How to wire 2 Transformers together to Double Current??

[Minion]

Hi Folks, I have another Stupid Newbie question for you Kind folks....

I am working of a couple channels of Green Pre but I realized that my Power transformer is Rated at only about 200mA to 250mA which I am afraid might not be enough current to run 2 Channels so I was thinking I could wire a couple of these Transformers together to get double the Current but since I am very new to this I have No idea how to procede...

The transformers are Center Tapped so they have a 3 wires going to the PSU and I know I have to wire them in Paralell to double the Current But when I comes to wireing Transformers in Parelell I can not seem to get my head arround it....I understand wireing Resistors and Caps in paralell and in series but I don"t know how I would do that with a Transformer were I have 5 Wires to deal with?? (2 input wires and 3 output wires on each Transformer..)

So if someone would be Kind enough to explain how I would do this on the Input and the Output I would be very greatfull.....(Unless 200mA is enough for 2 Channels of Pre??)


ThanX a Lot Guys!!!!


Cheers

 

[Brown Note]

just tie each wire to it's twin on the second transformer. ie: primary A on transformer 1 connects to primary A on transformer 2. same for the secondaries. once the wires are connected together wire to the psu as you would with one transformer.

if you get no power when the trafos are combined just flip the primaries of one trafo to correct the phase.

 

[Boswell]

Don't wire your transformer secondaries in parallel. It would only take a single turn difference in the turns ratios to set up large circulating currents through the pair of transformers, possibly leading to meltdown. If you got the phasing wrong, there would be a loud bang.

You didn't say what type of rectifier you plan to use, but if the original circuit had a bridge rectifier to get the + and - rails, you could change that to have a fullwave diode pair from one transformer for the + rail and the same (diodes facing other way) for the - rail. The centre-taps would still be ground.

Otherwise use the two transformers to build two separate power supplies, one for each channel. Don't interconnect the power rails of the two channels, but make sure the grounds are common.

 

[rafafredd]

Don't wire your transformer secondaries in parallel. It would only take a single turn difference in the turns ratios to set up large circulating currents through the pair of transformers, possibly leading to meltdown. If you got the phasing wrong, there would be a loud bang.

strange... I did it so many times. I´m sure I even got the phase wrong once or twice, but never got any loud bangs or meltdown transformers...

 

[Minion]

Well maybe what I will try since I have several of these small transformers is to Find the 2 that Match the Closest in Voltages and Try wireing them togehter (Without the PSU) and see how well they Play together ,I can monitor them for a couple of hours and see if they get hot (Which I guess would be a Symptom of them not getting allong)....

My PSU uses a Bridge rectifier but I believe I can Mod it to use 4 Diodes in a Bridge config....

Do any of you know exactly what the Current Draw is For each Channel of Green Pre (V1)??

Or how to Find out what the exact Current rateing is of my Transformers as they don"t have anything on them that tells me the Current rateing and I was under the Impression that I can"t just use my Digital Multi-Meter to Find out because Transformers only put out as much current as is being drawn from them and a DMM draws little to no current???


And I pretty much have to use these Transformers because were I live you can not buy that kind of stuff (Unless you want to pay Far more than it is Worth)....

What is Strange is that the Transformer in one of my 6 channel Line mixers uses a Smaller Transformer than these ones and it had 8 IC"s in it and Many LED"s so I thought these ones would work because they are Bigger....

Wait....Is it safer to wire Transformers in Series?? Because I can also get 2 x 13v out of these Transformers which should give me twice as much Current because I am running them at Half Voltage?? If so how would I do that with a 3 wire Transformer??


Any help would be Great!!!

PS: I can"t build another PSU because I don"t have the Room in my Rack enclosure.....

 

[producer4000]

I must agree with Rafa.
Parallelling (sp?) transformers is no big deal. Right here is a schematic
for a commercially produced video switcher/router power supply.
I can't imagine that they spend a lot of time or energy on matching the voltage outputs
on these power transfo's. So far, after some mods to get a negative rail, they work for me.

:guinness: :guinness:

Chris

 

[Minion]

Do you guys think it might be better to Wire them together not useing the Center tap??

I think I should be able to wire the 2 Common Leads of each Transformer together which would be my 0v and have the other 2 wires(one from each Transformer) going to the + 15v and a -15v??

If my understanding is Correct this should also work ...Right??


ThanXX

 

[syn]

hi
this link will tell you many things, how to measure e.t.c.

http://www.groupdiy.com/index.php?topic=16638

best

m

 

[Boswell]

Designers working for equipment manufacturers have the luxury of being able to specify their transformers accurately, so that they know that a pair from the same batch are identical in terms of parameters like numbers of turns on primary and secondary. It also means that they can afford to parallel up transformers to meet constraints such as physical height for a given electrical power.

I stand by my comments about DIY paralleling of supposedly similar transformers, especially if there is a chance of the phasing being wrong. I have seen it happen, with spectacular results. Luckily in that case, the fuses took the hit and there was no permanent damage done.

Series connection of secondaries is, of course, not a problem, and in this case, wrong phasing simply results in a near zero output. If I understand Minion's last question correctly, you can either use the two transformers to power the + and - power rails separately as I suggested last time, or you could use a separate bridge rectifier for each transformer and join the two sets of bridge outputs. This latter is less satisfactory, as there is no way to guarantee that the load is shared equally between the transformers.

The forum thread linked to by Syn contained a suggestion that you test a transfomer by measuring the short circuit current in the secondary. This is indeed a meaningful test but only if you slowly increase the volts on the primary with a Variac from zero until you get rated current in the shorted secondary. This will occur at typically 5% of the nominal primary volts, and is a test used by designers to measure the copper losses (I squared R) of the transformer. The magnetising (BH cycle) losses are estimated by measuring the primary current at rated input volts with the secondary open circuit. The sum of the two losses divided by the rated output power gives a good indication of the efficiency of the transformer when loaded.

 

[Minion]

Well what I ended up doing was I wired up 4 of my Small Transformers in Paralell and Left them on for several hours to make sure that they would play well together and after a couple of Hours they were still cool to the touch so I then connected them to the PSU PCB Board and did some Tests.....

While Trying to Incorrectly measure the Amps of the PSU i must of accidently touched something I shouldn"t have as I got a Big spark and then suddenly my Phantom power voltage Shot up to 88v so I thought I must have Fried something but I could not smell anything being burnt and the Board showed no signs of shorting....

I ended doing some Tests with my new $8 Digital Multi-Meter and could not Find any Fried Diodes or resistors so I figured it must be a Fried LM317 Regulator so I put in a new LM317 for the Phantom regulator and "POW" It worked, I am getting 47.75v for phantom power again and everything else checks out.....

Now I just have to Figure out how to integrate a Power on LED and a Phantom Power LED and I should have most everything Figured out....

Anyone know how to Incorporate a couple LED"s for Power and Phantom power hopefully somehow wired to the Power switches???

ThanXX

 

[SSLtech]

Search.

LED, phantom.

It's been kicked around plenty of times. Basically though, if you're 'struggling to make the rent' as far a phantom current goes, I reckon it's utterly foolish to feed an LED it's 20mA if you can't easily come up with 10mA to feed the mic...

Use a second pole of the switch and use a lower voltage to feed the LEDs. The rest of the 48V would just be spent making a resistor hot and getting you nowhere profitable.

If you just mean a single LED per rail to say that the PSU is actually operating, then get an LED and stick a 2k2-4k7 resistor in series with it, then wire it from 48V to 0V. For ±15-18V legs, use 1k0-1k8 resistors and do the same thing. Make sure that the diode arrowhead comes from the more positive leg and the flat bar goes to the more negative leg in every case.

Keith

 

[Minion]

Thanx for the Tips but since I am a Total Newbie most of it went right over my head.....

I"m thinking maybe I can just connect One end of the LED to a resistor and the end of the Resistor to one of the Poles on the Phantom power switch and then other end to Ground so when I turn on the Switch Hopefully the Led will Light up....

As for the Power switch I was thinking of connecting the Led to a Resistor and Connecting it to one of the 15v DC Rails of the Powert supply and the other end to ground so when I turn on the PSU the Light will come on....

Will this Work?? Or is that sort of what you told me to Do??(Remember it wen"t over my head)

I really suck at understanding what I read but I can understand a Picture much better (I think it is because I am Dislexic) so if what I said won"t work do you know were I can find a Picture that will show me what to do (A picture is worth a thousand words...Especially to me).....

This is probably why after going through allmost every Page on the Forum I still didn"t understand most of it......Ohh well, There aint much I can do about it....

ThanX

 

[syn]

phantom:

as for power led, should work. I used, think 1k.8 for 18v that way.
cheers
m