Help replacing 4 diodes with rectifier and improving old AT PSU

[Behemot]

Re: Help replacing 4 diodes with rectifier and improving old AT PSU

You can use halogens only if you put thermistor which lowers the starting current in series with the bulb.

[momaka]

Re: Help replacing 4 diodes with rectifier and improving old AT PSU

if we switch off both of secondary rectifier (12v to 3045 and 5v to 1645), or use bigger rectifier at 12v rail, will it raise the amperage capability of 12v rail?

Yes, it will.

But I doubt you would be able to pull the full 30A (half-bridge) or 20-24 A or so on forward-converter (or single-transistor forward, if you like) topologies. Most likely, you will be able to squeeze only a few more Amps.

It is not just the transformer that is the limitation. The output toroid inductor also matters. On older PSUs or PSUs with a weak 12V rail, the winding on that toroid inductor for the 12V rail will also have thinner wire. This means you won't be able to pull as much current through it as you can from the 5V rail.

That said, increasing the capacity of the output rectifiers is still a good idea, because then the rectifiers will have a lower forward voltage drop. This means less losses, better efficiency, and thus less heat in the power supply.

Another thing worth mentioning is to pay attention to the power supply topology before changing the output rectifiers. For half-bridge topology, you should NOT use a rectifier on the 12V rail with a rating less than 60V.

Are you sure the 12V rail uses a 1645 rectifier (i.e. 16A, 45V rectifier?)

Also, I WOULD replace the primary capacitors, not because it's necessary, but simply as an upgrade I would use something like Panasonic TS-ED for very high ripple handling.

It is not needed.
Good quality Japanese caps on the input will easily last 20+ years without problems.

[Logistics]

Re: Help replacing 4 diodes with rectifier and improving old AT PSU

I agree with many of the previous posters, that using low-impedance capacitors was not the way to go as you can change the way the circuit behaves, entirely. Also, I WOULD replace the primary capacitors, not because it's necessary, but simply as an upgrade I would use something like Panasonic TS-ED for very high ripple handling.

[stj]

Re: Help replacing 4 diodes with rectifier and improving old AT PSU

halogen has a very low resisance when cold so the startup current will be huge.

[Quaddro]

Re: Help replacing 4 diodes with rectifier and improving old AT PSU

Usually the transformer can output more A than the rectifiers used. So, the output rectifiers are the bottleneck and updating them will update the output rail's power and make the psu run colder (that has to do with the forward voltage drop of the new rectifier. the lowest the better)

I don't agree with that goodpsusearch.

Here's my thoughts on this...

The power supply may be capable of more current on 12v than what those output rectifiers are capable of, but most of the time the systems that use such power supplies don't even use that much current.
The transformer inside the psu is still a limitation, so if in theory you could raise the current but it would be by a little amount.

Replacing the output rectifier with a higher current one may be a good idea (and make the psu capable of more current) for another reason, a bit sideways... if the new rectifiers could handle higher temperature.

See for example a typical 3045, look at page 5 in the datasheet: https://cdn.badcaps-static.com/pdfs/...61aa51f969.pdf

This particular rectifier's current capability is only true at up to 100C, if the temperature goes up the rectifier can do less current.
On cheap power supplies the rectifiers are mounted on lousy heatsinks that don't dissipate heat well, the rectifiers will be often very hot (i'd say close to 80-100c).
When you do something power intensive (playing a game for example), the standard cheap power supply could overheat its rectifiers so much that they'll cause resets (tripping protections) or even blow the rectifiers.
If you happen to choose a rectifier that can do more current at such temperatures, that could mean the power supply won't blow up when playing a heavy game for a long time.

All you have to do is to use dummy load and draw the current up to the power supply then goes beyond that and see if it still maintain the 12V within the spec, The transformer has impedance which will limit the amount of current you can draw from the transformer.
Lower Vf diode will help reduce the power dissipation on the Diode. All power supplies/power sources have output impedance, the lower the impedance the higher the current capability.

well..very interesting discussion about it..

With bigger secondary rectifier, we can squeeze transformer amperage capability.

Can we use car/motorcycle halogen lamp as a dummy loader..?
after read budm post , i test my psu.

i have old enlight en8307 300w with 10A capacity at 12v rails (in the label).
i've replace the 12A rectifier with 30A one.
Load it with 2 halogen lamp @100w for 12v rail, and 1 halogen for 5 rail, since this psu is group regulated.
voltage regulation still solid at 12,02v (idle 12.2v).

but i'm too afraid to add more load (halogen)...
explosion is scary.

my question..
1. If we use 2 pieces 100w halogen lamp as a load, can we assume we have 200w load for the psu?
Since this is just lamp, with 100w number advertised..

2. If the transformer already pushed to the the limit, what will happen?
fail voltage regulation or simply burnt (or blow up) ?

thanks for the response.

[budm]

Re: Help replacing 4 diodes with rectifier and improving old AT PSU

All you have to do is to use dummy load and draw the current up to the power supply then goes beyond that and see if it still maintain the 12V within the spec, The transformer has impedance which will limit the amount of current you can draw from the transformer.
Lower Vf diode will help reduce the power dissipation on the Diode. All power supplies/power sources have output impedance, the lower the impedance the higher the current capability.

[mariushm]

Re: Help replacing 4 diodes with rectifier and improving old AT PSU

I don't agree with that goodpsusearch.

Here's my thoughts on this...

The power supply may be capable of more current on 12v than what those output rectifiers are capable of, but most of the time the systems that use such power supplies don't even use that much current.
The transformer inside the psu is still a limitation, so if in theory you could raise the current but it would be by a little amount.

Replacing the output rectifier with a higher current one may be a good idea (and make the psu capable of more current) for another reason, a bit sideways... if the new rectifiers could handle higher temperature.

See for example a typical 3045, look at page 5 in the datasheet: https://cdn.badcaps-static.com/pdfs/...61aa51f969.pdf

This particular rectifier's current capability is only true at up to 100C, if the temperature goes up the rectifier can do less current.
On cheap power supplies the rectifiers are mounted on lousy heatsinks that don't dissipate heat well, the rectifiers will be often very hot (i'd say close to 80-100c).
When you do something power intensive (playing a game for example), the standard cheap power supply could overheat its rectifiers so much that they'll cause resets (tripping protections) or even blow the rectifiers.
If you happen to choose a rectifier that can do more current at such temperatures, that could mean the power supply won't blow up when playing a heavy game for a long time.

[goodpsusearch]

Re: Help replacing 4 diodes with rectifier and improving old AT PSU

Usually the transformer can output more A than the rectifiers used. So, the output rectifiers are the bottleneck and updating them will update the output rail's power and make the psu run colder (that has to do with the forward voltage drop of the new rectifier. the lowest the better)

[Quaddro]

Re: Help replacing 4 diodes with rectifier and improving old AT PSU

Not unless you also raise the transformer capacity.

owh..
thankyou for the information..

[budm]

Re: Help replacing 4 diodes with rectifier and improving old AT PSU

mmm, i have a question..

all old atx 1 version is usually have a big amperage at 5v rails..but low at 12v rail.
for example, 5v rail use 3045, and 12v rail use 1645

if we switch off both of secondary rectifier (12v to 3045 and 5v to 1645), or use bigger rectifier at 12v rail, will it raise the amperage capability of 12v rail?

Not unless you also raise the transformer capacity.

[Quaddro]

Re: Help replacing 4 diodes with rectifier and improving old AT PSU

mmm, i have a question..

all old atx 1 version is usually have a big amperage at 5v rails..but low at 12v rail.
for example, 5v rail use 3045, and 12v rail use 1645

if we switch off both of secondary rectifier (12v to 3045 and 5v to 1645), or use bigger rectifier at 12v rail, will it raise the amperage capability of 12v rail?

[stj]

Re: Help replacing 4 diodes with rectifier and improving old AT PSU

No idea what's that.

[budm]

Re: Help replacing 4 diodes with rectifier and improving old AT PSU

that's nice, but it means nothing for internal equipment wiring.

there are other non-standard "standards" too i see a lot.
for example, building wiring.
red for switched live, yellow for unswitched.
very common when i see light fittings with integral battery backup.

Heck you can use any color you want when you do not really care about the standard or safety, then put on fake stickers and then hope that no one get hurt, and you better have lots of money for the law suit, but who cares, right?
"that's nice, but it means nothing for internal equipment wiring." So the internal wiring for the primary circuit from the IEC inlet to the AUX IEC oulet does not matter then right as far as the color code concern? Look at the so call UPS outlets that you that you say you have experience with, did they use just any color wires on the outlet and the inlet? "but it means nothing for internal equipment wiring." really?
May be you should learn from this statement and EXPAND your experiences:
"i work mostly in industrial equipment and if you think SONY, TDK, Delta etc dont use white & black you need to expand your horizons beyond shitty $10 pc supplies."

[Behemot]

Re: Help replacing 4 diodes with rectifier and improving old AT PSU

No idea what's that.

[stj]

Re: Help replacing 4 diodes with rectifier and improving old AT PSU

into?
you repair what your asked to repair.

you know megatouch arcade quiz machines? well most of those are AT.

[Behemot]

Re: Help replacing 4 diodes with rectifier and improving old AT PSU

Into old AT units? Really?

[stj]

Re: Help replacing 4 diodes with rectifier and improving old AT PSU

I would not stick such low-ESR caps as FR/FC into two decades old PSU, / Who knows what especially FR would do in there..

i'm not just saying it, i'm doing it.
the psu's work fine.

now what i wouldnt do is use FR in "tray" style psu's that use parallel banks of 6 or more caps on the same rail.

[goodpsusearch]

Re: Help replacing 4 diodes with rectifier and improving old AT PSU

I think it's a terrible idea to use GP caps with a load life of 1000 / 2000 hours when you can use entry level low esr caps with 3000-10.000 hours. I never had any problem with the series I mentioned above. Smps are not picky with caps if you pay attention not to use motherboard grade capacitors.

[Behemot]

Re: Help replacing 4 diodes with rectifier and improving old AT PSU

Are you kidding me? Have you even looked in the datasheet? Yes, YXF would be suitable even though it is still superior than anything from that era. FC has about 2/3 the ESR of YXF. And FR is different league with ESR 1/3 of the YXF's.

[goodpsusearch]

Re: Help replacing 4 diodes with rectifier and improving old AT PSU

So if they used general purpose caps, you also should use general purpose caps...

No. Entry level low esr caps such as FC, YXF would work fine in places where General purpose caps were used before.