Toroidal Transformer Question

[quantumtangles]

I intend to connect the brown (live) and blue (neutral) wires comprising the primary input of a toroidal transformer to a fused 3 pin plug.

When I plug it into the mains, I expect the secondary wires (coloured yellow and red and black and grey respectively) to output 2 x 35v AC.

Then I intend to connect the ground or earth wire (here in the UK it will be a green and yellow wire) from the mains lead directly to the metal cabinet of my home brew amplifier (which requires a 2 x 35v AC power supply).

This is the first time I have made a hot device so I am a little anxious about it.

Aside from testing the secondary outputs (very carefully with a DMM) once I plug in the transformer (taking into account that the voltage output could be highly dangerous if for any reason I have got the primary and secondary the wrong way round), should the device be entirely safe provided I solder the ground/earth wire issuing from the mains lead to the metal body of the amplifier cabinet.

Have I missed any safety issues here or should it be safe to proceed here with great caution?

Sincere thanks in advance for any responses.

 

[(*steve*)]

What you're doing sounds fine.

I would hope you don't confuse the primary and the secondary as that's a pretty major sort of fubar.

Of course, I should note that depending on where you are, the work you're doing may need to be checked (or even done) by someone qualified and perhaps appropriately licensed. And also "mains can kill".

Proceeding with great caution is always a good thing when you're doing anything involving the mains or other high energy power sources.

 

[quantumtangles]

Thanks Steve. I will get it checked by a certified electrician when the build is finished.

 

[duke37]

13A plugs in the UK can be fitted with various fuses and if a fuse goes, the plug is likely to be fitted with a 13A fuse. This far too big.
I suggest that you fit a 1A fuse inside the equipment.

If you fit a IEC soket on the equipment, then a standard mains lead can be used.
If you fit a cable, make sure that the cable is securely anchored and exits through a grommet, preferably with a flexible tail.

The secondary connections may need to be phased correctly depending on the equipment.

 

[JimW]

I am not a transformer expert by any means, so defer to others when in doubt. But... Toroidal transformers are generally used for higher frequency ranges. Still, they can be used for 50Hz power mains conversions, but it is at the lower end of their efficiency. Assuming you are looking at approximately 50VA, your picture looks to have a fairly small transformer. It may not be sufficient for your needs.

Here is some general info on power mains toroid use:
http://www.raftabtronics.com/TECHNO...oidalTransformerBasics/tabid/112/Default.aspx

JimW

 

[duke37]

JimW the link you provide gives good information.

Toroidal transformers are smaller and more efficient but difficult to wind and the core can saturate on turn on, giving a large current pulse. My recommendation of a 1A fuse may need to be increased because of this. Use slow blow (T).

The core material will determine the optimum frequency range

 

[Arouse1973]

From safety point of view. I think you should be using either an isolation transformer or an RCD . I use an isolation transformer if I am working on equipment which have large filter capacitors from live to ground which would normally trip an RCD.rating than your You have to remember that the earth lead on equipment is not there to protect you. It is there to blow the fuse in a fault or overload condition. At 340V ac pk even a 1 amp slow blow fuse wouldn't blow before it potentially killed you. Also the MCB alternative to old cartridge fuses still won't blow, your still dead. You need an RCD and an overload protection fuse which by default you should have in your mains supply and the fuse in the lead powering your equipment.

Thanks
Adam

 

[davenn]

I am not a transformer expert by any means, so defer to others when in doubt. But... Toroidal transformers are generally used for higher frequency ranges. Still, they can be used for 50Hz power mains conversions, but it is at the lower end of their efficiency. Assuming you are looking at approximately 50VA, your picture looks to have a fairly small transformer. It may not be sufficient for your needs..................

JimW

toroidal transformers are in abundant use in the mains power section of commercial audio power amplifiers these days, probably at least 80% of them. Of course its also common to see them in the audio output stage for supplying the 8 Ohm, 50V, 75V and 100V speaker outputs


Dave

 

[duke37]

Arouse1973
You do not say what country you are in.
The residual current breaker (RCD) has nothing to do with overload.
In the UK overload trips are being used in preference to fuses since they are faster.
The toroidal transformer will act as an isolating transformer but I always use an isolating transformer if working on the mains side of equipment. The isolating transformer will reduce the chance of fuses or trips acting since it adds a resistance in the supply so limits the current if your load transformer saturates or very high capacitor charge currents are involved. There is something very wrong if capacitor charging trips an RCD.

I have fitted some of my higher powered equipment with a start resistance which is shorted after a fraction of a second. This protects the transformer windings from high forces when the transformer saturates.

 

[Harald Kapp]

@Adam:

the earth lead on equipment is not there to protect you. It is there to blow the fuse in a fault or overload condition.

This is not entirely correct. By blowing the fuse the phase is shut off and the device falls into a safe mode. Therefore protection of the user is the purpose of an earth lead. In the relevant standards (IEC, UL etc.) it is therefore called "protective earth", short PE, for a reason. In order to be protective some requirements for a PE-connection are put in these standards like minimum current carrying capability, means of attaching PE to the device etc.

@quantumtangles:
Soldering earth to the metal housing is probably the second best way of attaching it. I suggest you use a cable lug securely attached to the earth lead and screw the lug to the housing without using a washer.
Also make sure the earth lead is a few centimeters longer than the mains leads. This way, if the cord comes loose and is being stretched, the mains leads will break first while the earth lead is still securely connected to the housing and can protect you from an electrical shock.

As for the inrush current: This is a known issue with toroidal transformers, specially if on the secondary side there are big electrolytic capacitors, as is typically the case for an amplifier.
You may consider adding a soft start circuit to your transformer. Some useful information is here. You can buy this as a module, e.g. here.

 

[Arouse1973]

Arouse1973
You do not say what country you are in.
The residual current breaker (RCD) has nothing to do with overload.
In the UK overload trips are being used in preference to fuses since they are faster.
The toroidal transformer will act as an isolating transformer but I always use an isolating transformer if working on the mains side of equipment. The isolating transformer will reduce the chance of fuses or trips acting since it adds a resistance in the supply so limits the current if your load transformer saturates or very high capacitor charge currents are involved. There is something very wrong if capacitor charging trips an RCD.

I have fitted some of my higher powered equipment with a start resistance which is shorted after a fraction of a second. This protects the transformer windings from high forces when the transformer saturates.

Yes I know, please read my post again.
Thanks Adam

 

[Arouse1973]

@Adam:

This is not entirely correct. By blowing the fuse the phase is shut off and the device falls into a safe mode. Therefore protection of the user is the purpose of an earth lead. In the relevant standards (IEC, UL etc.) it is therefore called "protective earth", short PE, for a reason. In order to be protective some requirements for a PE-connection are put in these standards like minimum current carrying capability, means of attaching PE to the device etc.

@quantumtangles:
Soldering earth to the metal housing is probably the second best way of attaching it. I suggest you use a cable lug securely attached to the earth lead and screw the lug to the housing without using a washer.
Also make sure the earth lead is a few centimeters longer than the mains leads. This way, if the cord comes loose and is being stretched, the mains leads will break first while the earth lead is still securely connected to the housing and can protect you from an electrical shock.

As for the inrush current: This is a known issue with toroidal transformers, specially if on the secondary side there are big electrolytic capacitors, as is typically the case for an amplifier.
You may consider adding a soft start circuit to your transformer. Some useful information is here. You can buy this as a module, e.g. here.

Sorry Harald that is without being rude, rubbish. Electronic equipment have a fuse rating that corrosponds to the size of the cable. So if its true we should only work on equipment that only works with a 1A fuse, do you still think you wouldn't fry working on a heater with a 13A fuse and you would be safe. The fuse is to protect the equipment, why would my H and S department give me so much grief otherwise. You willing to touch live with a 13A fuse and hope its ok. I'll get the sausages.
Adam

 

[davenn]

Electronic equipment have a fuse rating that corrosponds to the size of the cable

sorry Adam, but that is rubbish

electronic equip has a fuse that is rated in regards to the current drawn by the equipment. A given sized cable can handle a wide range of currents. eg pretty much100% of the power cables to appliances in your home will all have the same sized cable TV, toaster, fan, DVD player, computer, printer, microwave oven etc all of which have a wide range of current requirements and they will ( except probably the toaster and fan) have various ranges of fuses internally according to their current requirements

The toaster, fan, heater all rely on the fuse back at the fuse box at the power distribution panel
There will be likely only one item that has a much thicker cable and a much larger fuse at the main fuse box and that is the range ( rangetop/oven).

Dave

 

[Arouse1973]

sorry Adam, but that is rubbish

electronic equip has a fuse that is rated in regards to the current drawn by the equipment. A given sized cable can handle a wide range of currents. eg pretty much100% of the power cables to appliances in your home will all have the same sized cable TV, toaster, fan, DVD player, computer, printer, microwave oven etc all of which have a wide range of current requirements and they will ( except probably the toaster and fan) have various ranges of fuses internally according to their current requirements

Dave

Nope in the uk I can only put a max of a 10 amp fuse for a cable rated at 10amp over a given distance. This is regardless of the current that the equipment draws but you would be stupid to put a 13A fuse to protect a 1A cable although it would be ok. I don't think people understand cable ratings. A 1A rated cable is rated 1A for the voltage drop of that cable and for the heating effect that will occur over a length of cable. In a fault condtion it is the cable that the fuse is protecting not the user.
Might be diiferent in the US?,
Adam

 

[davenn]

all the standard power cables are rated to ~ 10 amp, so your arguent doesnt hold

As I said, the fuse in the TV radio, DVD player etc is rated in relation to the current drawn by the equip

think about it a minute ..... there's not a bunch of different sized cables huh
the only places you will see larger cables in on stuff like the range/oven maybe the electric water heater that draw much higher current

Dave

 

[davenn]

The fuses at the main house fuse box are rated to the max allowed for the internal house wiring they are protecting. This can be MUCH higher, say the 10 Amp max rating for that circuit, than some small appliance plugged into one of the house outlets on that circuit that is only drawing 0.5 A. The equip drawing 0.5 A continuous is likely to have a 1 to 1.5 A fuse in it, NOT a 5 or 10 Amp

Dave

 

[Arouse1973]

Example.
Human body model 1.5K ohms.Touching for long enough 240V rated13amp fuse equals dead person in th UK. Connect 1.5A cable from 32A fuse in house to multiple heaters drawing 25A. Fuse ok heaters ok wire on fire call fire brigade. Fuse didn't protect wire.
Adam

 

[Arouse1973]

The fuses at the main house fuse box are rated to the max allowed for the internal house wiring they are protecting. This can be MUCH higher, say the 10 Amp max rating for that circuit, than some small appliance plugged into one of the house outlets on that circuit that is only drawing 0.5 A. The equip drawing 0.5 A continuous is likely to have a 1 to 1.5 A fuse in it, NOT a 5 or 10 Amp

Dave

The equipment supplier can't be assured as to what your plugging into as to fuse rating. What if somebody put a bolt in the fusebox they can't trust anyone.

 

[Arouse1973]

The fuses at the main house fuse box are rated to the max allowed for the internal house wiring they are protecting. This can be MUCH higher, say the 10 Amp max rating for that circuit, than some small appliance plugged into one of the house outlets on that circuit that is only drawing 0.5 A. The equip drawing 0.5 A continuous is likely to have a 1 to 1.5 A fuse in it, NOT a 5 or 10 Amp

Dave

Sorry Dave
That made no sense to me. Please clarify. I said the fuse is to protect the wiring, which is what you just said?
Adam

 

[davenn]

I give up

believe what you like


Dave