did not exceed what the components could survive. No smoke, you may be OK.

1. If the current measurement is say 20-25 ohms...what do I do with that? Put another resistor in series or replace the resistor with a new one of higher value?
Those are electrically equivalent, although 2 resistors can dissipate more power than 1. It's more elegant to use a single, proper resistor.

2. Is 20mA to a LED the brightest it will go? And putting 21-30mA is ok but no brighter and only likely to decrease the life of the LED? Or does it make it brighter and decrease its life?
There might be some incremental brightness increase but it will be hard to see. You'll see a big difference going from 5 to 10mA, and a smaller difference from 15 up to 20mA. IMHO, I never run mine above about 15mA or so. They'll last longer, survive boo-boos better, and are plenty bright for indicators. The only reason to add more is for illumination, like for reading or for a flashlight.
 

Thread Starter

dbov22

Joined Aug 28, 2012
24
Thanks you two!!

Yes, i did mean mA and not ohms...silly me!

Right, are you ready for your next questions? this is potentially a risk...

1. You can all see the 19V DC Power supply i'm using, just so i know what i'm dealing with is this enough to give me a serious electric shock if the ends touched my skin? (i have read the post on this forum and can't work out whether it's deadly). I DO NOT plan on touching it, just in case though.

2. I plan on using a small plastic box and mounting 7 DC connectors into it in order to split the DC 19V PSU into 7 coonectors so I can neatly and quickly unplug each LED panel. Is this OK?

3. The female DC connectors (that will be mounted into box and linked upto DC PSU wires) have 3 legs on the back do they need to be earthed? or are they OK as shown in picture because the PSU is earthed?


By squeekybean at 2012-09-04
 

wayneh

Joined Sep 9, 2010
17,496
1. You can all see the 19V DC Power supply i'm using, just so i know what i'm dealing with is this enough to give me a serious electric shock if the ends touched my skin?
It's not too risky. Touching it with dry fingers, I doubt you could even detect it. Sticking it onto your tongue would be another matter. :eek: It IS enough to do damage IF a solid connection could be made, such as a large conductive pad on either side of the chest of a person with a weak heart. But again, incidental touches of small connectors with dry skin are no worry. I do it all the time.
 

Audioguru

Joined Dec 20, 2007
11,248
19V is a fairly low voltage. Dry skin conducts a low current from a low voltage so you do not feel anything.
But if your skin is soaked in salt water (it conducts electricity well) then 19V will probably give a shock.
 

elec_mech

Joined Nov 12, 2008
1,500
Question 1 has already been answered, so I'll hit the other two.

2. I plan on using a small plastic box and mounting 7 DC connectors into it in order to split the DC 19V PSU into 7 coonectors so I can neatly and quickly unplug each LED panel. Is this OK?
Yup.

3. The female DC connectors (that will be mounted into box and linked upto DC PSU wires) have 3 legs on the back do they need to be earthed? or are they OK as shown in picture because the PSU is earthed?
Sounds like these might have a switch built in. This is common for applications where a circuit receives power from batteries and a line source. The batteries get disconnected when the line source is plugged into the jack to prevent the line source from frying the batteries.

As this is not a concern with your application, you can ignore the extra connection.
 
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