Bridge rectifiers and wires

[justin66]

Hey guys im curently working on a simple power supply project. Take mot(microwave oven transformer) replace secondary windings put it through bridge rectifiers put some caps and have something like 15-17v unregulated. Purpose is to power car amps for subs. I have super simple logical questions someone could answer

First question whats the minimal capacitance do i need to have a linear output for 600w for entire system but i plan to upgrade so whats the minimal for 1kw or 2kw load?

Second. I will calculate how much i need later but is it better to have two bridge rectifiers rated for 30amps connected in paralel or is it better to have one 60 amp rated BR?

Third i know i have 1000va transformer  but when i rewind it how do i calculate how much power im geting? My target is as of right now to power 600w subs with as little voltage drop as possible. When i wound it with test wire i found an interesting thing the wire in transformer is 1cubic centimeter thik its output is 12.4v ac and almost 17v dc i tried powering subs with somewhere close to 300w rms voltage droped below 11v why? This wire is capable of passing 30amso the bridge rectifier is rated 15 amps but still i dont think it was the problem. Was it? When i upgraded dc wire from BR to thik amp wires voltage raised from 10 to 11.3v and it was no longer issue there. How do i calculate what BR i need and if i need something like 2kw at 16v i guess its no no. Unless i use like 3 MOTs or a big transformer right? But than comes foth question

Forth whats the best way to deliver high power? And i reccon if mot is capable of 600w continueous and subs are not constant power device so sould i put a battery as a buffer? But than its hard coz its not Conventional voltage.

Fifth question why car capacitors are so big if i managed to get 0.3f farads with a bunch of 2200uf caps but its nowhere close the size and also higher voltage where is the catch?

Sixth question what do you guys recommend for powering power car amps from mains?

Ps sorry for the long text

[T3sl4co1l]

At 12V and line frequency, the rule of thumb is something like 2mF per ampere.  So I'd suggest 100mF (= 100,000uF = 0.1F) rated 16 or 25V.  It will probably be a "computer grade" type, costing around $40, if you're shopping for new parts.

Don't use one of those "boost cap" (0.5F +) things, they're gimmicks, they're not rated for this much ripple, and they don't do much even in the best of cases.

Obviously, you need a big rectifier.  A single part is better.  Diodes don't play well together, in parallel.  At this current, you will need a heatsink, too.

#8 AWG wire is good enough for wiring.

MOTs are only good for about 500VA.  They get very hot at higher power levels.  Heck, they get hot enough just sitting there, without any load at all.  The rectifier spends about half that, so don't expect to use an amplifier over 250W or so.

BTW, did you remove the magnetic shunts?  You want to knock those out, otherwise your output voltage will sag a lot.

You'll need several MOTs, and a 240V 15A or larger mains circuit, to run a full 2kW of amplifiers.  You're better off just getting a mains powered amplifier.

Tim

[justin66]

At 12V and line frequency, the rule of thumb is something like 2mF per ampere.  So I'd suggest 100mF (= 100,000uF = 0.1F) rated 16 or 25V.  It will probably be a "computer grade" type, costing around $40, if you're shopping for new parts.

Don't use one of those "boost cap" (0.5F +) things, they're gimmicks, they're not rated for this much ripple, and they don't do much even in the best of cases.

Obviously, you need a big rectifier.  A single part is better.  Diodes don't play well together, in parallel.  At this current, you will need a heatsink, too.

#8 AWG wire is good enough for wiring.

MOTs are only good for about 500VA.  They get very hot at higher power levels.  Heck, they get hot enough just sitting there, without any load at all.  The rectifier spends about half that, so don't expect to use an amplifier over 250W or so.

BTW, did you remove the magnetic shunts?  You want to knock those out, otherwise your output voltage will sag a lot.

You'll need several MOTs, and a 240V 15A or larger mains circuit, to run a full 2kW of amplifiers.  You're better off just getting a mains powered amplifier.

Tim

Thanks for your reply. The best BR i can get is 1000v 60A rated. So if i comnect two of them its not gonna be good? Why? Thanks for capacitance info i will try using ten 10mf caps to save money i will use 16v rated is there a way to decrease voltage when i dont use subs and than just kick in full 17v when i use subs and load will drop the voltage? but how about i increase primary windings? By 30turns(currently its turn per volt 220 of them) and also i wanna put two E parts together am i getting at least 600w? If its like 1.1kVA and 800w microwave oven? Also primary windings are 17awg wire.

Currently im using hacked atx psu at 14v and i used full 600w for an hour or so 7ah battery was almost full how? The psu is rated 12v 16A

[mariush]

If you have a lot of current, it may make sense to use one of those ideal diode bridge controllers and four mosfets, see for example LT4320 : http://www.linear.com/product/LT4320

You don't have 0.7-1.1v per diode voltage drop inside the rectifier (or about 2w per amp lost as heat in the bridge rectifier), you only deal with a few mV per mosfet if you choose them right. 

Also seems like the extra couple of volts would reduce the required capacitance for your project

I'm afraid when it comes to microwave transformers customized, i'm clueless. I don't even understand some parts of your message so I'm staying out of that.

ps. I sincerely doubt it's a good idea to use 16v rated capacitors for this project.

[Audioguru]

I agree that a microwave oven is made as cheaply as is possible so a 1000W oven uses a transformer that gets hot (hey who cares, it is in an oven) and has its voltage drop a lot when it is loaded with 600W.
I do not know which amplifiers you powered but if they get hot then they are drawing a lot more power from the power supply than their output power to the speakers.

How are you measuring the amplifier output power? Many car amplifiers rated at "600 Whats! (600W)" actually produce only 100 real Watts.

[justin66]

I agree that a microwave oven is made as cheaply as is possible so a 1000W oven uses a transformer that gets hot (hey who cares, it is in an oven) and has its voltage drop a lot when it is loaded with 600W.
I do not know which amplifiers you powered but if they get hot then they are drawing a lot more power from the power supply than their output power to the speakers.

How are you measuring the amplifier output power? Many car amplifiers rated at "600 Whats! (600W)" actually produce only 100 real Watts.

Well its hard to mesure accurately but first of all i read only rms power second i know with inductors like speakers there is box raise also known as increased resistance. I assume im using somewhere arround 300w rms because there is two speakers rated at 150rms each when i power them they get close to full excursion so i assume its give or take 300w out.
My amp has a pretty big radiator and it gets hot to the touch after an hour of playing but o can still hold my hand on it

[justin66]

If you have a lot of current, it may make sense to use one of those ideal diode bridge controllers and four mosfets, see for example LT4320 : http://www.linear.com/product/LT4320

You don't have 0.7-1.1v per diode voltage drop inside the rectifier (or about 2w per amp lost as heat in the bridge rectifier), you only deal with a few mV per mosfet if you choose them right. 

Also seems like the extra couple of volts would reduce the required capacitance for your project

I'm afraid when it comes to microwave transformers customized, i'm clueless. I don't even understand some parts of your message so I'm staying out of that.

ps. I sincerely doubt it's a good idea to use 16v rated capacitors for this project.

Im sorry im not native english speaker

Basic idea is to modify mot and power the amps. I need to figure out the the best cost efficient methood witch will work fine

My supplyer has no lt4320. It seems like an awesome idea to use mosfets instead of diodes can you suggest some other parts like that i will search for it in www.evita.lt . but how about the waveform of mosfet BR is it going to become squere or not?

How about i increase primary windings to increase the efficiency thats how it works right? There is 17awg or about 1cubic mm wire as primary it is one turn per volt 220turns so if i add a couple of turns for primary i will end up increasing its efficiency or not?

Or how about adding in those separators i will decrease its output and heat as well right?


There is one more idea get an other mot and put two secondaries in series it will be two turns per volt this way efficiency should rise and heat drop. But now i dont have one handy

Yet an other idea is to submerge it in oil

[macboy]

Thanks for your reply. The best BR i can get is 1000v 60A rated.

Use a lower voltage rectifier. Rectifiers with higher reverse voltage ratings also have a higher forward voltage drop compared to ones that have a lower reverse voltage rating. If you can find a 100 V or even 50 V one, that would be ideal. Or use four separate Schottky rectifiers which have significantly lower forward voltage drop than silicon types. I salvage these from dead SMPS power supplies (like PC supplies). You can find some that will pass tens of Amps with barely 0.3 or 0.4 V drop, but are only rated for 30 or 40 V reverse voltage.  These are used on those supplies for the same reason you want them: lower voltage drop for better efficiency. With low voltage secondaries, you need to save every mV you can.

[pmbrunelle]

Here's another idea...

Use an AC motor (mains powered) to drive an automotive alternator. A belt drive should not be too difficult to figure out.

The power levels here are such that this may actually be reasonable...

[T3sl4co1l]

More primary turns will keep it running cooler.  But then you need more secondary turns, too.

You want the metal spacers (magnetic shunts) out.

Tim

[AG6QR]

The best BR i can get is 1000v 60A rated. So if i comnect two of them its not gonna be good? Why?

Generally, it's not a good idea to put diodes in parallel.  Putting two bridge rectifiers in parallel is basically just putting four sets of diodes in parallel.

Why not put diodes in parallel?  To a first approximation, it seems like it should work.  The diodes in a parallel pair will have equal voltage across them, therefore equal current, right?  Maybe not.  A problem is that a diode's I-V curve depends on temperature, and if you hold voltage constant, a slight increase in temperature will result in a significant increase in current. 

When you have two diodes in parallel, inevitably one will get just a little bit more current than the other (or one may get a little warmer than the other, which ends up causing the same problem).  The one with more current will then heat up more than the other.  This heating will cause it to grab still more current relative to its neighbor, which will in turn cause more heating, causing more current, etc..  This is a self-reinforcing positive-feedback loop.  It's known as "current hogging".  One diode will normally end up carrying a large majority of the current, while the other diode loafs along, carrying very little.  If the current in the "hogging" diode is more than the rating of the diode, the diode will probably fail open circuit eventually.  That leaves the other diode to suddenly start carrying all the current, and it will then fail.

There are strategies to minimize this. You can put both diodes close together on the same heatsink to keep their temperatures close.  You can put very low value, high power ballast resistors in series with each diode to try and balance out the currents.  But if you can use a single part rated for the full current, that generally works better than trying to split the current between two parts each rated to carry half.

Note that this same effect happens to LEDs, not just rectifier diodes.