I have three 60VA iron core transformers out put is 20v-0-20v. can the three xfmrs be put in parallel to get 180VA?
should there be a voltage matching resistor on the outputs before tying them together?
like using a very small, high wattage resistor (.1 ohm 20watt) in series with each out put . that would take care of any voltage differences, while letting the current add together. right?
should there be a voltage matching resistor on the outputs before tying them together?
like using a very small, high wattage resistor (.1 ohm 20watt) in series with each out put . that would take care of any voltage differences, while letting the current add together. right?
Same problem. You have low impedance outputs, one WILL tend to source all the current drawn. Whether at the AC output or after the bridge. Consider that the slightly higher voltage transformer will charge the filter capacitor to a voltage greater than the other transformers, their diodes won't even switch on!
Current equalizing resistors will work, but you lose regulation. Up to you.
Tim
Current equalizing resistors will work, but you lose regulation. Up to you.
Tim
If the transformers are identical then go ahead and parallel the primaries (watch your phasing).
You can then series or parallel your secondaries to get the voltages you need (watch the phasing).
There will be some mis-match between the transformers but they will sort themselves out (might run a little warmer than normal)
I have never run 3 in parallel but have run 2 often.
I'd be inclined to do this before the rectifier because I'd rather waste a bit of power in the transformer than in the rectifier.
Good Luck
You can then series or parallel your secondaries to get the voltages you need (watch the phasing).
There will be some mis-match between the transformers but they will sort themselves out (might run a little warmer than normal)
I have never run 3 in parallel but have run 2 often.
I'd be inclined to do this before the rectifier because I'd rather waste a bit of power in the transformer than in the rectifier.
Good Luck
If the transformers are identical i.e. if they have exactly the same core and the same number of turns on the primary and secondary then the AC voltages on the secondary will be the same. I doubt more than 1% difference.
In my experience commercially manufactured transformers that are the same probably share current better than transistors and diodes that are the same. In percentage terms I have seen larger variances between diode bridges than I have seen from identical transformers.
In my experience commercially manufactured transformers that are the same probably share current better than transistors and diodes that are the same. In percentage terms I have seen larger variances between diode bridges than I have seen from identical transformers.
Aunkst said:
I am curious what you find. If you can provide data around temp rise in the cores or current mismatch at one of the trafo's primaries etc.
Ok Here's something stupid.
I have a Japanese amp (Kenwood M2) that is rated at 220wpc at 8 ohms. It has 91 volt rails ! You'd think it could do 450 watts with those rails but as usual the trafo's are tinny.
I wanted to squeeze more power into my 8 ohms subs, so I took a 300VA trafo rated at 65-0-65 (87vdc) and created an add-on power supply to the Kenwood's own PSU. I connected the + gnd and - to the main amp. (the auxiliary PSU has its own bridge recifier and supply caps, so basically i have paralelled another PSU to the amp's own PSU) Everything works well. (yes the main amp works at 91vdc and the add-on only provides 87vdc) so as per Brian's post, for the most part the auxiliary add-on PSU's diodes are not even switched on till I load the Kenwood enough for it's rails to fall below 87vdc when the Aux PSU kicks in.
I have had no problems with this set-up (despite everyone warning against doing something like this) and while i have not gotten big improvements, the amps does clip at 270 watts at 8 ohms up from 242 watts without the add-on...
Whats the "realistic" risk with what I am doing?
ok, It took me a couple of extra days, but hooked up and running now. 3 identical xfmrs, in parallel before the rectifier.
works great.
no un-called-for heat. no problem at all.
now i have a power supply that I can run just about any of my amps off of with no worries. lots of back up power.
I might make some measurements before the new year if anybody is really interested.
works great.
no un-called-for heat. no problem at all.
now i have a power supply that I can run just about any of my amps off of with no worries. lots of back up power.
I might make some measurements before the new year if anybody is really interested.
Primaries in series
I have a pair of identical transformers that I'm going to try running with their primaries in series, so as to get a lower voltage from each. Cheaper than buying a new transformer of the right voltage.
I could wire the secondaries in parallel, or use them to power each channel of a Class-A amp. With the primaries in series, it will be important to load each transformer identically so perhaps I will try the parallel connection, perhaps after their bridge rectifiers. Or maybe I will just run them separately.
If I wire them together, I will use small-value resistors to let them share the load equally. Interesting thing about series primaries is that if they power two independent loads, the load drawing more current will get less voltage.
I have a pair of identical transformers that I'm going to try running with their primaries in series, so as to get a lower voltage from each. Cheaper than buying a new transformer of the right voltage.
I could wire the secondaries in parallel, or use them to power each channel of a Class-A amp. With the primaries in series, it will be important to load each transformer identically so perhaps I will try the parallel connection, perhaps after their bridge rectifiers. Or maybe I will just run them separately.
If I wire them together, I will use small-value resistors to let them share the load equally. Interesting thing about series primaries is that if they power two independent loads, the load drawing more current will get less voltage.
Re: Primaries in series
I have always been of the opinion that using resistors for load sharing in Trafo's is not a good solution. (Its more of a necessary evil.) However in this case if the secondaries are paralell connected via ballast resistors then while one may have more voltage drop than the other (in case of a mismatch) at a given load (especially with a class-A amp) you will have equilibrium overall. Especially in a class-A amplifier, the loading will be sufficiently high that percentage wise difference will be low between the two trafos.
What I would not do is putting the primaries in series and having separate secondaries running to each channel, that is dangerous as if one channel is loaded than the other, the unloaded channel's rails could rise up higher than spec and blow the amp.
paulb said:
I have always been of the opinion that using resistors for load sharing in Trafo's is not a good solution. (Its more of a necessary evil.) However in this case if the secondaries are paralell connected via ballast resistors then while one may have more voltage drop than the other (in case of a mismatch) at a given load (especially with a class-A amp) you will have equilibrium overall. Especially in a class-A amplifier, the loading will be sufficiently high that percentage wise difference will be low between the two trafos.
What I would not do is putting the primaries in series and having separate secondaries running to each channel, that is dangerous as if one channel is loaded than the other, the unloaded channel's rails could rise up higher than spec and blow the amp.
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