Trannies in parallel

Dan2

Member
2005-09-17 1:24 pm
I was just reading up on an amp i want to build, and they have 2 power supply setups:

for mono amp - 30-0-30 transformer

for stereo amp(2 mono's) 2x 22-0-22 transformer connected in parallel so 22-0-22 out.

Now this got me wandering - i need a PSU for a car radio, and i have a good couple of 12v transformers (small ones about 1A). would i be able to hook like 5 of them up in parallel to give me a power supply that can deliver about 5A???

I also have a few computer psu's - could i do the same with 2 or 3 of them to give me a nice rugged PSU???
 

mag

Member
2008-01-23 9:22 pm
Chiasso
In principle you can but at the condition that all the transformer are the same. I you pick up some random transformers an you connect the secondary in parallel there will be for sure a voltage difference between them. This will create a huge current between the transformer that is likely to blow up everything.
 

mag

Member
2008-01-23 9:22 pm
Chiasso
You means that each transformer has its own diode bridge and its own linear voltage regulator to 12V?
Then you connect in parallel the outputs of the regulators?

In principle you can, a standard voltage regulator consist of a pass PNP transistor with the emitter connected at the input and the collector at the output. Basically it can only source current and not sink any current.

All the regulators are different and if you connect the outputs in parallel the one with the higher output voltage will regulate and give you all the current.

Basically you will be in a situation where almost the total current will be sourced by only one regulator and almost nothing from the other.

Don't do this, even if you can make it work by fine adjusting the regulators output it will be a very bad design.

Go fo an higher power transformer, higher power brigde and higher power LDO. It is safer, much more reliable and much more cost effective.
 

Dan2

Member
2005-09-17 1:24 pm
Mag, thanks for clearing up the regulator question - which also answers the question i had about connecting computer PSU's in parallel. I guess some things just can't be so simple

:bawling:

Well i will just use my computer power supply for now for my car radio - i did put a rather large cap on the 12v rail which has given me some more bass.
 

mag

Member
2008-01-23 9:22 pm
Chiasso
Connecting power supplies in parallel is never a good idea... unless the power supplies are designed to allow this.

Don't try to parallel computer PSU! A computer PSU can have more than 15A on the +12V rail. Why do you need to put them in parallel? How much current do you need from this 12V.

A normal car radio does not need more than 6-7A of current with 4 speakers connected at full power. A computer PSU can do a good job.

Take care putting big capacitor in parallel with the PSU output.
How many uF did you put?
Computer PSU are designed to accept capacitve loads up to 5000uF on the +12V rail. If you add more there is the possibility that is becomes unstable.

ciao
 

Dan2

Member
2005-09-17 1:24 pm
i put a 2200uf cap - but its rated at mains voltage (220) so its physically quite big. while i am on the subject - the guy who gave me this cap told me that the bigger the cap, the better it is for amplifiers (but he was talking about the size not the rating) but is this true???

As for the power supply becoming unstable - how do you mean?? Computer PSU's are switch mode - correct? so what happens if i put a cap on that is too big??(just for interest's sake)

the car radio i have on the PSU now is a rather old sony 25x4 with a cd changer - the front display dimms when changing cd's and thats why i thought the PSU was not delivering enough current.

i have since got a brand new 50x4, but haven't hooked it up yet. I don't need the system to be super powerfull - because i am going to build a 50W rms amp to run a seperate sub.
 

mag

Member
2008-01-23 9:22 pm
Chiasso
If you are using a 200V rated capacitor at 12V is really a bad choice. Electrolytics voltage rating is the working voltage not the max voltage they can whitstand.

For example at 12V the better choice will be a 16V electrolytic or maximum 25V.

Electrolytics must always be used at at least 50% of they rated working voltage to maintain electrolyte chemical properties.

For example a 200V rated electrolytic is a good choice for voltages betwenn 100V and 200V, at 12V it will have a short life and its capacitance will be reduced after some years.

In your case a 2200uF cap can do the job (even if I doubt that it is really needed). If you want to add that capacitor use a 2200uF 16V or 25V cap. Remove your 200V rated capacitor it is totally useless.

The benefit of adding capacitance is also thanks to the ESR (equivalent series resistance), a 200V rated cap has higher ESR than a 16V cap at a given capacitance value; thus the ability to give you current spikes when needed is much better with a 16V cao rather than a 200V cap (at 12V).

For computer PSU check this link http://www.formfactors.org/developer/specs/ATX12V_PSDG_2_2_public_br2.pdf
you will find all the infos about ATX power supply.

If you add too much capacitance it can become unstable in the sense that when you have a sudden load change (for example a bass trasient) the output voltage can vary and it takes some time to come back to the 12V.

Don't trust the wattage ratio of car radio, normally they are only marketing BS. In the majority of case a 20Wx2 is exacltly equivalent to a 50Wx2. The amplifer is always made in the same way (12V powered bridge mode amplifier) but is seems that in the years the advertised output power increases so a 10 years old radio can be 20Wx2 and a brand new radio 50Wx2 even if they are using exacly the same amp with exacly the same real output power.

At 12V you can have a maximum output of 12V peak (in briged
mode configuration). 12Vpeak on 4ohm load means 18Wrms and this is the absolute maximum power you can expect from an amplifier powered at 12V on a 4ohm load.
The things change when an SMPS is used to step up the 12V to an higher voltage and then the amp is powered by this higher voltage rails
 

Dan2

Member
2005-09-17 1:24 pm
i know most of it is BS but i do know that the radio i am currently using is a bit weak compared to a 50x4 frontloader (from when it was used in a car). I just used the 50x4 as a reference so every1 would know what i am using.

thanks a ton for the computer PSU info!! and the info about the caps - i always thought that as long as the voltage rating was higher than the voltage you were using then all would be good.
 

Dan2

Member
2005-09-17 1:24 pm
Just got hold of 2 video machine power supplies - 1 is smps the other a transformer. both had this big chip which i found out is just a multiple voltage regulator. does any1 know how much current these can supply and what the voltages are??

I have a car amp (250W) which i want to hook up a power supply to and use with mains power (ie bypass the smps in it).

the power supply in the amp has 4 voltage regulators -> 2 are + 15v and 2 are -15v and it says 5A on each regulator (although the regulators are pretty small - the size of a small transistor). so that means i need a transformer that can deliver 20A at 30v. am i right?? OR in this case could i use 2 power supplies of 15v 10A?? (maybe even 2 computer PSU's??)
 

Dan2

Member
2005-09-17 1:24 pm
About the amp - i am very wrong. i just checked and the transistors i thought were voltage regulators are in fact transistors - but 1 set has +15V across it and the other set has -15v. i haven't taken the PC board out 'cos i don't have heatsink paste to put the transistors back, but this side of the board is quite empty with a lot of wire jumpers, so i guess there may be some stuff underneath.

i also checked the output transistors - their absolute maximum current rating is 8A. so for 2 channels (gonna bridge them) the amp can theoretically draw a maximum of 16A. is my reasoning about right?