Dumb Q: Why few shottky bridges?

[Ancient_Hacker]

So I'm looking over a pallet of dead Tek 4xx scopes. If this bunch is
like the last bunch, 95% of the dead scopes need new power supply
capacitors, have at least one shorted tantalum cap and consequent
burned-up micro filter chokes

Not much choice as to what to do about the capacitors, just replace
them.

And the bridge rectifiers have been running much too hot for too many
years, leading to really brown PC boards where there should be green.
About 20% of the bridge rectifiers have half-failed, so they're
basically half-wave rectifiers. Tek didnt realize it seems that diodes
send most of the heat down the leads to the PC board. Most of the
traces going to the bridges are NOT fat, so they they traces don't help
much in carrying away the heat. This is a really marginal design, as
evidenced by the many bad bridges and overheated boards.

So for the bridges, the thought comes to mind to replace them with
cooler-running schottky bridges.

So I look on Digi-Key, and find exactly two bridges, schottky flavor,
200ma IIRC.
Same for Mouser.

Any idea where I can find schottky bridges? or is there some really
good reason these arent made?

Yes, I know, I can wire up a foursome, but I'd really rather not cobble
something up, especially when there's four or five bridges to replace.

Regards,

George

 

[Roger]

Any idea where I can find schottky bridges? or is there some really
good reason these arent made?

Very few applications. In general they are incorporated into IC's where
they would be useful.

Also, a low voltage 200mA device can serve most small signal
requirements. More powerfull apps would use discretes. The concept of
prepackaged bridges is nothing more than encapsulating 4 diodes in a
little box, but most power schottkys are not used in std. bridges. Far
more likely to find them used in pairs and YES, there are a lot of
schottky pairs available! So perhaps 2 of them?

 

[Ban]

i.e. Diotec has a 1A series CS10D to CS 50D. They are made, but the market
is small.
They are for low voltages only, because Schottky is only up to 100V and then
there are some Si-carbite for higher voltages too.
The reason is that you do not need them for mains transformer rectifiers,
standard recovery is sufficient there. And in SMPS you have almost always a
dual winding(fewer turns needed) and use only two diodes (higher
efficiency). Also they are too expensive, compared with the 1N400X series.
They also have a very high reverse current, 1mA at room temp and 10mA at
100deg. (1N5817). This can lead to thermal runaway and self destruction.

Very few applications. In general they are incorporated into IC's
where they would be useful.

Also, a low voltage 200mA device can serve most small signal
requirements. More powerfull apps would use discretes. The concept of
prepackaged bridges is nothing more than encapsulating 4 diodes in a
little box, but most power schottkys are not used in std. bridges. Far
more likely to find them used in pairs and YES, there are a lot of
schottky pairs available! So perhaps 2 of them?

No they have only common cathode ones, you cannot make a bridge out of
those.

 

[g. beat]

So I'm looking over a pallet of dead Tek 4xx scopes. If this bunch is
like the last bunch, 95% of the dead scopes need new power supply
capacitors, have at least one shorted tantalum cap and consequent
burned-up micro filter chokes

Not much choice as to what to do about the capacitors, just replace
them.

And the bridge rectifiers have been running much too hot for too many
years, leading to really brown PC boards where there should be green.
About 20% of the bridge rectifiers have half-failed, so they're
basically half-wave rectifiers. Tek didnt realize it seems that diodes
send most of the heat down the leads to the PC board. Most of the
traces going to the bridges are NOT fat, so they they traces don't help
much in carrying away the heat. This is a really marginal design, as
evidenced by the many bad bridges and overheated boards.

So for the bridges, the thought comes to mind to replace them with
cooler-running schottky bridges.

So I look on Digi-Key, and find exactly two bridges, schottky flavor,
200ma IIRC.
Same for Mouser.

Any idea where I can find schottky bridges? or is there some really
good reason these arent made?

Yes, I know, I can wire up a foursome, but I'd really rather not cobble
something up, especially when there's four or five bridges to replace.

-------------
George -

AT least they are repairable with available parts.
While I can understand replacing capacitors, especially electrolytic, in
equipment that is 30 years old -- when it happens to new computer
equipment - people get upset !
http://www.badcaps.net/

Many good motherboards (and computers) dumped because of this issue. I have
rescued a few -- but there are tens of thousands of them out there !

gb

 

[jakdedert]

So I'm looking over a pallet of dead Tek 4xx scopes. If this bunch is
like the last bunch, 95% of the dead scopes need new power supply
capacitors, have at least one shorted tantalum cap and consequent
burned-up micro filter chokes

Not much choice as to what to do about the capacitors, just replace
them.

And the bridge rectifiers have been running much too hot for too many
years, leading to really brown PC boards where there should be green.
About 20% of the bridge rectifiers have half-failed, so they're
basically half-wave rectifiers. Tek didnt realize it seems that diodes
send most of the heat down the leads to the PC board. Most of the
traces going to the bridges are NOT fat, so they they traces don't help
much in carrying away the heat. This is a really marginal design, as
evidenced by the many bad bridges and overheated boards.

So for the bridges, the thought comes to mind to replace them with
cooler-running schottky bridges.

So I look on Digi-Key, and find exactly two bridges, schottky flavor,
200ma IIRC.
Same for Mouser.

Any idea where I can find schottky bridges? or is there some really
good reason these arent made?

Yes, I know, I can wire up a foursome, but I'd really rather not cobble
something up, especially when there's four or five bridges to replace.

Regards,

George

Your question is answered already, but I thought I'd add: if the bridge
recitifiers are not dissipating heat as you believe they should; why
don't you add a small heatsink to the package?

IIRC, there is heat-transfer epoxy available. Just glue a the sink to
the top of the package. It couldn't hurt.

jak

 

[Joerg]

Hello George,

So I'm looking over a pallet of dead Tek 4xx scopes. If this bunch is
like the last bunch, 95% of the dead scopes need new power supply
capacitors, have at least one shorted tantalum cap and consequent
burned-up micro filter chokes

Not much choice as to what to do about the capacitors, just replace
them.

And the bridge rectifiers have been running much too hot for too many
years, leading to really brown PC boards where there should be green.
About 20% of the bridge rectifiers have half-failed, so they're
basically half-wave rectifiers. Tek didnt realize it seems that diodes
send most of the heat down the leads to the PC board. Most of the
traces going to the bridges are NOT fat, so they they traces don't help
much in carrying away the heat. This is a really marginal design, as
evidenced by the many bad bridges and overheated boards.

So for the bridges, the thought comes to mind to replace them with
cooler-running schottky bridges.

So I look on Digi-Key, and find exactly two bridges, schottky flavor,
200ma IIRC.
Same for Mouser.

Any idea where I can find schottky bridges? or is there some really
good reason these arent made?

Yes, I know, I can wire up a foursome, but I'd really rather not cobble
something up, especially when there's four or five bridges to replace.

It might make sense to do a "cobbling up" to dissipate more of the heat
into the air instead of the board. Obviously they haven't done their
homework in the due diligence after the design. A thermal test should
have been completed before releasing that scope design.

It's easy to buy powerful single Schottky diodes. If you want to keep it
neat I'd look at regular silicon bridge rectifiers and let them ride a
bit higher than normal, assuming this is just the 60Hz rectification and
there are no stability issues. Take some that are larger than the
previous ones for more "air contact".

Question: What's the exact type of the scopes and where did you find
them? Could be a nice bargain if these have chronic problems and die
prematurely.

Once I bought a nice shortwave transceiver for a song. The manufacturer
had botched the design of the driver stage and nobody knew how to fix
it. I guess it'll be the same for classic photography freaks since some
of the otherwise really nice Minolta cameras have poor electrolytics in
them that should now be failing by the bucket load. That causes the
shutter to quit for good. Easy fix, but the repair shops will likely
charge so much that people rather give the things away. Some might not
even know how to fix these.

 

[Sofie]

A shottky bridge will not run any cooler than a regular bridge when it is
rectifying 50 or 60 Hz (line or mains) power.... the schotky application is
for high frequency switching power supplies and the like where the faster
recovery time of the schotky is required.
- - - - - - - - - - - - -

 

[Joerg]

Hello Sofie,

A shottky bridge will not run any cooler than a regular bridge when it is
rectifying 50 or 60 Hz (line or mains) power.... the schotky application is
for high frequency switching power supplies and the like where the faster
recovery time of the schotky is required.

Huh? Schottkys drop 200mV less than silicon rectifiers. That's about
25-30% less dissipation. Nothing to sneeze at I'd say.

 

[Sjouke Burry]

Hello Sofie,





Huh? Schottkys drop 200mV less than silicon rectifiers. That's about
25-30% less dissipation. Nothing to sneeze at I'd say.

They also leak in the reverse mode,
and cant stand much voltage,
and might be expensive.

 

[Joerg]

Hello Sjouke,

They also leak in the reverse mode,
and cant stand much voltage,
and might be expensive.

Leakage isn't so bad anymore these days. I have never seen it crimp the
efficiency much in my designs. No high voltage, of course. But yes, they
do cost a lot which is why I only use Schottky if I really have to.

 

[Ban]

Hello Sjouke,



Leakage isn't so bad anymore these days. I have never seen it crimp
the efficiency much in my designs. No high voltage, of course. But
yes, they do cost a lot which is why I only use Schottky if I really
have to.

It's good that you are so blissfully unaware of the shortcomings of
Schottkys, because Mr. Know-All has designed in such bombs as bridge
replacements.
Look at Tmax. operating, K/W j-a, capacity and especially reverse voltage
vs. power dissipation. The 1N5819 cannot even have 13V reverse in 0.5 duty
cycle and will produce a thermal runaway. And a 0.5 duty cycle is what
happens in a bridge rectifier. And this diode is rated for 50V!

 

[Ancient_Hacker]

se Mr. Know-All has designed in such bombs as bridge

replacements.
Look at Tmax. operating, K/W j-a, capacity and especially reverse voltage
vs. power dissipation. The 1N5819 cannot even have 13V reverse in 0.5 duty
cycle and will produce a thermal runaway. And a 0.5 duty cycle is what
happens in a bridge rectifier. And this diode is rated for 50V!

Funny, but the specs say 10mA max leakage at 100 C. With 13 volts,
that's only 0.13 watts. Unlikely to cause thermal runaway. Well, then
again it might, if you have the diode clipped onto those little
Radio-Shack clip-leads with the number 28 wires.

When hooked up to actual terminals, or soldered onto reasonable-size PC
board traces, these shouldnt run away.

BTW shottky's are not all that expensive anymore, Fairchild lists the
1N5819 at 9 cents in quantity.

 

[Ancient_Hacker]

Question: What's the exact type of the scopes and where did you find
them? Could be a nice bargain if these have chronic problems and die
prematurely.

You can sometimes find pallet-loads of these when a company upgrades
(downgrades) to more modern scopes. Check out the usual electronic
liquidator sites. I tend to go for the 465/475/485 variety. I'd stay
away from the 22xx series or the 455's, they're of much lower quality
than the other ones. Also avoid the 78xx and 79xx series-- they're
excellent scopes, but have fragile and unobtainable IC's in them.


Even so it's an iffy thing.

If the scopes have been well-treated and kept in a lab, and were not
left on 24/7, they can often be resuscitated with new capacitors, new
diodes, and a general cleaning.

But many have been bouncing in a car trunk for 20 years, or run 24/7
in a salt-water factory, or slid off from on top a coke machine, or
shipped loose on a pallet, or worse.

You can lessen the overall risk by buying a bunch of them, then using
the hopeless ones for knobs, CRT's and other misc unobtainable parts to
fix the better ones.

Just don't pay more than $40 each for the good-looking ones, or $20 for
the hopeless ones, m and you can come out ahead, if you consider fixing
scopes as "fun" and not drudgery.

 

[Ban]

se Mr. Know-All has designed in such bombs as bridge

Funny, but the specs say 10mA max leakage at 100 C. With 13 volts,
that's only 0.13 watts. Unlikely to cause thermal runaway. Well,
then again it might, if you have the diode clipped onto those little
Radio-Shack clip-leads with the number 28 wires.

When hooked up to actual terminals, or soldered onto reasonable-size
PC board traces, these shouldnt run away.

BTW shottky's are not all that expensive anymore, Fairchild lists the
1N5819 at 9 cents in quantity.

Get the Philips datasheet. With 0.13W and 100K/W (standard mounting) this
will result in +13K, more than doubling the the leakage, etc.
Read a datasheet first pls.

 

[Eeyore]

Get the Philips datasheet. With 0.13W and 100K/W (standard mounting) this
will result in +13K, more than doubling the the leakage, etc.
Read a datasheet first pls.

This begs the question why anyone would use them according to your logic. Yet
they do so. I think you must have made some wrong assumption somewhere ! Think
about thermal equilibrium.

Graham

 

[Ban]

This begs the question why anyone would use them according to your
logic. Yet they do so. I think you must have made some wrong
assumption somewhere ! Think about thermal equilibrium.

Graham

Did you download that datasheet? I was astonished myself about this. Look at
figure11, maybe my interpretation is wrong, but it seems to indicate that
fact. And look how bad the Rth is and with a 125deg. max junction temp. you
come to a usable dissipation of less than a watt for 55° ambient.

 

[Ancient_Hacker]

Get the Philips datasheet. With 0.13W and 100K/W (standard mounting) this

will result in +13K, more than doubling the the leakage, etc.
Read a datasheet first pls.

I did, apparently you didnt look too closely.

on the Fairchild datasheet, the spec is 1ma at room temp, 10mA at 100C.


At a more typical 80K/W, and with the leakage going down with voltage,
and the voltage being half a sine wave, applied only half the time,
that only raises the temp a very very few degrees, much less than the
13 you suggest.

Still worth doing the runaway calculation though!. The "Onsemi"
datasheet for this diode goes into this with charts graphs, equations
and tables.

 

[Ancient_Hacker]

Get the Philips datasheet. With 0.13W and 100K/W (standard mounting) this
will result in +13K, more than doubling the the leakage, etc.
Read a datasheet first pls.

Look a little closer. The 10mA is at 100C, at room temp it's only 1mA.

And it's about half that for the newer diodes, which are only 38 cents
for a 100V diode, so runaway isnt an issue.

And it will be even less in a typical rectifier circuit, where the peak
voltage only occurs for part of a cycle, every other half cycle.

Still one should glance at the runaway diagrams,
"onsemi" has particularly nice ones.

 

[John Fields]

Did you download that datasheet? I was astonished myself about this. Look at
figure11, maybe my interpretation is wrong, but it seems to indicate that
fact. And look how bad the Rth is and with a 125deg. max junction temp. you
come to a usable dissipation of less than a watt for 55° ambient.

 

[Ban]

I did, apparently you didnt look too closely.

on the Fairchild datasheet, the spec is 1ma at room temp, 10mA at
100C.


At a more typical 80K/W, and with the leakage going down with
voltage, and the voltage being half a sine wave, applied only half
the time, that only raises the temp a very very few degrees, much
less than the 13 you suggest.

Still worth doing the runaway calculation though!. The "Onsemi"
datasheet for this diode goes into this with charts graphs, equations
and tables.

12-volt dc supply using a bridge circuit with capacitive filter

such that IDC = 0.4 A (IF(AV) = 0.5 A), I(FM)/I(AV) = 10, Input

Voltage = 10 V(rms), RJA = 80°C/W.

Step 1. Find VR(equiv). Read F = 0.65 from Table 1,? VR(equiv) =
(1.41)(10)(0.65) = 9.2 V.

Step 2. Find TR from Figure 2. Read TR = 109°C@ VR = 9.2 V and RJA = 80°C/W.

Step 3. Find PF(AV) from Figure 4. **Read PF(AV) = 0.5 W@I(FM)I(AV)= 10 and
IF(AV) = 0.5 A.

Step 4. Find TA(max) from equation (3). TA(max) = 109 - (80) (0.5) = 69°C.

**Values given are for the 1N5818. Power is slightly lower for the1N5817
because of its lower forward voltage, and higher for the 1N5819
I copied this from the ON semi datasheet. You can see the equivalent reverse
voltage is *only 9.2V* and thermal runaway occurs at 69 degrees. And this is
a 10Vrms only secondary. Do it for 15V and see what comes out.

In reality they didn't consider transformer losses and mains overvoltage,
which would give a lot less margin. That is probably why so many of those
TEK power supplys failed.