Overload Protection for Transformers (was Cab Control)

[John Isherwood]

I intend to operate my DC layout using common return cab control.

 

I have installed two https://www.rapidonline.com/vigortronix-vtx-126-050-212-chassis-transformer-230v-50va-12v-12v-88-3930 chassis transformers; (though I subsequently wondered if one of these would have sufficed, as they have twin 12V AC outputs).

 

As I understand common return cab control - and please correct me if I am mistaken - the 12V AC supplies to the controllers must be from separate transformer windings. Does this imply two separate secondary 12V AC windings on the same core as a single primary 240v winding; or two entirely separate 240V / 12V AC transformers?

 

If the former, I assume that I could use the twin 12V AC outputs of a single https://www.rapidonline.com/vigortronix-vtx-126-050-212-chassis-transformer-230v-50va-12v-12v-88-3930 ; if the latter, I would need to use one output each from two https://www.rapidonline.com/vigortronix-vtx-126-050-212-chassis-transformer-230v-50va-12v-12v-88-3930 ?

 

As to cab control itself, the two 12V AC supplies  - from whatever source - will feed a pair of voltage regulator controllers as per

The DC outputs from these two controllers will be used for the cab control system, by using a single return bus wire, with two separate 0-12V supply bus wires - one from each controller. The supply to each track section will be from one or the other controller, selected by two-way, centre-off toggle switches.

 

I am checking all of this as I have just managed to destroy my first two transformers, due to an inadvertent short circuit on one of the 12V AC bus circuits !!

 

I now find that the https://www.rapidonline.com/vigortronix-vtx-126-050-212-chassis-transformer-230v-50va-12v-12v-88-3930 chassis transformers are out of stock, so I have had to order a pair of https://uk.rs-online.com/web/p/chassis-mounting-transformers/0504672/

 

I will double-check for short circuits in the two 12V AC bus circuits BEFORE connecting up the replacement transformers!!!

 

All comments gratefully received!

 

John Isherwood.

Edited August 11, 2021 by cctransuk

[Pete the Elaner]

2 separate windings, not necessarily 2 transformers.

If the outputs meet on the track (which they do with common return), the windings need to be electrically separated.

[AndyID]

The secondary (low voltage) windings can be on independent transformers or share a primary on one core. What matters is that there must be no electrical connection between them.

 

However, two windings on one core is a bit safer. With two transformers there is a risk that they can be connected in such a way that one of them steps down and the other one steps back up to a potentially lethal voltage across its primary connections.

[John Isherwood]

On 09/08/2021 at 16:51, cctransuk said:

I intend to operate my DC layout using common return cab control.

 

I have installed two https://www.rapidonline.com/vigortronix-vtx-126-050-212-chassis-transformer-230v-50va-12v-12v-88-3930 chassis transformers; (though I subsequently wondered if one of these would have sufficed, as they have twin 12V AC outputs).

 

As I understand common return cab control - and please correct me if I am mistaken - the 12V AC supplies to the controllers must be from separate transformer windings. Does this imply two separate secondary 12V AC windings on the same core as a single primary 240v winding; or two entirely separate 240V / 12V AC transformers?

 

If the former, I assume that I could use the twin 12V AC outputs of a single https://www.rapidonline.com/vigortronix-vtx-126-050-212-chassis-transformer-230v-50va-12v-12v-88-3930 ; if the latter, I would need to use one output each from two https://www.rapidonline.com/vigortronix-vtx-126-050-212-chassis-transformer-230v-50va-12v-12v-88-3930 ?

 

As to cab control itself, the two 12V AC supplies  - from whatever source - will feed a pair of voltage regulator controllers as per

The DC outputs from these two controllers will be used for the cab control system, by using a single return bus wire, with two separate 0-12V supply bus wires - one from each controller. The supply to each track section will be from one or the other controller, selected by two-way, centre-off toggle switches.

 

I am checking all of this as I have just managed to destroy my first two transformers, due to an inadvertent short circuit on one of the 12V AC bus circuits !!

 

I now find that the https://www.rapidonline.com/vigortronix-vtx-126-050-212-chassis-transformer-230v-50va-12v-12v-88-3930 chassis transformers are out of stock, so I have had to order a pair of https://uk.rs-online.com/web/p/chassis-mounting-transformers/0504672/

 

I will double-check for short circuits in the two 12V AC bus circuits BEFORE connecting up the replacement transformers!!!

 

All comments gratefully received!

 

John Isherwood.

 

Having now received the replacement transformers that I ordered, they will be installed this afternoon.

 

I spent yesterday afternoon ensuring that the cable joints in the wiring connected to the two halves of the LV AC circuitry are sufficiently separated, to avoid the short-circuit which destroyed the original transformers recurring.

 

However, I am now acutely aware of the fact that a short circuit in that side of the wiring could be fatal for the transformers, and I would like to install a fail-safe device that will interrupt any such overload before damage is done. Ideally, a resettable circuit breaker would, I imagine, serve the purpose - though a replaceable fuse might also work?

 

The transformers in question are https://uk.rs-online.com/web/p/chassis-mounting-transformers/0504672/

 

I would be grateful if someone could provide a link to a suitable component that could save me the expense of replacing these transformers for a second time! I am assuming that such a device would be installed in series within the LV AC circuitry?

 

Many thanks in anticipation.

 

John Isherwood.

[AndyID]

Hi John,

 

I confess I am really quite concerned. We only have the relatively wimpy 120 volt mains here which is bad enough but 240 volts is FOUR TIMES more lethal (it's the square of the voltage).

 

Please try to have a qualified electrical engineer inspect your supply before you switch it on or at the very least post a circuit diagram and a lot of photos here so we can offer advice.

 

A much safer approach would be to ditch the transformer altogether and use two self contained 12 volt output switched-mode power supplies. There are plenty of them and they are not very expensive.

 

Andy

[John Isherwood]

2 hours ago, AndyID said:

Hi John,

 

I confess I am really quite concerned. We only have the relatively wimpy 120 volt mains here which is bad enough but 240 volts is FOUR TIMES more lethal (it's the square of the voltage).

 

Please try to have a qualified electrical engineer inspect your supply before you switch it on or at the very least post a circuit diagram and a lot of photos here so we can offer advice.

 

A much safer approach would be to ditch the transformer altogether and use two self contained 12 volt output switched-mode power supplies. There are plenty of them and they are not very expensive.

 

Andy

 

Have no fear - the transformers are safely encased in purpose-made commercial cases, and are properly earthed.

 

The twin 12V AC outputs are attached to the cases, and comprise proper electrical screw terminals.

 

These are by no means the first power units that I have constructed in my 70+ years; I am fully aware of the potential hazards; an0d I proceed with due caution.

 

As part of this due caution, I am now more aware of the risks of a short circuit on the LV side, and this why I am seeking to install overload protection.

 

CJI.

[AndyID]

1 hour ago, cctransuk said:

 

Have no fear - the transformers are safely encased in purpose-made commercial cases, and are properly earthed.

 

The twin 12V AC outputs are attached to the cases, and comprise proper electrical screw terminals.

 

These are by no means the first power units that I have constructed in my 70+ years; I am fully aware of the potential hazards; an0d I proceed with due caution.

 

As part of this due caution, I am now more aware of the risks of a short circuit on the LV side, and this why I am seeking to install overload protection.

 

CJI.

 

Hi John,

 

Well OK, as long as you know why the two transformers were destroyed.

 

Without knowing a lot more about what the transformer is driving it's not possible to give much advice about overload protection. You could just put an underrated fuse (100mA perhaps) in the primary and be prepared to increase the value if it blows in normal operation. (Typically that would be during power-up.)

 

Andy

[DCB]

I would as a baseline put a 1 amp glass case fuse in each AC circuit and a 1/2 amp Polyfuse in the DC outputs.   For proper overload protection on the DC side raid an old Triang power unit with a pop up overload button.  That's proper protection, about 200 times the cost of a polyfuse, but it might save a good few quid over a few years.

[John Isherwood]

1 hour ago, DavidCBroad said:

I would as a baseline put a 1 amp glass case fuse in each AC circuit and a 1/2 amp Polyfuse in the DC outputs.   For proper overload protection on the DC side raid an old Triang power unit with a pop up overload button.  That's proper protection, about 200 times the cost of a polyfuse, but it might save a good few quid over a few years.

 

David,

 

Thank you for this - very helpful.

 

I must confess that I was thinking in terms of the old Tri-ang cut-outs when considering how I should provide output overload protection for the transformers. Are there no modern components that can limit the current flowing in a LV AC circuit, by breaking the circuit if a preset current is exceeded?

 

I do wonder if 1 amp would be sufficient for my purposes - some of my locos have 1960s / 70s motors and I will be using them for double-heading; that was one reason that I bought transformers with a rating of 50VA for two 12V output circuits.

 

The controllers that I use are based on variable voltage regulator circuits - see

 

Having used them for some time now, they seem to cope with the occasional short-circuit within the output to rail circuitry without adverse effect, so I had not considered the need for overload protection in that area.

 

Any further thoughts concerning resettable overload protection within the LV AC circuitry would be gratefully received.

 

John Isherwood.

[Nearholmer]

A polyswitch self-resets once the current is below threshold. No need for a pushbutton.

 

You can still obtain press-button resetable devices for 12V dc, mainly for use in automotive applications, but they typically aren't available with ratings less than 5A - if you ferret around there might be 2A ones, but I'm not sure.

 

The old "Triang type" ones might be easy to use, and rather fun, but the protection they provide is very coarse, and I suspect that the automotive ones may be the same, although I haven't delved into that. "Coarse" in this context means that the amount of energy they let through to a fault, before actuating, is high, so potentially a lot of damage can occur before the protection clears the fault. The sensitive electronics in a modern car are almost certainly protected by quick-blow fuses and/or polyswitches downstream of the resettable devices which are there to deal with faults in wiring and in robust components.

 

Personally, I wouldn't run the low voltage side, either the AC part, or the DC part, without putting protective devices as close to the output terminals as possible, and routing the connections from output to protective device very carefully, because although the nominal currents that can flow before the primary-side protection operates should, if you've got that right, be low, they will still be sufficient to cause the sort of heating that can start a fire in the tinderbox that is a model railway. Something as simple as a screwdriver left on the layout, making slightly iffy contact with the rails could cause enough heat to ignite the corner of the instruction leaflet that is laying next to it .......

Edited August 11, 2021 by Nearholmer

[John Isherwood]

12 minutes ago, Nearholmer said:

A polyswitch self-resets once the current is below threshold. No need for a pushbutton.

 

You can still obtain press-button resetable devices for 12V dc, mainly for use in automotive applications, but they typically aren't available with ratings less than 5A - if you ferret around there might be 2A ones, but I'm not sure.

 

The old "Triang type" ones might be easy to use, and rather fun, but the protection they provide is very coarse, and I suspect that the automotive ones may be the same, although I haven't delved into that. "Coarse" in this context means that the amount of energy they let through to a fault, before actuating, is high, so potentially a lot of damage can occur before the protection clears the fault. The sensitive electronics in a modern car are almost certainly protected by quick-blow fuses and/or polyswitches downstream of the resettable devices which are there to deal with faults in wiring and in robust components.

 

 

Thank you - very helpful.

 

On the face of it, then https://uk.rs-online.com/web/p/resettable-wire-ended-fuses/7127496/  this might be suitable? (I assume that they function OK in AC circuits)?

 

If so, I'll get a couple ordered straight away.

 

Thanks again,

John Isherwood.

[AndyID]

31 minutes ago, cctransuk said:

 

David,

 

Thank you for this - very helpful.

 

I must confess that I was thinking in terms of the old Tri-ang cut-outs when considering how I should provide output overload protection for the transformers. Are there no modern components that can limit the current flowing in a LV AC circuit, by breaking the circuit if a preset current is exceeded?

 

I do wonder if 1 amp would be sufficient for my purposes - some of my locos have 1960s / 70s motors and I will be using them for double-heading; that was one reason that I bought transformers with a rating of 50VA for two 12V output circuits.

 

The controllers that I use are based on variable voltage regulator circuits - see

 

Having used them for some time now, they seem to cope with the occasional short-circuit within the output to rail circuitry without adverse effect, so I had not considered the need for overload protection in that area.

 

Any further thoughts concerning resettable overload protection within the LV AC circuitry would be gratefully received.

 

John Isherwood.

 

Hi John,

 

You can short the output of a LM317 to ground all day without doing it any harm. It has built-in overload protection.

 

Any fusing arrangements you include only need to be concerned with excessive current prior to  the 317 regulator.

 

Cheers,

Andy

[Nearholmer]

17 minutes ago, cctransuk said:

On the face of it, then https://uk.rs-online.com/web/p/resettable-wire-ended-fuses/7127496/  this might be suitable? (I assume that they function OK in AC circuits)?

 

Make sure you click on the data sheet link and read it through, especially the time-current curves - these things have a "hold" and a "trip" current. The "hold" they will supply continuosly, the "trip" is where they will trip near-instantaneously, so you need to choose accordingly.

 

 

Edited August 11, 2021 by Nearholmer

[John Isherwood]

Just now, Nearholmer said:

 

Make sure you click on the data sheet link and read it through - these things have a "hold" and a "trip" current. The "hold" they will supply continuosly, the "trip" is where they will trip near-instantaneously, so you need to choose accordingly.

 

Good advice - and the https://uk.rs-online.com/web/p/resettable-wire-ended-fuses/7127496/  has too long a trip interval at 15.6 seconds.

 

I note that some of the polyswitches specify the voltage rating as being DC?

 

On reflection, https://uk.rs-online.com/web/p/resettable-wire-ended-fuses/5176900/  seems to meet my requirements, and no mention of DC! (I assume that polyswitches will function on AC)?

 

Your views would be appreciated.

 

John Isherwood.

[BR60103]

What I see used here are 12V automobile bulbs between controller and layout.  Mine go on the common rail side. These will light up on a short. They also start to glow if your train draws a bit more current than normal. (A trio of HD locos will light a room.)

I suspect that it could be used in the AC circuit as well.

 

Best to have them in a socket, although some of mine have had the leads soldered to the bulb for 30 years.

I like the double filament bulbs as they have less resistance. (combined brake and turn signal) Incandescant not LED.

[AndyID]

5 hours ago, BR60103 said:

What I see used here are 12V automobile bulbs between controller and layout.  Mine go on the common rail side. These will light up on a short. They also start to glow if your train draws a bit more current than normal. (A trio of HD locos will light a room.)

I suspect that it could be used in the AC circuit as well.

 

Best to have them in a socket, although some of mine have had the leads soldered to the bulb for 30 years.

I like the double filament bulbs as they have less resistance. (combined brake and turn signal) Incandescant not LED.

 

Yes, they are very effective thermistors. Their resistance increases with current, quite significantly as they start to glow. Being purely resistive they work equally well on DC or AC.

[John Isherwood]

5 hours ago, BR60103 said:

What I see used here are 12V automobile bulbs between controller and layout.  Mine go on the common rail side. These will light up on a short. They also start to glow if your train draws a bit more current than normal. (A trio of HD locos will light a room.)

I suspect that it could be used in the AC circuit as well.

 

Best to have them in a socket, although some of mine have had the leads soldered to the bulb for 30 years.

I like the double filament bulbs as they have less resistance. (combined brake and turn signal) Incandescant not LED.

 

Thanks for that - but what I need on the AC output circuit is something that will interrupt the current instantaneously - not be reliant on human intervention. As I have found to my cost, a short-circuit across the AC output terminals produces serious overheating, and death to the transformer.

 

Unless someone tells me otherwise, https://uk.rs-online.com/web/p/resettable-wire-ended-fuses/5176900/ seems to be what I need.

 

John Isherwood.

[DCB]

Looks like a polyfuze to me,  I may be wrong.    They are a bit slow to trip,  

[SamThomas]

One thing I have not seen mentioned is the possibility of using 2 x wire wound transformers with separate A/C plugs is the fact that under some/rare circumstances if one transformer is unplugged it is possible for a potenially lethal voltage to be present on the exposed pins.

 

It does  take a number of conditions for this to happen & I'm not going to launch into a long winded technical explanation & then engage in the enivitable ping pong.

 

Some stand alone analgue controllers have a warning to this effect on them.

[Grovenor]

Doesn't matter if they are a bit slow so long as they react before the transformer gets too hot. The LM317 will protect the DC side so you are only protecting against a direct short in the LV AC wiring. Choose a polyfuse that will carry the transformer rated load continuously, looks like 2A for yours.

[John Isherwood]

12 minutes ago, Grovenor said:

Doesn't matter if they are a bit slow so long as they react before the transformer gets too hot. The LM317 will protect the DC side so you are only protecting against a direct short in the LV AC wiring. Choose a polyfuse that will carry the transformer rated load continuously, looks like 2A for yours.

 

Thanks for that - https://uk.rs-online.com/web/p/resettable-wire-ended-fuses/5176900/ ordered.

[AndyID]

4 hours ago, cctransuk said:

 

Thanks for that - https://uk.rs-online.com/web/p/resettable-wire-ended-fuses/5176900/ ordered.

 

Hi John,

 

Please do not use any sort of resettable fuse on the transformer secondaries or primary. The 317 protects the psu from any external overload which means that an overload on the transformer is the result of a serious fault in the internal wiring in the psu.

 

A resettable fuse can mask an intermittent wiring fault. It can lead you to believe you have resolved a problem when in fact you have not.

 

It's better to use a single conventional fuse in the primary circuit. That will protect both the primary and secondaries and it should only ever blow in the event that there is a serious insulation issue internal to the psu in which case that must be identified and resolved before power is restored.

 

You might find it necessary to use a slow blow fuse if the regulators take a large in rush current.

 

Cheers,

Andy

[John Isherwood]

2 hours ago, AndyID said:

 

Hi John,

 

Please do not use any sort of resettable fuse on the transformer secondaries or primary. The 317 protects the psu from any external overload which means that an overload on the transformer is the result of a serious fault in the internal wiring in the psu.

 

A resettable fuse can mask an intermittent wiring fault. It can lead you to believe you have resolved a problem when in fact you have not.

 

It's better to use a single conventional fuse in the primary circuit. That will protect both the primary and secondaries and it should only ever blow in the event that there is a serious insulation issue internal to the psu in which case that must be identified and resolved before power is restored.

 

You might find it necessary to use a slow blow fuse if the regulators take a large in rush current.

 

Cheers,

Andy

 

I'm afraid that you are misunderstanding my project; the LV output from the transformer feeds an AC ring main around the entire railway room. This feeds several plug-in points for voltage regulator controllers which convert the AC to controlled DC.

 

Therefore there are ample opportunities for inadvertent short-circuits in the LV AC wiring - as I found out the hard way. No fault whatsoever within the PSU.

 

I am convinced of the need for overload protection within the LV AC circuitry, and not convinced that a low value fuse within the HV primary circuit would solve the overload problem.

 

Let me set out my layman's perspective on this matter - and I am quite happy to be told that it is hogwash!

 

The primary circuit, fitted with a 5 amp fuse, will produce 240 x 5 = 1200 Watts of power.

 

That power, via electro-magnetism, is converted into power in the secondary circuit; in this case at 12V.

 

Thus 1200 watts could potentially generate 1200 ÷ 12 = 100 amps of current; surely enough to destroy any secondary windings?

 

As I said - probably hogwash - but basic physics nearly sixty years ago, plus some logic, is the way that my brain works!

 

CJI.

[John Isherwood]

DUPLICATE POST

Edited August 12, 2021 by cctransuk

[AndyID]

Hi John,

 

Ah yes! I had forgotten that. Yes, some sort of interrupters in the secondary circuits might be a good plan. Bear in mind that poly-fuses will still pass a small amount of current even in "open circuit" mode. Actual circuit breakers might be better but possibly hard to find. A quite effective method (as someone already suggested) is the old car brake-light bulb. That would give you an immediate visual warning as well. You can put multiple filaments in parallel to get the desired current limit and they will definitely save your transformers.

 

To protect the primary you should not use anything as large as a 5A fuse. That's more than enough current to start a serious conflagration. Most people don't realize that the fuse in the plug is there to protect the cable to the equipment, not the equipment itself. The worst case (probably very unlikely) for you would be some sort of partial breakdown in the transformer that caused it to draw say 2 amps. That would dissipate 480 watts in the transformer and it would quite quickly become very hot but the fuse would never blow.

 

For each 50 VA transformer I'd suggest you use a 250mA slow-blow fuse. That would also protect the secondaries. In any event I would not use anything greater than 500mA for each transformer.

 

The max current you mention in the secondaries is impossible for that transformer. The secondary current is limited by the impedance of the magnetic circuit between the primary and secondary and the magnetic flux in the transformer's core will saturate and limit the secondary current long before those sort of currents/power are produced. A more likely scenario is that the transformer's primary winding will overheat, fuse and go open-circuit. You could measure the resistance of the primaries on your defective transformers to see if they are open-circuit.

 

Cheers,

Andy