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Author Topic: Need unregulated 5Vdc Help!!  (Read 665 times)

KD2E

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Need unregulated 5Vdc Help!!
« on: June 06, 2022, 08:55:02 AM »

For a line voltage to 4-20mA sensor, I need a 5 volt supply that does NOT regulate. I WANT it to fall and rise above 5 volts as the line voltage varies.
It seems those very light, junky wall wart switching jobs do a very good job of regulation. I put one on a variac. But the old, heavier ones from the 'ol days...before switching supplies would be the key. They do not regulate very well!  But I doubt I'll find one of them for 5Vdc. Anyone have any ideas, or do you think my best choice would be to put together a transformer, cap or two, few diodes..and make a ultra junky 5 V supply?  I only need the voltage reference, and no current capability..so it can be very weenie.
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K1KIM

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Re: Need unregulated 5Vdc Help!!
« Reply #1 on: June 06, 2022, 09:55:15 AM »

I must have 40 of those wall wart supplies in my junk box. Some are at least 20-25 years old

If you can tell me what to look for I gladly send you one.
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WB6BYU

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Re: Need unregulated 5Vdc Help!!
« Reply #2 on: June 06, 2022, 12:58:40 PM »

Older TV power transformers used to have a 5V winding for the
rectifier filament (5U4, 5Y3, etc.)  But that's probably overkill.

You can always just use a resistive divider across a higher voltage
transformer to get your 5V reference.  That should be simple to
adjust, since you don't need to draw much current.

Otherwise, we used to modify transformers by rewinding the
secondary winding to get the desired voltage.

KD2E

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Re: Need unregulated 5Vdc Help!!
« Reply #3 on: June 06, 2022, 01:46:27 PM »

I must have 40 of those wall wart supplies in my junk box. Some are at least 20-25 years old

If you can tell me what to look for I gladly send you one.

Actually, if you have any that have the output as AC...like 6VAC, or 9VAC, or something like that...I think that will work for this project.
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KD2E

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Re: Need unregulated 5Vdc Help!!
« Reply #4 on: June 06, 2022, 01:50:34 PM »

To all...I looked at how others do this (https://www.acscout.com/)  and learned that what I need to do
is find an AC wal wart....and rectify that. This will then have no regulation, and the DC voltage I'm left with will wander in response to the AC rising and falling.  So, I just gotta search around for a wal wart with an AC output. I guess the voltage doesn't really matter...I can use a resistor to bring it down!
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K1KIM

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Re: Need unregulated 5Vdc Help!!
« Reply #5 on: June 06, 2022, 01:55:41 PM »

To all...I looked at how others do this (https://www.acscout.com/)  and learned that what I need to do
is find an AC wal wart....and rectify that. This will then have no regulation, and the DC voltage I'm left with will wander in response to the AC rising and falling.  So, I just gotta search around for a wal wart with an AC output. I guess the voltage doesn't really matter...I can use a resistor to bring it down!

I probably have at least one of those also
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K1KIM

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Re: Need unregulated 5Vdc Help!!
« Reply #6 on: June 06, 2022, 02:54:55 PM »

I must have 40 of those wall wart supplies in my junk box. Some are at least 20-25 years old

If you can tell me what to look for I gladly send you one.

Actually, if you have any that have the output as AC...like 6VAC, or 9VAC, or something like that...I think that will work for this project.

I'll look later this evening. I know I have some very large 12VAC outdoor lighting ones. But they weigh about 15# and are surely overkill.
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K1KIM

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Re: Need unregulated 5Vdc Help!!
« Reply #7 on: June 06, 2022, 03:19:31 PM »

Email sent with photo.
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N7EKU

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Re: Need unregulated 5Vdc Help!!
« Reply #8 on: June 06, 2022, 03:36:39 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
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W5FYI

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Re: Need unregulated 5Vdc Help!!
« Reply #9 on: June 06, 2022, 03:54:14 PM »

My go-to source for wall warts are thrift stores. They usually have a bin or two of ac and dc varieties in assorted voltages and wattages. The same bins are also good places to find plugs of various sizes, too.

73, Stew
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W9AC

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Re: Need unregulated 5Vdc Help!!
« Reply #10 on: June 07, 2022, 04:20:06 AM »

To all...I looked at how others do this (https://www.acscout.com/)  and learned that what I need to do
is find an AC wal wart....and rectify that. This will then have no regulation, and the DC voltage I'm left with will wander in response to the AC rising and falling.  So, I just gotta search around for a wal wart with an AC output. I guess the voltage doesn't really matter...I can use a resistor to bring it down!

Or, spend $3.95 + shipping for a 5 VDC/800 mA unregulated wall cube from Jameco:

https://www.jameco.com/z/950-00143-Jameco-Reliapro-5VDC-800mA-Unregulated-Linear-Wall-Adapter-2-5x5-5mm-Center-Positive_2231443.html

Paul, W9AC
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AC2EU

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Re: Need unregulated 5Vdc Help!!
« Reply #11 on: June 07, 2022, 07:56:42 AM »

Won't the filter cap mask the rapid and small fluctuations?

K8AXW

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Re: Need unregulated 5Vdc Help!!
« Reply #12 on: June 07, 2022, 09:39:35 AM »

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
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N7EKU

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Re: Need unregulated 5Vdc Help!!
« Reply #13 on: June 07, 2022, 11:39:29 AM »

Won't the filter cap mask the rapid and small fluctuations?

It would depend on the value of the cap and thus the time constant it gives you.  And also it depends on the meter response time and what one wants to actually observe.  A lot of unknowns at this point.

73
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Mark -- N7EKU/VE3

KD2E

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Re: Need unregulated 5Vdc Help!!
« Reply #14 on: June 16, 2022, 03:38:25 PM »

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.
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