Power Supply for AZ-EQ6 Mount

[Ant-33]

I've recently built a power supply for my AZ-EQ6 mount, so I wanted to share with you why and how I did so.

Normally Skywatcher advise using the mount with their battery power tank - but what if you want to use it in your back garden and don't want the nuisance of maintaining a battery power supply? The Skywatcher manual for the mount, on the topic of power supply requirements, reads as follows:

My first step was to use a 12V 10A adaptor purchased online - one that looks somewhat like a notebook computer power supply. That was great - it worked, and the mount worked fine. There was just one problem. I kept getting small electric shocks from the exposed metal on the mount. Nothing life-threatening, but uncomfortable, and worrying. I raised the topic with the manufacturer of the mount, and they told me this:

The slight shock might be caused by the AC-to-DC adapter which does not have good insulation between the DC output and AC input.

If an AC-to-DC adapter is to be used to power Skywatcher's GOTO mount,

1. Use an AC-to-DC adapter which has an grounding pole on its AC side plug, and the negative pole of the DC output should also connect to the grounding pole of the AC plug. Make sure the AC power outlet is grounding properly.

or

2. If grounding is not available, users should use high quality AC-to-DC adapters which has good insulation between AC input and DC output.

Okay - but I couldn't find a power supply in the UK that specified its grounding in a way that conformed with this. Here's the problem:

Grounding is not just for fun. It's not just to stop you from getting tingly feelings from the exposed metal of an appliance. It's there to save your life in the event of a fault developing in the appliance or power supply. For example, if the exposed metal is floating to some voltage that isn't zero, and the power supply goes faulty, that exposed metalwork could suddenly be at 240V ac. That would probably be fatal, If it's properly grounded the fuse would blow and your life would be safe. Unless, of course, the earth went faulty at the same time, but that's two simultaneous faults, which is really unlikely.

So I set about building myself a power supply to meet all these requirements. I wanted it to be a 15V switched mode supply capable of 5A. The real work is done by an RS-75-15 switched mode 15V 5A power supply, stock number 644-6979 from RS:

You can see this power supply on their website here: Embedded Switched Mode Power Supply

I mounted it in an equipment box from Maplin:

As you can see, I used crimped connectors for most of the wiring. You'll see if you look closely that I've connected the mains earth to the 15V -ve terminal to ground the negative line.

[EDIT: after some of the discussion below, and more communications with both RS and Synta, I have disconnected this earth connection between AC gnd and DC -ve. The RS power supply should meet the requirement of good isolation between AC and DC, so it seems that removing this connection is the safer option.]

The black unit you see on the back panel in this photo is a switched/fused IEC mains connector socket (Maplin again), which you can see from the other side here:

I fused it at 5A on the 240V side. On the front panel I have a mains indicator light, a 5A fuse for the 15V, a connector for the lead to the mount (more about this in a moment), and an ammeter so I can see how much current my mount is drawing:

For the power lead to the mount I was faced with a challenge. I wanted to use a connector that is suitable for power and rated for at least 5A. The socket on the front panel has to be female, as an exposed male connector shouldn't have live power on it. I opted for a 3-pin din connector, partly because I had the plug already and partly because it's ideal for the job.

The GX12 2-pin connector to connect to the mount is easily available on Ebay, and I used two core wire rated at 10A.

The RS power supply has a voltage adjuster, which I adjusted to read 15V on my voltmeter. When connected to the mount, and the mount is powered up all works well, and the mount handset reads the power voltage as 14.7V. I've soak tested this arrangement, and mount and power supply both seem happy.

[jambouk]

Cool. Nice work.

I run mine from a modified laptop charger, also at 15v I think, and like you used to, I get the occasional "tingle" - far from ideal I know. I'm always surprised the mount just doesn't earth itself as the feet are on the ground.

James

 

[Neil27]

Wow great solution, is this similar to the design of the Maplins one rated at 13.8v?

Also when are you starting production ?

[Owmuchonomy]

I took the easy route and purchased the ready to go 5A 13.8V continuous regulated fused supply from Maplins for £40 for my AZEQ6.

[MarkyD]

On 10/04/2016 at 21:39, Ant-33 said:

 You'll see if you look closely that I've connected the mains earth to the 15V -ve terminal to ground the negative line.

Not a good idea.  These PS have isolation transformer in them and you have just shorted out the isolation to the secondary.  They also have Y suppressor capacitors across the L-N, N-E and L-E to remove any spikes/surges and you have just introduced these spikes to the isolated ELV output possibly putting a few 100 volts or even 1000 volts into the DC circuit which will make toast out of your mount. It will also corrupt the feedback circuit for the PWM control.

Also if these capacitors fail (which they do occasionally and when they do they usually short) you can have the situation of 230V AC on the ELV side.  If your 230V supply is not up to current 17th edition regs (30mA RCD's etc) or has an open circuit on the CPC you have the possibility of having 230V AC potential on the mount and casing.  Electricity is lazy and will find the shortest/easiest route of least resistance back to it's source which is usually the via the human body in a fault situation

Remove the earth from the Output

--

Mark

[Ant-33]

2 hours ago, jambouk said:

Cool. Nice work.

I run mine from a modified laptop charger, also at 15v I think, and like you used to, I get the occasional "tingle" - far from ideal I know. I'm always surprised the mount just doesn't earth itself as the feet are on the ground.

Thanks, James - I guess the white finish on the feet stops any connection to ground.

1 hour ago, Neil27 said:

Wow great solution, is this similar to the design of the Maplins one rated at 13.8v?

Also when are you starting production ?

Not sure what design Maplins use, but I'm pretty sure the DC -ve isn't grounded, so there's still potential for the floating voltage problem

1 hour ago, Owmuchonomy said:

I took the easy route and purchased the ready to go 5A 13.8V continuous regulated fused supply from Maplins for £40 for my AZEQ6.

Yes, I nearly did that - plenty of people do, and FLO used to sell them on. I think it's the one the FLO people use on the bench, but they could verify that.

[Ant-33]

1 hour ago, MarkyD said:

Not a good idea.  These PS have isolation transformer in them and you have just shorted out the isolation to the secondary.  They also have Y suppressor capacitors across the L-N, N-E and L-E to remove any spikes/surges and you have just introduced these spikes to the isolated ELV output possibly putting a few 100 volts or even 1000 volts into the DC circuit which will make toast out of your mount. It will also corrupt the feedback circuit for the PWM control.

Also if these capacitors fail (which they do occasionally and when they do they usually short) you can have the situation of 230V AC on the ELV side.  If your 230V supply is not up to current 17th edition regs (30mA RCD's etc) or has an open circuit on the CPC you have the possibility of having 230V AC into the mount and casing.  Electricity is lazy and will find the shortest route back to it's source which is usually the via the human body in a fault situation

Remove the earth from the Output

Hi Mark - thanks for this feedback. You obviously know a lot more about how the RS power supply is designed than me. When I was planning this project I contacted RS and asked them whether it's okay to connect the DC -ve to the mains ground. This was their reply:

Re: 644-6979 -  floating grounds
Thank you for your switch mode power supply enquiry.
The output negative could be grounded to the mains ground to prevent a floating voltage.
However it should be noted the electrical isolation between the input and output will no longer exist due to this direct electrical connection. 

I guess I'd be a little disappointed to discover that this reply really means that the grounding is dangerous to the mount. I've raised your concern with them, so I'll post again when I get a reply.

Referring back to the Skywatcher power supply requirements, they specifically say use a switched mode power supply and connect the DC -ve to the mains ground - and that's what I've done. If you test the connections on the mount itself, you will see that the esposed metal of the mount is connected directly to the DC -ve pin of the power connector. If the exposed metalwork isn't earthed, that strikes me as a classic example of a dangerous non-earthed appliance. Normally when you have an appliance that uses an un-earthed supply, there is no exposed metal that has any direct connection to the power + or - (is this called 'double-insulated'?). That's not the case with the AZ-EQ6.

Thanks again for your reply. I'll get back to you when I hear back from RS

[jambouk]

Ant,

Should I drop a highly conductive cable from some metallic part of the mount into a spike in the ground when I set up? Would this help to earth the mount and limit the chance of me getting a little shock, or worse?

James

 

[Ant-33]

Just now, jambouk said:

Ant,

Should I drop a highly conductive cable from some metallic part of the mount into a spike in the ground when I set up? Would this help to earth the mount and limit the chance of me getting a little shock, or worse?

James

 

Hi James, maybe - I know that some other people do this - but for safety's sake I was trying to stick with what Skywatcher recommend, so for me to suggest otherwise may be otherwise than wise - if you get my meaning. 

This raises a problematic issue. As far as I can tell, there's no power supply commercially available in the UK as a finished unit that meets the Skywatcher specification. Most people seem to settle with the little electrical tingles, but I'm not convinced that's safe. I'm going to check my solution with Skywatcher before I commit one way or another - after all, they sell the mount in the UK, so they must have a useful response on how to power it.

[jambouk]

I'll devise some earthing cable.

James

 

[Ant-33]

5 minutes ago, jambouk said:

I'll devise some earthing cable.

James

 

Yes. I've sent the question off, so hopefully I can get a response from Skywatcher on this. Earthing is a thorny issue.

[blinky]

I used to get tingles from my mount, managed to blow it 3 times..... Turns out there was a direct connection from the pcb to +12v not ground, when the metal body of my dew band phono plug touched the mount it shorts it out! My last pcb I made an insulating washer and it's now isolated

 

[Ant-33]

1 minute ago, blinky said:

I used to get tingles from my mount, managed to blow it 3 times..... Turns out there was a direct connection from the pcb to +12v not ground, when the metal body of my dew band phono plug touched the mount it shorts it out! My last pcb I made an insulating washer and it's now isolated

 

So are you saying that on your AZ-EQ6 the +12V was connected to the chassis - as supplied? I've done a continuity test on mine, and the DC -ve is connected to the chassis. Strange if they're different.

[blinky]

I have an HEQ5 Pro, the circuit board where the screw connects it to the mount body, has the metal 12v plane exposed. I checked with a meter and the chassis of my HEQ5 Pro was indeed at +12v!

[MarkyD]

You should not need to earth the mount.  You don't earth your car? and you don't earth the mount when powered by a battery.  The wall socket power supplies that come with the mounts do not have an earth either.  The PS input is isolated from the ELV side by the transformer and is called SELV (Seperated Extra Low Voltage).  The double insulated items are called class II and really only applies to LV items (50V - 1000V).  If you are getting a "tingle"  from the mount I would say there is a higher potential than 12V or you are sticking your tongue on it

Well designed earthing is good but a badly designed earth system is as lethal as one that has no earth with Earth leakage faults (I'm talking about 230V installations here)

These are the three types of ELV circuits (<50V)

Selv - Separated Extra Low Voltage, by far the most common in most transformers etc etc, no potential to earth on the secondary side.

Pelv - Protective Extra Low Voltage, secondary has a Potential to earth, never seen anything employing this, Im sure there is just Ive never come across it.

Felv- Functional Extr Low Voltage, Has a functional earth, usually carrying leakage current. you wont see these unless you work for BT, the Functional conductor is normally Cream in colour.

--

Mark

[Ant-33]

9 minutes ago, MarkyD said:

You should not need to earth the mount.  You don't earth your car? and you don't earth the mount when powered by a battery.  The wall socket power supplies that come with the mounts do not have an earth either.  The PS input is isolated from the ELV side by the transformer and is called SELV (Seperated Extra Low Voltage).  The double insulated items are called class II and really only applies to LV items (50V - 1000V).  If you are getting a "tingle"  from the mount I would say there is a higher potential than 12V or you are sticking your tongue on it

Well designed earthing is good but a badly designed earth system is as lethal as one that has no earth with Earth leakage faults (I'm talking about 230V installations here)

These are the three types of ELV circuits (<50V)

Selv - Separated Extra Low Voltage, by far the most common in most transformers etc etc, no potential to earth on the secondary side.

Pelv - Protective Extra Low Voltage, secondary has a Potential to earth, never seen anything employing this, Im sure there is just Ive never come across it.

Felv- Functional Extr Low Voltage, Has a functional earth, usually carrying leakage current. you wont see these unless you work for BT, the Functional conductor is normally Cream in colour.

--

Mark

That makes good sense Mark - but Skywatcher said it should be earthed - that's as well as their recommendation to use a switched mode supply - so considering I don't want those tingly shocks, and don't want a risky installation, I'm not sure where that leaves me. Given my description of my supply above, do you think that it would be unsafe as shown, but safe if I removed the earth link from the DC -ve?

[Ant-33]

53 minutes ago, blinky said:

I have an HEQ5 Pro, the circuit board where the screw connects it to the mount body, has the metal 12v plane exposed. I checked with a meter and the chassis of my HEQ5 Pro was indeed at +12v!

Ah - interesting. Not sure I like the idea of it floating around at 12V - also, what if the power supply went faulty? When the chassis metalwork isn't insulated from the power supply it strikes me there's a risk, with a faulty power supply, of 240vac on the chassis.

[MarkyD]

10 minutes ago, Ant-33 said:

That makes good sense Mark - but Skywatcher said it should be earthed - that's as well as their recommendation to use a switched mode supply - so considering I don't want those tingly shocks, and don't want a risky installation, I'm not sure where that leaves me. Given my description of my supply above, do you think that it would be unsafe as shown, but safe if I removed the earth link from the DC -ve?

I personally would not earth the secondary as it defeats the object of the isolation in the PS transformer (SELV).  But in my profession (Qualified Electrician) you should always follow the manufactures advice and installation instructions.  If it all goes tits up and you have followed these then the manufacturer is at fault and not you - same comes with using a different wall adapter than the one supplied by SkyWatcher.  Also make sure that the instructions apply to the UK market and not say the USA who don't use earths in there domestic wiring but run at 110V with center tapped ground so the max fault voltage is 50 - 55V (the same as those yellow boxes you see powering tools on building sites)

--

Mark

[MarkyD]

Just as a note I use one of these to power my NEQ6. dew heaters etc.  I haven't earthed the output but if you look at the block diagram in the pdf there are decoupling capacitors from the V+ and V- to earth internally which would be bypassed

http://www.rapidonline.com/mean-well-sp-200-12-200-4w-12v-active-pfc-enclosed-power-supply-85-3847

--

Mark

[Ant-33]

16 hours ago, MarkyD said:

I personally would not earth the secondary as it defeats the object of the isolation in the PS transformer (SELV).  But in my profession (Qualified Electrician) you should always follow the manufactures advice and installation instructions.  If it all goes tits up and you have followed these then the manufacturer is at fault and not you - same comes with using a different wall adapter than the one supplied by SkyWatcher.  Also make sure that the instructions apply to the UK market and not say the USA who don't use earths in there domestic wiring but run at 110V with center tapped ground so the max fault voltage is 50 - 55V (the same as those yellow boxes you see powering tools on building sites)

--

Mark

I've got a message in with Skywatcher, so I'll see what they come back with. I've given them a description of what I've done and why, and of the concerns that have been expressed - I'll post back here when I hear from them.

16 hours ago, MarkyD said:

Just as a note I use one of these to power my NEQ6. dew heaters etc.  I haven't earthed the output but if you look at the block diagram in the pdf there are decoupling capacitors from the V+ and V- to earth internally which would be bypassed

http://www.rapidonline.com/mean-well-sp-200-12-200-4w-12v-active-pfc-enclosed-power-supply-85-3847

--

Mark

Yes, I see what you mean. I'm not sure whether the NEQ6 suffers from the tingly shocks that people have experienced with the AZ-EQ6 - do you notice any on your mount? I just don't want to use it unearthed and discover that I still get shocks from the mount body. It defeats the object of buying this power supply.

[MarkyD]

22 hours ago, Ant-33 said:

I've got a message in with Skywatcher, so I'll see what they come back with. I've given them a description of what I've done and why, and of the concerns that have been expressed - I'll post back here when I hear from them.

Yes, I see what you mean. I'm not sure whether the NEQ6 suffers from the tingly shocks that people have experienced with the AZ-EQ6 - do you notice any on your mount? I just don't want to use it unearthed and discover that I still get shocks from the mount body. It defeats the object of buying this power supply.

I get no tingles from my mount.  Below are the AC and DC readings I get from the mount to my Ring Main earth.  Nothing to worry about.  After reading up on the tingling people are experiencing it seems to be those that are using cheap Power Supplies or those wall socket adapters.  Neither have isolation transformers and usually tie the neutral to the ground as a common internally.  Easiest way to check is to see if you have continuity between the output ELV and the input.  I would never use a wall adapter or a cheap PS outside especially as we are using these when dew is most likely to form,  even with an RCD protected system.  Those that aren't RCD protected are dicing with death using any 230V appliance outside as the only protection they will have is overload protection which is a minimum of 32A (on a absolutely 100% perfect system) on a final ring circuit before any thing fuses or trips.  BTW a 32Amp MCB takes up to 160A to trip (240Amps for a 30A rewirable fuse).  It will not trip at 33Amps but will sit there getting warm.  This is why we have to test the Earth loop Impedence of wiring to ensure that the resistance is low enough to allow enough current through to trip the devices in 0.4secs or less - in a fault situation the larger the current the better to ensure that the devices trip or blow

This also worries me when I see others building there obs with no regard to power design and just "laying" some cables in the ground!

HTH

--

Mark

[Ant-33]

9 hours ago, MarkyD said:

I get no tingles from my mount.  Below are the AC and DC readings I get from the mount to my Ring Main earth.  Nothing to worry about.  After reading up on the tingling people are experiencing it seems to be those that are using cheap Power Supplies or those wall socket adapters.  Neither have isolation transformers and usually tie the neutral to the ground as a common internally.  Easiest way to check is to see if you have continuity between the output ELV and the input.  I would never use a wall adapter or a cheap PS outside especially as we are using these when dew is most likely to form,  even with an RCD protected system.  Those that aren't RCD protected are dicing with death using any 230V appliance outside as the only protection they will have is overload protection which is a minimum of 32A (on a absolutely 100% perfect system) on a final ring circuit before any thing fuses or trips.  BTW a 32Amp MCB takes up to 160A to trip (240Amps for a 30A rewirable fuse).  It will not trip at 33Amps but will sit there getting warm.  This is why we have to test the Earth loop Impedence of wiring to ensure that the resistance is low enough to allow enough current through to trip the devices in 0.4secs or less - in a fault situation the larger the current the better to ensure that the devices trip or blow

This also worries me when I see others building there obs with no regard to power design and just "laying" some cables in the ground!

HTH

--

Mark

Many thanks, Mark. You've been a lot more helpful than Synta. I've now heard back from them, but basically they say it's a matter of compromising between different risks, and that a  good quality grounded power supply will help eliminate leakage AC power to the mount body. That doesn't address any of the issues raised above (not usefully, anyway). For now, I'm going to go with the suggestion of disconnecting the link between DC -ve and AC gnd, and I'll see if I experience any problems. I'll also take measurements similar to the ones you showed. If all seems okay, I'll go ahead using it that way. After all, their original advice did include the following:  If grounding is not available, users should use high quality AC-to-DC adapters which has good insulation between AC input and DC output. The supply I'm using certainly qualifies as a high quality AC-to-DC adaptor, and it should have good insulation between AC input and DC output. In my comment above I quoted RS as saying (of connecting AC gnd to DC -ve),  However it should be noted the electrical isolation between the input and output will no longer exist due to this direct electrical connection.  So I can infer from their comment that without the link, there is good electrical isolation.

Thanks again for your help

Cheers, Tony

[jambouk]

I've been to look at my mount; I wonder if I notice the "tingles" more when I am using my plastic feet (anti-vibration, but I din't use them for that reason). I've just put the metal feet on plastic to isolate the mount from the ground and stuck the multimeter on the mount and the voltage fluctuates from -0.2v to +0.2v over the course of about 15 seconds. I've left it on to see the mount "charges up" and I'll check it again later. I'd be surprised if the tingles I have felt are caused by 0.2v! It's felt more like the 15v the mount is supplied with from the laptop charger.

james

[jambouk]

After an hour or so there was no change.

[Ant-33]

11 hours ago, jambouk said:

After an hour or so there was no change.

Fair enough. Mine has painted feet, so I think it's insulated from the ground I stand it on, with or without an insulating mat - I guess if that weren't the case, there'd be no tingles. I'm curious - earlier you said yo use a modified laptop charger - in what way is it modified?