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LED Red to Green CB Hi Cap

Stew B

Stew B

Doh !
Help !

I have a very basic knowledge of electrics and soldering skills. I was trying to change the Red LED on my CB Hi Cap to Green. I cannot get the Green LED to light.

Supply voltage to the resistor appears to be 25.6 volts and for the Red LED it is using a 3K3 resistor measured at 3.28 kOhms on my basic meter.

The Red Led Diode measurement said 1.55V though I cannot get my meter to measure this anymore for some reason, the Green LED said 1.9V. I also measured a voltage on the Red LED of 1.66V but god knows how I did that.

So I am confused in that to get more than 1.55 Volts (1.66V measured) through the original Red LED using a 3K3 resistor then the current is only 7.3 mA ??

From looking at past posts it says for a Green LED I would need approx 2V at around 20mA ? Is this correct? Therefore to get a 23.6 V drop (25.6 - 2) at 20mA would need a 1.18kOhm resistor or for 7.3mA then 3.23kOhm resistor.

When I just change the Green for Red LED using the3.3kOhm resistor it doesn't light up, I had a 3.2kOhm resistor but it still doesn't light using that. Not having an endless supply of resistors I wondered what I might actually need as I cannot seem to measure the amps flowing so have no clue whether is should be 7.3mA or 20mA.

So I am confused and not sure what to do.

Any advice would be appreciated thanks
 
if you have very basic knowledge of electrics and soldering you should not be doing this.

Rgds
Stuart
 
I assume you've got the LED the right way around? 3k3 should do the trick, it has for me in the past when I've done the same swap. 7.3mA should be enough to light the LED, do you have a part number of the LED? Manufacturer's number or supplier's stock number would suffice.
 
If you were measuring across the LED then now you could either have a dry joint/open circuit or the LED has gone short circuit.

If you have a diode test option on your meter try that on the LED (with the power off) as there is normally enough voltage coming out of the meter to make it glow weakly (when the right way around).

Can you post the part number of the new LED so that we can take a look at the datasheet?

In the past I have more than quadrupled the ‘required’ in line resistor to adjust the intensity so any close value should be OK.
 
Check your wiring and make sure the LED is wired in the right way around. I would expect the 3.3kohm resistor to do the job assuming that you are using a standard indicator type green LED. The LED should have a small flat on the side of the LED body, the leg nearest to the flat should be taken to the (-) supply and the other leg to the resistor and the resistor to the positive supply (25.6V)
 
Th RS components part number is 228-6004 and yes the inline resistor is wired to the anode and the cathode is wired to te negative, I have tried it both ways to make sure and the cathode does have the flat side of the LED bulb. When I try the meter across the LED on it's own it does light up dimley when set to continuity / Diode mode though I do not get a forward voltage reading on the meter. OK it's a cheap meter and may be faulty now.. The RED LED works fine and I changed the existing 3K3 resistor for a new one just in case but it is still the same, the Red LED works fine, nothing from the Green one with either a 3.3kohm or 3.2kohm resistor.
 
I wonder if you've damaged the green led? You say it lights when testing with the diode tester but not in circuit so I wonder if putting pressure on the legs with test probes makes it work but not when in circuit. I can't think of any other reason why it works 'under test' but not 'in-circuit'. With about 24v across the resistor, 22k gives 1mA which will be quite dim. 3k3 gives you about 7mA and 1k2 would give 20mA. Depends how bright you want it TBH.
 
The LED in Hicaps is driven from the raw rectified supply. 3k3 is the usual dropper resistance, approx 10mA of current.
Beware that I've seen some LEDs where the cathode is the long lead rather than the anode. The lead next to the flat edge is always the cathode.
 
I took a look at the datasheet and 1.18K Ohms is correct by my calculation too.

“The Red Led Diode measurement said 1.55V though I cannot get my meter to measure this anymore for some reason”

Without a working meter it’s going to be impossible to try to work out what is going on. Can you use it to measure the voltage of a battery?

If that works I would check the voltage from the supply side of the resistor to the cathode of the LED. If that shows a voltage then measure the voltage across the resistor and across the LED. If there is no voltage across the resistor then you have an open circuit somewhere (cracked track?).
 
Stew B said:
the inline resistor is wired to the anode and the cathode is wired to te negative, I have tried it both ways to make sure and the cathode does have the flat side of the LED bulb.
Click to expand...

It's worth bearing in mind that in the Hicap, the voltage for the LED is derived from the -ve terminal of one of the bridge rectifiers so the driving voltage is negative wrt Ov. In this particular case that means the anode goes towards the 0v line and the cathode goes towards the rectifier (-ve). As you say, you've tried it both ways so maybe not the issue in this case.
 
Thanks for your help guys, I did get a 1.2kOhm resistior yesterday and will try it out at the weekend. I did connect the Green LED up to a 6V DC supply and used a 220 ohm resistor to drop voltage to the 2V forward voltage of the LED and it lit up fine so will have a play again and hope it works out with the 1.2k Ohm resistor on the Hi Cap. The original RED LED is connected the same was as I did with the Green i.e. inline resistor to the Anode as the cathode leg does have the flat on the LED body. Well i am sure that is correct, but no harm in checking it all again carefully. Laugh is when I reconnect the RED LED with either the old or the now new 3K3 resistor fitted it lights up fine. I am a mechanical design engineer by profession, I did do electrical engineering as part of my ONC many many years ago and now know why I did Mechanical only for my HNC. I can and have rewired house electrics with success but that is now restricted to what you can and cant do, or be bothered with, but electronics still baffle me past the basic Ohms law stuff etc. It's stuff like the -ve you get from the bridge rectifier wrt 0v that I cant get my head round, I can solder in through the hole resistors into circuit boards and inline like this but stop well short of complete circuit boards like some of you can do.
 
Forget the exact value of the resistor! It couldn't be less critical! There is something odd like like a bad solder joint etc here as it should light even with 100K resistor (dimly!)
 
Just FYI - I own a CB NAP140 with an original green LED, and a CB Hicap and CB NAP32.5 both with red LEDs. I changed the Hicap from red to green and matched it with the NAP140, here is what I used for Hicap raw DC supply (LED supply) - 25.4VDC

LED that I used which is a good color match with the NAP140 - Broadcom HLMP-3507 (4.7mcd, 2.1V, 10mA, 569nM)
https://canada.newark.com/broadcom/...n-4-2mcd-569nm/dp/40K0061?ICID=I-RP-STM7REC-1
Resistor to match intensity with NAP140 - 27K ohm 1/2W
.
That being said the NAP140 is 35 years old so possibly the LED in it might have faded with age, or LEDs back then just weren't as bright, also I prefer the LEDs to be dimmer so they are not distracting. The new green LED I used was only 4.7mcd at rated current and much too bright compared to the NAP140 green LED with the 3K3 resistor, with 27K it mathced the NAP140 perfectly and also the rough intensity of the original red LED.
To come up with this R value I just used a 50K trimpot which I had lying around and aligator clip test leads on the green LED and adjusted the pot to taste, then soldered in a fixed resistor - simples.I didn't bother to measure V across resistor or LED because it's basically irrelevant.
NAC32.5 I used same green LED and 27K resistor, supply was 24.15VDC.
On both new green LEDs current was just under 1mA to get my preferred intensity.
 
Last edited:
Stew B said:
Thanks for your help guys, I did get a 1.2kOhm resistior yesterday and will try it out at the weekend. I did connect the Green LED up to a 6V DC supply and used a 220 ohm resistor to drop voltage to the 2V forward voltage of the LED and it lit up fine so will have a play again and hope it works out with the 1.2k Ohm resistor on the Hi Cap. The original RED LED is connected the same was as I did with the Green i.e. inline resistor to the Anode as the cathode leg does have the flat on the LED body. Well i am sure that is correct, but no harm in checking it all again carefully. Laugh is when I reconnect the RED LED with either the old or the now new 3K3 resistor fitted it lights up fine. I am a mechanical design engineer by profession, I did do electrical engineering as part of my ONC many many years ago and now know why I did Mechanical only for my HNC. I can and have rewired house electrics with success but that is now restricted to what you can and cant do, or be bothered with, but electronics still baffle me past the basic Ohms law stuff etc. It's stuff like the -ve you get from the bridge rectifier wrt 0v that I cant get my head round, I can solder in through the hole resistors into circuit boards and inline like this but stop well short of complete circuit boards like some of you can do.
Click to expand...

I think you might find that will be much too bright with a 1.2K resistor (20mcd at 10mA). Also just FYI on my CB hicap the resistor is connected to the cathode (black wire), and the anode (gray wire) goes straight to the 0V link bar on the smoothing caps.
As for the LED/resistor supply voltage just remember in this case it's all relative, if you get -25.6VDC on the rectifier then 0VDC is +25.6VDC higher than the rectifier negative so the LED/resistor supply is 25.6VDC and the anode is connected to the higher voltage (in this case the 0VDC wrt GND) and the cathode connected to the lower voltage (in this case the negative -25.6VDC o/p of the rectifier). Simply put for a LED providing you have enough relative voltage to forward bias the LED (ie. turn it on = 2.2VDC for your LED) the series resistor is used to adjust the current which adjusts the brightness of the LED.
 
Well I started again today and now it works, I resoldered all the joints and made sure the cathode was connected to the resistor in the black wire feed as Loz828 said above. Checking the red LED this was how it was wired too. So I must be stupid and got it the wrong way round, but I am sure I tried it both ways before. Anyway maybe I was wrong and that will teach me to be more thorough. So happy man, the 3k3 resistor works fine.
I have made a new stick on front panel to make a hybrid Olive Hi Cap to match the 102/180. Only had the CB HiCap serviced last year by Class A so didn’t really wanted to have to find an Olive one and have that serviced again. It looks fine to me and I am happy with it.

 
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