Help identifying glass diode?

[DaveC]

Partial view here (it's V12, lower left in photo):

http://home.covad.net/~tmcenter/peninsula/pcb3.jpg

Measured out-of-circuit, almost zero ohms, both ways. Printing (barely
legible: "1(?)018P" (that first "1" could be any character or number with a
vertical element).

How do you tell if this is a simple signal diode or a zener? Do zeners come
in this package?

Any help identifying this and recommending a substitute would be greatly
appreciated.

Thanks,

 

[DaveC]

Examining other similar diodes in the circuit, it seems to be a 1N4148.

Thanks,

 

[legg]

Partial view here (it's V12, lower left in photo):

http://home.covad.net/~tmcenter/peninsula/pcb3.jpg

Measured out-of-circuit, almost zero ohms, both ways. Printing (barely
legible: "1(?)018P" (that first "1" could be any character or number with a
vertical element).

How do you tell if this is a simple signal diode or a zener? Do zeners come
in this package?

Any help identifying this and recommending a substitute would be greatly
appreciated.

This appears o be a diode used in a gate drive or current sensing
circuit.

To check the diode, you will have to desolder one end and remeasure
the diode and the empty position to determine it's integrity and
locate the real short, if the unpopulated location still looks low
impedance.

You must read and report lettering on the part to identify it's type.

If used to prevent overvoltage on a current sensing position, it may
have been a zener intended to short on sensor open circuit, to prevent
damage to the more sensitive sections of the control circuit.

If it is used in a gate drive circuit, it may have been a zener or
diode that failed short after the failure of the power device being
driven - in which case the driven power device and other components
attached to the gate and source will measure incorrectly and need
replacing.

Failed zeners can usually be identified by the body marking.

RL

 

[Paul Burridge]

I've got some 3V9 zeners that look remarkably like that.

 

[DaveC]

I've got some 3V9 zeners that look remarkably like that.

And I've got some that are just plain switching diodes.

How do I determine what this diode is (was) without some markings?

Helllllp!

 

[DaveC]

I see some application notes for the SMPS controller IC this PS uses, and
there is a zener diode at the FET's gate. The diode is "USD1120", but I can't
find *any* data on the net about it.

What diode is a USD1120?

Thanks,

 

[Sam Goldwasser]

If it's across gate to source, it's a zener protection diode. Probably
18 V, a 1N4746, looks like a fat glass diode.

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[legg]

I see some application notes for the SMPS controller IC this PS uses, and
there is a zener diode at the FET's gate. The diode is "USD1120", but I can't
find *any* data on the net about it.

What diode is a USD1120?

What is the lettering on the body of the part in question?

Unless the app note is specific to this commercial device, it will
only be usefull as general information.

RL

 

[Winfield Hill]

DaveC wrote...

I see some application notes for the SMPS controller IC this PS uses,
and there is a zener diode at the FET's gate. The diode is "USD1120",
but I can't find *any* data on the net about it.

What diode is a USD1120?

The USD1120 is a 20V 1A axial Schottky diode made by Unitrode,
Solid State Devices, and Microsemi. I have a two-page Microsemi
datasheet. It's not a zener diode.

Thanks,
- Win

(email: use hill_at_rowland-dot-org for now)

 

[Leeper]

Based on the lettering alone, could it be a 1N918, which happens to be
somewhat similar to a 1N4148?

Possibly the P is a B or something like that?

 

[DaveC]

If it's across gate to source, it's a zener protection diode. Probably
18 V, a 1N4746, looks like a fat glass diode.

Not *directly* across G-S; the current sense resistor is between ground and
source. It looks like this:

^
|
|
G |--+ D
-+--| |<---+ IRFP340B
| |----+ S
| |
| |
- \
^ / 0.22 ohm 2w
| \
| /
| |
| |
+ --------+-
///

The diode in question is not shown as a zener (as don't know if it is). Also,
there is a protection diode internal to the FET between S and D, according to
the data sheet.

The glass diode (which looks very much like a 1N4148 nearby) has some partial
markings which are: "C18P?" where the ? is an illegible character. Since the
markings wrap around the diode, this character could be at the beginning or
end of the number.

The only hit I come up with when I search the net for this number is ComChip.
They make SMD diodes, and one of the CZRM27 series is the CZRM27C18P. It is
an 18v "voltage regulator diode", according to their data sheet.

Other ideas? Where to find a replacement? Use just any 18v zener of 1w or so?

Thanks,

 

[DaveC]

Turns out I actually have an 18v diode lying around. I'll use that.

Thanks to all who piped-in.

 

[Clint Sharp]

The glass diode (which looks very much like a 1N4148 nearby) has some partial
markings which are: "C18P?" where the ? is an illegible character. Since the
markings wrap around the diode, this character could be at the beginning or
end of the number.

The only hit I come up with when I search the net for this number is ComChip.
They make SMD diodes, and one of the CZRM27 series is the CZRM27C18P. It is
an 18v "voltage regulator diode", according to their data sheet.

Other ideas? Where to find a replacement? Use just any 18v zener of 1w or so?

Sounds OK, but I'd try and size match it, 1N4148 size devices tended not
to be of the 1W rating when I was repairing this stuff though, more like
300mW.

Don't forget, if you don't push enough current through the zener, it
won't!

The Zener is referenced in one of the UC384x datasheets (Not
necessarily Unitrode's) and the reasons for it's existence are
explained, IIRC you select the zener voltage to suit the FET but there
seemed to be two 'standard' values for this part, 15v and 18v whichever
PSU you opened.

 

[legg]

The glass diode (which looks very much like a 1N4148 nearby) has some partial
markings which are: "C18P?" where the ? is an illegible character. Since the

So its a BZX55C18 (BZX79C18) or similar 18V 400mw(500mW) zener.

You have not reported on impedance measurements of the part's vacant
position, or the disonnected part.

RL

 

[DaveC]

So its a BZX55C18 (BZX79C18) or similar 18V 400mw(500mW) zener.

Fairchild semi's data sheet now in hand.

You have not reported on impedance measurements of the part's vacant
position, or the disonnected part.

Diode is shorted in both polarities. Vacant position is relatively high
impedance (10K ohms).

Thanks,

 

[DaveC]

Sounds OK, but I'd try and size match it, 1N4148 size devices tended not
to be of the 1W rating when I was repairing this stuff though, more like
300mW.

Don't forget, if you don't push enough current through the zener, it
won't!

I'm no electronics professional, but mW rating is maximum power dissipation.
I think I could use a 10W device here and I'd still get 18v out of it, no? I
leave the "pushing enough current" to the circuit designers.

The Zener is referenced in one of the UC384x datasheets (Not
necessarily Unitrode's) and the reasons for it's existence are
explained, IIRC you select the zener voltage to suit the FET but there
seemed to be two 'standard' values for this part, 15v and 18v whichever
PSU you opened.

The "18" on the diode defines it as an 18v device, to my satisfaction.

 

[GPG]

The USD1120 is a 20V 1A axial Schottky diode made by Unitrode,

Solid State Devices, and Microsemi. I have a two-page Microsemi
datasheet. It's not a zener diode.

page 7 http://www.metatech.com.hk/datasheet/aic/dcdc_pdf/AIC3843.pdf

 

[DaveC]

Sounds OK, but I'd try and size match it, 1N4148 size devices tended not
to be of the 1W rating when I was repairing this stuff though, more like
300mW.

Matching by eye (which I find generally reliable), the original zener diode
matches up with the 500mW form factor.

 

[Winfield Hill]

GPG wrote...
Correction, my datasheet is from Unitrode.

page 7 http://www.metatech.com.hk/datasheet/aic/dcdc_pdf/AIC3843.pdf

I suspect that's AIC's poorly drawn symbol for a Schottky diode.
It's commonly used to insure that no improper substrate current
flow can be induced by a fast-switching-induced negative gate-
voltage spike. You can use any old 1n5817, etc., for the task.

Thanks,
- Win

(email: use hill_at_rowland-dot-org for now)

 

[DaveC]

The Zener is referenced in one of the UC384x datasheets (Not
necessarily Unitrode's) and the reasons for it's existence are
explained, IIRC you select the zener voltage to suit the FET but there
seemed to be two 'standard' values for this part, 15v and 18v whichever
PSU you opened.

I've got 4 UC3843 data sheets, and none say anything about zeners (except an
internal reference voltage for the IC), and they all use the USD1120 shottky
diode at the gate of the FET.

I'd like to see the explanation (although all I'm doing is replacing what the
circuit designer put in there originally), out of curiosity. Which data sheet
are you looking at?

Thanks,