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Power Supply

Richard Novak
 

As I wait to order an 1100GTO, I want to buy and set up the electrical equipment.  The mount will be secured to a cart - on an AP 42” Portable Pier.   It will carry a load of about 40 pounds:  Steallarvue SVX152T, ASI2600MC, guide scope, ASI174MM guide camera, focus motor, Power Box Advance and NUC mini-PC.  

 

 

I’ve read the messages about power supplies and wiring issues and would like to build the system correctly the first time. The power supplies and block will be attached to a small board which will be hung on the pier or be attached to the legs.  I rarely image at temperatures below 30 degrees F. 

 

 

I’ve attached a simple diagram.  I’d like input on these issues:

1 - Is the MFJ (PS138V15A) power supply sufficient for the 1100GTO with my system?

2 – Powerwerx only sells the coaxial plugs with 18 gauge wire so I plan to run 16 gauge wire and connect to about 6 inches of the 18 gauge that is connected to the plugs.  Is this overkill and I should just use the 18 gauge all the way back to the distribution block?  Are there better options?

3 – Anything else?

Wiring.jpg

Mike Dodd
 

On 3/17/2023 8:37 AM, Richard Novak wrote:

1 - Is the MFJ (PS138V15A) power supply sufficient for the 1100GTO with my system?
Oh, absolutely! The mount draws only a few amps maximum, and that when slewing at high speed.
2 – Powerwerx only sells the coaxial plugs with 18 gauge wire so I plan to run 16 gauge wire and connect to about 6 inches of the 18 gauge that is connected to the plugs.Is this overkill and I should just use the 18 gauge all the way back to the distribution block?
I think #18 is file all the way, given the equipment you show connected to the D-block.

#18 copper wire has a resistance of 10.5 ohms per 1,000 feet, so 4 feet would have 0.042 ohms. If your imaging equipment draws, say, 5 amps (might be high; I don't know about your dew heaters), you'd see a voltage drop of 0.21 volts at the Power Box Advance. If you're comfortable with that, go with #18 wire. #18 is fine for the NUC.

BTW, I think #10 wire from the power supply to the D-box is waaay overkill. For only 12", you could go with #14 or #16.

Hope this helps.

--- Mike
Linwood Ferguson
 

On Fri, Mar 17, 2023 at 09:55 AM, Richard Novak wrote:
Is this overkill
It's hard to have wire too large.  :)  I've actually got 14AWG in places you have 16AWG, but 16AWG should be fine. 

The one thing I would suggest is connect the common (black, negative) together between the two power supplies. This ensures any floating voltage on the output side is tied to the same reference and (for example) if you had a USB cable between CP4 and NUC, the shield on the USB will then not carry current and so will have less noise (and with that said, I recommend not using USB but use ethernet to the CP4 for all sorts of reasons).  Whether this is really needed depends on exactly how and if the power supplies ground their output or let it float, but generally speaking it can never hurt, and may help.  It's also important if you ever used two batteries instead of two power supplies.

What I did was set up two of the powerpole distribution blocks, one for the mount, one for everything else (you can see below).  While used with power supplies the mount one is is not really "distributing" anything (well, other than the common tie), it does give  you a quick way to move power around, say to switch to one battery powering both, or if one power supply fails you can power both by tying them together.  It looks a bit ugly as I velcro everything to the tripod, not screw it in; the blocks are mounted on a piece of plastic that started life as a cutting board.  Avoiding holes in my nice wood tripod. 

I also have fuses in the feed wires (not shown), but some people get the larger distribution blocks where each connection has its own fuse. The power supplies have over-current protection so fuses may not really be required but old habits die hard. 

There is also a powerwerx power supply with powerpole outputs if you want to avoid the banana plugs or screw-on (both of which have a way of working loose).  I think AP has it on their site as well.  But these look fine and nice and tiny, just check that they are screwed down periodically. 

The other thing i would consider is a travel router, though it depends on your networking setup.  If you ever plan to take this away from your home network it would provide an access point to which to connect.  Even at home, a lot of people find that wifi in NUC's (depending on the NUC) have poor range.  I have had good luck with a travel router to provide connectivity, and just use a short ethernet cable to go to the NUC (and the CP4). 

Final comment - I have three of the Pocket Powerbox Advances (one on each OTA), so I like them, but they have one real "gotcha".  In the dark, at night (or even in the daytime when you might be looking at it upside down).... the power input and the variable voltage output are in the exact same place on opposite sides.  If you accidentally plug the input into the output it smokes.  I have done it, quite a few people have done it. That output is primarily for something like a DSLR.  I suggest covering the variable output with gaffer tape, it will save your PPBAdv a trip to Greece. 

Linwood
Richard Novak
 

Thanks for all the input.

After plugging the power supply into the variable voltage socket on my first Pocket Powerbox, I taped the socket.  Surprisingly, there was no damage.  I taped the socket as soon as the new one arrived.  Pegasus should either ship already taped or have different plug sizes.

I'll follow all the suggestions.  I especially like the second D-block.  
Brian Valente
 

>>> After plugging the power supply into the variable voltage socket on my first Pocket Powerbox

I have done that too. It’s so easy. Good idea to tape it

On Fri, Mar 17, 2023 at 11:25 AM Richard Novak <rfnovak11@...> wrote:
Thanks for all the input.

After plugging the power supply into the variable voltage socket on my first Pocket Powerbox, I taped the socket.  Surprisingly, there was no damage.  I taped the socket as soon as the new one arrived.  Pegasus should either ship already taped or have different plug sizes.

I'll follow all the suggestions.  I especially like the second D-block.  

--

DFisch
 


Would you then recommend that your two PowerWerx PD8 blocks get a common ground if powered by two sources? Tom

On Fri, Mar 17, 2023 at 14:30 Brian Valente <bvalente@...> wrote:
>>> After plugging the power supply into the variable voltage socket on my first Pocket Powerbox

I have done that too. It’s so easy. Good idea to tape it

On Fri, Mar 17, 2023 at 11:25 AM Richard Novak <rfnovak11@...> wrote:
Thanks for all the input.

After plugging the power supply into the variable voltage socket on my first Pocket Powerbox, I taped the socket.  Surprisingly, there was no damage.  I taped the socket as soon as the new one arrived.  Pegasus should either ship already taped or have different plug sizes.

I'll follow all the suggestions.  I especially like the second D-block.  

--

--
TJF MOBILE
M Hambrick
 

Hi Richard

If you set up your gear in the same spot every time you may want to consider grounding your pier. I sank one of those 10 ft ground rods right under my pier and have a cable that attaches to the base of the pier. It is (somewhat) visible in the photo. The ground rod is underneath that green valve box cover.

Hopefully, the soil is not rocky where you live.

Mike
Harley Davidson
 

Very interesting Mike.

I have several spots on my property that I setup on. This is a great idea that I could put these grounding rods at those locations.

1---Shall I assume that you "pull" the cover to expose the ground rod which is below the grass surface to facilitate cutting grass? Although from your picture it doesn't look like the cover needs to be pulled.

2---Could I just do the same but without the cover? Your thoughts?

3---Would you have the cover brand name and model number?

4---What gauge wire do you recommend? Yours appears to be 14 or 16 gauge? I have plenty of stranded 12 gauge wire.

5---Does the wire connect to the grounding rod by an alligator clip for easy attachment and removal?

I have clay soil but with all the wet weather I've had now is the time to do it.

thanks  tony



On 3/17/2023 8:45 PM, M Hambrick wrote:

Hi Richard

If you set up your gear in the same spot every time you may want to consider grounding your pier. I sank one of those 10 ft ground rods right under my pier and have a cable that attaches to the base of the pier. It is (somewhat) visible in the photo. The ground rod is underneath that green valve box cover.

Hopefully, the soil is not rocky where you live.

Mike
ROBERT WYNNE
 

I don't believe upsizing a portion of a circuit to a heavier gage wire is advisable or effective. The main reason being is that you are always limited by the weakest link in the electrical circuit - the 16 gage wire. -Best, Robert

On 03/17/2023 5:37 AM Richard Novak <rfnovak11@...> wrote:


As I wait to order an 1100GTO, I want to buy and set up the electrical equipment.  The mount will be secured to a cart - on an AP 42” Portable Pier.   It will carry a load of about 40 pounds:  Steallarvue SVX152T, ASI2600MC, guide scope, ASI174MM guide camera, focus motor, Power Box Advance and NUC mini-PC.  



I’ve read the messages about power supplies and wiring issues and would like to build the system correctly the first time. The power supplies and block will be attached to a small board which will be hung on the pier or be attached to the legs.  I rarely image at temperatures below 30 degrees F. 



I’ve attached a simple diagram.  I’d like input on these issues:

1 - Is the MFJ (PS138V15A) power supply sufficient for the 1100GTO with my system?

2 – Powerwerx only sells the coaxial plugs with 18 gauge wire so I plan to run 16 gauge wire and connect to about 6 inches of the 18 gauge that is connected to the plugs.  Is this overkill and I should just use the 18 gauge all the way back to the distribution block?  Are there better options?

3 – Anything else?
Greg McCall
 

I don't think it's about any limitation as such which to me implies current limits.
In these cases, it's about voltage drop and making sure any gear being operated does not fall below the desired voltage under it's full load (which you can measure to check)

I also agree with other comments where the resistance and appropriate voltage drop has been calculated and the comments re length of the run.
(the shorter the run, the thinner cable you can get away with)
I see no issue regarding upsizing other than the cost of cable or the practicalities of having connectors on too thick a wire. 

Cables should have fuses to protect the source from a cable fault such as a short circuit. While a limited current source may sound OK, it's not a substitute for some form of fusing (close or at the source). Worse case, having a current limited may protect the power supply but say worse case with a short and using a cable on the thinner side which might not handle the protected current without heating up so the cable could eventually heat up enough to start a fire so why take the chance? 
BTW, fuses protect the cable and not the downstream gear.

On Sat, Mar 18, 2023 at 12:51 PM ROBERT WYNNE <robert-wynne@...> wrote:
I don't believe upsizing a portion of a circuit to a heavier gage wire is advisable or effective. The main reason being is that you are always limited by the weakest link in the electrical circuit - the 16 gage wire. -Best, Robert
On 03/17/2023 5:37 AM Richard Novak <rfnovak11@...> wrote:


As I wait to order an 1100GTO, I want to buy and set up the electrical equipment.  The mount will be secured to a cart - on an AP 42” Portable Pier.   It will carry a load of about 40 pounds:  Steallarvue SVX152T, ASI2600MC, guide scope, ASI174MM guide camera, focus motor, Power Box Advance and NUC mini-PC.  



I’ve read the messages about power supplies and wiring issues and would like to build the system correctly the first time. The power supplies and block will be attached to a small board which will be hung on the pier or be attached to the legs.  I rarely image at temperatures below 30 degrees F. 



I’ve attached a simple diagram.  I’d like input on these issues:

1 - Is the MFJ (PS138V15A) power supply sufficient for the 1100GTO with my system?

2 – Powerwerx only sells the coaxial plugs with 18 gauge wire so I plan to run 16 gauge wire and connect to about 6 inches of the 18 gauge that is connected to the plugs.  Is this overkill and I should just use the 18 gauge all the way back to the distribution block?  Are there better options?

3 – Anything else?
Greg McCall
 

If you are using a MAINS supply and your power supply's earth is connected somehow with the 12v negative, depending on the county and how your electricity supply is wired, I would not automatically ground your gear as you may inadvertently cause ground loops.
Unless you know for sure it is safe in your MAINS (which is what we call our AC supply from the street) countries wiring, it's safer not too.
(I also can't see a reason to want to do it either, even if you just used a stand-alone battery) 

On Sat, Mar 18, 2023 at 12:23 PM Harley Davidson <astrocnc@...> wrote:
Very interesting Mike.

I have several spots on my property that I setup on. This is a great idea that I could put these grounding rods at those locations.

1---Shall I assume that you "pull" the cover to expose the ground rod which is below the grass surface to facilitate cutting grass? Although from your picture it doesn't look like the cover needs to be pulled.

2---Could I just do the same but without the cover? Your thoughts?

3---Would you have the cover brand name and model number?

4---What gauge wire do you recommend? Yours appears to be 14 or 16 gauge? I have plenty of stranded 12 gauge wire.

5---Does the wire connect to the grounding rod by an alligator clip for easy attachment and removal?

I have clay soil but with all the wet weather I've had now is the time to do it.

thanks  tony



On 3/17/2023 8:45 PM, M Hambrick wrote:
Hi Richard

If you set up your gear in the same spot every time you may want to consider grounding your pier. I sank one of those 10 ft ground rods right under my pier and have a cable that attaches to the base of the pier. It is (somewhat) visible in the photo. The ground rod is underneath that green valve box cover.

Hopefully, the soil is not rocky where you live.

Mike
M Hambrick
 

Hi Tony

To your questions:

Although 12-gauge wire will work, I used a 4-gauge grounding cable because I happened to have a piece about 3 feet long. The clamp that attaches to the grounding rod is designed for the heavier grounding cables that are used in residential installations. It is attached. with a wrench. 

The end that attaches to the pier is a crimp-on copper ring terminal designed for 4-gauge wire with a 1/4" ID ring. I also got it at Home Depot. I have a 1/4" brass bolt attached to the pier for connecting the ground cable.

Yes, you have to remove the cover to get to the grounding cable. I pull the cover off, get the cable, and run it through the hole, then attach the cable to the pier with a 7/16" wrench. The whole process takes less than a minute.

I got the valve box with cover at Home Depot (see below). I keep the ground cable permanently attached to the grounding rod. The cable is about 3 feet long, and I just coil it up and put it in the valve box when I am not using it. I buried the grounding rod about 6 inches below ground level and dug out the dirt to install the valve box. I keep a rubber stopper in the hole when I am not using it. This keeps the "critters" from making a home in the dark space under the cover.

There are a number of You-Tube videos that demonstrate how to bury a 10 ft long ground cable. The best one I saw had you starting by just jamming the ground rod into the ground, removing it, and filling the hole with water. Then repeat the process until it is buried as deep as you want. I was amazed at how fast it was. It took less than 5 minutes to sink a 10 ft rod to about 6 inches below the surface, but there are absolutely no rocks in the soil where I live.

Mike

 

Harley Davidson
 

THANKS for this detailed information Mike.

I in fact myself years ago put a grounding rod in when I updated my electric service box. I used a 3 lb hand drilling hammer to put it in the ground, very easy.

The clamp is the very same I used. And like you said only takes an extra minute to connect so I will do the same.

tony

On 3/17/2023 11:48 PM, M Hambrick wrote:

Hi Tony

To your questions:

Although 12-gauge wire will work, I used a 4-gauge grounding cable because I happened to have a piece about 3 feet long. The clamp that attaches to the grounding rod is designed for the heavier grounding cables that are used in residential installations. It is attached. with a wrench. 

The end that attaches to the pier is a crimp-on copper ring terminal designed for 4-gauge wire with a 1/4" ID ring. I also got it at Home Depot. I have a 1/4" brass bolt attached to the pier for connecting the ground cable.

Yes, you have to remove the cover to get to the grounding cable. I pull the cover off, get the cable, and run it through the hole, then attach the cable to the pier with a 7/16" wrench. The whole process takes less than a minute.

I got the valve box with cover at Home Depot (see below). I keep the ground cable permanently attached to the grounding rod. The cable is about 3 feet long, and I just coil it up and put it in the valve box when I am not using it. I buried the grounding rod about 6 inches below ground level and dug out the dirt to install the valve box. I keep a rubber stopper in the hole when I am not using it. This keeps the "critters" from making a home in the dark space under the cover.

There are a number of You-Tube videos that demonstrate how to bury a 10 ft long ground cable. The best one I saw had you starting by just jamming the ground rod into the ground, removing it, and filling the hole with water. Then repeat the process until it is buried as deep as you want. I was amazed at how fast it was. It took less than 5 minutes to sink a 10 ft rod to about 6 inches below the surface, but there are absolutely no rocks in the soil where I live.

Mike

 

DFisch
 

Mike what a great share. Simple concrept but the solution is so clear when you write a great recipe, thanks!
Tom

On Sat, Mar 18, 2023 at 10:33 Harley Davidson <astrocnc@...> wrote:
THANKS for this detailed information Mike.

I in fact myself years ago put a grounding rod in when I updated my electric service box. I used a 3 lb hand drilling hammer to put it in the ground, very easy.

The clamp is the very same I used. And like you said only takes an extra minute to connect so I will do the same.

tony

On 3/17/2023 11:48 PM, M Hambrick wrote:
Hi Tony

To your questions:

Although 12-gauge wire will work, I used a 4-gauge grounding cable because I happened to have a piece about 3 feet long. The clamp that attaches to the grounding rod is designed for the heavier grounding cables that are used in residential installations. It is attached. with a wrench. 

The end that attaches to the pier is a crimp-on copper ring terminal designed for 4-gauge wire with a 1/4" ID ring. I also got it at Home Depot. I have a 1/4" brass bolt attached to the pier for connecting the ground cable.

Yes, you have to remove the cover to get to the grounding cable. I pull the cover off, get the cable, and run it through the hole, then attach the cable to the pier with a 7/16" wrench. The whole process takes less than a minute.

I got the valve box with cover at Home Depot (see below). I keep the ground cable permanently attached to the grounding rod. The cable is about 3 feet long, and I just coil it up and put it in the valve box when I am not using it. I buried the grounding rod about 6 inches below ground level and dug out the dirt to install the valve box. I keep a rubber stopper in the hole when I am not using it. This keeps the "critters" from making a home in the dark space under the cover.

There are a number of You-Tube videos that demonstrate how to bury a 10 ft long ground cable. The best one I saw had you starting by just jamming the ground rod into the ground, removing it, and filling the hole with water. Then repeat the process until it is buried as deep as you want. I was amazed at how fast it was. It took less than 5 minutes to sink a 10 ft rod to about 6 inches below the surface, but there are absolutely no rocks in the soil where I live.

Mike

 

--
TJF MOBILE
Linwood Ferguson
 

I think it is worth highlighting Greg's post and asking what problem you are trying to solve by using a separate earth ground at the tripod? 

If this is a "separately derived" system, such as a battery or generator it may make sense. 

If your home AC power is involved, a lot depends on how the power supply handles the AC ground that comes from your house.  Some AC->DC power supplies may ground the common (almost always -) output to the earth ground, some may isolate it completely and let it float, some may couple it less completely. Then there are power supply cases which may be in contact with metal on the tripod and may or may not be earth grounded through AC. 

When you ground the pier/tripod (and cases of some equipment connected to it) together, this MAY mean that there is now a path from that earth ground at the tripod back to your home's earth ground, which may not be a good thing. In a permanent observatory you probably have already crossed this appropriately and to code, but spontaneously establishing a new earth ground on a tripod may or may not be a good thing. 

Consider this - go outside and drive two ground rods 100' apart, and run an insulated cable between them, and you will likely get a significant voltage between them which will vary with weather, water content, etc. Connect that wire between them and current will flow trying to equalize large portions of ground to the same potential. It's the kind of flow that can cause noise, and it can even affect safety devices like GFI breakers. 

When one builds a separate building you cross this, and presumably do it to code - you might run the ground and neutral from the house and not separately ground it, you might separately ground it in the new building. How everything is connected in either case (for AC) is clearly but not simply specified in code, and if solar and generators are involved it gets even more complicated.  But doing this ad hoc for an outdoor tripod should include a similar thought process to make sure you do not inadvertently cause issues. It may involve some investigation into how/if your power supplies are grounded as well.

And note most of my thoughts here relate to US AC power, your country may have different code and even different home wiring. Note I am not an electrician, so take all this as basically saying "be sure you do it right" not "this is how you do it".

Linwood
M Hambrick
 

Hi Linwood

I am not sure I understand anything you said.

There was a thread on the forum a while back where it was suggested that some issues that someone was having might be due to grounding. The suggestion to ground the pier or tripod was along the lines of "It may not solve the problem, but it will not hurt anything either".

Mike
Linwood Ferguson
 

On Sat, Mar 18, 2023 at 12:46 PM, M Hambrick wrote:
There was a thread on the forum a while back where it was suggested that some issues that someone was having might be due to grounding. The suggestion to ground the pier or tripod was along the lines of "It may not solve the problem, but it will not hurt anything either".
Hopefully someone more knowledgeable about both code and noise can answer more fully.

I am sure that bonding DC common's together fall into that category (may not help, won't hurt). 

Earth grounding in different physical locations when there is also a grounding conductor between them is more complicated. 

Anyone with credentials or knowledge in this area care to dive in? 

Linwood
Jeffc
 

No credentials here.  

My understanding is you can have a ground loop problem (current through the ground) if the ground rods of (for example) the house and ground rods used by other equipment is not “bonded” (ie the ground rods are not connected with an electrical conductor.) 

I have this problem now with one of the ham antennas here.   The antenna support mast is effectively “grounded”.   When I disconnect the antenna coax, there’s a small current between the coax shield and the antenna mast. The only time I “notice” this current is if I have my hand on the mast and touch the disconnected coax outer part of the connector.  (There’s a small tingle.)
That coax shield goes back to the radio, and the radio is bonded to another ground on the other side of the property.  (It’s an unfortunate setup.)

As for the portable pier… I assume there’s a house there with property grounded electric.  With the portable pier in the backyard, if it is self powered (eg battery) then there’s no other path back to other (house) ground rod system.  Afaict no problem.  

If the portable pier equipment is powered by AC from the house electric (eg AC/DC power supply) , and that equipment is grounded at the pier (only), then I think you can have a ground loop between house and pier.  
The two ground rods (house and pier) should be tied together somehow.  
Probably what’s going on is the electric run to the pier has a ground wire (all modern wiring does afaict).  Perhaps the pier ground rod is tied to that electrical supply ground which then ties the ground rods together. (Tho I’ve always seen the bonding shown as a separate wire buried in the ground connected directly to the rods.)

HOWEVER:

A) if you are installing an outlet in the backyard somewhere you should be using a licensed electrician. 

B) Said licensed electrician can provide the proper locality code requirements for grounding, and do the job correctly. 

Seems to me running permanent AC to a pier in the backyard would be the same as installing AC outlets outside, or in an outbuilding.  

Now if the ground rod is only attached to the pier… I would wonder why.    I could see that if there was something fluke RF going on, or concerns about lightning or static electricity.  But in that case I would want to make sure the electrical ground (AC or AC/DC) is isolated from that ground.   Just my opinion.  Not advice. 


On Mar 18, 2023, at 10:02 AM, ap@... wrote:

On Sat, Mar 18, 2023 at 12:46 PM, M Hambrick wrote:
There was a thread on the forum a while back where it was suggested that some issues that someone was having might be due to grounding. The suggestion to ground the pier or tripod was along the lines of "It may not solve the problem, but it will not hurt anything either".
Hopefully someone more knowledgeable about both code and noise can answer more fully.

I am sure that bonding DC common's together fall into that category (may not help, won't hurt). 

Earth grounding in different physical locations when there is also a grounding conductor between them is more complicated. 

Anyone with credentials or knowledge in this area care to dive in? 

Linwood

M Hambrick
 

Well, this is all very interesting discussion. Thanks for the comments Greg, Jeff, and Linwood

The AC power to the house is properly grounded.

When I am set up in the back yard, I am running off of two of the Powerwerx 25 Amp variable voltage power supplies that are plugged into an AC outlet at the house. Each power supply feeds into a RigRunner that is attached to the pier. You can see one of the RigRunners in the photo that I posted above. One of the RigRunners supplies power to the mount and camera(s), and the other powers all of the other equipment (dew heaters, laptop, etc).

The ground rod that I sunk next to where I set up my pier is not connected to the ground rod at the house.

Based on the comments above, it sounds like there is no need to connect my pier to the ground rod, and in fact, doing so could cause more harm than good. Is my understanding of this correct ?

Mike
Linwood Ferguson
 

On Sun, Mar 19, 2023 at 09:04 AM, M Hambrick wrote:
Based on the comments above, it sounds like there is no need to connect my pier to the ground rod, and in fact, doing so could cause more harm than good. Is my understanding of this correct ?

My goal was to raise the issue more than say I knew the definitive answer, hoping someone would definitively say. 

Here is what I think I do know: 

In the US, this is all covered in the very convoluted NEC section 250. 

The earth ground (ground rods) for the house must be bonded to the neutral conductor at the service entrance and ONLY at the service entrance. Up until 2008 if you put in a separate building (say an observatory powered from the house) it was permitted to put in separate grounding and bond neutral and ground at the observatory as well as at the main house. That is no longer permitted, neutral and ground can be bonded ONLY at the single service entrance (generators and separately derived systems are different). 

Outside outlets must have all three conductors (for 110v), ground, neutral and load. The ground there must come from the house, and any metal case of that outlet must be bonded to the ground conductor from the house.  It is not (any longer for some time) permitted to run just neutral and load. 

Different areas of earth may have different potentials due to water content, atmospheric induced charges, etc.  If two such are bonded together with a ground conductor, there is current flow between them.

If you put in a separate ground rod you MUST NOT connect it to the neutral conductor in the field, period (again, unless a generator/etc is involved). 

Local building inspectors can make up their own rules to supplement the NEC, so local rules govern, including which revision of the NEC they adopt. 

What I almost know: 

I can find no prohibition nor requirement for separate ground, so if you have a ground rod outside, I cannot find anything that says you may not connect it to the ground conductor from the house (note that "I cannot find" doesn't mean it is not in there, but I did look), nor requiring you to do so, but as @Jeffc advised if you do not, you may have a significant potential difference between them.

Now speculating: 

If the NEC is silent on the issue it says to me that it is safe either way.  The NEC is about not dying and not catching on fire, it doesn't specify "best" only safe. 

If you have your grounding conductor from the house AND a ground rod at the site, where ever they touch (if they touch at all), there is likely to be some amount of current flow from natural potential differences (conversely by touching they pull those areas into the same potential so you do not risk the kind of unexpected current flow @Jeffc described)

If you have a metal pier sunk in the ground you already have this situation regardless and it would be difficult to avoid it (e.g. insulating cases to keep them from touching it). So doing it with a tripod is basically the same thing.

I would THINK that such current flow is of very low frequency and unlikely to generate objectionable noise, but I really do not know. 

---- 

I am not trying to say do not do this, nor saying do this (well, other than "do not bond neutral to that ground" that is unsafe and illegal). Just that there is an implication for a ground loop and current flow that it brings that one should understand.

My suggestion of bonding the DC commons together is completely different, I think that is always a good thing.

I'm sorry if this tangent was confusing. 

Linwood

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