Bestec ATX-300-12Z REV.EHR

[POM_MJ]

Re: Bestec ATX-300-12Z REV.EHR

Not quite got the idea yet.
I'll give explaining another shot.....

~~
Within a series the ESR varies with the can size.
In the cheap caps that end up in PSU's a ___uF 10v cap is usually physically larger than the same uF cap in 6.3v.
Because the 10v is physically bigger it has lower ESR.
-
That situation is so common that when they use cheap caps they tend to just use 10v by default.
-
But... By switching to a better 'grade' of cap [the MBZ in this case] the 6.3v cap now has lower ESR than the cheap 10v cap did and is a significant upgrade without needing a bigger cap.

Here's an example:
Look at the CapXon KF 6.3v and 10v 1500uF [A typical PSU type cap.]
6.3v is 10x16mm and ESR is 0.140
10v is 10x20mm and ESR is 0.062 [same series, bigger can, better ESR]

Now look at a better grade cap, the MBZ 1500uF
the 6.3v is only 10x12.5mm but ESR is WAY lower at 0.026

That's what I was trying to get across.
You don't need a 10v cap.
The grade change already accomplished the same thing....
.... and in fact upgraded you beyond why the 10v was used.

~~
And related.
~~
Now look at the MBZ chart again.
You will notice that REGARDLESS of the voltage....
All the 10x16mm have the same ESR and Ripple
All the 10x20mm have the same ESR and Ripple
All the 10x23mm have the same ESR and Ripple
- That's why I say -within a series- ESR changes with can size, not voltage or uF.
- That is typical of good brands of caps.
It's because their ratings are an 'at least' or 'no less than' kind of number with a realistic margin for error. Very good way to rate things for use in the real world. As a bonus, when they do things with a margin that way the true value is probably better than what you see in the table.

If you look at the same thing in the KF chart the ESR is all over the place for a given can size.
- That's typical of crap brands of caps.
It's probably due to the ratings in their table being an exact number for THAT cap with zero margin for error. Not a good way to do things because in the real world the actual conditions won't match the test conditions exactly.
.

Ok, I get understand now and with replaced by good caps Rubycon, I don't need to recaps again.
Thankyou very much for your explainations.

[glen65]

Re: Bestec ATX-300-12Z REV.EHR

Hello,
I have an Emachine with a Bestec 20 pin supply that giving me
problems and would like to try testing this computer with
a supply out of another machine. However there is a
slight difference. The Bestec supply has a pink wire connected
to the red wire on pin 11 and there is no wire on pin 13.
But the supply in the other machine has no pink wire and
does have a white wire connected to pin 13. Can this supply
still be used in the Emachine?

Thanks

[POM_MJ]

Re: Bestec ATX-300-12Z REV.EHR

Hello,
I have an Emachine with a Bestec 20 pin supply that giving me
problems and would like to try testing this computer with
a supply out of another machine. However there is a
slight difference. The Bestec supply has a pink wire connected
to the red wire on pin 11 and there is no wire on pin 13.
But the supply in the other machine has no pink wire and
does have a white wire connected to pin 13. Can this supply
still be used in the Emachine?

Thanks

Hi
The pink wire on red wire is +5V sense wire for return to regulation circuit inside the PSU, some of PSU has this wire due to good circuit design of the PSU, doesn't affect to the system.

The white wire is -5V, thats used in very old system, not see them for long time ago.

Yes, The PSU has or has no white wire can be used.

[PCBONEZ]

Re: Bestec ATX-300-12Z REV.EHR

Correct.
The white wire [-5v] has been optional for several years now.
The pink wire is also optional, it 'measures' [senses] the voltage on the red wire.
[Kind'a stupid. They do that to measure at a point closest to the mobo to be more accurate. [To account for the voltage drop due to the resistance of the red wire.] The flaw in that plan is the pink wire also has resistance so they might as well measure from inside the PSU anyway. - Method was probably thought up by a wire salesman with a lot of pink wire.]
.

[stj]

Re: Bestec ATX-300-12Z REV.EHR

the pink wire has virtually no loss because it handles no current draw.

the design has been around forever - i see stuff from 1978 with it all the time.

[everell]

Re: Bestec ATX-300-12Z REV.EHR

Hello,
I have an Emachine with a Bestec 20 pin supply that giving me
problems

What is model number of your Bestec power supply? Check the label attached to the power supply.

[PCBONEZ]

Re: Bestec ATX-300-12Z REV.EHR

the pink wire has virtually no loss because it handles no current draw.

the design has been around forever - i see stuff from 1978 with it all the time.

Yeah, and the voltage drop in 18" of copper wire is so HUGE!!!
LOL !!!

[stj]

Re: Bestec ATX-300-12Z REV.EHR

th volt-drop over 18" of copper wire will be relative to the diameter & current-flow.

[PCBONEZ]

Re: Bestec ATX-300-12Z REV.EHR

You forgot to include the resistivity for specific alloy of copper used, the diameter of the individual strands of wire, and the temperature of the wire at the time.
- Yeah, I know how it works.
It's insignificant and not -practically- measurable unless you are measuring to 3 or 4 decimal places.
.

[ve3tlc]

Re: Bestec ATX-300-12Z REV.EHR

OK Toasty - I'll take a shot at it!

First I would like to say that most of my work with the Bestec power supplies was problems in the 5vsb circuit. My solution to the Bestec ATX-250 12E was a bit extreme, but it worked. As for the other Bestec models - I have not seen the same kind of 5vsb failures due to the pwm chip in the later models. Most of these failures have been bad caps - just like the other brand power supplies.

The Bestec ATX-300 12Z we are discussing here does not appear to have a 5vsb problem. One more piece of proof that the later models are indeed good power supplies when recapped.

The problem here is that both 5v and 12v rails are low. They are both regulated by the same regulator chip IC3 which is a AZ431 (or equivalent). The resistor from the 12v rail to the reference pin is R57 which is 24K. The resistor from the 5v rail to the reference pin is R61 which is 5.1K. The resistor from reference pin to ground is a parallel combination of R59 and R60. R59 is 3.00K and R60 is 27.0K. This parallel combination calculates to 2.70K.

So why two resistors in parallel between reference pin and ground? As a cost savings, rather than putting in a variable resistor to set the 5v and 12v voltage values, they put in a higher resistance than what is needed (in this case R59 which is 3.00K). Then they bridge a parallel resistance to bring the voltages into spec. So if you play with the R60 value, you will probably be able to put those voltages where you want them.

These resistors are surface mount resistors on the bottom of the pc board. You may or may not want to attempt changing them. Also, the R60 value on your board may or may not be 27.0K. The R59 value probably is 3.00K.

I realize this was an older post but it had the information I have been looking for. I have been given the BESTEC 300W supplied referenced in the post and would like to use it to power an HF radio. I need to adjust the 12 v rail to 13.6 V. It seems that changing R60 should do the trick. I don't need the other voltages so it doesn't really matter how they are affected. I am interested in the capacitor changes that are mentioned. Is it safe to assume that I should look at beefing up the 12V related caps? Is there any way of determinig what capacitors should be upgraded?

[370forlife]

Re: Bestec ATX-300-12Z REV.EHR

As long as they are 16V caps (which are generally what are always used on the 12v rail,) they should be fine.

[PCBONEZ]

Re: Bestec ATX-300-12Z REV.EHR

You may have to put load on the other rails to get the PSU to start and/regulate.
Because of that the over-all efficiency will be in the pooper.

ATX spec limits Ripple on +12v to 120 mV max.
Don't need to 'beef caps' unless that's too much noise for your application.
.