Benchtop Power Supply Question

[megaraider]

Re: Benchtop Power Supply Question

Leave the how matter it is special , for me , and try to answer my question ... Though I won't tell you when you're right or wrong .

Really No question to answer
Wishing you good luck exceeding individual max rated current using parallel diodes with you're special heatsink…

why not just use a 25a rectifier from an old pc psu?

it's because i'm leaving the always possibility to add more diodes/lms in case i want to put more laptops on the table.

Not a problem... add more rectifiers in parallel

[jiroy]

Re: Benchtop Power Supply Question

why not just use a 25A rectifier from an old pc psu?

It's because I'm leaving the always possibility to add more Diodes/LMs in case I want to put more laptops on the table . When I'll draw the circuit , I'll leave that possibility for lot of other parts too . So nothing against rectifiers basically other than that ..
I left the core calculations for the last touch for such reasons .

[jiroy]

Re: Benchtop Power Supply Question

No secret ... IMO a great mistake.
The heatsink, no matter how 'special' it is, only works to keep the diodes under temperature limits, and if common to keep them all at about the same temperature.
Nothing else, sorry!



Yes!

Leave the how matter it is special , for me , and try to answer my question ... Though I won't tell you when you're right or wrong .

[stj]

Re: Benchtop Power Supply Question

why not just use a 25A rectifier from an old pc psu?

[megaraider]

Re: Benchtop Power Supply Question

It's like pushing me to reveal more secrets I would prefer to keep it hidden!! (...) the special heatsink is making them symetric with high precision ? .

No secret ... IMO a great mistake.
The heatsink, no matter how 'special' it is, only works to keep the diodes under temperature limits, and if common to keep them all at about the same temperature.
Nothing else, sorry!

The trick is to either go ahead and match them else current limit them, just like bjts.

Yes!

[eccerr0r]

Re: Benchtop Power Supply Question

The trick is to either go ahead and match them else current limit them, just like bjts.

[jiroy]

Re: Benchtop Power Supply Question

In practice what @stj claims is TRUE! You'll watch this behavior when the overall current exceeds the maximum rate per diode (plus a unknown value).

It's like pushing me to reveal more secrets I would prefer to keep it hidden!! Well fine megaraider ... How the overall current would know which one is the first when the special heatsink is making them symetric with high precision ? .
Yet , I do agree that if something is arranged chaotically , of course it will rise many different problems ..

[megaraider]

Re: Benchtop Power Supply Question

Come on stj; (...) and the picture's background initially should look vintage

parallel high current diodes doesnt work btw,
the one with the lowest voltdrop takes more of the current - then fails leaving the remaining ones overloaded.
then they fail.
happens all the time in some cheap tv psu's that try to do it.
also kills a lot of early Ryobo battery chargers

I really doubt that when they're each rated 6A. and fyi , they'll be equipped with special heatsinks similar to old CRT monitors ...
So you deserve another ...lol

In practice what @stj claims is TRUE! You'll watch this behavior when the overall current exceeds the maximum rate per diode (plus a unknown value).

[jiroy]

Re: Benchtop Power Supply Question

@stj;

That's how my dust bins look like ,

[jiroy]

Re: Benchtop Power Supply Question

that picture looks like my dustbin after a repair

Come on stj; I'm in the middle of hundreds of projects , including a 30 km FM transmitter , and the picture's background initially should look vintage

parallel high current diodes doesnt work btw,
the one with the lowest voltdrop takes more of the current - then fails leaving the remaining ones overloaded.
then they fail.

I really doubt that when they're each rated 6A. and fyi , they'll be equipped with special heatsinks similar to old CRT monitors ...
So you deserve another ...lol

[stj]

Re: Benchtop Power Supply Question

that picture looks like my dustbin after a repair

parallel high current diodes doesnt work btw,
the one with the lowest voltdrop takes more of the current - then fails leaving the remaining ones overloaded.
then they fail.

happens all the time in some cheap tv psu's that try to do it.
also kills a lot of early Ryobo battery chargers

[jiroy]

Re: Benchtop Power Supply Question

I have almost finished a diagram's prototype for the laptops PSU project . So far , my choice went for a parallel series of LM338 , backed up by parallel diodes as illustrated in an image previously , while leaving a chance for additional ones , as necessary . Right now , I estimate LM338 power at 5A maximum , to stay on safe side , and for each another 5A , I'll add another one ..

When the project is finished , although the delays and distractions were more than to count , It will be available only for some particular friends , and through PM's .

In the meantime and for the people who liked my backups parts , my Sony Cam is back on line after some repairs , so I may re-post the previous images which were very bad , and yet , you know there are some size limits for uploads , so I'll resize them in PS as necessary .
Here's a sample of my vintage capacitors .

[redwire]

Re: Benchtop Power Supply Question

You need a reference voltage for the comparators, not sure what he's using for that. Hitachi 555's are pretty ooollldddd

[eccerr0r]

Re: Benchtop Power Supply Question

Well, the CMOS TLC555 can go down to 2v or so, if needed?

[jiroy]

Re: Benchtop Power Supply Question

I finally finished the overvoltage protector and its main features :


-Voltage comparison : LM339 is used in applications where a comparison between two voltage signals is required. In addition with four of those comparators on board the device can compare four pairs of voltage signals at a time which comes in handy in some applications.

-Timer : IC HA17555 timer is a highly stable integrated circuit. It can function as an accurate time delay generator and free running multivibrator as well. When it is used as an oscillator, its frequency and duty cycle can be accurately adjusted with the help of external components like resistors and a capacitor.What is the minimum voltage for 555?
+4.5 volts
Supply- voltage operating range for the 555 is +4.5 volts (minimum) to +16 volts (maximum), and it is specified for operation between +5 volts and + 15 volts. The device will operate essentially the same over this range of voltages without change in timing period.
I only wish if they make a 3v. version of 555...

- 2 Zeners in parallel to feed 4 indicator leds .

- Drop down voltage through mylar 1mf 400 volts and 4 diodes 1n4007 rectifying and 10 kohms resistor ,etc ..The main DC voltage is 12v .

- 3 potentiometers , 1 for time delay and two for high and low AC.

[jiroy]

Re: Benchtop Power Supply Question

Interesting

Enjoy these gems ...

[stj]

Re: Benchtop Power Supply Question

20mm fuses are common on 240v,
but remember that ceramic ones often contain sillica or boron to stop it just plating the tube with metal oxide.

now glass fuses are a different story, i wont use glass for mains voltage.

[eccerr0r]

Re: Benchtop Power Supply Question

The explanation I heard is assume this: 240V supply, two 120V fuses shorted across it. One of the fuses blows. How man volts does the burned fuse need to withstand?

That said, that Samsung TV I had used a 5mmx20mm 250V fuse to protect the Vboost line of the APFC boost circuitry...

[stj]

Re: Benchtop Power Supply Question

Going back to the discussion of fuses...
I was wondering if using more than 1 fuses in series can achieve a higher voltage protection.

i would say no.

once one fuse blows if it arc's across then your hoping the second fuse blows.
it probably wont because of reduced current, but even if it does it may still arc too.

sillica filled fuses are best - and not expensive.
HRC fuses usually arent the fastest though.

[momaka]

Re: Benchtop Power Supply Question

Going back to the discussion of fuses...
I was wondering if using more than 1 fuses in series can achieve a higher voltage protection.

If you put a GFCI after an isolation transformer, earth ground no longer is part of the "hot"/"neutral" of the secondary (after an iso transformer, both poles are "hot" sort of ... or neither because neither pole has any relation to GND due to isolation) so no current will flow from the iso secondary to GND. And since no current flows, it will never trip.

If you have a iso transformer plugged into a GFCI, it likewise will not trip either, same problem.

Note that neither case is a loss of protection, just that it wouldn't trip, or at least I don't see how it could trip as no current would flow to ground as there's no connection and thus you wouldn't get electrocuted either - and the GFCI won't protect you anyway if you put two fingers into both holes of a wall outlet.

Hmmm...
What you say is true up there, and I also thought of that in my last post.
Nonetheless, I was thinking about this in bed yesterday morning, and I realize I may indeed have had a brain fart with thinking that a GFCI plugged in after an isolation transformer would do anything. Indeed you're right - it won't. And again, that has to do with the fact that a GFCI measures difference in current flows between "Live" and "Neutral" lines (which like you said, neither of which can be said to be Live nor Neutral, since neither is referenced to ground.) After all, you can't have current flowing out of one line from the ISO traffo and not have this current come back in through the other - that would violate KCL!

The only way a GFCI will work after being connected to an ISO transformer is if one of the output lines of the ISO transformer is connected to ground (or referenced with a low-enough resistance) BEFORE/upstream of the GFCI. But then that completely defeats the purpose of having the ISO transformer in the first place.

So yeah... GFCI after an ISO transformer = no gain in safety.

DannyX has that groundless building I think.

Many buildings in [former] Soviet Block countries were built like this - particularly the "high-rise" concrete condos. Neutral was tied to ground only at one point, (typically in the basement where the power comes in) for the entire building. From that point forward, I think those buildings only have Neutral and 3 AC phases going to the apartments (each apartment only gets 1 phase though, IIRC.) It was also common practice in such buildings for electricians to tie the ground pin on outlets to Neutral - something that would be a hard violation both here and there now in modern buildings. It's not well-checked, though, so some electricians still wire it this way even in new buildings.