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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 22nd Mar 2014, 10:00 am   #41
Tubeglow
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Default Re: Fuses

I haven't noticed this in the thread,

Looking at voltage ratings, think about HT+ on valve circuits etc.
The disconnection time is linked to the arc time within the fuse after the element breaks.
So things to think about are.

At fault current how much current can flow before the fuse disconnects.
What is the Arc distance and at what voltage of operation.

Then how is the arc quenched to increase the speed of operation.

So at fault, the fault draws as much current as the supply can deliver until initial element break. Then fault current will continue to flow until the gap (+ ionised gas) is big enough for the arc to stop.

So if you think about sand filled fuses as the element breaks the sand falls into the gap. If the element is under tension from a spring it will pull apart faster (arc distance) There is always more (if you have a DC fuse in a circuit you can use a cap)

Just a thought. If you use surge suppression for transformer magnetisation current you can fuse closer to the operation current of the circuit.

Just for fun.
Tubeglow.

Last edited by Tubeglow; 22nd Mar 2014 at 10:10 am.
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Old 22nd Mar 2014, 11:11 am   #42
Tubeglow
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Default Re: Fuses

One more quick thought,

the type of voltage across the fuse is obviously different with AC and DC.
So the AC voltage is following a sine wave + whatever transients are on it.
The fuse element is heated with the combination of V & A so this is varying on an AC supply. The clearance time is dependant upon the arc quench with a drop in voltage this is helped (depending where on the cycle the fault occurs).
Fault current is dependant upon impedance of supply and VD.<<how much current can you draw from a fuse board..a fuse is a piece of wire a CB is two contacts.

On DC the fault has a clearance time dependant upon Arc and distance gap of the fuse element. There is no change in supply voltage except possible fast release of current from supply caps.

The fuse rating is different on different fuse types cartridge fuse 1.5 times rated value. The clearance time is dependant upon fault resistance path.
If you have a "Bad" earth then the clearance time will be slower. You need enough current to flow as fast as possible to make the fuse element operate.
This is known as ADS(automatic disconnection of supply)
A fuse not exceeding its operation point could allow extreme damage. heating of both earth cables and supply cables possible shock from elevated voltage on a chassis. Waiting for the fuse to blow..

Slow blow for things like magnetisation current of a transformer. (Surge at turn on..)
Quick blow for electronics or supplies with low current rating that will not stand high fault current..

Over rating of fuses to allow for surge like magnetisation current is a bad idea..when its value would exceed well above operational current.

Just for fun..

Tubeglow.

Last edited by Tubeglow; 22nd Mar 2014 at 11:26 am.
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Old 22nd Mar 2014, 11:42 am   #43
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Default Re: Fuses

General rules I adopt:

Use current rating 25-50% above the maximum steady-state current the equipment can draw.

Ensure voltage rating is at least equal to the circuit voltage.

Use anti-surge (T) types if there is a switch-on surge, otherwise normal (F).

If the supply impedance is low (ie operates from mains) use ceramic, sand-filled high-breaking-capacity (HBC) fuse. Otherwise, glass types so you can see if they've failed.

Use 1.25" fuse rather than 20mm whenever possible.

If in doubt, arm yourself with a dozen spare fuses and replicate/simulate the possible faults. Ensure the fuse does its job each time. (Makes a mess of the equipment, but great fun).

I do confess to using a 250V-rated fuse (1.25" type) on a 400V 10A DC rail, but it was to hopefully save the PCB in case of a downstream fault, not as a primary means of safety protection. As it happened, it was never put to the test.
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Old 22nd Mar 2014, 11:49 am   #44
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Default Re: Fuses

Quote:
Originally Posted by kalee20 View Post
I do confess to using a 250V-rated fuse (1.25" type) on a 400V 10A DC rail, but it was to hopefully save the PCB in case of a downstream fault, not as a primary means of safety protection. As it happened, it was never put to the test.
What type of circuit was it?

Its always been a bit difficult to protect HT+ in valve amps. You can use microwave fuses but they are a bit big.
I know in the past any fuse is better than no fuse..I do cap quench on B+.
The HT windings are often mA rated, so its quick blow. Then you have the cap charge to deal with at power up.
Some people put fuses in the cathode of OP valves because its low voltage.
But if you get a supply cap short or OP Tx fault its another story.

Tubeglow.

Last edited by Tubeglow; 22nd Mar 2014 at 11:57 am.
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Old 5th Oct 2015, 1:42 pm   #45
Alistair D
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Default Re: Fuses

Earlier in this thread mention was made of lower current draw equipment with no mains fuse. In my test equipment collection I have a number of 50s and 60s instruments that fall into the same category. I realise I can modify them and fit an internal fuse but a simple and elegant solution would be plug fuses of less than 1Amp. So far internet searching has drawn a blank so I presume such things do not exist. Does anyone know of a source?

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Last edited by Alistair D; 5th Oct 2015 at 2:09 pm. Reason: missed a
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Old 5th Oct 2015, 1:59 pm   #46
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Default Re: Fuses

I assume that it's taken as read that the BS1362 fuse is there to protect the cable, rather than the appliance internals. The appliance is expected to have further pertinent protection for itself, so to speak. Thus, there's no theoretical need for less than 3A rated fuses in the plug, though 2A and even 1A ceramic 1" x 0.25" are available.

I respect the philosophy of not hacking vintage kit around, but I always place primary side safety as an exception to this- safety always comes first, even if PVC insulation, heatshrink sleeving, insulating covers, and currently-compliant fuse-holders wouldn't have been original fitment. It's unusual that some sort of entirely safe fuse-holder can't be fitted to a piece of kit.
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Old 5th Oct 2015, 3:05 pm   #47
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Default Re: Fuses

I suppose if you want to make no internal modifications to the vintage device you could fit a fuseholder in a plastic box and wire the unit's mains cable into that (fuse in the live wire of course). That shouldn't be any more dangerous electrically than fitting a fuseholder in series with the live wire inside the set.

In the event of a catastrophic failure the plug fuse should still handle the high current available from the mains, and blow safely. For a more minor overcurrent that wouldn't blow the plug fuse, the added fuse will fail.
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Old 19th Dec 2015, 9:33 am   #48
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Default Re: Fuses

For the application of general valve equipment such as radios and amplifiers, the general trend is to add an AC fuse(s) on the power transformer secondary for B+ supply. That B+ usually supplies all circuitry, so is often the next best extension beyond just an AC mains side fuse. AC winding protection, compared to B+ DC fusing has benefits.

For most equipment, I can't see a benefit in heater supply fusing, as the heater winding is usually operating at close to rating, and so it would be difficult to generate a substantial multiplier in current above a fuse value that would typically have to be at least 20% above winding rating. In addition, the heater supply does provide quite a substantial turn-on overload that needs to be considered for the AC main side fuse, as well as any secondary side fusing.

PSUD2 is a good tool to simulate the turn-on current surge levels in a B+ supply. For a valve rectifier, that surge is delayed from mains turn-on surge (power transformer magentisation and heater surge current) unless hot-switched. But if an ss rectifier is used, then that turn-on surge is additive to the mains turn on surge.

A NTC part added to the mains supply input circuit is a very common modern addition to equipment. It is easy to retrofit in most vintage equipment, and reliable if properly selected. The NTC is commonly sized to manage the ramp up of a certain capacitor filter supply - so can be very applicable to alleviating turn-on surge current through the mains side fuse relating to PT magnetising current and ss diode supply current, but not for heater supply suppression as that is a much longer time-frame. As such, the NTC can allow a standard fuse, compared to a slow-blow, or a fuse value increment reduction.
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Old 19th Dec 2015, 10:33 am   #49
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Default Re: Fuses

Fusing heater supplies can be problematic - remember that the heaters have a significantly lower resistance when cold - providing a fuse which offers significant protection while not being vulnerable to the fatigue-failures caused by regularly being run close to fusing-point... OK, you can get slow-blow fuses but I still feel that fusing heaters is likely to compromise the overall reliability of the equipment.

[in equipment with valve rectifiers the first couple of seconds after power-on is likely to see the highest heater-current: not only are the heaters cold/low-resistance, but also until the rectifier(s) come up to emission and start drawing power the power-transformer has no other load on it so can potentially feed quite a bit more than its continuous-rating current to the heaters...]

Fuses in the B+ side I accept (not just one fuse in the centre-tap-to-earth; that won't save your transformer if a diode fails s/c!).
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Old 19th Dec 2015, 12:11 pm   #50
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Default Re: Fuses

Quote:
Originally Posted by G6Tanuki View Post
Fuses in the B+ side I accept (not just one fuse in the centre-tap-to-earth; that won't save your transformer if a diode fails s/c!).
Practicality tends to rule nowadays, with a minimal update of old equipment including a PT secondary side CT fuse, and either new ss diodes, or inserting ss diodes in series with valve diodes.

The addition of an ss diode in series with a valve diode provides protection against valve diode s/c.

Some equipment needs fusing in each leg of a full-wave CT rectifier circuit, such as when a bias supply is derived from one of the legs.
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Old 23rd Dec 2015, 9:57 am   #51
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Default Re: Fuses

Quote:
Originally Posted by trobbins View Post
A NTC part added to the mains supply input circuit is a very common modern addition to equipment. It is easy to retrofit in most vintage equipment, and reliable if properly selected. The NTC is commonly sized to manage the ramp up of a certain capacitor filter supply - so can be very applicable to alleviating turn-on surge current through the mains side fuse relating to PT magnetising current and ss diode supply current, but not for heater supply suppression as that is a much longer time-frame. As such, the NTC can allow a standard fuse, compared to a slow-blow, or a fuse value increment reduction.
One of my motives for using modern inrush limiters (but in a somewhat higher cold resistance than usually employed for SMPSUs), besides giving an easier time to DH rectifiers, dial lamps and elderly and possibly marginal switches) is that a tightly-rated quick-blow fuse can be employed, rather than the vagaries and inertia of delay types. I know that there's an argument against tightening the safety of primary side fitment over original construction lest it end up being frowned on in court but much original construction was minimal, lousy, hasty, over-thrifted and based on dubious premise and marginal or non-existent legislation anyway. I'd rather head off the problem in the first place, rather than sit back to the consequence of fire or exposing an innocent party to hazard, all the while smugly muttering, "I never touched it, guv". (But I knew that it was inadequate....)

The familiar centre-tap fuse for HT secondaries always struck me as a minimum-hassle after-thought really- it probably stopped a few roasting smoothing/decoupling resistors setting fire to waxies, but as Tanuki points out, it doesn't guard against possible rectifier failure modes. To fuse both secondary limbs would imply using 1.25" types, i.e. considerable HV withstand capability, and that's getting both bulky and (from the accountant's viewpoint) pricey, though it is possible in some stuff. When I joined the world of electronics maintenance, it wasn't unusual for individual ICs to be fed through flame-proof fusible resistors as it wasn't unkown for them to go internal short- and some PSUs had impressive current capability. So, when overhauling valve kit, I prefer to use power film resistors in place of small wirewounds- I don't mind a puff of smoke and a bit of repair work over possible more serious and unheralded consequences.
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Old 23rd Dec 2015, 11:54 am   #52
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Default Re: Fuses

No doubt this has been mentioned before, but it always struck me as strange that few British manufacturers fitted internal fuses in any domestic equipment, whereas most, if not all, continental setmakers fitted them, sometimes in mains primary windings, HT secondaries, and even LT (Heater) windings. It surely cannot be due to the differences in mains connectors because, as we all know, the old British 5 & 15Amp round pin plugs did not have fuses, besides which, the once ubiquitous lamp adaptor, again unfused was often used
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Old 23rd Dec 2015, 9:30 pm   #53
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Default Re: Fuses

5x20mm fuses come with up to 500VAC rating - sure, 250VAC is most common, and the valve holder also needs to be equivalently rated (unless using an axial leaded part, which is pretty nifty, but again not too common). A secondary side fuse, even in the CT, has to have the same voltage rating as an arm fuse.

Given the amount of circuitry that is supplied from the diodes onwards, and the general reliability of modern ss diodes, I'd suggest that a CT fuse is an excellent choice compared to no fuse. If the equipment uses a valve rectifier, and each plate is additionally protected by an ss diode then that provides some additional assurance.

Some bias supplies are capacitively coupled off one side of a power transformer secondary V-0-V HT winding - they are susceptible to stress if a CT fuse blows, and so fusing the secondary winding arms would be appropriate.
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Old 1st Feb 2016, 9:11 pm   #54
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Default Re: Fuses

Link to an article that tries to cover the design and awareness needed for fusing in valve amps, and in particular the power transformer high voltage secondary. Ciao, Tim

http://dalmura.com.au/projects/Valve%20amp%20fusing.pdf
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