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Old 11th Nov 2018, 10:25 pm   #1
bikerhifinut
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Default Toroidal Transformers

Working out how to mount a couple of toroids on a chassis in order to save space.
Ok I know its possible to stack them together, I now know what precautions to take so that areas covered.
However.
The issue i will have is height above the chassis if I top mount both as I intend to get a nice round cover to make it look neater. probably will drill a few ventilation holes in it too.
So, I was drawing things out and I thought, can I have one transformer underneath the aluminium chassis with its mounting plate and rubber washer and the other on the opposite side likewise with insulated washer and mounting plate, and use a single bolt through both effectively clamping them together through the chassis plate?
Probably a daft question, but I thought it would be wise to ask the knowledgable ones.
For information purposes, one transformer will be a low voltage one to supply 6.3V valve heaters and will be a 50VA job but supplying 6A total. The other is a big beastie of a HT job which will be used to provide 450V DC at around 300mA. It's conservatively rated to be able to provide twice that so it should run cool.
My plan is to put the HT toroid on the chassis top and the heater one underneath as it will be smaller in diameter and give me a bit more room underneath the chassis. I see no issues there.
I'm assuming also that I need to put separate varistors across the primaries of each transformer, unless that's overkill and a single one across the paralelled primaries is enough?
Andy.
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Old 12th Nov 2018, 12:14 am   #2
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Default Re: transformers

Can't see why that wouldn't work. Other than cooling and mechanical damage, the only "interesting" thing about mounting toroids is to avoid creating a shorted turn with the mounting hardware.
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Old 12th Nov 2018, 12:29 am   #3
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Default Re: transformers

One toroid transformer above the chassis and one below has been done in commercial gear before, so yes, it is fine.

One long bolt from one through the other will work, but I would be more inclined to use threaded rod.

Use a nut and lock washer either side of the chassis to secure the rod, it will be more secure and less chance of damaging one of the toroids from over tightening a bolt.
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Old 12th Nov 2018, 12:47 am   #4
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Default Re: transformers

thanks chaps.
I think that will be the plan then.

A.
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Old 12th Nov 2018, 11:04 am   #5
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Default Re: transformers

There's no problem in stacking toroidal transformers!

They have a very low external field (theoretically zero if perfectly uniformly wound) so there's virtually zero interaction between them.

However, each will create some heat, so whether in direct contact, or with a chassis between, each is effectively sitting on something warm. You may need to consider this, as a general practice, though you do say you are de-rating your transformers so doubt it will be a problem in your situation.
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Old 12th Nov 2018, 1:05 pm   #6
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Default Re: Toroidal Transformers

Noted kalee.
Yes I have considered the heat build up issue and i figure to drill some holes in the mounting plates/insulating washers so that the interior of the core gets a bit of convection airflow in conjunction with a set of holes drilled around the mounting hole on the chassis plate. I am looking out for a UK source for a cover for the toroid on top of the chassis and should i get one, that too will get some cooling holes drilled in it. I figure the big toroid will be running at less than 50% of its rated capacity anyway and the smaller one at around 75-80% so I think it may be wiser to put the big un below and the little un above but either way I don't expect a lot of heat. The base of the amp case work will be made of a wire mesh material so again plenty of airflow around the circuitry.
Thanks for your inputs folks, I am happy that with the reasonable precautions that all will be good.
I could of course just go and buy a custom or off the shelf transformer to suit the circuit but as I have the big toroid already and the small heater one will cost me about £13 from RS online it's going to be a LOT cheaper than a new EI transformer from the usual manufacturers.
By the way, am i right in thinking that I need to divide the AC current rating by roughly 1.5 in order to get a transformers actual current delivery at DC from a FW rectifier? i.e. because the rectified voltage is 1.414 x the RMS AC volts then to achieve the same power rating the transformer must be delivering LESS current and thus it must be taken into account when planning a circuit?

Andy

Last edited by bikerhifinut; 12th Nov 2018 at 1:14 pm. Reason: extra question
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Old 12th Nov 2018, 1:14 pm   #7
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Default Re: Toroidal Transformers

Don't know if it should be a new thread but does anyone here know a good source of isolation toroids, ie 1 to 1 turn ratio but at a VA rating more suited to a pair of EL84? I got the idea from Morgan Jones' book a while back as using silicon rectifiers one gets more or less the perfect HT for EL84.
A
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Old 12th Nov 2018, 2:28 pm   #8
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Default Re: Toroidal Transformers

Quote:
Originally Posted by bikerhifinut View Post
By the way, am i right in thinking that I need to divide the AC current rating by roughly 1.5 in order to get a transformers actual current delivery at DC from a FW rectifier? i.e. because the rectified voltage is 1.414 x the RMS AC volts then to achieve the same power rating the transformer must be delivering LESS current and thus it must be taken into account when planning a circuit?
Probably worse than that, to be honest!

A rectifier operating directly into a reservoir capacitor is a difficult circuit to analyse. Peak current is massively dependent on parasitic series resistances (such as transformer winding resistances, capacitor ESR, rectifier characteristics) and inductances. Fortunately, it is possible to apply a few engineering assumptions...

Consider a 100V secondary voltage. After rectification, you'll have about 140V. So if you load the DC at 1A, you're drawing 140W from the transformer.

A rough 'rule of thumb' is that the rectifier / capacitor-input filter gives a Power Factor of around 0.65. So for your 140W being supplied, the transformer throughput VA is 140/0.65 = 215VA.

So if you are loading the DC output at 1A, you really need a transformer rated at 100V 2.2A RMS.

However, to muddy the water, the transformer manufacturer may have already taken some of this into account when they assign ratings to their products - so check whether the secondary load current is AC RMS or DC after rectification!
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Old 12th Nov 2018, 2:49 pm   #9
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Default Re: Toroidal Transformers

Battery chargers especially for dryfit cells are notorious for giving transformers a hard time. A 50% de-rating is commonly recommended in data sheets.
I did some experiments with adding an SMPSU choke in series with the rectifier but it still got hot with longer pulses at a lower current so there was no reduction in resistive heat being generated.
I once had a non technical boss who told me to use the wrong transformer even after I warned him about it. He listened to me after an insurance claim letter hit the works door mat.
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Old 12th Nov 2018, 4:58 pm   #10
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Default Re: Toroidal Transformers

Re toroid placement the only thing I can add to that already said is if using an aliminium chassis or in reference to shielding is to place the heater tfmr in the optimum position as it will create the most hum/interference.

"By the way, am i right in thinking that I need to divide the AC current rating by roughly 1.5 in order to get a transformers actual current delivery at DC from a FW rectifier?" I've found it better to measure than to calculate if possible where tfmr's are concerned.

Lastly for things like this, where your building a new amp,it's also best to fix/solder joints etc so that they can be easily undone at first, save cursing later.

Tother Andy.
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Old 12th Nov 2018, 5:02 pm   #11
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Default Re: Toroidal Transformers

AC VA rating vs. DC W drawn depends to some extent on the duty cycle. If continuous I would go for a factor of 2.
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Old 12th Nov 2018, 11:11 pm   #12
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Default Re: Toroidal Transformers

Cheers.
for the purposes of the present exercise I don't need to worry about the transformers capacity as it is designed to comfortably supply twice the amount of HT current that I intend to utilise.And as i won't be using much of the other secondaries current, maybe an amp and a half tops then its going to be running way below its design parameters.
But for future reference I shall bear in mind the think of a number and double it rule for DC current draw to keep in the safe zone.
I imagine this rule applies to frame transformers as much as toroids even though they saturate differently.
I have been trying to find out suppliers of HT transformers recently and those that are still in business are charging eye watering prices for them. And VVT aren't taking any orders until 2019 due to a backlog of orders.
This is one of the main reasons I am considering toroids as alternatives, but its not so easy to find ones that can give the sort of HT neeeded for the bigger valves. Easy for EL84 as a standard isolation transformer will give a 330 volts DC from silicon rectifiers.
What's becoming apparent is the sheer acreage of chassis top that I need just for the mains and output transformers and its looking a bit tight for the valves and PSU capacitors through the top. Never mind where i shall mount the circuit tagboards. And I do like to have a good bit of space to work in on the wiring and keep it neat and accessible.
I don't think i will be attempting any large valve power amplifier projects again once this one is finished.
Andy.
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Old 13th Nov 2018, 4:06 am   #13
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Default Re: Toroidal Transformers

Transformer secondary DC current into a full wave rectifier is in the region of 0.62 X A ac.

There is a handy pdf here: http://download.altronics.com.au/fil...ansformers.pdf

And some other handy ones: https://www.altronics.com.au/technicaldata/
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Old 13th Nov 2018, 1:03 pm   #14
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Default Re: Toroidal Transformers

And yet another question.
As far as i can see, theres no reason why I can't connect a pair of identical toroids secondaries in series to get a desired AC voltage if unable to get a Vigh enough value?
Eg 2x75V secondaries in series on one transformer gives 150V, so if you get another identical toroid thats 150+150 = 300?
Not Ideal i grant you but possible at a pinch?

A.
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Old 13th Nov 2018, 1:13 pm   #15
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Default Re: Toroidal Transformers

I love toroids because they are so easy to add extra windings.

So it is easy, if a bit tedious, to get whatever voltage you want from them!
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Old 13th Nov 2018, 1:43 pm   #16
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Default Re: Toroidal Transformers

Quote:
Originally Posted by bikerhifinut View Post
And yet another question.
As far as i can see, theres no reason why I can't connect a pair of identical toroids secondaries in series to get a desired AC voltage if unable to get a high enough value?
Eg 2x75V secondaries in series on one transformer gives 150V, so if you get another identical toroid thats 150+150 = 300?
Yes. In fact they don't even have to be identical. You could get 270V with one 150V secondary and another 120V secondary.

The time they DO have to be identical is if you want to draw more current than a single winding by itself can supply - then you can connect another in parallel, but only if identical. Otherwise, you have a small difference in voltage, which will give rise to circulating currents.
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Old 13th Nov 2018, 3:46 pm   #17
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Default Re: Toroidal Transformers

Quote:
but its not so easy to find ones that can give the sort of HT needed for the bigger valves
Big 50V (twice) ones are quite inexpensive and a voltage doubler/tripler/quadrupler is easy these days with the cheap HUGE capacitors available.
 
Old 13th Nov 2018, 6:11 pm   #18
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Default Re: Toroidal Transformers

Its a thought but something tells me its not really good practice and puts a large strain on the capacitors. As you say, caps are cheap enough and of course you don't need to have the working voltage the same as the desired HT until the next filter.
All the same, I think I'd rather start off with the AC volts I needed first especially as this is for power amplifier use with currents up to half an amp being drawn.
It's just my take on things.

A
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Old 13th Nov 2018, 6:31 pm   #19
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Default Re: Toroidal Transformers

Quote:
Originally Posted by kalee20 View Post
Yes. In fact they don't even have to be identical. You could get 270V with one 150V secondary and another 120V secondary.

The time they DO have to be identical is if you want to draw more current than a single winding by itself can supply - then you can connect another in parallel, but only if identical. Otherwise, you have a small difference in voltage, which will give rise to circulating currents.
Thats interesting.
So for instance, say I need around 350VAC at 200VA then getting a 2 x 115 (1x 230) plus a 2x 65V (130V AC) all rated at 200vA and series connecting the windings i will get 345V AC at 200 VA? and still able to stack them together if space is tight on a chassis?
That would get a decent transformer at a fraction of the price of the custom wound valve jobbies, even taking into account the need for a separate heater transformer.

I havent yet investigated the costs of getting a toroid custom wound for a valve amp but i was thinking its got to be a lot cheaper than a conventional frame one? Has anybody had this done recently?

Its not just academic as I have some projects in the queue that will need mains transformers.

A.
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Old 13th Nov 2018, 6:43 pm   #20
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Default Re: Toroidal Transformers

Quote:
Originally Posted by bikerhifinut View Post
Its a thought but something tells me its not really good practice and puts a large strain on the capacitors. As you say, caps are cheap enough and of course you don't need to have the working voltage the same as the desired HT until the next filter.
All the same, I think I'd rather start off with the AC volts I needed first especially as this is for power amplifier use with currents up to half an amp being drawn.
It's just my take on things.
I think the capacitors won't be overstressed if chosen for suitable ripple current ratings, but the transformer utilisation factor may be the bigger problem.
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