Forums

Wires...?

Started by fungus November 27, 2011
Spurred on by my first LED light setup, I'm now doing the hallway...

The setup is this: There's a 12V switching power supply connected to a
small current regulator board ("CRB"). The CRB fixes the output
current to 850mA. After that come the LEDs.

I did the initial test on the table. The LED current was 850mA as it
should be, output from the CRB was 10.5 Volts. I left it running for a
day, all the voltages/currents were good.

So ...  I moved into the hallway and started putting it together. I
put the PSU on the floor and ran a length of lamp wire up to the CRB
which is at ceiling level. When I switched it on it looked a bit dim.
I measured the LED current and it was only 450mA. Weird.

On investigation I found that the voltage at the PSU was 12V but at
the other end of the piece of wire it was only 11V. The CRB didn't
have enough voltage margin to power the LEDs properly.

The PSU has a little trimmer pot on it to adjust the output voltage. I
tweaked it up to 13V and everything started working properly again.

Why would I get this voltage drop?  I measured the resistance of the
wire and it's 0.1-0.2 Ohms - background noise level for my multimeter.
I thought it might be some sort of coupling issue because the CRB is
switching on/off quickly so I looked the CRB"s input voltage with an
oscilloscope. It was perfectly flat, nothing to see. I even tried
putting a ceramic capacitor across the CRB input to see if it would
help and it made no difference. The voltage drop along the wire was
still a whole volt.

(Note: The output voltage from the CRB is also perfectly flat - it's
not outputting a PWM signal to the LEDs)

As a second test I moved the CRB next to the PSU (a six inch piece of
wire) and put a longer wire between the CRB and the LEDs (this wire
was slightly shorter/thicker then the lamp wire).

The voltage drop from the PSU to the CRB went down to 0.1V - much
better! (but still measurable)

But ... the voltage output from the CRB went up from 10.5V to 11.5V. I
didn't gain anything. I just moved the missing Volt from one wire to
another!

Can anybody shine a light on what's happening here?

This setup works and I assume the PSU will be OK outputting 13V (it's
only at 40% of rated current) but it bothers me. Either copper wire is
nowhere near as good as I imagined (in which case how does the phone
company stay in business?) or there's something going on that I don't
know about.
On Sun, 27 Nov 2011 04:04:16 -0800 (PST), fungus
<openglMYSOCKS@artlum.com> wrote:

>Spurred on by my first LED light setup, I'm now doing the hallway... > >The setup is this: There's a 12V switching power supply connected to a >small current regulator board ("CRB"). The CRB fixes the output >current to 850mA. After that come the LEDs. > >I did the initial test on the table. The LED current was 850mA as it >should be, output from the CRB was 10.5 Volts. I left it running for a >day, all the voltages/currents were good. > >So ... I moved into the hallway and started putting it together. I >put the PSU on the floor and ran a length of lamp wire up to the CRB >which is at ceiling level. When I switched it on it looked a bit dim. >I measured the LED current and it was only 450mA. Weird. > >On investigation I found that the voltage at the PSU was 12V but at >the other end of the piece of wire it was only 11V. The CRB didn't >have enough voltage margin to power the LEDs properly. > >The PSU has a little trimmer pot on it to adjust the output voltage. I >tweaked it up to 13V and everything started working properly again. > >Why would I get this voltage drop? I measured the resistance of the >wire and it's 0.1-0.2 Ohms - background noise level for my multimeter. >I thought it might be some sort of coupling issue because the CRB is >switching on/off quickly so I looked the CRB"s input voltage with an >oscilloscope. It was perfectly flat, nothing to see. I even tried >putting a ceramic capacitor across the CRB input to see if it would >help and it made no difference. The voltage drop along the wire was >still a whole volt. > >(Note: The output voltage from the CRB is also perfectly flat - it's >not outputting a PWM signal to the LEDs) > >As a second test I moved the CRB next to the PSU (a six inch piece of >wire) and put a longer wire between the CRB and the LEDs (this wire >was slightly shorter/thicker then the lamp wire). > >The voltage drop from the PSU to the CRB went down to 0.1V - much >better! (but still measurable) > >But ... the voltage output from the CRB went up from 10.5V to 11.5V. I >didn't gain anything. I just moved the missing Volt from one wire to >another! > >Can anybody shine a light on what's happening here? > >This setup works and I assume the PSU will be OK outputting 13V (it's >only at 40% of rated current) but it bothers me. Either copper wire is >nowhere near as good as I imagined (in which case how does the phone >company stay in business?) or there's something going on that I don't >know about.
With a little back of the envelope math: Assuming you're using AWG 14 zip cord, that's about 2.5 ohms/1000 ft. Assuming a 20-ft run from the PSU to the CRB, each leg would be about 0.05 ohms. Swagging some efficiency for the CRB to make the math easy, assume it's pulling 1 A to supply 0.85 A. The expected drop across each feeder leg would then be 0.05 V. So, 12 V across the PSU should drop to 11.9 V across the CRB. Either you're running AWG 24 (25 ohms/1000 ft -> 0.5 V/leg @ 1A -> 11 V) or you're pulling 10 A through the CRB or you've got a high resistance connection somewhere. -- Rich Webb Norfolk, VA
On Nov 27, 1:40=A0pm, Rich Webb <bbew...@mapson.nozirev.ten> wrote:
> > Either you're running AWG 24 (25 ohms/1000 ft -> 0.5 V/leg @ 1A -> 11 V) > or you're pulling 10 A through the CRB or you've got a high resistance > connection somewhere. >
I did suspect the screw terminals but they've been connected and disconnected several times over the last day or so and nothing changed in any of the measurements. Maybe I could check that out in more depth. PS: I've always wondered how things like heaters need 13 amp cable (here in Europe) but the plugs and switches they use to connect to the supply seem to have quite small contact areas...
On Nov 27, 1:40=A0pm, Rich Webb <bbew...@mapson.nozirev.ten> wrote:
> > Either you're running AWG 24 or you're pulling 10A > through the CRB or you've got a high resistance > connection somewhere. >
Aha! I was using a piece of solid-core telephone wire because the last few inches will be visible so I wanted a thin wire going up the wall instead of a huge chunk of mains cable. I just measured it with my micrometer and googled "AWG". It comes in at around AWG 25. It's not 20 feet long but between that and all the screw connections it probably adds up. So...if I wanted to fix it I could run a thicker wire up to the last few inches then switch to the thin wire. OTOH I can leave it as-is. It works, I know nothing sinister is happening and the CRB stays cooler the closer the input&output voltages are to each other. Anyway, mystery solved - I'm an ignoramous who needs to keep the AWG chart handy.
fungus wrote:
> Spurred on by my first LED light setup, I'm now doing the hallway... > > The setup is this: There's a 12V switching power supply connected to a > small current regulator board ("CRB"). The CRB fixes the output > current to 850mA. After that come the LEDs. > > I did the initial test on the table. The LED current was 850mA as it > should be, output from the CRB was 10.5 Volts. I left it running for a > day, all the voltages/currents were good. > > So ... I moved into the hallway and started putting it together. I > put the PSU on the floor and ran a length of lamp wire up to the CRB > which is at ceiling level. When I switched it on it looked a bit dim. > I measured the LED current and it was only 450mA. Weird. > > On investigation I found that the voltage at the PSU was 12V but at > the other end of the piece of wire it was only 11V. The CRB didn't > have enough voltage margin to power the LEDs properly. > > The PSU has a little trimmer pot on it to adjust the output voltage. I > tweaked it up to 13V and everything started working properly again. > > Why would I get this voltage drop? I measured the resistance of the > wire and it's 0.1-0.2 Ohms - background noise level for my multimeter. > I thought it might be some sort of coupling issue because the CRB is > switching on/off quickly so I looked the CRB"s input voltage with an > oscilloscope. It was perfectly flat, nothing to see. I even tried > putting a ceramic capacitor across the CRB input to see if it would > help and it made no difference. The voltage drop along the wire was > still a whole volt. > > (Note: The output voltage from the CRB is also perfectly flat - it's > not outputting a PWM signal to the LEDs) > > As a second test I moved the CRB next to the PSU (a six inch piece of > wire) and put a longer wire between the CRB and the LEDs (this wire > was slightly shorter/thicker then the lamp wire). > > The voltage drop from the PSU to the CRB went down to 0.1V - much > better! (but still measurable) > > But ... the voltage output from the CRB went up from 10.5V to 11.5V. I > didn't gain anything. I just moved the missing Volt from one wire to > another! > > Can anybody shine a light on what's happening here? > > This setup works and I assume the PSU will be OK outputting 13V (it's > only at 40% of rated current) but it bothers me. Either copper wire is > nowhere near as good as I imagined (in which case how does the phone > company stay in business?) or there's something going on that I don't > know about.
Your meter is not telling you the truth about the wire? You are border lining your current limiting board due to the 1 diode drop voltage (0.7) that is going to take a sharp drop. I only assume you're using a bipolar type limit circuit ? So in one case you may have a high R cord and then next case insufficient voltage range to make up for the loss in the CRB. Remove 1 led out of the string. Jamie
On Nov 27, 3:54=A0pm, Jamie
<jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote:
> > Your meter is not telling you the truth about the wire? >
Maybe...I'll have to revisit that.
> =A0 =A0 You are border lining your current limiting board due to the 1 di=
ode
> drop voltage (0.7) that is going to take a sharp > drop. I only assume you're using a bipolar type limit circuit ? >
I dunno. It's a prebuilt board (based on the MBI6651 if you want to google it).
> =A0 =A0So in one case you may have a high R cord and then next case > insufficient voltage range to make up for the loss in the CRB. >
Looks like it was the cord. I hadn't realized ten feet of copper had such a high resistance (and my meter wasn't measuring it for some reason).
> =A0 =A0 Remove 1 led out of the string. >
Not really possible... I've already got the lamps in place. (Some cute little lamps I bought in Marrakesh last week... :-)
"fungus" <openglMYSOCKS@artlum.com> schreef in bericht 
news:194e69b3-8aab-4f55-b1a1-719f3938c1fa@gi1g2000vbb.googlegroups.com...
> Spurred on by my first LED light setup, I'm now doing the hallway... > > The setup is this: There's a 12V switching power supply connected to a > small current regulator board ("CRB"). The CRB fixes the output > current to 850mA. After that come the LEDs. > > I did the initial test on the table. The LED current was 850mA as it > should be, output from the CRB was 10.5 Volts. I left it running for a > day, all the voltages/currents were good. > > So ... I moved into the hallway and started putting it together. I > put the PSU on the floor and ran a length of lamp wire up to the CRB > which is at ceiling level. When I switched it on it looked a bit dim. > I measured the LED current and it was only 450mA. Weird. > > On investigation I found that the voltage at the PSU was 12V but at > the other end of the piece of wire it was only 11V. The CRB didn't > have enough voltage margin to power the LEDs properly. > > The PSU has a little trimmer pot on it to adjust the output voltage. I > tweaked it up to 13V and everything started working properly again. > > Why would I get this voltage drop? I measured the resistance of the > wire and it's 0.1-0.2 Ohms - background noise level for my multimeter. > I thought it might be some sort of coupling issue because the CRB is > switching on/off quickly so I looked the CRB"s input voltage with an > oscilloscope. It was perfectly flat, nothing to see. I even tried > putting a ceramic capacitor across the CRB input to see if it would > help and it made no difference. The voltage drop along the wire was > still a whole volt. > > (Note: The output voltage from the CRB is also perfectly flat - it's > not outputting a PWM signal to the LEDs) > > As a second test I moved the CRB next to the PSU (a six inch piece of > wire) and put a longer wire between the CRB and the LEDs (this wire > was slightly shorter/thicker then the lamp wire). > > The voltage drop from the PSU to the CRB went down to 0.1V - much > better! (but still measurable) > > But ... the voltage output from the CRB went up from 10.5V to 11.5V. I > didn't gain anything. I just moved the missing Volt from one wire to > another! > > Can anybody shine a light on what's happening here? > > This setup works and I assume the PSU will be OK outputting 13V (it's > only at 40% of rated current) but it bothers me. Either copper wire is > nowhere near as good as I imagined (in which case how does the phone > company stay in business?) or there's something going on that I don't > know about.
Well, Ohms law stands. R=U/I=1/450=2R2 or 2.2 Ohm, which has to account for the wire (twice), the ampmeter and various contactresistances. Guess you have not taken them all when measuring ohms, besides this meters are not known for there lack of accuracy measuring below 10 Ohm. (Unless you have a very good=expensive and callibrated one.) If you want to get an idea of the wire resistance you better use a curent source of - let's say - 1A or 2A. Then short the wires at one side, connect the source at the other and measure current and voltage simultanuously at that side. Be sure to measure the voltage at the wires side, not at the source side. Supposing you have reasonable good meters you'll find the resistance using Ohms law as mentioned. Be aware that doubling the thickness of the wire will reduce its resistance by four. As for telephone companies, they use currents way below 100mA (depending on where you live) and voltages up to 48V. But they sure have to be aware of line losses. The phenomenon you encountered is well known and power supplies are often equiped to deal with it. They have separate sense lines to measure the voltage near the load and regulate the power output to be correct at that point. petrus bitbyter
"petrus bitbyter" <petrus.bitbyter@hotmail.com> schreef in bericht 
news:4ed25ab5$0$16304$e4fe514c@dreader37.news.xs4all.nl...
> > "fungus" <openglMYSOCKS@artlum.com> schreef in bericht > news:194e69b3-8aab-4f55-b1a1-719f3938c1fa@gi1g2000vbb.googlegroups.com... >> Spurred on by my first LED light setup, I'm now doing the hallway... >> >> The setup is this: There's a 12V switching power supply connected to a >> small current regulator board ("CRB"). The CRB fixes the output >> current to 850mA. After that come the LEDs. >> >> I did the initial test on the table. The LED current was 850mA as it >> should be, output from the CRB was 10.5 Volts. I left it running for a >> day, all the voltages/currents were good. >> >> So ... I moved into the hallway and started putting it together. I >> put the PSU on the floor and ran a length of lamp wire up to the CRB >> which is at ceiling level. When I switched it on it looked a bit dim. >> I measured the LED current and it was only 450mA. Weird. >> >> On investigation I found that the voltage at the PSU was 12V but at >> the other end of the piece of wire it was only 11V. The CRB didn't >> have enough voltage margin to power the LEDs properly. >> >> The PSU has a little trimmer pot on it to adjust the output voltage. I >> tweaked it up to 13V and everything started working properly again. >> >> Why would I get this voltage drop? I measured the resistance of the >> wire and it's 0.1-0.2 Ohms - background noise level for my multimeter. >> I thought it might be some sort of coupling issue because the CRB is >> switching on/off quickly so I looked the CRB"s input voltage with an >> oscilloscope. It was perfectly flat, nothing to see. I even tried >> putting a ceramic capacitor across the CRB input to see if it would >> help and it made no difference. The voltage drop along the wire was >> still a whole volt. >> >> (Note: The output voltage from the CRB is also perfectly flat - it's >> not outputting a PWM signal to the LEDs) >> >> As a second test I moved the CRB next to the PSU (a six inch piece of >> wire) and put a longer wire between the CRB and the LEDs (this wire >> was slightly shorter/thicker then the lamp wire). >> >> The voltage drop from the PSU to the CRB went down to 0.1V - much >> better! (but still measurable) >> >> But ... the voltage output from the CRB went up from 10.5V to 11.5V. I >> didn't gain anything. I just moved the missing Volt from one wire to >> another! >> >> Can anybody shine a light on what's happening here? >> >> This setup works and I assume the PSU will be OK outputting 13V (it's >> only at 40% of rated current) but it bothers me. Either copper wire is >> nowhere near as good as I imagined (in which case how does the phone >> company stay in business?) or there's something going on that I don't >> know about. > > Well, Ohms law stands. R=U/I=1/450=2R2 or 2.2 Ohm, which has to account > for the wire (twice), the ampmeter and various contactresistances. Guess > you have not taken them all when measuring ohms, besides this meters are > not known for there lack of accuracy measuring below 10 Ohm. (Unless you > have a very good=expensive and callibrated one.) > > If you want to get an idea of the wire resistance you better use a curent > source of - let's say - 1A or 2A. Then short the wires at one side, > connect the source at the other and measure current and voltage > simultanuously at that side. Be sure to measure the voltage at the wires > side, not at the source side. Supposing you have reasonable good meters > you'll find the resistance using Ohms law as mentioned. > > Be aware that doubling the thickness of the wire will reduce its > resistance by four. > > As for telephone companies, they use currents way below 100mA (depending > on where you live) and voltages up to 48V. But they sure have to be aware > of line losses. > > The phenomenon you encountered is well known and power supplies are often > equiped to deal with it. They have separate sense lines to measure the > voltage near the load and regulate the power output to be correct at that > point. > > petrus bitbyter >
Oops. These meters are known for their lack of accuracy. These meters are not known for their accuracy. petrus bitbyter
On Nov 27, 4:43=A0pm, "petrus bitbyter" <petrus.bitby...@hotmail.com>
wrote:
> > Well, Ohms law stands. R=3DU/I=3D1/450=3D2R2 or 2.2 Ohm, which has to acc=
ount for
> the wire (twice),
Yes of course...it'll be double the resistance. That's where the other missing volts are. My wire is twice as long as I think it is...
fungus wrote:

> On Nov 27, 3:54 pm, Jamie > <jamie_ka1lpa_not_valid_after_ka1l...@charter.net> wrote: > >>Your meter is not telling you the truth about the wire? >> > > > Maybe...I'll have to revisit that. > > >> You are border lining your current limiting board due to the 1 diode >>drop voltage (0.7) that is going to take a sharp >>drop. I only assume you're using a bipolar type limit circuit ? >> > > > I dunno. It's a prebuilt board (based on the MBI6651 if you want > to google it). > > >> So in one case you may have a high R cord and then next case >>insufficient voltage range to make up for the loss in the CRB. >> > > > Looks like it was the cord. I hadn't realized ten feet of > copper had such a high resistance (and my meter > wasn't measuring it for some reason). > > > > >> Remove 1 led out of the string. >> > > > Not really possible... I've already got the lamps in place. > > (Some cute little lamps I bought in Marrakesh last week... :-)
I didn't find much of a PDF on that however, what I did find does state the on switch resistance of being 0.45 ohms. So it looks like you're losing some head room there and you should count that value in on your calculations. Also, for 850ma, using 24 awg is way to small! Jamie