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smr

Dew strap set on "fire"

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Hi,

I need some help please!

I've been running my imaging rig quite happily for some time now but I had to order a 2" dew strap for my guidescope the other day.

So today I connected everything up and connected the new guidescope strap.

Whilst setting up, and fortunately I noticed it whilst I was setting up and not in the house, my telescope dew strap (4") started emitting some wisps of smoke. Luckily I realised it wasn't my breath under my head torch and was indeed smoke, then I noticed a small orange flame lighting up where the cable goes into the heater band. 

It's melted that part of the cable.

Something a bit awry with power and amperage then I assume. 

I run my rig off the mains. So I have an extension cable from the house (going into an RCD adapter) out into my dribox into a 4 way plug extension socket. 

Plugged into the 4-way is a 240 Mains to 12V voltage converter. Then I have this 12v multisocket thing below and then a hitec astro 2 port dew controller plugged into that (also below)

So what's a better and safer way of connecting my dew straps to something?

 

Here's what I have....

 

81Z3AbLv37L._AC_SL1500_.jpg

 

 RMS3.jpg

 

Manual Mini Dew Controllers

Edited by smr

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IMG_9751.thumb.jpg.c452b943cbc30e7e1bd5aec96aedd663.jpgI've had this happen twice in the last 3 months - both times where the cable attaches to the heater element. My 3" heater actually caught fire as I ripped it off the scope - the 4" just got very hot and started to smell but I noticed it before it actually burst into flames.

I was using them with the HitecAstro four channel controller. I then started using a 365Astronomy element with the HA controller and that suddenly stopped working as well - it just didn't heat up. The led indicators came on but the heater didn't warm up irrespective of which socket I plugged it in to. I contacted HA and they asked me to send the controller back for repair (at no charge), however, I received a message from HA saying there was no fault to repair - I am still awaiting return of the unit to try again.

So I am now using the 365Astronomy strap connected directly to a 12.5 volt supply and hoping it behaves itself whilst wrapped around my Canon 200mm!

Adrian

P.S. I cannot remember the make of the heater straps - I think I bought all three from FLO a couple of years ago. The weakness in the design is definitely where the cable atatches to the strap - the strain relief is completely inadequate.

 

Edited by Adreneline
Typo + clarification + photo added

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Whooooo!!!

That's scary!!

I use a very similar power set-up with AstroZap dew heaters and controller (#AZ-720).

Never experienced anything like that.

 

Edited by Merlin66

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I would guess you have a short circuit at the cables going into the strap, therefore bypassing the heater wire in the strap and getting very hot.

How many amps does that psu supply, looks like it could be 5?

For 1 dew strap, I would say <0.5A is enough, but if you can't change your psu you really should have a fuse somewhere in that circuit, say a 1A or something.

Do you have an ohm meter or multimeter? Measure the resistance of the dew strap and if it's reading 0ohms or near as dammit, it's likely a damaged strap.

A good heater strap will be a few ohms, not 0.

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8 minutes ago, Jonk said:

For 1 dew strap, I would say <0.5A is enough, but if you can't change your psu you really should have a fuse somewhere in that circuit, say a 1A or something.

The 365Astronomy strap I have draws 0.48 amps at 12.5 volts so R must be around 26 ohms.

The HA controller does not contain any fuses (it was my first thought when it stopped working - I opened it to have a look!) and although I am  no electronics expert I think it uses triacs to limit the current. If it does then it doesn't limit it enough otherwise I would not have had two straps fail in this manner.

A strap melting around a metal dew shield is one thing - a strap melting around a plastic lens hood is quite another.

Adrian

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I'm constantly amazed at the posts on SGL of blown equipment costing hundreds of pounds, with nary a sign of a fuse in the power lines.

A 13 amp fuse in the mains plug isn't going to hack it......

Michael

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2 minutes ago, michael8554 said:

with nary a sign of a fuse in the power lines

One might ask why dew heaters are not supplied with inline fuses in the supply cables? They all seem to come with a phono plug for direct connection to the controller.

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4 minutes ago, smr said:

What could have caused this ?

In my case I presume it was just flexing of the cable connection to the heater element eventually gave rise to a short circuit and with no effective current limiting it took as much current as was available from the supply - mine will potentially supply 12A at 12V. I have now bought some in-line connectors so plan to fit an in-line fuse.

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Couple of mine shorted at the end as well, it's bad design.

I cut mine open and reinsulated with adhesive glass tape we use at work.
All work as normal now.

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The triple 12v socket can be used for a maximum of 8amps. But surely two heater bands don't use 8amps?

It's just a coincidence that the day I used a second dew strap for the first time, the first dew strap (which had been absolutely fine for a good year or so) started melting itself.

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Scary, does sound like a damaged strap though.

I have been wondering about my DIY one and thinking it has little protection so will be ordering some inline fuse holders I think. Something like ref=pd_sbs_107_18?_encoding=UTF8&pd_rd_iref=pd_sbs_107_18?_encoding=UTF8&pd_rd_ithis maybe?

In a perfect world I guess the controllers would have fuses built in, but no harm in adding one between the controller and strap, especially if it saves other stuff connected.

 

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10 hours ago, Merlin66 said:

I use a very similar power set-up with AstroZap dew heaters and controller (#AZ-720).

Never experienced anything like that.

Neither have we. We have of course experienced the occasional failure but fortunately nothing as spectacular or scary as that experienced by smr. 
 

11 hours ago, Adreneline said:

P.S. I cannot remember the make of the heater straps - I think I bought all three from FLO a couple of years ago. The weakness in the design is definitely where the cable atatches to the strap - the strain relief is completely inadequate.

I cannot find an order from you for heater tapes so I don’t think they were ours 🙂

FLO only sells AstroZap heater tapes. We have been selling them for as long as I can remember, probably at least a decade. I think in the UK most heater tapes in use today are AstroZap. All are covered by a ‘two year no fault replacement warranty’ so if anyone reading this has a problem with theirs they are welcome to contact us for a replacement. We won’t even ask how it happened... 😎

HTH, 

Steve 

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Funnily enough I've just sent you an email 2 minutes ago Steve, asking if the strap is covered under warranty. 

I ordered it from you in October 2018. 

Not sure why it happened but from posting how I connect my equipment together and the replies it seems to me that I'm connecting things ok.

I also noticed the cable itself getting very warm when connected, compared to the cable I've only just ordered from you this week (the 2" version) which was cold to the touch when in use.

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One of mine has burned up like that before too, for no apparent reason.  An Astrozap strap and an astrozap controller.  The other strap that was connected at the same time was fine.

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10 hours ago, michael8554 said:

I'm constantly amazed at the posts on SGL of blown equipment costing hundreds of pounds, with nary a sign of a fuse in the power lines.

A 13 amp fuse in the mains plug isn't going to hack it......

Michael

Quite right Michael, the fuse in the mains plug should be changed to a 1 or 2 amp.

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Even a 1 amp fuse in the mains plug means the unit could still deliver well over 10 amps at 12V.

If the supply was 100% efficient then in theory a 240V supply could supply around 20 amps at 12V.

Now nothing is 100% efficient, far from it and there is maths involved when converting AC to DC but for sure a 1 amp fuse would not necessarily prevent this.

A 1 amp fuse in each output to the dew straps would be the best  thing to do.

Steve

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9 minutes ago, smr said:

Funnily enough I've just sent you an email 2 minutes ago Steve, asking if the strap is covered under warranty. 

I ordered it from you in October 2018. 

Yes, no problem. It is only a year old so well within the two-year warranty period 🙂

Where is your HitecAstro dew controller now? I think we had best replace that too. 

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1 minute ago, FLO said:

Yes, no problem. It is only a year old so well within the two-year warranty period 🙂

Where is your HitecAstro dew controller now? I think we had best replace that too. 

Thank you very much Steve.

Still got the HitecAstro dew controller, not binned it, if that's what you mean?

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5 minutes ago, smr said:

Thank you very much Steve.

Still got the HitecAstro dew controller, not binned it, if that's what you mean?

Yes. I think we had best replace that too please.

It is the weekend but I will find your email when we reopen Monday then will arrange collection/replacement of both items. 

HTH, 

Steve 

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1 minute ago, FLO said:

Yes. I think we had best replace that too please.

It is the weekend but I will find your email when we reopen Monday then arrange a collection/replacement of both items. 

HTH, 

Steve 

Thank you Steve. :)

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The fuse in a 13A plug should always be rated with consideration for the expected load. Given that the smallest fuse you can buy is 1A, that equates to 240watts.
That is a lot of scope power, dew heating, etc.

What is actually needed for dew heater fire prevention is protection in the 12V side of things.

Take this hypothetical setup....

You have a mains to 12V 10A power supply. That is 120watts
You might think a 1A mains fuse should be good. 240V & 120W = 0.5A.
In practice, inrush current and fast acting mains plug fuses (long explanation so just accept it) mean you may have to go up to a 2A or 3A plug fuse.
In other words the plug fuse is only useful for serious failures within the power supply.
The power supply should (but may not) have some sort of current limt on the output circuit. If you overload the output it may well limit around 15A.
Or it may struggle along as best it can (20A? 30A?) until an internal thermal trip or big fuse goes.

If you consider a 4 channel dew heater, without taking it apart you often do not know what internal current limt measures are in place.
For example, you might have a controller claiming to deal with 5A per channel with a car cigar plug on the inlet.
The cigar plug may or may not have a fuse. If it does, it is probably going to be 20A.
In practice, wire fuses tend to blow in a few to 10 seconds if the current is double the rating. In other words with a 20A fuse, you need 40A for a reasonably prompt blow.
If you exceed a fuse rating by 50% it will probably taqke all day before it thinks about blowing.
But if your power supply can't deliver enough current to blow the fuse, it fuse will never blow.

So what about the dew heater output channels? The situation depends on the controller.
The cheapest way to build a controller is to simply have a transistor that is turned on/off to determine the heating.
No fuses. No thermal protection. No current limit circuit.
In general, if you overload a transistor and cause it to fail, it will fail permanently on. Not always. But that is the most dangerous condition.
There are measures that can be taken at design to shut down a channel if there is an external fault. But these cost money and we all look for the cheapest box.

With my hypothetical setup, if the wires at the dew heater short together, then you will get a fire.
Quite simply there will be 10A and more flowing through (in all probability) a few strands of wire and you will get localised intense heating.
Even if there is not significant localised heating, the wires running from the dew control to the heater will get warm, possibly starting to melt insulation.
If you want to know what 120 watts (or less) feels like, just get hold of a light buld. That is a filament lamp. Yes you will get burnt.

Unless you know the innards of your power supply and dew heater controller, you need to add your own protection at the outputs of the controller.
That could be a small box containing 4 fuseholders. Put a fuse in each holder that is rated a bit higher than the heating element.
If you change your setup, you can change the fuse value.
If you use 20mm or 1.25" fuses and holders then you can buy fuses in any value to suit the application.
Alternatively you can make something using PTC thermistors but these mean soldering.
Or if you ask the controller manufacturer about his overload protection?
 

Is a flaming dew heater an issue? In itself probably not. It is outside and low cost. But do you want to melt your guidescope, telrad, OTA lens cell, etc?

If you look back 15-20 years, there was little scope electrical wiring. As it was more specialised, users tended to think more and ask more questions.
Nowadays there is lot more scope wiring. This gives much more opportunity for problems. There is pressure on equipment price so effective overload protection may be unaffordable.
Further, there is tendency for us to buy a power supply, a heater control and the heaters without thinking of the complete assembly

15 years ago we might have bought everything from one dealer and had the over the counter advice about wiring techniques, protection, etc.
Today we are more likely to buy the items online, from different places.

Each 'box' or part of the system may well be built to a standard and be CE (or whatever) stickered.
But is the combination of devices fit for purpose?

David.

 

 

 

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