Power Supply Meter Problem

[woody3]

Hello - this thread has gone very quiet - everybody soldering??
Some help ,please:
While waiting for my main PSU pcb to arrive I've been putting together an auxillary psu for my two LED voltage and current meters and a 12v supply for a fan I want to put in the PSU box. I've used two 7805s off the same tx which gives me 5v fixed and a 7805 with a divider which gave me 5- 14 volts which I've fixed at 12v by a preset. The two meters work fine on test except when I run them together. Voltmeter display drops to 0.3v (actual volts remain at 12) and current display fluctuates reading 3-4 amps (which is not right for a 250mA tx! Can someone tell me what will fix it.
Othe Q is that the little tx runs Very hot. Meters draw 40 mA each and fan draws 110 mA - so less than 200 all up. Do these little transformers run hot anyway or am I heading for a small bonfire.
Thanks
Woody

 

[audioguru2]

Hi Woody,
You're overloading the little transformer with all the LEDs in your meters. If you have two 3-digit meters they have a total of up to 34 LEDs that could be lighted when displaying 1.88. Assuming a not very bright current of 10mA for each LED, their total current is 340mA! Plus current for all the other stuff.
If the transformer is a DC adapter, its little filter cap must have a lot of ripple, going right through the regulators and affecting the meters' logic.
Maybe the meters draw 40mA each when not lighting. If that's their operating current, then the total transformer current is 540mA.
You're darn right you'll have a bonfire!
Use a more powerful adapter or LCD meters.

 

[woody3]

Hi Audioguru
Thank you for the reply.
The LED meters each draw approx 41mA (measured) when lit (3 1/2 digit) and functioning. I'm using a 4,700uF cap for smoothing. Fan is drawing 110mA making a total of 190mA. Total is well within tx spec.
Any advice on the 'mutual interference' of the two meters? ???
Thanks
Woody

 

[audioguru2]

Hi Woody,
You are correct. I checked the LED current for new LED digital meters and it is spec'd at 41mA, but they don't say how many segments are lighted. Therefore your little transformer is operating close to its limit. I have found that very small transformers are very inefficient, not enough wire I suppose. I have a little 9VDC/200mA adapter here that is hot without a load. Its load calculates as 1.8W, but its input is rated at 4.4W. Should I fully load it to see if it smokes? You might get some smoke too. Or a bonfire! ;D

What are you measuring with the meters? Many meters aren't allowed to measure their own supply voltage because then their input exceeds their common-mode limit. They are only supposed to measure the difference between a voltage source's terminals (or current, with a shunt) that is isolated from their supply, which happens when they are powered by a separate battery. ???

 

[woody3]

Hi Audioguru
I'm measuring voltage of a separate (different) PSU output at its panel terminals and current being drawn from the same source to run the fan - just to give me a measurable load. Meters are powered by the little PSU at 5v. So I'm not trying to get the meters to measure their own circuit.
Woody

PS You are also right - this little tx gets quite warm with no load! (I do have a LED at the output to remind me its on - but thats only drawing 8mA).

 

[audioguru2]

Hi Woody,
Your meters' displays are probably multiplexed and might be radiating RF interference to each other. Try small capacitors across their inputs and/or a grounded shield between them.
Long ago there was a multiplexed display IC that had the rising and falling edges of the LED driver pulses slowed down to avoid interference, the same method that many modems use.

 

[MP1]

Woody,
How about a drawing for those of us who read schematics a little better than words? ;D

MP

 

[woody3]

Hi MP/Audioguru
Not exactly a scematic but hope it serves the purpose
Woody

 

[woody3]

Sorry - trying again
woody

View attachment 36555

 

[MP1]

I am not sure if I am looking at this right. You should isolate the 12 volt supplies if I am looking at this correctly. Check to see how much voltage is actually getting to each display under the test condition. It looks out of balance.

MP

 

[woody3]

Hi MP
Thanks for the reply. The meters need a 5v supply to function. The other psu is purely serving as a source of an imput to be measured. What about putting a link between the -ve 5v supply and the -ve input? as per attached?
Sorry to be a pain but I need these working for my new 0-30v psu now under construction.
Woody

View attachment 36561

 

[audioguru2]

Hi Woody,
Don't link the input to its own supply.
Have you asked the meters' manufacturer what they recommend?

 

[woody3]

Hi Audioguru
Thanks for the response. Problem is I bought them from a seller on eBay located in California who seems to have got them from a Chinese? source. - and I'm in the UK! > That having been said they seem good quality.
You are right there is RF flying around as I get a fluctuating display on the current meter with only one input lead connected.
Re your earlier post what value caps do you advise - 100pF or similar?
Thank you for all the help
Woody

 

[audioguru2]

Hi Woody,
Since when is Chinese stuff bought at e-Bay good quality?
You aren't measuring AC at high frequencies so try 0.01uF ceramic caps across the inputs right at the meters.

 

[MP1]

Woody, as I had guessed, I was not reading your diagram as you had intended.

You should perform a search on the web for the data sheet of the devices you are using. To comment further, I would only be guessing like audioguru, without the data sheet.

By the way, I have also found high quality stuff made in China. The bad name for Chinese equipment was caused by the quality of merchandise purchased by the importers. Their profit margin was greater on the junk. They are the ones who should be faulted for the quality, not the Chinese manufacturers.
As a frequent International traveler, I have seen this with other countries as well.

MP

 

[audioguru2]

The bad name for Chinese equipment was caused by the quality of merchandise purchased by the importers. Their profit margin was greater on the junk. They are the ones who should be faulted for the quality, not the Chinese manufacturers.
MP

Yeah, but the importers sell the junk on e-Bay.
What about the counterfeit semiconductors that come from China?
Remember a few years ago when Chinese (made in Taiwan, actually) faulty electrolytic capacitors caused grief to many manufacturers of equipment?
Recently I bought 20 Chinese ultra-bright LEDs from my local parts store. 10 of them were no good! Their replacements were from India and have excellent quality.