Hi. I am working on a project and planning to replace the amplifier in a portable boombox radio to a higher powered one. The existing transformer can't be used because it was originally a european import (240v) and with our USA 120v mains, only outputs about 1/2 the necessary voltage. So I do have a bunch of NOS transformers in my spare part bin but they are smaller than I'd like to use for this application (12.6v @2.5a). The new amplifier is based on the TA8210AH chip. I think 2.5a will be insufficient to provide good performance.

I would prefer not to buy a new 5+amp transformer (1) to save money (2) because I have some transformers here available to use and (3) larger transformer might concentrate too much weight and stress at that corner of the portable radio.

So... asking if it's OK to install 2 of my 12.6v @2.5a transformers and run the secondaries in parallel to increase the available output current.

 

It is commonly done in industry.

Make sure that;
1. both transformer secondaries measure the same AC volts (ie 2 matching transformers)

2. connect 1 secondary wire first between the 2 transformers, then measure AC volts between the 2 remaining loose wires of the secondaries. The voltage should be about zero volts. That proves the 2 transformers are in phase and it is safe to connect the last 2 wires.

I hope that makes sense! Be careful...

 

Thanks. The transformers I wish to use are all brand new, Triad F6-12.

Here is the datasheet:
http://www.alliedelec.com/Images/Products/Datasheets/BM/PARALLAX/967-7093.PDF

I have about a dozen and they are all identical. The primaries and secondaries are all labeled with numbered tabs. I presume that if I connected all the numbered tabs together, both transformer windings should be truly parallel and in-phase? Or do I need to check/verify due to potential differences introduced in the manufacturing process?

 

You might consider using the power brick from an old laptop. It'd have the power you need and be lighter and more efficient than a transformer or two. You could probably get one free anywhere they recycle old computer parts.

 

Thanks Wayneh but I've tried those lightweight switching power supplies before and they do not perform well with larger audio amps, nor do they appear to be nearly as reliable as the old fashioned linear transformered supplies. I've also seen cases where the AM radio function become nearly useless due to noise from the PS but admittedly, the quality of the supply was probably the source.

In fact, here is a comment/review of the Coby HDR-650 radio. I purchased a couple and can confirm that what the reviewer said is true:

".....I got the Coby HDR-650 HD radio as an xmas gift, as I was very interested in HD radio. The looks and features of the radio were very attractive - I like the display, but what use is a good looking radio if it can't pick up stations? The power supply that came with the radio (a KY Industries model from China) radiates so much radio noise that I could not pick up ONE station on the AM band. I know it's the power supply, because whenever the KY power supply is plugged in, every AM radio in the room loses all reception whether the Coby is turned on or not..."

I also tried a universal laptop supply that claims to be capable of 15v @4A output. But when plugged into the DC port of the radio, it began going into protection mode almost immediately when the radio was on. This was an over the counter, new item. Not sure why but I always just presumed that these switching supplies just don't like being in applications where current demands are high or maybe the ratings were optimistic. In any event, this same supply worked fine when powering a laptop computer.

 

Thanks. The transformers I wish to use are all brand new, Triad F6-12.

Here is the datasheet:
http://www.alliedelec.com/Images/Products/Datasheets/BM/PARALLAX/967-7093.PDF

I have about a dozen and they are all identical. The primaries and secondaries are all labeled with numbered tabs. I presume that if I connected all the numbered tabs together, both transformer windings should be truly parallel and in-phase? Or do I need to check/verify due to potential differences introduced in the manufacturing process?

Never take such things on trust if you don't have to. If one transformer were defective or incorrectly labelled,you could end up destroying both. Since you are concerned to save money on parts, you presumably don't want to see that happen.

You should also beware of the possibility that the transformers could have slightly unequal turns ratios, which could set wasteful circulating currents even if the secondaries were paralleled the right way round. That has been known to cause overheating, so you would do well to check that the voltage difference is really quite low.

One get-around if that were not the case would be to use separate bridge rectifiers and reservoir capacitors for each transformer, and then connect their outputs via a pair of resistors of a fraction of an ohm each to help them share the load...Frankly though, you might be better off looking for a good deal on a 12V 5A transformer.

 

Ok guys. I might yet pick up a larger transformer. I figured it's fair to investigate the practicality of whether parts on hand can be made to work in a particular application before expending new money.

In any case, I did connect 2 of my transformers in the manner suggested to see whether they are matching. This is what I came up with:

Transformer A: 15.56v across the secondaries
Transformer B: 15.57v across the secondaries
Both fluctuates a teeny amount during measurement so it's not certain that there is exactly .01v difference.

With one of the secondaries tied together, I measured voltage across the 2 remaining secondary wires. Each time I connected, the meter reads a small voltage which immediately drops precipitously. Probably not actual readings but rather the autoranging function of the meter hunting for proper range. The reading continues dropping until after about a minute or so, the reading stabilizes and reads .0003 to .0004 volts where it remains.

Not sure exactly how low is sufficient but this voltage difference seems to me to be rather insignificant, right? So is it safe to say that these transformers are "matching?" Also, I'm not opposed to wiring each transformer with it's own bridge rectifier and reservoir capacitor but the recommended load resistors, if I didn't calculate wrong, would need to be 30w+ each and those are neither cheap or insignificant in size, so that really isn't a practical approach.

 

You have got hold of the wrong end of the stick there. The resistors would be in series with the output from each reservoir cap, they might have been maybe about 0.22Ω, and at 2.5A per output they would have dissipated about 1.4W each.

In any case, as the error is only 0.4mV or so, you just join the windings up in parallel.

 

Thanks Adjuster. Had another brain hiccup there.

In any case, being able to parallel them is a huge relief. The cost of a new transformer with the associated shipping costs could easily approach $40-$50 so this is a huge savings for me.

 

...switching power supplies... do not perform well with larger audio amps

Interesting. I wonder if someone here knows why? Could it be because they're designed to supply DC, and when the load is audio AC, their self-regulation is all screwed up? If that theory were true, and maybe somebody can offer a better one, you may "just" need decoupling - a big low pass filter. Of course RF noise is another issue.

 

...
Transformer A: 15.56v across the secondaries
Transformer B: 15.57v across the secondaries
Both fluctuates a teeny amount during measurement so it's not certain that there is exactly .01v difference.

With one of the secondaries tied together, I measured voltage across the 2 remaining secondary wires. Each time I connected, the meter reads a small voltage which immediately drops precipitously.
...

That's all perfect. They can be paralleled. The main risk(!) is that you had the secondaries out of phase, in which case the 2 wires will read DOUBLE the secondary AC volts or about 30vAC.

Don't worry too much about the small voltage mismatch. Mains transformers have quite a lot of "voltage sag" (maybe 10% to 20%) as current is drawn and you can parallel transformers even if they are a few percent different in voltage compared to each other. Your two test transformers are as close to perfect as it gets.

Wayneh- Re SMPS supplies modern ones are smart and will shutdown if the output ripple gets too large. Even if they don't shutdown there is a chance that the AF current pulses from the load will destabilise the SMPS regulation circuit. And yes a large PSU cap right at the amp can be a good fix in most cases especially if there is a small resistance in front of it.