DC cable....twin voltage?

[MG1]

Is it possible to run 12vdc and 5vdc within the same cable?

I'm sure the answer is probably a massive NO, but just wanted to check.

I was thinking that if I could use a 4mm 3 core flex to carry 12vdc, 5vdc and ground over a 10Meter run to the scope I could keep all of the 12v > 5v circuitry in my power box and minimise what's happening out at the scope.

Just a thought.

Oh and am I right in thinking I can use one common ground, rather than needing a ground for each voltage?

Sorry for being a numpty...I am looking for an amateur electronics course around my parts...

Cheers,

Michael

[malc-c]

Yes, you can run 12v on one core, 5v on another, and depending on how you generate the different voltages you could common the ground return. However, running low voltage over long distances is often not practical as the voltage drops due to the resistance of the cable.

[MG1]

Cheers Malcolm... Is there a formula I can use to calculate the voltage drop?

I've measured the voltage I'm getting at the end of my run to the scope on the 12vdc supply and it's only falling from 13.8 to 13.71, but am I right in thinking that lower voltages drop quicker?

Would that require a larger or small cross section?

I was using the calculator on this site as a rough guide: DC Cable Sizing Tool - Wire Size Calculator - MM2 & AWG - solar-wind.co.uk

as for the common ground.... If I use two different AC transformers would that need separate grounds, whereas if I use my 13.8v AC transformer and a voltage regulator circuit could I use a common ground?

This is all because I wish to power a USB hub at the mount. Currently I'm not getting enough power, and when I measured the voltage across the USB ports I realised I'm only getting 3.2v...so am in danger of under voltage damage maybe?

Thanks again

[malc-c]

I'm sure there is some form of calculator on the net somewhere, and sometimes it's not all about having more cross section of cable. It also depends on the current the load draws, so whilst you get a drop of around 10th of a volt, under load the voltage drops, as you found with supply for the USB hub.

Best advice for a supply would be to use a 6 - 0 - 6 volt transformer, two rectifiers, and a couple of regulators. Provided you are not going to draw more than an amp then normal 7805 and 7812 would do. Best to mount them on a heatsink to be sure. The attached sketch might help.

[Naz]

You could always use a larger diameter cable, the greater the diameter the less the resistance. Maybe not very practical.

[MG1]

I think I require higher loads than 1A. Around 6A on 12v & 2A on 5V I think (plus building in a little more for future proofing)

Should I be considering switching regulators such as LM2576S and LM2576T

I couldn't find any linear regs with higher load capacity.

Although I've just found a fairchild LM7805CT that handles 2.2A at 5v and is only 50p!

will look at this a little over the weekend I think as i'm shattered now.

Thanks for t he help though.

Michael

[malc-c]

If you intend running at that amperage then switching regulators are the only way to go. If you were running linear the heatsink would have to be fairly substantial IMO.

USB ports can only source 500mA (half an amp) which may well explain why you were only getting such a low voltage before.

[yesyes]

someone else on here recently recommended these:

24V / 12V to 5V, Step-down, 5A, 25W DC-DC Converter on eBay (end time 20-Apr-11 08:05:13 BST)

I ordered 2 and used one yesterday. Very neat solution. Hopefully the seller will relist this item.

It's a bit more expensive but it's a very neat and sealed solution with a proper heat sink and mounting holes.

[r3i]

Cheers Malcolm... Is there a formula I can use to calculate the voltage drop?

I've measured the voltage I'm getting at the end of my run to the scope on the 12vdc supply and it's only falling from 13.8 to 13.71, but am I right in thinking that lower voltages drop quicker?

Thanks again

There are shed loads of on-line voltage drop calculators: Google

I've been wanting to do the same thing to carry 12V with an ampage of around 7A over about a 15m distance, but with the cable that I tried (can't remember the spec) the voltage drop was too much. What thickness of cable did you try?

[MG1]

I'm currently running two lengths of thick bell wire, prob around 1.5mm2 approx in total and that's giving a drop off of around 700mV, so that's still within the input voltage required for my kit.

I was planning to run 5v the same distance, but have decided I am better off putting a voltage regulator at the scope and bringing in down to 5v there... I think it I was running any longer runs I'd probably run 24v out to the scope and then step down to 12 and 5... voltage drop seems to be less with higher voltages.

As soon as I work out my solution I'll post something up.

[Psychobilly]

someone else on here recently recommended these:

24V / 12V to 5V, Step-down, 5A, 25W DC-DC Converter on eBay (end time 20-Apr-11 08:05:13 BST)

I ordered 2 and used one yesterday. Very neat solution. Hopefully the seller will relist this item.

It's a bit more expensive but it's a very neat and sealed solution with a proper heat sink and mounting holes.

Think it might have been me who suggested it...

You can get them from here as well and Cheaper to... but not by much when you add on the postage..

DC/DC Converter 12V/24V Step down to 5V ? 25W | eBay UK

When you need reasonable ammounts of current at lower voltages then local regulation is the way to do it... which is where modules like this come in...

Peter...

[MG1]

Ah, yes...I think that's what you recommended to Daz...I wonder how he's got on?

Do these do away with the need for a smoothing circuit?

I'm looking at a LM2576-6 with a 100uF capacitor smoothing the input, 100uH coil and IN5822 diode on the feedback and 1000uF cap on the output.

It's a little cheaper, except that minimum quants mean I will probably have to make 5!

However if those encapuslated thingamijigs are as simple as 12v in 5v out then it would be daft not to go with that.

Chris, can you let me know if they get hot?

I'm also making a headphone amp at the moment so may be able to combine some bits...haven't checked the parts list yet.

CHeers

[Photosbykev]

Personally I would run at 12v (13.8v) until you get local to the hub and then rectify the 12v down to 5v, it seems slightly silly to drop the 12v to 5v at the supply end. Some 16A mains rated cable would easily handle 12v @ 10A+ and the volts drop shouldn't be too significant. You should be checking your voltages when the system is under load as that is when you will see any volts drop.

[yesyes]

Think it might have been me who suggested it...

Ah, yes, of course! Thanks for that. It was just what I was looking for at the time, just not for the same reason.. ;-))

(it went into my observatory clock project and powers a matrix of 512 LEDs)

[Psychobilly]

If ever somebody needed serious amounts of LV DC in the obs then I have been snapping up these for our burn in rigs in work... got 5 of them now...

Rapid Electronics - Electrical & Power > Power Supplies > Bench PSUs

These have remote voltage sense and can be setup to compensate for the voltage drop in the main current carrying wires....

Just a pity they dont sell the 15V 60A version...

[yesyes]

Ah, yes...I think that's what you recommended to Daz...I wonder how he's got on?

Do these do away with the need for a smoothing circuit?

I'm looking at a LM2576-6 with a 100uF capacitor smoothing the input, 100uH coil and IN5822 diode on the feedback and 1000uF cap on the output.

It's a little cheaper, except that minimum quants mean I will probably have to make 5!

However if those encapuslated thingamijigs are as simple as 12v in 5v out then it would be daft not to go with that.

Chris, can you let me know if they get hot?

I'm also making a headphone amp at the moment so may be able to combine some bits...haven't checked the parts list yet.

CHeers

They are as easy as 12V in and 5V out. I run mine with input and output on common ground.

Mine has 18.5V in and the LEDs draw about 1.2Amp at 5V. It's really over-spec'ed for what I'm using it for. But the 7805 regulator I used before did get WAY too hot having to drop 13.5V, even with a big heat sink (from an old CPU). This module is switch mode and does not even get warm (at least at 1.2Amp-ish).

There's no data sheet but I'm pretty sure it has a capacitor across the output to smooth the voltage. No need for external components.

[Psychobilly]

IIRC they are 90%+ efficient so even at full load your only going to be producing around 3W of heat and the diecast case should be more than ammple to heat sink that away especially when your "outdoors" ...

I use thousands of LM317's a year and heatsinks but for this app I'd just buy one of the modules...

Peter...

[MG1]

Personally I would run at 12v (13.8v) until you get local to the hub and then rectify the 12v down to 5v, it seems slightly silly to drop the 12v to 5v at the supply end. Some 16A mains rated cable would easily handle 12v @ 10A+ and the volts drop shouldn't be too significant. You should be checking your voltages when the system is under load as that is when you will see any volts drop.

Ah yes...I was measuring during load, but I'd forgotten that the greatest load is whilst slewing....and even then I'd imagine it's greater still with a full load.

So with the Kendrick drawing 2.4A (5", 3" & 2" at 100%) and the (unloaded mount) slewing at full speed in both directions the voltage drop is 1.88volts... and that's with a load of probably just over 4Amps (I'm assuming the 2A rating of the mount is only reached at capacity)

This brings the voltage down to 11.85v and the light on the mount flashes.

hummm....So I def need to uprate the cable if I'm going to add a 2or3Amp 5v regulator at the scope end.

[MG1]

IIRC they are 90%+ efficient so even at full load your only going to be producing around 3W of heat and the diecast case should be more than ammple to heat sink that away especially when your "outdoors" ...

I use thousands of LM317's a year and heatsinks but for this app I'd just buy one of the modules...

Peter...

I think you're right there...especially if they are as easy to use as Chris says.

right...ebay it is then!

then to source some fatter but flexible cable....ho hum!