Bauman PSU Troubles!!!! (the troubles that never ends)

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Jonkan

Well-known member
Joined
Jun 7, 2004
Messages
733
Location
Sweden
Why cant a newbie get a break....PANG! (=the sound of the +5v rail caps on the api preamps exploding)

My +5 volt regulator (7805) just crapped out, after three hours of use!!! Now i get 28volts out on the +5 rail.

This happened with only two relays engaged!!! It wasnt even very hot when i touched the heatsink btw.

I have four channels connected at the same time. I just built a new psu, as my old one wasnt working properly (i will fix the old one when i have more time)

I did the mod, where you take the power for the relays from the r1/c1 junction.
no r10, just a wire link from input of 7805 to r1/c1.

the toroid gives is a 2x25va and i measured 28v ac in to the psu before regulated/rectified.
r11 is 1k (not shure if this can matter? (how do i decide which value here is appropriate?)

Could it be i got a bad 7805 regulator from the store?

Heres the schematic for the bauman psu: http://www.thediypill.phx.com.br/forumfiles/psu312sch_v1.pdf

What the hell is wrong here? Im going totally crazy, and im seriously doubting my abilities as a diy builder. I have never had so much trouble, and worked so hard for anything to work before in my life...
But i just cant seem to get a break, however much i try to fix this damn pre.

Please help a diy brother out, im so very depressed so i cant even think about anything else at the moment.

sincere regards,
Jonas (probably the unluckiest diy builder around)
 
Hi Jonas:

If I understood you correctly, you connected the input of the 7805 IC directly to the 48V output. According to the 7805 IC datasheet, maximum input voltage is 35V, and that's probably why it exploded.

I'd suggest using a half-wave rectifier ( 1 diode) connected to one secondary output that will supply the 7805 IC. I am willing to edit your schematic if you think that might help you.

If your relays are rated for 5V power supply, I don't think you can connect them to the main rectifier (where you have 35V).

Don't give up. We've all had our shineless moments.

Regards,
Milan
 
hi jonas,
don't give up yet. these set backs should always be viewed as learning moments.
i haven't built my api yet, but i've been reading through the threads to prepare for what lies ahead. i do beleive that you need a resistor before the input leg of the 5V regulator. the jumper suggested from junction C1/R1 is for use with 12V or 24V regulators and 12V or 24V relay combination.
from reading the api help thread posts it seems to me that using the 12V or 24V regulator with the jumper from junction C1/R1 and 12V or 24V relays would be the best option. forget the 5V relays and regulator and go with the higher voltage ratings. higher rated caps for the relay supply might be an option, but if you're going to replace them anyway, just go for the higher voltage relays/relay supply option and be on the safe side. less current draw and cooler running everything. hang in there. you'll be so happy when things work out and relish in the fact that you've learned for the next time.
kind reagards,
grant
 
R10 will bring the 48 down, but you need load current to size it.

I feel this pain every five minutes, then I fix it, then the pain starts all over again, a vicious cycle, known only to the last dying breed of analog techs working on thru hole with real soldering irons cooking all the time.
When's the last time you walked into a company and saw a bunch of Weller's burning>
Happens very rarely around here anymore.
What was the question?
Oh yeah.

I do not see how you killed the reg, it sitson another reg with short protection, your reg has short protection,also.

I got a bucn of bad regs from the sirplus one time.
Usually off brands.
If it says National Semi on it, it will probably last forever.

Can anybody build a power supply with the right tranaformer feeding the right regulator, or should we just kludge things up like a Nun in a Radio Shack?
 
"When's the last time you walked into a company and saw a bunch of Weller's burning?"

This morning.. except for my personal Metcal.. :green: no wonder i get more done than anyone else!
 
I think you may have misunderstood,

If I understood you correctly, you connected the input of the 7805 IC directly to the 48V output

I think I had the 7805 connected to the input of the 48v regulator, not to the output. atleast, that was my plan.

So it should be well in the operating range i think.


Bauman was the one who actually suggested that one should get rid of r10 altogether and connect it to the c1/r1 junction.

And hey, it worked for three hours!

Maybe i missed the part where it had to be 12 or 24v relays.

I really need a solution where the 5v relays will work, because i cant afford new relays.



At first, i used a 470r 5w resistor for r10 connected to the r1/c1 junction. But when you enabled more than two relays, the voltage got so low, that it didnt work. Thats why i changed it.

Im thinking, could it be that the cap on the api micpre that exploded had gone bad, and shorted itself between +/- legs after it stopped working?

Then the regulator would have been shorted out, and that would explain why the voltage suddenly got so high. And also why that cap suddenly exploded.

the caps on the other channels +5 rail were 25 volts, and seemed to atleast not explode with 27volts on them...

Could they still be working fine even though they got +2v volts more than they were rated for?

And what bout the relays, are they toast or can they handle a couple of seconds of 27volts?

/J
 
[quote author="Jonkan"]At first, i used a 470r 5w resistor for r10 connected to the r1/c1 junction. But when you enabled more than two relays, the voltage got so low, that it didnt work. Thats why i changed it.[/quote]
Strange, I'm using a 450R 5W resistor for R10 coming from the R1/C1 junction (with 5V relays), and it's works "ok" with 4 relays engaged. Granted, R10 and the reg gets very hot, but it works. If I build another pair, I will definitely use 12V or 24V relays for less current draw.

What are you measuring at the input to the 5V? If it's in the 20V range, that reg is gonna start cooking. I'd recommend putting R10 back in. The idea is to drop the raw DC down a bit at the input.
 
2 relays worked fine with the 470r r10, but when i enabled more than that, it didnt work.

The voltage dropped to almost zero, and the relays "let go" of their intended position.

Remember also, i have 4channels.
Two api, and two Bauman N72.
 
Which toroid are you using (is it an Amveco part)? The relays and the N72s are going to draw a lot from the positive side post rectifier (the API is bipolar). Perhaps you are asking more of the power transformer than it can deliver.

A P
 
[quote author="Jonkan"]
the caps on the other channels +5 rail were 25 volts, and seemed to atleast not explode with 27volts on them...

Could they still be working fine even though they got +2v volts more than they were rated for?
[/quote]

when i built my green pre psu i had over 70V on a cap rated for 63V for a while on my +48V side while i tried to figure out the problem. found my error and fixed it and the psu powered up with the same 63V cap in. caps have a tolerance over the rated value, but by how much depends on the quality of the cap and the volts going in. but that's just my experience and it don't count for much since i'm still new at this stuff.

remember jonas, it's not about who built the shiniest or best looking mic pre, it's about the journey of replacing parts when they blow up (or something like that).
:wink:
keep at it, when you finally get it working... :guinness: :guinness: :guinness: :guinness: :guinness: :guinness: :guinness: :guinness: :guinness: !
-grant
 
Its a 2x25VA transformer, is that enough?


it's not about who built the shiniest or best looking mic pre, it's about the journey of replacing parts when they blow up (or something like that).

Then im the king of that!

My pre looks like ass, and basically everything has blown up at one time or another. I just scratched my frontpanel when i had to unmount all the switches etc...So good looks is out of the question.


I just need some help to get this show on the road again, without worrying about this happening again.

So any more ideas?
/J
 
My point was that I'd still leave R10 in... perhaps you can use a different value that doesn't drop as much voltage.
 
That's 48V input on the 7805, far to high...
The resistor voltage drop will change depending on current, not a good idea.
Small relays pull very little current, I would install several diodes in series to lower the voltage a little, .7 Volts regardless of current.
Why not use a 7824 and use 24 volt relays, it thats all its driving.
The schematic also shows 1N4148 diodes on the relays, those should be 1N4002.
If it was me, I would never use the phantom to drive relays...
 
The 7805 is not on the output of the +48v regulator as on the schematic, its connected to the r1/c1 junction. so it should be alot less than 48v.

It should be the same as the 28vac rectified..And of course i forgot how much that is, or how to calculate it (can you diy better memory somehow?)

/J
 
Whats the voltage across C1?
I would still put 2 1N4004 diodes in series between the C1/R1 junction and the 7805 input.
It will give some isolation between the 2 circuits.
 
When you get to the output trans part make sure you notice which wire goes to ground and output cap, it can make a big difference...
Just a thought..
 
Dude,

I see you are doing the C1/R1 thing, cool. That what I do today.
BUT, you need to use a nice heatsink on the 7805. I use the rack case itself, isolate the regulator with Mica and it works for hours with all the switches turned on. That means all relays and all the leds on, and I use some current on leds :green:

It will heat cos you are taking 24V and regulating down to 5V, so the heat will be high and will burn the part if you dont cool it down.

Some folks have used 24V relays and 7824 instead, clever move. I just have tons of 5V relays so... I let the heatsink do the job :)

try it out! lemme know!!
hey dont give up on this stuff, you are doing alright as far as I know
:thumb:



:guinness: :guinness:
Fabio
 
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