over voltage protection

Bill Wright · Mar 7, 2014

Can any kind person advise me of a simple way to provide over-voltage
protection on a 12V DC PSU? The current drawn by the equipment is approx
1A. I've looked at MOVs but they don't seem to protect below about 30V,
which I think might be higher than than the PSU output when the PSU
regulation has failed. Ideally I suppose I'd like something that would
provide protection at about 20V.

Bill

Phil Hobbs · Mar 7, 2014

On 03/07/2014 09:22 AM, Bill Wright wrote:

Can any kind person advise me of a simple way to provide over-voltage
protection on a 12V DC PSU? The current drawn by the equipment is approx
1A. I've looked at MOVs but they don't seem to protect below about 30V,
which I think might be higher than than the PSU output when the PSU
regulation has failed. Ideally I suppose I'd like something that would
provide protection at about 20V.

Bill

The usual method is a polyfuse in series followed by a transzorb to
ground. Use a through-hole polyfuse--their switching behaviour is much
more predictable. Tyco makes laminated combinations of the two, called
PolyZens, which are nice because the dissipation in the transzorb makes
the polyfuse switch much faster and more accurately.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

RobertMacy · Mar 7, 2014

On Fri, 07 Mar 2014 07:22:16 -0700, Bill Wright <bill@invalid.com> wrote:

Can any kind person advise me of a simple way to provide over-voltage
protection on a 12V DC PSU? The current drawn by the equipment is approx
1A. I've looked at MOVs but they don't seem to protect below about 30V,
which I think might be higher than than the PSU output when the PSU
regulation has failed. Ideally I suppose I'd like something that would
provide protection at about 20V.

Bill

Are you 'shorting out' excess? Or, 'opening up' to excess?

Stating 1A sounds like you want a 'crowbar' above 12Vdc

If your supply current limits, you might have a problem. If your supply
folds back current you can use an appropriate [and cheap] tranzorb.
They're fast. Probably digikey or such

You may get away with a raw zener, but don't recommend, because they're
slow to turn on. relatively.

Don't ever recommend MOV, since they're rated for 'lifetime of operations.
In other words, they disappear AS YOU USE THEM!

Phil Hobbs · Mar 7, 2014

On 03/07/2014 10:47 AM, RobertMacy wrote:

On Fri, 07 Mar 2014 07:22:16 -0700, Bill Wright <bill@invalid.com> wrote:

Can any kind person advise me of a simple way to provide over-voltage
protection on a 12V DC PSU? The current drawn by the equipment is
approx 1A. I've looked at MOVs but they don't seem to protect below
about 30V, which I think might be higher than than the PSU output when
the PSU regulation has failed. Ideally I suppose I'd like something
that would provide protection at about 20V.

Bill

Are you 'shorting out' excess? Or, 'opening up' to excess?

Stating 1A sounds like you want a 'crowbar' above 12Vdc

If your supply current limits, you might have a problem. If your supply
folds back current you can use an appropriate [and cheap] tranzorb.
They're fast. Probably digikey or such

You may get away with a raw zener, but don't recommend, because they're
slow to turn on. relatively.

Don't ever recommend MOV, since they're rated for 'lifetime of
operations. In other words, they disappear AS YOU USE THEM!

Zeners aren't slow, certainly not the avalanche ones (i.e. those whose
knees are above about 6 volts). TVS devices are sub-nanosecond.

Avalanche breakdown generates carriers in situ, so you don't have to
wait for them to diffuse through undepleted silicon as in normal PN
junctions.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

John Fields · Mar 8, 2014

On Fri, 07 Mar 2014 14:22:16 +0000, Bill Wright <bill@invalid.com>
wrote:

Can any kind person advise me of a simple way to provide over-voltage
protection on a 12V DC PSU? The current drawn by the equipment is approx
1A. I've looked at MOVs but they don't seem to protect below about 30V,
which I think might be higher than than the PSU output when the PSU
regulation has failed. Ideally I suppose I'd like something that would
provide protection at about 20V.

---
If the PSU output goes to 20V, what do you want to have happen?

John Fields

Bill Wright · Mar 8, 2014

John Fields wrote:

On Fri, 07 Mar 2014 14:22:16 +0000, Bill Wright <bill@invalid.com
wrote:

Can any kind person advise me of a simple way to provide over-voltage
protection on a 12V DC PSU? The current drawn by the equipment is approx
1A. I've looked at MOVs but they don't seem to protect below about 30V,
which I think might be higher than than the PSU output when the PSU
regulation has failed. Ideally I suppose I'd like something that would
provide protection at about 20V.

---
If the PSU output goes to 20V, what do you want to have happen?

John Fields

This is to protect a LED array. The LEDs will withstand 25V for a short
time (I don't know how long but I tried a sample LED and it was OK for
the 10 seconds I tried it. The PSU is rated at 3.3A but the load is only
1A, so I thought crowbar protection following a 2A quickblow fuse would
be OK.

Bill

Bill Wright · Mar 8, 2014

RobertMacy wrote:

On Fri, 07 Mar 2014 07:22:16 -0700, Bill Wright <bill@invalid.com> wrote:

Can any kind person advise me of a simple way to provide over-voltage
protection on a 12V DC PSU? The current drawn by the equipment is
approx 1A. I've looked at MOVs but they don't seem to protect below
about 30V, which I think might be higher than than the PSU output when
the PSU regulation has failed. Ideally I suppose I'd like something
that would provide protection at about 20V.

Bill

Are you 'shorting out' excess? Or, 'opening up' to excess?

Stating 1A sounds like you want a 'crowbar' above 12Vdc

That's what I assumed, from my position of ignorance.

Bill

RobertMacy · Mar 8, 2014

On Fri, 07 Mar 2014 09:14:06 -0700, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

...snip....
Zeners aren't slow, certainly not the avalanche ones (i.e. those whose
knees are above about 6 volts). TVS devices are sub-nanosecond.

Avalanche breakdown generates carriers in situ, so you don't have to
wait for them to diffuse through undepleted silicon as in normal PN
junctions.

Cheers

Phil Hobbs

I know you've said this before. And even had an explanation for why I had
terrible +/-12 volt square wave from using them in feedback of an Opamp.

But tell me again, I don't understand how I could barely get audio speed
square wave [without horrible overshoot spikes because the zeners weren't
coming on fast enough]. I didn't get into it much other than hearing from
other EMC and microwave people to not use zeners [confirmed by my
experience trying to make that sq wave generator] They said the zeners
turn on too slowly compared to products DESIGNED to turn on fast.

Phil Hobbs · Mar 8, 2014

On 3/7/2014 3:48 PM, RobertMacy wrote:

On Fri, 07 Mar 2014 09:14:06 -0700, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

...snip....
Zeners aren't slow, certainly not the avalanche ones (i.e. those whose
knees are above about 6 volts). TVS devices are sub-nanosecond.

Avalanche breakdown generates carriers in situ, so you don't have to
wait for them to diffuse through undepleted silicon as in normal PN
junctions.

Cheers

Phil Hobbs

I know you've said this before. And even had an explanation for why I
had terrible +/-12 volt square wave from using them in feedback of an
Opamp.

But tell me again, I don't understand how I could barely get audio speed
square wave [without horrible overshoot spikes because the zeners
weren't coming on fast enough]. I didn't get into it much other than
hearing from other EMC and microwave people to not use zeners [confirmed
by my experience trying to make that sq wave generator] They said the
zeners turn on too slowly compared to products DESIGNED to turn on fast.

I think you had a PN rectifier in series with the zener, and it was the
rectifier that was being slow.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

Jim Thompson · Mar 8, 2014

On Fri, 07 Mar 2014 10:47:19 -0500, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 03/07/2014 09:22 AM, Bill Wright wrote:
Can any kind person advise me of a simple way to provide over-voltage
protection on a 12V DC PSU? The current drawn by the equipment is approx
1A. I've looked at MOVs but they don't seem to protect below about 30V,
which I think might be higher than than the PSU output when the PSU
regulation has failed. Ideally I suppose I'd like something that would
provide protection at about 20V.

Bill

The usual method is a polyfuse in series followed by a transzorb to
ground. Use a through-hole polyfuse--their switching behaviour is much
more predictable. Tyco makes laminated combinations of the two, called
PolyZens, which are nice because the dissipation in the transzorb makes
the polyfuse switch much faster and more accurately.

Cheers

Phil Hobbs

Remove M1 from this...

<http://www.analog-innovations.com/SED/OverAndReverseVoltageProtection.pdf>

Adjust R2/R1 ratio for 20V, or whatever you want.

Add series R from the junction of R3, R4 to the cathode of the TL431
to keep it from frying.

Increase R4 & R5 to ~5.1K (not critical).

Listen to Larkin whine >:-}

...Jim Thompson
--
| James E.Thompson | mens |
| Analog Innovations | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| San Tan Valley, AZ 85142 Skype: Contacts Only | |
| Voice

Michael Black · Mar 8, 2014

On Fri, 7 Mar 2014, Bill Wright wrote:

RobertMacy wrote:
On Fri, 07 Mar 2014 07:22:16 -0700, Bill Wright <bill@invalid.com> wrote:

Can any kind person advise me of a simple way to provide over-voltage
protection on a 12V DC PSU? The current drawn by the equipment is approx
1A. I've looked at MOVs but they don't seem to protect below about 30V,
which I think might be higher than than the PSU output when the PSU
regulation has failed. Ideally I suppose I'd like something that would
provide protection at about 20V.

Bill

Are you 'shorting out' excess? Or, 'opening up' to excess?

Stating 1A sounds like you want a 'crowbar' above 12Vdc

That's what I assumed, from my position of ignorance.

The supply does't have a crowbar already?

A good one would. I remember bringing home endless switching supplies
from the local surplus store, and they all generally had a crowbar on
them, despite being various brands and models.

I'd hope a supply in a computer had some sort of protection.

I haven't actually looked at the switching supplies now used as "ac
adapters" to know if they include some protection.

Michael

Bill Wright · Mar 8, 2014

Thanks to those who responded. It was very helpful.

Bill

RobertMacy · Mar 8, 2014

On Fri, 07 Mar 2014 16:12:49 -0700, Phil Hobbs <hobbs@electrooptical.net>
wrote:

...snip....
I think you had a PN rectifier in series with the zener, and it was the
rectifier that was being slow.

Cheers

Phil Hobbs

From memory, when we used those 'slow' zeners for overvoltage protection,
we used to keep the zener slightly on with a bias current, so when you
needed it, it was already working. had HS blocking diode between zener and
circuitry. That sped it up the overvoltage protection clamp quite a bit.

The sq wave generator used two zeners 'facing' each other, using the
'built-in' forward diode. like a 'balanced' tranzorb.

Just how slow can forward conducting diode be? THAT slow? You mean, you
can get a huge forward voltage across the zener's junction before it
finally comes on? But it's really fast in the breakdown direction? That
means back to back diodes in the feedback path, albeit small output
voltage, would have done the same?

Maybe should have used a FW high speed rectifier [4 pack] in series with a
single zener? Would have been symmetrical for sure.

Any App Notes, or White Papers on the subject you can send me a 'specific'
URL that desribe that slow forward and fast breakdown?

Phil Hobbs · Mar 9, 2014

On 3/8/2014 9:02 AM, RobertMacy wrote:

On Fri, 07 Mar 2014 16:12:49 -0700, Phil Hobbs
hobbs@electrooptical.net> wrote:

...snip....
I think you had a PN rectifier in series with the zener, and it was
the rectifier that was being slow.

Cheers

Phil Hobbs

From memory, when we used those 'slow' zeners for overvoltage
protection, we used to keep the zener slightly on with a bias current,
so when you needed it, it was already working. had HS blocking diode
between zener and circuitry. That sped it up the overvoltage protection
clamp quite a bit.

The sq wave generator used two zeners 'facing' each other, using the
'built-in' forward diode. like a 'balanced' tranzorb.

Just how slow can forward conducting diode be? THAT slow? You mean, you
can get a huge forward voltage across the zener's junction before it
finally comes on? But it's really fast in the breakdown direction? That
means back to back diodes in the feedback path, albeit small output
voltage, would have done the same?

Maybe should have used a FW high speed rectifier [4 pack] in series with
a single zener? Would have been symmetrical for sure.

Any App Notes, or White Papers on the subject you can send me a
'specific' URL that desribe that slow forward and fast breakdown?

Forward conduction in PN diodes is slow, all right--there are app notes
on things like SMPS failures caused by huge turn-on overshoots, e.g.
http://www.linear.com/docs/27403. The reason is that the thick, highly
doped contact region shields out all the E field, so that carriers have
to diffuse their way out of that before they get to the depletion zone
and feel the electric field. Once the concentration gradient is
established, they conduct fine.

Avalanche breakdown is quick, as in avalanche transistor pulsers and
avalanche photodiodes. The carriers get generated right there in the
depletion zone, where there's no waiting. You can measure it by looking
at the modulation of the transmitted light. This paper claims that it
works up to >~ 10 GHz in silicon, which is reasonable considering how
fast avalanche PDs and transistors are.

http://ieeexplore.ieee.org/iel5/55/29365/01327716.pdf

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

John Fields · Mar 9, 2014

On Sat, 08 Mar 2014 07:02:02 -0700, RobertMacy
<robert.a.macy@gmail.com> wrote:

On Fri, 07 Mar 2014 16:12:49 -0700, Phil Hobbs <hobbs@electrooptical.net
wrote:

...snip....
I think you had a PN rectifier in series with the zener, and it was the
rectifier that was being slow.

Cheers

Phil Hobbs

From memory, when we used those 'slow' zeners for overvoltage protection,
we used to keep the zener slightly on with a bias current, so when you
needed it, it was already working. had HS blocking diode between zener and
circuitry. That sped it up the overvoltage protection clamp quite a bit.

The sq wave generator used two zeners 'facing' each other, using the
'built-in' forward diode. like a 'balanced' tranzorb.

Just how slow can forward conducting diode be? THAT slow? You mean, you
can get a huge forward voltage across the zener's junction before it
finally comes on? But it's really fast in the breakdown direction? That
means back to back diodes in the feedback path, albeit small output
voltage, would have done the same?

Maybe should have used a FW high speed rectifier [4 pack] in series with a
single zener? Would have been symmetrical for sure.

Any App Notes, or White Papers on the subject you can send me a 'specific'
URL that desribe that slow forward and fast breakdown?

---
http://www.onsemi.com/pub_link/Collateral/HBD854-D.PDF

Page 27.

John Fields

Tim Wescott · Mar 9, 2014

On Fri, 07 Mar 2014 14:22:16 +0000, Bill Wright wrote:

Can any kind person advise me of a simple way to provide over-voltage
protection on a 12V DC PSU? The current drawn by the equipment is approx
1A. I've looked at MOVs but they don't seem to protect below about 30V,
which I think might be higher than than the PSU output when the PSU
regulation has failed. Ideally I suppose I'd like something that would
provide protection at about 20V.

Bill

Why didn't anyone mention a using a Zener to turn on an SCR?

Just a honkin' big Zener after a fuse should do it, yes?

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com

Jim Thompson · Mar 9, 2014

On Sat, 08 Mar 2014 14:19:46 -0600, Tim Wescott
<tim@seemywebsite.please> wrote:

On Fri, 07 Mar 2014 14:22:16 +0000, Bill Wright wrote:

Can any kind person advise me of a simple way to provide over-voltage
protection on a 12V DC PSU? The current drawn by the equipment is approx
1A. I've looked at MOVs but they don't seem to protect below about 30V,
which I think might be higher than than the PSU output when the PSU
regulation has failed. Ideally I suppose I'd like something that would
provide protection at about 20V.

Bill

Why didn't anyone mention a using a Zener to turn on an SCR?

Just a honkin' big Zener after a fuse should do it, yes?

over voltage protection

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