Pages: [1]   Go Down

Author Topic: Calculate LED mA meter shunt R  (Read 265 times)

N2EYE

  • Posts: 108
    • HomeURL
Calculate LED mA meter shunt R
« on: July 14, 2022, 07:25:24 AM »

hello eham community.
I'm trying to replace a current meter with a string of 3 green, 24v LED's.
Ran out of space to mount another panel meter.

The purpose is to monitor Control Grid Current on a Tetrode running grid-driven w/DC grounded kathode.  I only need a general indicator of current as it should be nearly zero at all times.  I estimate 20mA would be equal to "full scale deflection" bright warning light to shut down.

Using an  online Current Limiting Resistance calculator, I arrive at a 3w 3.3k resistor.  So far so good but what about a parallel Shunt Resistor?  All the Control Grid current cannot go through the LED's, right? At least it sounds like a bad idea, I think.  IDK, never did this before and LED's are new to  me.

It would be prudent to add a Resistor in parallel with the LED string.  Anyone have a suggestion or comment?

Neil NYC
Logged

W9IQ

  • Member
  • Posts: 8866
Re: Calculate LED mA meter shunt R
« Reply #1 on: July 14, 2022, 07:42:43 AM »

Neil,

The concept sounds problematic but let's explore it a bit.

You say you have 24 volt green LEDs. That sounds like they have onboard current limiting. Do you have a link to the LEDs?

Is the grid current that you wish to monitor DC or RF?

- Glenn W9IQ
Logged
- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

N2EYE

  • Posts: 108
    • HomeURL
Re: Calculate LED mA meter shunt R
« Reply #2 on: July 14, 2022, 10:08:33 AM »

Thank you for your reply,
I do not have any data for the LED's. I ordered them from Surplus Sales of Nebraska a few years ago.  labeled "24vdc"  with polarity markings about 1.75" long silver cartridge panel mount, nice solid lens.  NOS. 

The current is DC and comes from the G1 Meter output on  the G3SEK Tetrode PCB.  I left the shunt out of the PCB.  Max grid voltage is -68vdc

I saw an LED used on a Pride Tetrode amp for this purpose I think that is where I got the idea.  Already have 3 panel meters with one doing double duty for Plate/Screen V (switched).  Not much  real estate left on the front panel so a string of 3 LED's fit nice.

I thought I'd run a parallel low resistance resistor on the way to the tube grid like metering shunt.  I had used 2 of these LEDs in a homebrew antenna-mast  relay box as a diode across the relay terminals.  They illuminate without any resistor  when connected to PSU so you may be correct about internal resistor   

neil
« Last Edit: July 14, 2022, 10:17:08 AM by N2EYE »
Logged

W9IQ

  • Member
  • Posts: 8866
Re: Calculate LED mA meter shunt R
« Reply #3 on: July 14, 2022, 11:35:28 AM »

Neil,

Unfortunately, those LEDs cannot be used in their present form since they already are regulating current based on a 24 volt supply. If it contains a simple dropping resistor and you can short it out or remove it, you may be able to make use of them. Otherwise you will need to purchase new "bare" LEDs.

Let me know how you would like to proceed.

- Glenn W9IQ
Logged
- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

W9IQ

  • Member
  • Posts: 8866
Re: Calculate LED mA meter shunt R
« Reply #4 on: July 14, 2022, 12:24:21 PM »

Neil,

After spending a little time looking at that supervisory board, I don't think the LED concept is easily incorporated.

You will need to install a shunt resistor (R1) in order for the grid over-current fault circuit to work. That shunt resistor is protected by two shunt diodes that will limit the maximum shunt voltage to around 1.2 volts. An LED is a current operated device but it will develop an ~2 volt drop over its usable current range. This means that the LED cannot be placed in series or in parallel with the shunt.

It would be possible to modify the circuit but I am not sure how far you would like to take this concept. It would involve adding more shunt diodes and possibly changing one pot.

- Glenn W9IQ
« Last Edit: July 14, 2022, 12:43:03 PM by W9IQ »
Logged
- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

N2EYE

  • Posts: 108
    • HomeURL
Re: Calculate LED mA meter shunt R
« Reply #5 on: July 15, 2022, 08:04:01 AM »

Thank you for being direct, you saved me time and effort at the bench. I'll have to find a small and traditional 10mA meter to fit somewhere.   'Back to the drawing board',  Thank you again.
neil NYC 
Logged

N2EYE

  • Posts: 108
    • HomeURL
Re: Calculate LED mA meter shunt R
« Reply #6 on: July 15, 2022, 08:44:34 AM »

Found a very small 20mA panel meter that will fit so I can conform to the original Tetrode Board plans.

Thanks again
neil , NYC
Logged
Pages: [1]   Go Up