Mains here is 220-240VAC 50/60Hz.
current load of the system
=========================
The amp will have a current load of 150V 80mA
power supply heaters:
rectifier tube: 5V 3A
tubes used for voltage regulation: 6.1V 2.8A
heaters used in the left/right channels:
there are 4 tubes with the heaters connected in parallel: 8.1V 2.55A
======================
There are 2 transformers in the amp. The primary of the 2 transformers is configured as 230VAC.
The heaters for the tubes used in the left/right channels is connected to the 2nd transformer. The 2 transformers will be sharing 1 IEC socket/fuse/power switch.
The fuse will be installed on the primary side of transformer.
Can someone help on determining the correct fuse amperage?
Thank you.
current load of the system
=========================
The amp will have a current load of 150V 80mA
power supply heaters:
rectifier tube: 5V 3A
tubes used for voltage regulation: 6.1V 2.8A
heaters used in the left/right channels:
there are 4 tubes with the heaters connected in parallel: 8.1V 2.55A
======================
There are 2 transformers in the amp. The primary of the 2 transformers is configured as 230VAC.
The heaters for the tubes used in the left/right channels is connected to the 2nd transformer. The 2 transformers will be sharing 1 IEC socket/fuse/power switch.
The fuse will be installed on the primary side of transformer.
Can someone help on determining the correct fuse amperage?
Thank you.
Start out with a 3 - 5 Amp fuse. With the amp running measure the actual current and select a fuse at 125 - 200% of this rating. Then test your selection to make sure that it does not blow on start up. It should be a "slow blow" fuse.
Sounds like a stupid way to do it... it is actually the "pro way" to go about it. A pro might start with a more precise value, but the measurement and testing would take place anyway.
Sounds like a stupid way to do it... it is actually the "pro way" to go about it. A pro might start with a more precise value, but the measurement and testing would take place anyway.
Hi jarthel,
One way to ballpark this is add up all your power draws. For example,
This figure is your expected current draw. There will be a turn on surge much higher than this, so take that figure and add poobah's (' = ownership! ) correction for a fuse value.
To measure the current, your can use a clamp on type current meter (It snaps around one wire of your supply), these are not very accurate but will give you a ballpark number. You can rig up an AC current meter with appropriate shunt in line, then plug it in. Those connections are at line potential so insulate everything. The last method is to insert a low value resistor (say, 0.1 R ) and connect an AC voltmeter across that. Insulate all connections and turn on. Note that you should use a wire wound type that can take a surge. Figure out your wattage and multiply this by 5 at least.
You need to be comfortable doing this, have another person near by. If your are not familiar with AC wiring, have a technician or electrician help you. Do not give your mains a chance to get you or others that may be near by!
-Chris
Edit: I have a box with an AC cord, AC ammeter and socket wired up and ready to go. That is safe.
One way to ballpark this is add up all your power draws. For example,
So 5V X 3A gives you 15W. Add the "watts" figures all together and divide the total by 0.85 (assuming your power transformers are 85% efficient). Divide this by your current (arrrrgh!), as in "at this time" voltage level to give you the current. W / V = A
This figure is your expected current draw. There will be a turn on surge much higher than this, so take that figure and add poobah's (' = ownership!
) correction for a fuse value.To measure the current, your can use a clamp on type current meter (It snaps around one wire of your supply), these are not very accurate but will give you a ballpark number. You can rig up an AC current meter with appropriate shunt in line, then plug it in. Those connections are at line potential so insulate everything. The last method is to insert a low value resistor (say, 0.1 R ) and connect an AC voltmeter across that. Insulate all connections and turn on. Note that you should use a wire wound type that can take a surge. Figure out your wattage and multiply this by 5 at least.
You need to be comfortable doing this, have another person near by. If your are not familiar with AC wiring, have a technician or electrician help you. Do not give your mains a chance to get you or others that may be near by!
-Chris
Edit: I have a box with an AC cord, AC ammeter and socket wired up and ready to go. That is safe.
It is also best to fuse the primary of each transformer seperately after the common switch. This allows each fuse to match each transformer.
Remember that the purpose of the fuse is to protect your expensive transformer and protect against fire. In most cases a fuse will act too slowly to protect the circuit itself, particularly if sold-state. So the actual fuse value is not too critical, provided it meets these two main aims. That is also why fusing the secondaries is usually a waste of time. Although in the past fuses have been used in the finals of audio amps, with mixed success.
There is also a slight typo above.
To clarify
Calculate all the various Watts, (volts times current), drawn from each Secondary winding of the transformer and add together.
Multiply by 0.85 to allow for transformer efficiency as suggested above
This gives total power going into the Primary of the transformer
Divide this by the mains voltage, (230v). This gives total current going into the transformer. Check this does not exceed the transformer VA rating.
Note that on switch-on there will be an initial surge of current, the magnitude of which varies acording to the design of the transformer, the type of load, and the instant at which switch-on occurs. The best answer for this is to use a slow-blow type fuse, in which case you can rate it as suggested above at 125% the current calculated above.
A simpler way is to take the VA rating of the transformer, divide by mains voltage, and this gives rated current. Use this for the slo-blow fuse value.
If not slo-blow, a certain degree of trial and error is required, but keep it as close to the values calculated above without nusance trips.
In case you are worried, a well designed transformer can run for a considerable period at a slight excess of its rated value. However it will get hot, and this shortens the life of the insulation, so eventually it will fail. The greater the excess, the greater the heat, and the shorter the life, so its not a good idea to over-run transformers, but they will tolerate short-term abuse as can happen during construction.
Take care of the 230v, use a proper shielded fuse holder, and protect all "hot" terminals. I have been bitten several times and don't like it.
Remember that the purpose of the fuse is to protect your expensive transformer and protect against fire. In most cases a fuse will act too slowly to protect the circuit itself, particularly if sold-state. So the actual fuse value is not too critical, provided it meets these two main aims. That is also why fusing the secondaries is usually a waste of time. Although in the past fuses have been used in the finals of audio amps, with mixed success.
There is also a slight typo above.
To clarify
Calculate all the various Watts, (volts times current), drawn from each Secondary winding of the transformer and add together.
Multiply by 0.85 to allow for transformer efficiency as suggested above
This gives total power going into the Primary of the transformer
Divide this by the mains voltage, (230v). This gives total current going into the transformer. Check this does not exceed the transformer VA rating.
Note that on switch-on there will be an initial surge of current, the magnitude of which varies acording to the design of the transformer, the type of load, and the instant at which switch-on occurs. The best answer for this is to use a slow-blow type fuse, in which case you can rate it as suggested above at 125% the current calculated above.
A simpler way is to take the VA rating of the transformer, divide by mains voltage, and this gives rated current. Use this for the slo-blow fuse value.
If not slo-blow, a certain degree of trial and error is required, but keep it as close to the values calculated above without nusance trips.
In case you are worried, a well designed transformer can run for a considerable period at a slight excess of its rated value. However it will get hot, and this shortens the life of the insulation, so eventually it will fail. The greater the excess, the greater the heat, and the shorter the life, so its not a good idea to over-run transformers, but they will tolerate short-term abuse as can happen during construction.
Take care of the 230v, use a proper shielded fuse holder, and protect all "hot" terminals. I have been bitten several times and don't like it.
OK,
OK 120VA +75VA = 195VA
195 VA / 207V = 0.95 Amps (we use 207 because that is 90% of line)
0.95 Amps X 125% ~= 1.25 Amps
Use 1.25 to 1.5 Amps and fuggeddabowdit.
Separate fusing for the primaries is best. You can use the same calcs above. This isn't strictly neccesary... usually, if something goes wrong you'll pull alot more than an Amp and a single fuse would open pretty quickly. The most important thing of all is that the current rating of your power cord is greater than the fuse rating.... no brainer. This is so you don't burn your house down.
Along as your chassis is solidly connected to ground... no one really cares if your amp goes up in smoke.
If you can get someone to measure your primary currents safely, then you could trim the fuse values down a bit... NOT an emergency and not worth spending a lot of $$$.
EDIT: Fusing is not a casual issue and we have not treated it casually here. Keep in mind, that if the power levels here were much greater, a much more serious approach would be prudent.
Calculating your total power draw would be a good exercise for you... and as Chris and AudioFreak have said... divide by 0.85. Then multiply your calculated amps by 125 - 200% for your fuse value
You are totally safe with 1.5 Amp fuse. Transformers can easily withstand a 200 - 400% overload long enough to clear a fuse without a bit of danger or damage. I just didn't want to give the impression that this is trivial. At lower power levels, there is just more "slack" in the calc's.
OK 120VA +75VA = 195VA
195 VA / 207V = 0.95 Amps (we use 207 because that is 90% of line)
0.95 Amps X 125% ~= 1.25 Amps
Use 1.25 to 1.5 Amps and fuggeddabowdit.
Separate fusing for the primaries is best. You can use the same calcs above. This isn't strictly neccesary... usually, if something goes wrong you'll pull alot more than an Amp and a single fuse would open pretty quickly. The most important thing of all is that the current rating of your power cord is greater than the fuse rating.... no brainer. This is so you don't burn your house down.
Along as your chassis is solidly connected to ground... no one really cares if your amp goes up in smoke.
If you can get someone to measure your primary currents safely, then you could trim the fuse values down a bit... NOT an emergency and not worth spending a lot of $$$.
EDIT: Fusing is not a casual issue and we have not treated it casually here. Keep in mind, that if the power levels here were much greater, a much more serious approach would be prudent.
Calculating your total power draw would be a good exercise for you... and as Chris and AudioFreak have said... divide by 0.85. Then multiply your calculated amps by 125 - 200% for your fuse value
You are totally safe with 1.5 Amp fuse. Transformers can easily withstand a 200 - 400% overload long enough to clear a fuse without a bit of danger or damage. I just didn't want to give the impression that this is trivial. At lower power levels, there is just more "slack" in the calc's.
Hi AudioFreak, poobah,
Thanks. I missed that divide changed to multiply thing.
Poobah's (' - ownership again) comments are right on. If you can, you should fuse for your normal current draw plus a safety. There is a thermal fuse inside your transformers. They open when the transformer gets too hot. You don't want to go there.
So jarthel,
Do you understand how to measure the current? If not, will you get some help? We would like to keep all our members!
-Chris
Thanks. I missed that divide changed to multiply thing.
Poobah's (' - ownership again) comments are right on. If you can, you should fuse for your normal current draw plus a safety. There is a thermal fuse inside your transformers. They open when the transformer gets too hot. You don't want to go there.
So jarthel,
Do you understand how to measure the current? If not, will you get some help? We would like to keep all our members!
-Chris
living on the edge
Hi,
Measuring the onload AC current of domestic equipment.
This works with plug tops that have a fuse carrier inside the plug top.
DANGER !
Switch off the socket outlet at the wall.
Remove the plug top cover, remove the plug top fuse.
Re-insert the plug top into the wall socket.
Set your Multimeter to AC current and highest range 5Aac to 12Aac.
Switch on the wall socket, switch on the equipment.
Attach the ammeter to each of the exposed fuse terminals.
The ammeter will read the current going into the equipment. Ask someone else (the witness at a fatal accident enquiry if all goes wrong) to turn up the volume and note the highest reading.
Do NOT allow yourself to be distracted. You don't want those probes to slip.
This can be done a little more safely by using insulated croc clips attached to the fuse carrier before powering up the wall socket outlet.
However the current capacity of either method will not suit high power equipment.
If you decide you are competent to carry out the above dangerous procedure you will be working with exposed LIVE terminals. Make sure you remove all children, animals and drunks from the vicinity before you start work.
Take care.
Alternatively,
solder a low value resistor into an old fuse. Insert the fuse into your plug top and read the AC volts drop across the fuse/dummy resistor. This allows the equipment to operate without needing to keep the multimeter in contact with the exposed terminals and also allows range setting to a more sensitive scale. A 1r0 600mW resistor will suit readings upto 500mA, 0r47 for upto 1200mA, 0r22 for upto 2A. 0r1 for upto 3A. There will be too much build up of heat inside the old fuse to allow much higher than this. Be carefull removing the dummy fuse, it might be hot!
The DANGERs using the alternative are just as risky.
Hi,
Measuring the onload AC current of domestic equipment.
This works with plug tops that have a fuse carrier inside the plug top.
DANGER !
Switch off the socket outlet at the wall.
Remove the plug top cover, remove the plug top fuse.
Re-insert the plug top into the wall socket.
Set your Multimeter to AC current and highest range 5Aac to 12Aac.
Switch on the wall socket, switch on the equipment.
Attach the ammeter to each of the exposed fuse terminals.
The ammeter will read the current going into the equipment. Ask someone else (the witness at a fatal accident enquiry if all goes wrong) to turn up the volume and note the highest reading.
Do NOT allow yourself to be distracted. You don't want those probes to slip.
This can be done a little more safely by using insulated croc clips attached to the fuse carrier before powering up the wall socket outlet.
However the current capacity of either method will not suit high power equipment.
If you decide you are competent to carry out the above dangerous procedure you will be working with exposed LIVE terminals. Make sure you remove all children, animals and drunks from the vicinity before you start work.
Take care.
Alternatively,
solder a low value resistor into an old fuse. Insert the fuse into your plug top and read the AC volts drop across the fuse/dummy resistor. This allows the equipment to operate without needing to keep the multimeter in contact with the exposed terminals and also allows range setting to a more sensitive scale. A 1r0 600mW resistor will suit readings upto 500mA, 0r47 for upto 1200mA, 0r22 for upto 2A. 0r1 for upto 3A. There will be too much build up of heat inside the old fuse to allow much higher than this. Be carefull removing the dummy fuse, it might be hot!
The DANGERs using the alternative are just as risky.
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