Can voltage drop be a problem?

[kerrylewis]

I have a mains power supply indoors to serve the AZ-EQ6 and because of its position and the scope location the cable is rather long- about 12 m. Is this likely to affect the operation of they mount? I find that alignment and tracking are not always as they should be. I'm thinking that I might try a different location for the psu with shorter cables.

[Stu]

I do think it can be an issue, mounts can start doing strange things if battery voltage drops so no reason to think it would be any different. I have certainly seen better performance since using a decent psu.

I have my supply next to the scope with short 12V cables, can you not do likewise?

Cheers,

Stu

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[sbooder]

I have a 20m length of cable running from the house to my mount and I have never had a problem.  I do use a 15V 5A power unit because I wanted the mount to receive 13.5V and I put my tester kit on the end and I get about that even when the cable freezes in winter but I do use the orange heavy duty extension cable.

[pete_l]

There are two problems that I have seen.

The first is that the current through a long cable causes a voltage drop. So you start off with 12 Volts at your supply and that has "mysteriously" dropped a Volt or two at the far end. Often when a motor starts it causes a surge which exacerbates the voltage drop (and isn't noticed when you measure the "steady state" voltage with a meter).

The second problem I have seen is with some manufactured cables. Sometimes they use crimped joints which are not properly crimped. That causes poor contact - sometimes after a long period as the joint corrodes. Not only do you get a voltage drop across the bad joint, but I've had one cable melt at the plug end due to the heat from the bad joint.

However, I should add that these issues have all been with low-voltage cables, not with mains (as the mains supply doesn't draw much current). Out here I'm right at the end of a rather dodgy electricity supply. That frequently drops to 210 V and when I turn the immersion heater on, 190V is not unknown. Luckily all the electronics has regulated supplies, so the droopy mains doesn't have an effect on the equipment  - it just causes the lights to go a little dim.

[kerrylewis]

Hello again Simon

And yes Stu I can put the supply closer, it will just need some fiddling around to set it up in my garage.

I used to have the supply next to the scope and plugged into a mains extension lead but I didn't like that arrangement especially when conditions were a bit damp.

I'll give the garage location a go I think

Cheers

Kerry

[kerrylewis]

And thanks Pete

I did measure the voltage 'steady state' and it seemed ok but, as you say, I wasn't confident that it gave the true picture when motors started whirring

[Paul M]

The voltage drop is a function of the cable resistance and the current being drawn by the equipment.

Ohm's Law is your friend.

Cable resistance is decided by its cross sectional area and length.

So a long thin cable will sufferer greater volt drop than a short heavy one.

The size of the power supply has no bearing on cable volt drop.

Without looking up cable sizes I can't give precise fingers but it's quite easy to drop a couple of volts on a long run of cable running a couple of Amps.

Resistance=current/volts

2 amps/ 1volt = 1 ohm

So a cable ristance of 1 Ohm will lose you 1 volt at your mount.

So that's only. 0.5 ohm's per conductor, positive and negative.

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[trazor]

I used to have the supply next to the scope and plugged into a mains extension lead but I didn't like that arrangement especially when conditions were a bit damp.

Kerry

You need one of these...

[kerrylewis]

Have moved the power supply to the garage now. The cable is still about 5m long but I'll see if there is any difference in the behaviour of the mount before trying other options

[Lonestar70]

The voltage drop is a function of the cable resistance and the current being drawn by the equipment.

Ohm's Law is your friend.

Cable resistance is decided by its cross sectional area and length.

So a long thin cable will sufferer greater volt drop than a short heavy one.

The size of the power supply has no bearing on cable volt drop.

Without looking up cable sizes I can't give precise fingers but it's quite easy to drop a couple of volts on a long run of cable running a couple of Amps.

Resistance=current/volts

2 amps/ 1volt = 1 ohm

So a cable ristance of 1 Ohm will lose you 1 volt at your mount.

So that's only. 0.5 ohm's per conductor, positive and negative.

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OOps!!... Not quite correct Paul...

Resistance = Volts / current : - 1 / 2 = 0.5 Ohm.

Voltage drop in cable = Current x Resistance : - for a 2 amp load = 2 x 0.5 = 1v drop between Battery + and Mount +.

There would also be a drop of 1v between Mount - and Battery - for the return wire... since the same 2 amp current is flowing through it but in the opposite direction and it's resistance would be the same.

Kirchhoff's Law... In a series circuit the sum of the current leaving the circuit is the same as that entering.

If the total voltage drop between Battery + and Battery - of 1v is required then the cable resistance would need to be 0.25 Ohm in each wire... i.e a total resistance of 0.5 Ohm.

To the OP...

The best way to achieve a safe mains powered set up is to keep the Mains/ DC Power Supply in doors (or under good cover) and feed a heavy cable out to a small distribution box close to the mount... this box can have multiple outputs using smaller DC connectors, or Cigarette type sockets... from which short smaller dia cables can be run to the mount etc. with little fear of any major voltage drop. 

The Mains/DC power supply should be capable of providing a stable 13.8v - 15v output voltage at a continuous load current of at least 30% higher than the total required load current... this should take care of motor start surges etc.

Hope this helps.

Best regards.

Sandy.

[Paul M]

OOps!!... Not quite correct Paul...

Resistance = Volts / current : - 1 / 2 = 0.5 Ohm.

Voltage drop in cable = Current x Resistance : - for a 2 amp load = 2 x 0.5 = 1v drop between Battery + and Mount +.

There would also be a drop of 1v between Mount - and Battery - for the return wire... since the same 2 amp current is flowing through it but in the opposite direction and it's resistance would be the same.

Kirchhoff's Law... In a series circuit the sum of the current leaving the circuit is the same as that entering.

If the total voltage drop between Battery + and Battery - of 1v is required then the cable resistance would need to be 0.25 Ohm in each wire... i.e a total resistance of 0.5 Ohm.

To the OP...

The best way to achieve a safe mains powered set up is to keep the Mains/ DC Power Supply in doors (or under good cover) and feed a heavy cable out to a small distribution box close to the mount... this box can have multiple outputs using smaller DC connectors, or Cigarette type sockets... from which short smaller dia cables can be run to the mount etc. with little fear of any major voltage drop. 

The Mains/DC power supply should be capable of providing a stable 13.8v - 15v output voltage at a continuous load current of at least 30% higher than the total required load current... this should take care of motor start surges etc.

Hope this helps.

Best regards.

Sandy.

I offered my contribution as an example of why one shouldn't post things that need thinking about after having already started the weekend. If you know what I mean...

I only got worse as the evening wore on, but at least I wasn't posting here..... at least..er...I don't think I was...!?

The shame of it is that it was my trade until 4 years ago  

[kerrylewis]

Good reply Paul!

I'm grateful for all the responses. I'm going to try the shorter cable set up and then possibly switch to a heavier cable as suggested by Sandy

Cheers

Kerry

[Lonestar70]

Good reply Paul!

I'm grateful for all the responses. I'm going to try the shorter cable set up and then possibly switch to a heavier cable as suggested by Sandy

Cheers

Kerry

Yes... excellent reply from Paul  ... I know the feeling my friend  

Kerry,

If you can get hold of some 'Arctic Blue' cable (2 core or 3 core) with 2.5²mm conductors this will be ideal.

Just use 2 of the cores if you get 3 the core type.

It is usually sold in most DIY stores as outdoor extension cable and can usually be bought by the metre... it may be marked up as 25 Amp cable.

This has a resistance of 0.00798 ohms per metre.

A 5 metre length @2amps would have a resistance of 0.0399 ohms and would drop 0.0798 volts per wire at 2 amps... this gives a total drop of 0.1596 volts between the battery/PSU + and Battery/PSU -

10 metres would double this voltage drop to 0.3192 volts total.

It is quite important to keep the total voltage drop to a minimum for the following reason: -

The voltage drop in the return wire (-) effectively places the mount electronics at this voltage above ground... this can create problems with noise immunity... and can often be the cause of loss of connection or poor data transmission with a PC via USB link... if allowed to get much more than 0.5 volts it can cause the mount to reset or do other unwanted/unpredictable things.

Hope this helps.

Best Regards.

Sandy.

[alan potts]

The mains voltage would have to drop a massive amount to affect a transformer at around 13 volts, according to my Father-in-law who was manager of the sub-station here. I have a 3 phase supply and that varies from 216- 235 volts depending on the phase, bear in mind these transformer are all the same with diffent plugs. On my mains unit having just checked on low phase I am getting 13.6 volts from my Power Mac transformer.

I think it is more likely to be the same issue I has poor quality connector at the scope, it is still not brilliant now and I have upgraded though I have the Pro 5 SW.

Alan.

[Lonestar70]

The mains voltage would have to drop a massive amount to affect a transformer at around 13 volts, according to my Father-in-law who was manager of the sub-station here. I have a 3 phase supply and that varies from 216- 235 volts depending on the phase, bear in mind these transformer are all the same with diffent plugs. On my mains unit having just checked on low phase I am getting 13.6 volts from my Power Mac transformer.

I think it is more likely to be the same issue I has poor quality connector at the scope, it is still not brilliant now and I have upgraded though I have the Pro 5 SW.

Alan.

Hi Alan,

Yes your father in law is correct... a good quality mains PSU should output a stable 13.6v - 13.8v DC over a wide variation in actual mains input voltage.

This is not quite the same problem as Kerry is having... his problem is the resistance of the connectiing cable between the PSU and the mount... i.e. his 13.6v -13.8v is reduced to a lower voltage by the cable... thus the mount is only receiving 12.6v to 12.8v... this voltage drop will increase with the amount of current being drawn and could easily bring the mount voltage down below the minimum required, even for a short period, when, for example, his mount motors both start up (surge currents can easily be 4 or 5 times running current) to slew the mount. 

This would certainly become a problem using a 12v battery for power... especially as it's output voltage dropped during use.

As to the quality of the connectors used... I agree, a poor quality one will add to the resistance or provide intermittent contact... or both.

Keep happy.

Best Regards.

Sandy.

[Paul M]

I'm tired and between shifts so I'm staying away from Ohm's Law in this post. But....

Volt drop due to cable resistance is always a greater concern at low voltage. The automotive standard 13.8v supply voltage isn't a great voltage for transmitting high power over long distances.

There is a very good reason why the National Grid is transmitted at such insanely high voltage - Losses! 

By the time it reaches the customer's house it's been stepped down to a more user friendly voltage which can still be carried over quite long cables without too much loss.

The Wiring Regulations require that Voltage Drop be verified as being within certain parameters on fixed wiring and the appropriate cable selected to ensure it is compliant.

So it is an issue at mains voltages but not something the consumer needs to worry about.

Anyway, most of the 12v DC (13.8) equipment we use will be supplied from regulated power supplies which will give a stable output across a range of output loading and also input voltage. So (ignoring Switch Mode power supplies which can be fussy about input voltage) your average Series Regulated power supply will give good performance over a huge range of input voltages. In the UK mains voltage should be between  216v and 253v so dropping a fraction of a volt over domestic circuits is not a huge problem for electronic equipment. Though you'd prefer your 8.5kW electric shower to have nearer the higher voltage limit at it's terminals!!

That's why it's best to get the PSU somewhere near the equipment and keep DC cables as short and chunky as possible. 

[jackrussell0232]

Threads like this always make me smile.  Everyone is really giving the same answer, but in an attempt to justify it, or just throw in an extra analogy, it's easy to get something the wrong way round.  So here's my attempt to throw myself on my sword and re-write the laws of physics courtesy of a typo or grammatical error.  (no digs at anyone intended   ).

'Volt drop' along a conductor (power cable) is proportional to the current (I) which flows through it and the resistance ® of said conductor (not applied voltage).  This is the V=IR bit.  But current is of course proportional to voltage and inversely proportional to the resistance.  I = V/R.   The power dissipated by the conductor is equal to the voltage drop across it multiplied by the current going through it (P=VI) or V squared/R or I squared x R.

This is why the national grid lines convert the voltage from generators up to 400,000 volts to travel the around the country, before transforming it back down to 240v via a series of substations.  The send a higher voltage at lower current because this means less electrons flowing (but with much more pressure behind them), so less heating of the cables and less volt drop.  Otherwise, the cables would need to be half a metre diameter!  A lot of electrickery is just like plumbing (inc kirchoff's), but this practice is quite the opposite - it would be like pumping water from a reservoir through very thin pipe at huge pressure, because it was more efficient. ( Water distribution can use fat pipes because they're hollow and go underground).   It is a bit like hydraulics though, where you can develop the same force (deploy huge energy) with very little fluid movement, if it's under huge pressure.

High resistance (weedy cable) and high current (motors starting up) are you enemy.  Just watch someone trying to jump start a car with comedy jump leads - 100+ amps being drawn means the 12v which the good car battery could supply is all lost across the cables.

If your AZ eq is like my old style eq6, it will draw about 3A peak while starting to slew both axes, and this will rapidly drop to just over 1A, but if the 3A surge causes the voltage to drop to 11v, you might have fun and games.  This might only need about 0.5 - 0.8 Ohms between all your cables, connectors and everything else and you can't measure that reliably without a specialist milliohmmeter.  So if you're running around 10m I'd be tempted to use 2.5mm cable for sure.  

Phew, is that the longest answer?

Bed time for me, exhausted now,

Jack 

[101nut]

With all this discussion about voltage drop why not just do the obvious? Use a mains extension lead (protected with an RCD at the *house* end) into a dri-box (or similar) at the mount end with a 4-way block containing the psu and anything else you need to keep dry (pick a box size to suit). No voltage drop worries, nice and neat and tidy.

AndyG

[kerrylewis]

Thanks everybody for your input.

The bottom line is that it is more for me to have the mains power supplies permanently set up, either in the house in or the garage. However, if the resulting longish runs of 12v cable mean that the mount is likely to behave erratically, then it's obviously not worth it.

I'm waiting to try out the current(!) layout with the PS in the garage and if that doesn't seem to work it will be back to Plan A with the ps near to the scope but with suitable protection devices. 

Cheers - Kerry 

[Stu]

Sorry if I've missed this in the thread Kerry, but have you upgraded the cable run from the PS to the mount? Thicker/heavy duty cable for the 12v section will reduce the voltage drop and give you best chance of success.

Stu

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[Vega]

12m is gonna cause voltage drop for sure. Longer AC and shorter at the DC end is definately the answer. . Keep the DC cables pretty hefty too. As for weather protection, there are some pretty neat outdoor junction boxes around online. At least your source is coming from the grid. I use deep cycle batteries to power my EQ6 which used to trip me up in the winter months (until I invested in 200w of solar panels  )