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smr

Dew strap set on "fire"

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hmm a 1A fuse on the mains side is quite likely to blow if you have a switch-mode PSU hanging off it tho as they have a fair surge current on power-up, 5A would be the lowest I'd go for the mains. Agree tho regarding line fuses on the 12v side, some sort of fuse/thermal protection would seem a good idea given the value of the equipment attached.

Edited by DaveL59
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It is worth remembering that a fuse is rated at a current it will blow only very slowly over a period of time as the fuse element heats up.

So a 3 amp mains fuse will happily pass 3 amp for some time before giving up the ghost

If you know the fusing factor of the fuse this tells you the current needed to instantly blow the fuse , typically 1.5 or 2 times the rated current can be passed for a few seconds , for a 3 amp fuse than can mean 6 amps or almost 1.5kw into a fault for a few seconds 😱

It typically takes more than 2x the rated current to give a fast break response.

 

 

You really need to fuse on the low voltage side to protect your equipment.

There are fast acting fuses in 20mm and 1 1/4 inch size that are a far better way to protect equipment than the slow responding mains fuses.

A small distribution box with some of these fuses on the low voltage outputs will not be that difficult to make and could save a lot of damage

Fast acting antisurge fuses may cost a little more but its a wise investment.

 

This kind of panel holder work well , just mount into an abs project box

https://www.amazon.co.uk/sourcingmap-Electrical-Panel-Mounted-Holder/dp/B007Q82F98

 

or you could always go a bit more creative with your fuse box

 

 

1_d184087609d1a1ed7215cf170c2059cc.jpg

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Thats crazy, heat strap actually catching on fire!!! As mentioned before, definitely caused by a short out.....

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3 minutes ago, markse68 said:

Polyfuses are cool for short circuit protection- just wire in series with outputs inside the box- should be there from the manufacturer really 

https://www.ebay.co.uk/itm/1-1A-1100mA-RGEF110-UF110-Resettable-Fuse-30V-PTC-Polyswitch-Polyfuse-12v-Z346-/323820428730

E27EC0E5-A7D4-43F0-97F1-9564919FBA60.png

Out of curiosity do these suffer from the same delay problems as mentioned earlier?

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wired inside the box is a tad inconvenient compared to a panel fuseholder on the outside where a few seconds lets you change a fuse

obviously don't change the fuse till you know why it blew in the first place

 

you can select the fuse delay response characteristics to suit your needs for most fuse types

from slow blow where motor starts take a big surge current to delicate electronics that want fast response to protect delicate semiconductor components

 

the kind you would use for a mount motor drive is not the same as one you would use for an astrocamera

 

fuse selection can get complicated but ANY fuse is going to be better than no protection , just try to match the type to what it is protecting.

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6 minutes ago, fifeskies said:

wired inside the box is a tad inconvenient compared to a panel fuseholder on the outside where a few seconds lets you change a fuse

Not really- they’re thermal fuses that reset when they cool down so don’t need changing though they do degrade if activated too often. They don’t completely break the circuit but limit the current to a safe level

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17 minutes ago, upahill said:

Out of curiosity do these suffer from the same delay problems as mentioned earlier?

They’re pretty fast acting I think as fuses go

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A quick look on the spec sheet for these gives some more information about them

 

1 amp type take 8 seconds to trip at 5 x overload , so much slower response than using classic fuses

They also don't break the supply as mentioned above , only limit the overload amount.

 

Could be useful for motor protection as they will limit any stall current

 

they are also very temperature sensitive , maybe not so good for out in the cold

(where their trip time gets delayed as they don't heat up so fast and trip to limit the current)

the full spec sheets shows their response to ambient temperature

 

 

poly.jpg

Edited by fifeskies

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As a rule of thumb, a fuse will pass the rated current indefinitely at 25C.
PTC polyfuses, PTC thermal fuses and similarly named devices have the same characteristic.
So do circuit breakers.

All the devices become more sensitive at higher temperature and less sensitive at low temperature.
That means a 1A fuse at room temperature becomes a 0.9A fuse in your central heating boiler and a 1.1A fuse inside your deep freezer.

For astro use, this is not a big issue. You are out to catch faulty wiring, not try to catch something going 10% over current.

If you apply a (nearly) short circuit to a fuse, it will blow at anything from a few seconds at 2x rated current, to milliseconds at 10x rated current.
You can buy fuses with different time lag characteristics. But remember the characteristics vary between manufacturers.
Many sellers describe fuses as 'anti surge' or 'time delay' but you can get a 2:1 variance on time by changing manufacturer.
If you buy a named fuse (for example Littelfuse) you can look up time/current data.
If you buy from a retailer/ebay etc, you have no idea what the characteristic is going to be.

A word of caution here. I have seen equipment sold on Amazon and made in China that had dangerous fake fuses in the mains side.
I always swap out mains fuses for something with known provenance if in doubt.

Circuit breakers are tending to fall out of favour. They can be expensive and require front panel work to fit them.
They have the benefit that it is easy to see when they have tripped and you can buy different time lag characteristics.

Looking at the polyfuse type of solution. These have a time/current characteristic determined by the manufacturer and are not generally available in different time lag variants.
These again tend to trip in a few seconds at 2x rated current and a fraction of a second at 10x rated current.
They are ideal for astronomy because if you get a wiring short, all you need to do is remove power, fix the short and away you go. No hunting for a replacement fuse.

All fuses, circuit breakers and polyfuse devices have a maximum fault current. That is the maximum current that is allowed under fault conditions before something unwanted happens.
If you take a glass fuse and have a big fault current (for example using it on the mains) then it can explode spectacularly.
Glass fuses are best thought of as having a fault capability in the tens of amps and low voltage.

Circuit breakers vary considerably in their ability to deal with a fault.
The (physically large) breakers in your fusebox at home can handle hundreds of amps fault current without failing.
Other breakers for inside equipment may have a life of tens of operation with a fault current of 30A or so.

Polyfuse type devices have a maximum voltage rating and a maximum current break that needs to be checked.
Very often a polyfuse will have 40A max breaking. But if you put one straight across a 12V lead acid battery you can get a couple of hundred amps.
Usually if you do this a few times the polyfuse has increased sensitivity as part of the body becomes open circuit. Eventually it fails completely.
If you exceed the voltage rating the device may break down can explode - a bit like a glass fuse.

Don't forget that when tripped, a polyfuse self heats with a low current through the load. The polyfuse body tends to remain about 125C or a little hotter until you remove the load (faulty wiring) or incoming supply.
Not hot enough to melt solder. But don't put it next to a bit of soft plastic.
 

HTH, David.

 

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The likely reason why dew straps fail where the cable is attached to the heating strap is because so many use screened audio cable for the phono cable and unless you're really careful during construction when attaching it to the heater, a strand of screen can easily break loose when manipulating the strap and float around, coming into contact with the 12V at some point. This single strand across 12V will heat up very quickly causing the smoking problem seen.

I shorten the cables on my dew straps to more convenient lengths and fit my own phono plugs. The Astrozap straps used screened cable so am always cafeful to not move the cable to much where it enters the strap. The last few straps I bought have been from dewheater on ebay who sells W&W Astro straps which she makes. These actually use fairly thick 2 core flexible cable moulded to the phono connector and not screened cable so is a more reliable solution in my opinion.

Alan

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8 hours ago, FLO said:

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

Apologies for the mistake. In that case I cannot remember who I bought them from - I'll have to see if I can find something amongst my records.

Adrian

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The box by Michael with its fused outputs is exactly the way to go.

Indicators to monitor each output is a very useful addition as he has done.  (I assume LED)

I am making a box to use with my pier mount and will use LED to monitor each output , however LED at the usual current are very bright (nominal 20mA)

It is worth using a far higher in line resistor to lower the LED current , trial and error will let you find what suits you , but I find even a few mA current is enough to make the LED show when looked at directly allowing you to confirm the fuse has not blown but will not cast a bright coloured glow around where you don't want it.

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