what fuse should i use?

[starship trooper]

hi there,

I am making a power station for my mount and other things and was wondering what fuse i should use inline for the mount which is a heq5pro and is pulling about 2.5amps so do i use a 2.5amp fuse or could i use a 3amp fuse?

Also if i was to have 2 or 3 rca running off of a 12v output should i wire them in series or parrallel?

Cheers dazz

[MLyons]

I wouldn't put a fuse with a rating above 3A in.

If your scope/mount is drawing 2.5A, then, unless you use a slow-blow fuse, a 2.5A fuse could pop with any spikes.

What happens when both motors are running together? Does it stay below 2.5A?

A fuse with a rating of 10% higher than your maximum normal current draw is adequate protection

Multiple outlets are parallelled (and preferably individually fused)

Martin

[Stephen]

The SkyTron cables from FLO have a 3A fast-blow fuse.

Dunno if this helps

[starship trooper]

2.5amps was the highest I saw the mount draw but I see what you mean and there could be power spike now and again so I think I might try a 3amp fuse and see how it goes.

Thanks dazz

[Carbon Brush]

Think of a fuse as being there to protect against fire if there is a serious fault. For example, if you short the wires from a battery, do you want a fuse to pop, or the wires to burn?

A fuse will not do anything about an equipment fault that causes a bit of excess current to be drawn. for example, a component failure in a motor drive.

In round figures, a fuse will cope with 2x it's rating for a very long time. If the overload is applied sometimes, then after a lot of heating/cooling cycles, the wire will fatigue and the fuse will open. But when a fuse has been fatigued in this way, it can fail under normal use.

Why not consider fitting the more expensive thermal overload type devices? These trip quickly on high fault current (like a fuse). But, unlike a fuse, if subject to small overload for a time, they trip relieably. They also have the benefit of having a reset button. No fiddling to find the right replacement fuse after fixing the damaged wiring.

Another option is a PTC thermistor. No moving parts at all.

If you go for a fuse, think about the holder. I have seen lots of problems caused by dodgy connections in cheap fuse carriers - including burnt plastic. Just the sort of thing you are trying to prevent!

David.

[Gina]

2.5amps was the highest I saw the mount draw but I see what you mean and there could be power spike now and again so I think I might try a 3amp fuse and see how it goes.

Thanks dazz

Yes, I would go for 3A or possible 5A as a maximum. It's often suggested to use a quick blow fuse at twice the normal maximum current. Or a bit lower rated slow-blow (or "thermal") fuse depending on the type of load. For electronics I would use a quick blow fuse.

[Carbon Brush]

Sorry Gina but 'electronics' does not necessarily warrant a quick blow fuse.

The choice of fuse type, fast/slow blow and all variants, depends on the load.

A DC drive motor on a mount will pull a high start current, reducing as it accelerates to speed.

A dew heater takes a high current that is turned on/off at intervals. The duty cycle determining the heater power. A meter in the line will read the average current only.

Both of these circuits will, given time, fatigue a correctly rated quick blow fuse. Hence the tempation to increase the fuse rating beyond the load current.

If you stick to a fuse in these circuits, a correctly rated slow blow is the obvious choice. Though the circuits are also obvious candidates for the correct type of circuit breaker, or a PTC thermistor.

Hope this is useful.

David.

[brantuk]

I use a Maplins 5a PSU to run a scope and two 4" dew bands if that's any help - but I'm no electronics buff - just never had any problems with that set up

[Gina]

Sorry Gina but 'electronics' does not necessarily warrant a quick blow fuse.

The choice of fuse type, fast/slow blow and all variants, depends on the load.

A DC drive motor on a mount will pull a high start current, reducing as it accelerates to speed.

A dew heater takes a high current that is turned on/off at intervals. The duty cycle determining the heater power. A meter in the line will read the average current only.

Both of these circuits will, given time, fatigue a correctly rated quick blow fuse. Hence the tempation to increase the fuse rating beyond the load current.

If you stick to a fuse in these circuits, a correctly rated slow blow is the obvious choice. Though the circuits are also obvious candidates for the correct type of circuit breaker, or a PTC thermistor.

Hope this is useful.

David.

Thank you David - yes, I agree, I oversimplified my post Yours is a good explanation.

[Macavity]

Hope this is useful.

I think you're spot on, David. Fuses have a "different purpose", and will not (overly) protect delicate electronics. As a random aside, I note that some plug / socket combos allow significant sparking, during insertion / withdrawl (Ooer!) - Especially with a 12V (unfused, high-current!) leisure battery. <G> Wide® spacing of "pins", good construction etc., seems advantageous.

In my main 15A battery cables, I will probably, eventually throw in a few 10A fuses... to protect the cable mostly?

[Carbon Brush]

Hi everyone. Think of a line fuse as being only to protect cable from fire.

A gelled lead acid battery, as used in many astro power supplies will easily deliver a couple of hundred amps if short circuited. More current than the hot end of an arc welder. A standard car battery will easily deliver twice this. I think that is enough to make us think carefully about line fuses.

As for sparks, etc on connect and disconnect. There are more issues here.

Very often, scope kit will have on the input circuit, components called capacitors. Why they are there is very involved. A capacitor stores electricity. When you connect, the capacitor is empty and current flows in a quickly as possible to fill it. You get a high current inrush. This all happens before you even think about turning on motors, etc. So any power connectors or switches have to be able to handle this high current (for a fraction of a second) without damage.

You may have seen instances of switches in cars or houses that have welded closed due to inrush or fault current. That is the sort of damage than you can get.

The next problem comes with the flimsly little DC connectors so popular on scope kit. These are chosen simply on cost. They cost next to nothing. They may carry the load current OK, but inrush is a different matter. Any sparking means you lose the plating on the contacts, so you get an increase in contact resistance, which can cause heating, or poor connection. Always turn the kit on/off using a seperate switch. Not by inserting the connector.

A friend once asked me look at an old Meade handset. The memory had been scrambled. He had been using a damaged DC plug/socket on the mount. It had lost it's plating and sometimes gave intermittent or poor connection. A design error by Meade had allowed the memory to get corrupted on a power 'brown out' condition. But it had been avoidable by looking after the power connector and switching on/off separately. In this case I soldered a power lead into the mount and told him to switch on/off elsewhere.

Sorry if it is rambling, but hopefully useful in part.

[Gina]

Very good point and a useful warning for everyone Thank you.

[Macavity]

The next problem comes with the flimsly little DC connectors so popular on scope kit.... Always turn the kit on/off using a separate switch. Not by inserting the connector.

Indeed, and precisely as recently observed. I am definitely persuaded to

(overcome laziness and) install / use more switches, closer to source.

[Gina]

If you're switching an inductive load it can also be worthwhile added a spark suppression circuit too. Just a resistor and capacitor costing pennies can stop destructive sparking and arcing.

[Macavity]

Well, in the spirit of general confession:

For miscellaneous amusement (backup?), I invested in a 12V/240V 300W Maplin inverter, for my otherwise "low-voltage" obsy. I bought myself a standard, wall-attaching, 240V BULB "Rose" holder. In the course of relocating the latter, I inadvertantly left it connected to the inverter... An ENLIVENING little jolt!! LOL. Probably not good for a "dicky ticker" though? I remember (worked with) "Valves" (Vacuum Tubes!), but how easily we forget current... and Voltage!

[Gina]

Yes, I've had a few shocks in my time too - in the era of "bottles"! And a few little burns on my fingers from high voltage sparks I was careful to minimise the risks though.

[starship trooper]

well thank you for all the advices i have managed to get a couple auto reset brakers so hopefully they should do the trick.

And heres some pictures of my scope hub.......