Dew heater band PS has died after only one month.

[Rusted]

Hi,

I bought a 230V - 12V DC 5A 60W power supply for £30 for my three [6" aperture] Astrozap dew heating bands only one month ago.

These are controlled by a HitechAstro 4-way controller. Rated and fused at 6A total.

I have only used the dew heating system for a few hours but the controller's LEDs did not come on today when I needed them after a cold night.

The LEDs in the cigar type plug and the PS did come on.

Checked the output from the PS cigar socket on my DMM at only 5V DC.  Oh dear!

What do the experts suggest these bands draw in real life on a setting of [say] 50% [12 o'clock} on the controller?

Astrozap says a little over 1A per band max. Have I cooked the PS by asking too much of it?

Is such a PS really the best way to run multiple dew bands? I have mains in the obs.

I have seen LED lighting PSs with a claimed 100W output. Should I invest in one of these instead?

Thanks

[fifeskies]

Astrozap site says the 6 inch bands are 1.4 amps each , so 3 will be 4.2 amps , however when cold starting they may well draw a higher start up current till the bands warm up (resistance increases with temperature).

I assume you have a switch mode style power brick , the ratings on these can be a bit optimistic.

Checking with a DVM off load will not give an accurate result as these supplies need a minimum load to regulate and may be clamping down to 5v off load.

Do you have a small 12v bulb you can put across the output to check the onload voltage (a 15w car side light bulb is perfect), but it does sound like the unit has been damaged.

 

The controllers (or mine anyway) use pulse width modulation so are in fact on full load then off then on again at high frequency. Your power brick may not like the high current pulses.

I recommend a linear style PSU for all astronomical use , even dew bands, more reliable and no high frequency interference to have to worry about, can bleed onto USB cables and affect bandwidth. 

If you have a fixed obs then a big linear PSU will be an easy thing to have.

I have one in mine and have had no problems since installing it, a 13.8 radio ham type is probably best (mounts prefer the higher voltage). You will need to have some inline 12v regulation for the more sensitive equipment like cameras but my dew bands work fine direct from the 13.8 via the level controller.

 

 

[fifeskies]

used power supplies do pop up on here quite often.

I used a Maplin one at first then saw the bigger one for sale on a local site so upgraded.

 

Edited November 8, 2020 by fifeskies

[fifeskies]

[Dr_Ju_ju]

I run all my home garden system components i.e. computer, mount, cameras (3 off), hubs (2 off), focusers (2 off), dew controller etc. all off the 600w pc power supply, with no issues...

Personally, & dependent on the dew controller electronics, I'd just go for a 10A switched mode supply e.g. https://www.amazon.co.uk/Adapter-Charger-Monitors-Equipments-Connector/dp/B07JM7P3WZ/ref=sr_1_1_sspa?crid=2X5M96QJCSNV9 or one of the 'LED' power supplies that are used for 3D printers, looking for a good brand like Meanwell.... 

[Rusted]

Thank you very much for all the excellent information. I shall try the bulb test.

I have been using very old 12V 24VA H&M model railway power supplies for decades for other purposes.
Though they are only rated at 2A so can't manage all the dew bands simultaneously.

Sadly I have had two modern PS failures in the last few months. 

Investing in something more reliable, like a 10A bench PS,  to run everything in the observatory might be a good idea.

AWR drives + several DC motor focusers and controllers + dew bands & controller + PC monitor + LED desk lamp all add up to a lot of 12V PSs!

Unheated, cold and often damp accommodation may not be ideal for modern PSs. They clearly state "indoor use" only.

 

What about a modern 12V battery and "smart" charger? Is that a serious [affordable] option? Or something to avoid?

Thanks again

 

 

[fifeskies]

A decent battery and charging solution will work very well and give a stable supply.

With mains in my observatory I decided the big bench power supply was the easier solution as I don't have to consider battery health.

My supply is well above the rating of any use I will put it to and can just add 12v gear without having to consider the current draw issue.

(It can supply 25amps continuous).

 

One slight advantage of the big battery route (well 2 actually) , it can be used as a portable supply away from the mains (and 2nd it keeps the gear running during power outages).

I don't use my main rig away from the observatory so didn't need portability.

[Rusted]

Thanks. I have no need of portability.

One slight worry is the considerable expense of running a beefy PS continuously.
But then, my multiple PSs probably all add up to exactly the same thing.
I can't believe how many PSs I have running now. Both in and out of the observatory.

I had the AWR drives PS go a few months ago. Now the dew band PS.
Even the fibre Internet box PS gave up after a previous replacement died after two years.
So they still owe me one!

My secondhand, flea market bought, H&Ms have run continuously for 30 years!

10A might be a bit too conservative if I assume at least 5A continuous draw for the AWR drives + 5A for 3 heat bands.
The DC focusers are over and above that but very intermittent. Though they might still be the last straw.

Thanks again.

[Dr_Ju_ju]

If you already have mains power to the obsy, then forget about any batteries, unless you want to ensure power in the event of main failure (but that would also require additional kit e.g. Power Inverter.)

Don't worry about power supply running expense, as it will only incur big costs when actually doing loads of work (high current draw) else it's just trickling along...

[fifeskies]

The standby current of a big linear PSU is well below its output rating, once the initial current pulse charges the (usual) big output stabilising capacitors.

In use apart from a slight thermal loss , you will only be paying for the power you actually use, and this is liable to be similar to the losses in a bunch of powerbrick individual units , and perhaps actually less.

My big beefy bench PSU does not get very warm in use , I am sure if I was drawing anything like its output rating it would get fairly warm but I am only using a small percentage of that on average. 

[Rusted]

Thanks again.

I have just found a Mean-well RS-150 12V 12.5A 150W PS in a ventilated/perforated case. About £35 equivalent over here in DK.

That should have enough welly for all my needs with a reserve. Good guarantee.

RS-150.cdr - RS-150.pdf

I like the screw down terminals. Rather than messing with countless miniature jack & cigarette lighter plugs.

Any thoughts on this one? Or should I raise my sights a bit? Other specific recommendations?

Thanks

[fifeskies]

Looks to be a good match , a switch mode of course but nothing wrong with that ( I am old school and prefer the linear type ).

Will need put into some sort of external ventilated box to protect it from damp.

My only concern on these is the convection cooling aspect , my supplies above 100watt are all fan cooled, usually with a thermal switched or controlled fan that only runs when needed.

I would add a 12v fan run from the output to blow cool air over it, fit this inside the external box and it will stop it overheating.

 

You will be able to trim the output voltage a bit if you want to.

 

Use nice thick hook up cables and you will be fine , I use 2 x 0.75mm cables most of the time , these are nominal 6 amp rated , but still small enough to fit the common connectors.

 

 

[fifeskies]

While short circuit protected , you might still want to consider in-line fuses to protect your astro gear, especially drive motors. If they stall they will draw damaging high currents and without any in-line fuse this supply will continue to deliver the high current to the motors.

A small fuse panel with individual protected outputs is easily added

 

I have small inline boxes on all my cables with a fuse protecting the line. Use anti surge fuses to avoid surge nuisance problems.

(my supply can deliver up to 50amps into a stalled motor so very much needed in my case) 

 

Edited November 9, 2020 by fifeskies

[Rusted]

Thanks. Does the same problem apply to stepper motors? 

[fifeskies]

Stepper motors will be managed by the driver circuitry and stall current may well be limited by the controller, They will probably try a bigger current if they detect the motor not turning but not overdrive it (assuming a decent design).

The drive transistors may well fail before there is any motor risk as the boards tend to have small electronics that are less robust than the actual motors.

I would still go for an inline supply fuse myself just to be sure.

My inline box and fuse cost me only about £2 or £3, a lot cheaper than swapping out a damaged motor or driver board.

 

This is the one inline with my NEQ6 mount, fuse is near to the mount end for easy access , though it has not needed replaced so far.

 

 

Edited November 9, 2020 by fifeskies

[Rusted]

Thanks again.

Your knowledge of the subject obviously exceeds my own by an order of magnitude.

I would have no idea how to choose suitable fuse ratings for each bit of kit.

[fifeskies]

For dew straps , they should never draw significantly over the nominal current. So I use 2 amp fuses for my 4 inch dew straps rated at 8 watt (0.7amp) , but could actually probably use 1 amp.

My heaters work through a control box with 2 channels and 1 fuse each output at 2 amps , the contol box input cable has a 5a fuse

I have a 5 amp inline fuse for my NEQ6 to allow for a surge if both RA and DEC motors start together, and that seems fine , tracking the current is far below that.

1.5 times to 2x the nominal steady current is usually about right , though anything very sensitive to overload might need closer tolerance.

 

I have LED strip lighting in my observatory on 12v , that only needs a fuse slightly above the load rating as it is usually dimmed well down. So for a 4 amp length of LED strip (when on full) 5a can protect the circuit.

 

Photo shows Moon and Venus from the observatory with the LED lights on full for the photograph (usually dim red).

 

 

Edited November 9, 2020 by fifeskies

[Rusted]

Thanks for being so patient and helpful. All good stuff!

I use two, single 1W white LED overhead bulbs for maintenance floodlighting. Red 1W overhead LED bulb for imaging.
A red LED string provides almost no real light but provides useful boundary indicators around the dome base ring.

I lived in Rosyth for a couple of years as a 7-8 year old. All rebuilt now. Though Camdean school is still there. 

[fifeskies]

I'm about 30 mins from Rosyth.

Not that I have any call to go there.

For a while they ran the Rosyth to Zeebrugge car ferry and I took it once on holiday. Shame its gone as it made Europe a lazy sail away overnight for us.

Been to Denmark but as a touring student many many years ago.

 

I have a warm room for imaging so dont need to be at the scopes except when I'm enjoying some actual visual views , which I will always do.

 

 

Always happy to help out anytime.

[Rusted]

Wow! Impressive! Umpteen screens and everything is so tidy and well organised! 
I'm definitely not doing it right.   Only one, 25" AOC high res screen mounted on the massive pier.

My "warm room" is doing solar imaging when it is well over 80F outside!  87F shade air temperature is memorable.
I haven't done any night time imaging. Except for the Moon and Venus when well placed: High and early but dark enough! 

I have just ordered the MW RS-150W-12V-12.5a from RS. No MW dealers I could find in DK. 
It looks as if it has to be mounted to a big alloy plate. No problem there.
I shall have to shield the mains terminals for safety's sake. Perhaps add a fan and a digital thermometer sensor.
My present wiring "loom" looks an awful mess! 

[fifeskies]

The alloy plate will be needed as a heatsink I suspect.

Tidy now as just had a big clear out ready for the winter

 

You can never get away from the wires tho, my baby rig has them running in all directions.

 

(Its easier to be tidy when I have my bigger scopes on the pier)

 

 

 

Edited November 9, 2020 by fifeskies

[Rusted]

You have an excellent set-up.

And just as many trees as I have it seems.

I have discovered blade type fuses and holders are readily available at low cost.
Not sure if 12V blade fuses are fast blow or slow.
The marine types have transparent covers and even LEDS and switches.

[Dr_Ju_ju]

FYI, you can get resettable blade fuses e.g. https://www.ebay.co.uk/itm/Circuit-Breaker-Blade-Fuse-12V-24V-Resettable-5-30A-Marine-Rally-Automotive/321268071405 or even these panel mounting types that replace the cartridge fuses https://www.ebay.co.uk/itm/Resettable-Thermal-circuit-breaker-fuse-available-in-5-10-15-20-30A-35A/142027504609

[fifeskies]

Blade fuses (car fuses) tend to be slow acting, but good enough if you choose a slightly lower value than you would for a cartridge fuse,  not twice running current for certain.

I avoided too many LED monitored fuses as they add to light pollution for night imaging , but have my dew heater outputs on LED so I can see how high they are turned up.

 

Have been reducing my treeline with a reciprocating saw to reduce height a bit but have neighbouring trees I can't avoid out to the East.

Edited November 9, 2020 by fifeskies

[fifeskies]

Most resettable fuses have a trip factor of 2 so again try to choose a slightly lower fuse rating if you used resettable fuses.

Most cartridge fuses trip at 1.5x factor

 

ie a 5 amp cartridge will break at 7.5 amps  , a 5 amp resettable will trip at 10amp , in this case use a 4 amp resettable to trip at 8 amps.

 

resettable types are best for protecting more robust items , ideal for dew heaters , less so for mounts where I would prefer a fuse.

Fuses will trip much faster on a very high fault.

 

Edited November 9, 2020 by fifeskies