Pages: 1 [2]   Go Down

Author Topic: Need unregulated 5Vdc Help!!  (Read 666 times)

W9IQ

  • Member
  • Posts: 8866
Re: Need unregulated 5Vdc Help!!
« Reply #15 on: June 16, 2022, 04:01:22 PM »

You neglected to say how you plan on reporting/displaying the line voltage based on a 0 to 5 volt signal.

- 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: Need unregulated 5Vdc Help!!
« Reply #16 on: June 16, 2022, 04:55:48 PM »

You neglected to say how you plan on reporting/displaying the line voltage based on a 0 to 5 volt signal.

- Glenn W9IQ

A link to the sensor would be helpful as well.

- 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.

N7EKU

  • Member
  • Posts: 1471
Re: Need unregulated 5Vdc Help!!
« Reply #17 on: June 16, 2022, 05:39:33 PM »

Hi,

Just head down to your local goodwill/salvation army with your DMM in hand and check out what they have.  They usually price them at just a couple bucks.

Or get an old doorbell transformer which will bring the house AC down to a safe level, then you can rectify it and use a resistor divider network to bring the voltage down to the midpoint of you sensor.  You would have to do that anyway with an unregulated wallwart.

73

to all....I'm using this to monitor AC line voltage for sags and swells. It is not a PQ (power quality) monitor....that takes a lot more. Just used for an alert for brown-outs etc. . I was told the sensor is standard 4-20mA loop, but when I got the sensor...it does indeed have the 4-20 input, but ALSO has a 0-5 vdc sense.
I bought a 0-5 vdc to 4-20 ma adapter board on Amazon, which I planned to use...but now that I see the sensor already HAS the 0-5.....I can skip that!!  I grabbed one of the wal warts mentioned here from Jameco. It is advertised as a 5Vdc UNREGULATED supply.  That would be PERFECT!!!!   Got it today, but checked it out...it's pretty darn well regulated!   shows 5.023 volts from 80-130 ac volts input.  RATS!!!  So, I'm back to my original idea of using the AC wal wart...a diode and cap and a little box, to get me my crappy 5 volts unregulated for the 0-5 volt sensor.  Then the application lets you match up, for example 5 volts is 130, 4 volts is 120, 3 volts is 110.....something like that.
Logged
Mark -- N7EKU/VE3

AA4HA

  • Member
  • Posts: 2689
Re: Need unregulated 5Vdc Help!!
« Reply #18 on: August 14, 2022, 06:30:52 PM »

Your "span" on the 0-5 volt DC input will be very limited;

Since you are looking for sags (drops) and swells (rises) you want to re-span the input.

For example, so an 75 volt line input provides you with a 0 volt output, and a 150 volt line input gives you a 5 volt output.

Your "zero" is 75 volts and your "span" is 75 volts.

Full scale is 150 volts.

Maybe just a little bit of capacitance to filter out the ripple but not enough to eliminate sags and swells.

(I was an instrumentation and controls engineer for part of my career)

You can do great things with an op-amp, a few resistors and a cap or two.
Logged
Ms. Tisha Hayes, AA4HA
Lookout Mountain, Alabama

WB6BYU

  • Member
  • Posts: 20896
    • Practical Antennas
Re: Need unregulated 5Vdc Help!!
« Reply #19 on: August 14, 2022, 08:58:15 PM »

Quote from: AA4HA

..You can do great things with an op-amp, a few resistors and a cap or two.



And sometimes a zener diode in the feedback loop to offset the voltage.

WA3SKN

  • Member
  • Posts: 8126
Re: Need unregulated 5Vdc Help!!
« Reply #20 on: August 15, 2022, 05:11:34 AM »

If you want to monitor the 120v/240v power, why not just monitor the power?

-Mike.
Logged

K8AXW

  • Posts: 7391
    • HomeURL
Re: Need unregulated 5Vdc Help!!
« Reply #21 on: August 15, 2022, 10:14:38 AM »

Sometimes building  something around a component or even a module becomes exasperating.  Most of the time it's better to simply build from an existing circuit.

I built my AC line monitor from an old Heathkit circuit. 
Logged
A Pessimist is Never Disappointed!

KD2E

  • Member
  • Posts: 349
Re: Need unregulated 5Vdc Help!!
« Reply #22 on: August 23, 2022, 06:55:44 AM »

Generally,

Most old-style transformer wall warts are not regulated anyway.  They just had a transformer, rectifier, and smoothing cap.  So likely any "heavy" wall wart would do fine.

They normally weren't regulated because the transformer was sized to just big enough for the device they came with.  So when the device loaded the power supply, it's voltage would drop to whatever the device needed.  That's why it was important to be careful with those supplies and not use one rated for a high load, on a low-load device -- the voltage wouldn't drop so the device was exposed to over-voltage.

You can check this by measuring the unloaded voltage from an old wall wart -- it will be much higher than what the label says!  Then load it to draw the rated current indicated and the voltage should drop to what it says on the label.

73

Mark...so an old one that says 3vdc @200mA.   Unloaded, it reads almost 6 volts.  So, 3 divided by .2 gives me 15 ohm resistor to use, and 3 times .2 gives me .6 watts....  So about a 10 ohm resistor at a watt or two across the wall wart would be a good place to start??   Does that sound roughly correct??
Logged

WB6BYU

  • Member
  • Posts: 20896
    • Practical Antennas
Re: Need unregulated 5Vdc Help!!
« Reply #23 on: August 23, 2022, 08:54:15 AM »

That depends on what you are trying to accomplish.

If you want to drop the voltage to the specified output of the transformer,
then with a 15 ohm load it should put out 3V at 200 mA.

But if you want to use it as a 5V (or 3V) source for an indicator of
some sort, then you don't need to draw full power.

For example, if it puts out 6V no load, you could stick 6 each 1K
(or even 10k) resistors across the output, and take your 5V
reference off one resistor from the top of the stack, because the
output voltage under that load won't drop much.

Or you could experiment to find what load resistance drops the
output to 5V:  a good starting point might  be to assume the
internal resistance of the transformer is 15 ohms, so a load
resistor of 5 x 15 = 60 ohms.

But in your application, using higher value resistors so the
supply is lightly loaded, then tapping across the resistor chain
to get the desired voltage, dissipates less power is probably
is easier to adjust.  (You could, of course, just stick a 10K pot
across the output of the supply and adjust it for the desired
voltage.

That's assuming that your detector / indicator circuit has a
high input impedance, of course.

N7EKU

  • Member
  • Posts: 1471
Re: Need unregulated 5Vdc Help!!
« Reply #24 on: September 05, 2022, 01:31:23 PM »

Generally,

Most old-style transformer wall warts are not regulated anyway.  They just had a transformer, rectifier, and smoothing cap.  So likely any "heavy" wall wart would do fine.

They normally weren't regulated because the transformer was sized to just big enough for the device they came with.  So when the device loaded the power supply, it's voltage would drop to whatever the device needed.  That's why it was important to be careful with those supplies and not use one rated for a high load, on a low-load device -- the voltage wouldn't drop so the device was exposed to over-voltage.

You can check this by measuring the unloaded voltage from an old wall wart -- it will be much higher than what the label says!  Then load it to draw the rated current indicated and the voltage should drop to what it says on the label.

73

Mark...so an old one that says 3vdc @200mA.   Unloaded, it reads almost 6 volts.  So, 3 divided by .2 gives me 15 ohm resistor to use, and 3 times .2 gives me .6 watts....  So about a 10 ohm resistor at a watt or two across the wall wart would be a good place to start??   Does that sound roughly correct??

Hi,

From your first post, it sounds like your sensor wants to read 4-20mA?  I don't know what the load of your sensor is, however if you want a current in range, you could try 6V/0.012A = 500 ohms and place a resistor of that value in series with your sensor.  Then measure what current is running at that point and adjust the resistor down in value until you get ~12mA of current going.

You would want to load the transformer as least as possible so that it isn't "self-regulating" by having the secondary voltage reponse dictated by both your resistor and the resistance of the windings/loss in the core.

One thing to realize is that the range is likely to be low.  Assume that your AC line voltage will vary by about +/- 10%.  The transformer is changing 120VAC to 6VAC.  So the voltage on the secondary side will be about 5.4 to 6.6 and the current will then only range from 10.8mA to 13.2mA.  But I assume you can scale that in software somehow?

73,  Mark


Logged
Mark -- N7EKU/VE3

KD6VXI

  • Member
  • Posts: 1022
Re: Need unregulated 5Vdc Help!!
« Reply #25 on: September 06, 2022, 12:54:10 PM »

KD2E:

I can't help you but I'd appreciate it if you would help me.

I was a power plant operator for many years, many years ago.  Since most of our
instrumentation operated using 4-20ma. sensors, your question got my attention.

As I recall, (!) we powered the instrumentation with well regulated 12vdc and the 4-20 sensors monitored the current (4-20ma) drop across a 270 ohm resistor.  The purpose of the 4-20ma. instruments is that the monitoring of minute current flow was a great deal more accurate than dealing with the voltage fluctuation in the very long runs of instrument wiring. Understand, I'm working with a 22 year old operator, not instrumentation, memory. 

With this in mind, would you be so kind to offer some insight of what you want to do.

Al

I'm responsible for over 500 4-20ma sensors.

We have them that run on 12V and 24v.  Depending on what they are measuring, their age, the manufacturer, etc.

The device he's looking at, I doubt it monitors reactive vs real power.  I guess maybe it doesn't matter, but the PF will change readings.

--Shane
WP2ASS / ex KD6VXI
Logged

KD6VXI

  • Member
  • Posts: 1022
Re: Need unregulated 5Vdc Help!!
« Reply #26 on: September 06, 2022, 01:00:33 PM »

Get a 3.5 volt AC or so xformer (3.5VAC will give 4.949 volts rectified and unloaded)   and a 1/2 uF cap.  You could probably get by with a .1 uF cap.

The larger the capacitor the longer it will take to register a change in voltage....  Depending on the load resistance, which is probably very small.  Hence my statement probably being able to use a .1 uF.

--WP2ASS / ex KD6VXI
Logged
Pages: 1 [2]   Go Up