Crowbars...

[Cursitor Doom]

Gentlemen,

I need to protect some expensive RF transistors from over-voltage.
They\'re fed from a linear supply which is notorious for shorting its
pass transistors and putting 42V across these devices instead of the
required 28.5V. Until such time as I get around to finding a better
PSU, I thought the simplest expedient would be to rig up a
straightforward crowbar circuit based on sensing the voltage on the
wiper of a preset resistor in a divider chain between the 28.5V and
GND. Would this be okay for votage sensing? I don\'t really want to get
involved with op amps and precision voltage references unless it\'s
really necessary (but if it is, then so be it as these trannies aren\'t
cheap and are becoming hard to come by).

CD.

 

[John Walliker]

On Sunday, 23 July 2023 at 10:39:55 UTC+1, Cursitor Doom wrote:

Gentlemen,

I need to protect some expensive RF transistors from over-voltage.
They\'re fed from a linear supply which is notorious for shorting its
pass transistors and putting 42V across these devices instead of the
required 28.5V. Until such time as I get around to finding a better
PSU, I thought the simplest expedient would be to rig up a
straightforward crowbar circuit based on sensing the voltage on the
wiper of a preset resistor in a divider chain between the 28.5V and
GND. Would this be okay for votage sensing? I don\'t really want to get
involved with op amps and precision voltage references unless it\'s
really necessary (but if it is, then so be it as these trannies aren\'t
cheap and are becoming hard to come by).

CD.

You probably need a fast response, so a thyristor crowbar may be too slow.
Maybe a TL431 and a beefy PNP or PMOS transistor to augment the current
handling.
John

 

[Jan Panteltje]

as ten vltage does not rise spo fast.
On a sunny day (Sun, 23 Jul 2023 03:00:03 -0700 (PDT)) it happened John
Walliker <jrwalliker@gmail.com> wrote in
<e3f3b152-a776-4809-8d8f-c808dfac9131n@googlegroups.com>:

On Sunday, 23 July 2023 at 10:39:55 UTC+1, Cursitor Doom wrote:
Gentlemen,

I need to protect some expensive RF transistors from over-voltage.
They\'re fed from a linear supply which is notorious for shorting its
pass transistors and putting 42V across these devices instead of the
required 28.5V. Until such time as I get around to finding a better
PSU, I thought the simplest expedient would be to rig up a
straightforward crowbar circuit based on sensing the voltage on the
wiper of a preset resistor in a divider chain between the 28.5V and
GND. Would this be okay for votage sensing? I don\'t really want to get
involved with op amps and precision voltage references unless it\'s
really necessary (but if it is, then so be it as these trannies aren\'t
cheap and are becoming hard to come by).

CD.

You probably need a fast response, so a thyristor crowbar may be too slow.
Maybe a TL431 and a beefy PNP or PMOS transistor to augment the current
handling.
John

Thyristor crowbar should work if there are any big filter caps
as then voltage does not rise so fast.
Some inductors present in the down switcher circuit would limit rise-time to.
I have seen thyristor crowbars used in color TV power supplies.

 

[Lasse Langwadt Christensen]

søndag den 23. juli 2023 kl. 11.39.55 UTC+2 skrev Cursitor Doom:

Gentlemen,

I need to protect some expensive RF transistors from over-voltage.
They\'re fed from a linear supply which is notorious for shorting its
pass transistors and putting 42V across these devices instead of the
required 28.5V. Until such time as I get around to finding a better
PSU, I thought the simplest expedient would be to rig up a
straightforward crowbar circuit based on sensing the voltage on the
wiper of a preset resistor in a divider chain between the 28.5V and
GND. Would this be okay for votage sensing? I don\'t really want to get
involved with op amps and precision voltage references unless it\'s
really necessary (but if it is, then so be it as these trannies aren\'t
cheap and are becoming hard to come by).

how much current do they need and how much over voltage can they handle?

 

[John Larkin]

On Sun, 23 Jul 2023 10:39:44 +0100, Cursitor Doom <cd@notformail.com>
wrote:

Gentlemen,

I need to protect some expensive RF transistors from over-voltage.
They\'re fed from a linear supply which is notorious for shorting its
pass transistors and putting 42V across these devices instead of the
required 28.5V. Until such time as I get around to finding a better
PSU, I thought the simplest expedient would be to rig up a
straightforward crowbar circuit based on sensing the voltage on the
wiper of a preset resistor in a divider chain between the 28.5V and
GND. Would this be okay for votage sensing? I don\'t really want to get
involved with op amps and precision voltage references unless it\'s
really necessary (but if it is, then so be it as these trannies aren\'t
cheap and are becoming hard to come by).

CD.

I recently saw a Diodes Inc chip that is a combination overcurrent
fuse and overvoltage limiter.

But fix the bad supply! Or add yet another regulator after it.

A giant SCR crowbar would be easy. A zener and an SCR. And a fuse
maybe.

What RF transistor is it?

 

[server]

On Sun, 23 Jul 2023 04:50:53 -0700, John Larkin
<jlarkin@highlandSNIPMEtechnology.com> wrote:

On Sun, 23 Jul 2023 10:39:44 +0100, Cursitor Doom <cd@notformail.com
wrote:

Gentlemen,

I need to protect some expensive RF transistors from over-voltage.
They\'re fed from a linear supply which is notorious for shorting its
pass transistors and putting 42V across these devices instead of the
required 28.5V. Until such time as I get around to finding a better
PSU, I thought the simplest expedient would be to rig up a
straightforward crowbar circuit based on sensing the voltage on the
wiper of a preset resistor in a divider chain between the 28.5V and
GND. Would this be okay for votage sensing? I don\'t really want to get
involved with op amps and precision voltage references unless it\'s
really necessary (but if it is, then so be it as these trannies aren\'t
cheap and are becoming hard to come by).

CD.

I recently saw a Diodes Inc chip that is a combination overcurrent
fuse and overvoltage limiter.

But fix the bad supply! Or add yet another regulator after it.

A giant SCR crowbar would be easy. A zener and an SCR. And a fuse
maybe.

What RF transistor is it?

If the RF transistors are feeding a nearby antenna, make sure the
crowbar triggering circuit does not pick up some of the radiated RF.
Such random RF pickup could fire the SCR, shutting down the power
supply .

After a few blown fuses, I replaced the fuse with some soldering tin.
As an additional bonus I also got a smell indication that the crowbar
had operated. Moving the antenna outside solved the nuisance
triggering problem.

 

[Cursitor Doom]

On Sun, 23 Jul 2023 04:50:53 -0700, John Larkin
<jlarkin@highlandSNIPMEtechnology.com> wrote:

On Sun, 23 Jul 2023 10:39:44 +0100, Cursitor Doom <cd@notformail.com
wrote:

Gentlemen,

I need to protect some expensive RF transistors from over-voltage.
They\'re fed from a linear supply which is notorious for shorting its
pass transistors and putting 42V across these devices instead of the
required 28.5V. Until such time as I get around to finding a better
PSU, I thought the simplest expedient would be to rig up a
straightforward crowbar circuit based on sensing the voltage on the
wiper of a preset resistor in a divider chain between the 28.5V and
GND. Would this be okay for votage sensing? I don\'t really want to get
involved with op amps and precision voltage references unless it\'s
really necessary (but if it is, then so be it as these trannies aren\'t
cheap and are becoming hard to come by).

CD.

I recently saw a Diodes Inc chip that is a combination overcurrent
fuse and overvoltage limiter.

But fix the bad supply! Or add yet another regulator after it.

A giant SCR crowbar would be easy. A zener and an SCR. And a fuse
maybe.

What RF transistor is it?

MRF 485. Motorola don\'t make \'em any more. There was a company called
Eleflow that was re-manufacturing some obsolete semis including this
one, but they seem to have gone out of business unfortunately.

 

[Cursitor Doom]

On Sun, 23 Jul 2023 04:35:29 -0700 (PDT), Lasse Langwadt Christensen
<langwadt@fonz.dk> wrote:

søndag den 23. juli 2023 kl. 11.39.55 UTC+2 skrev Cursitor Doom:
Gentlemen,

I need to protect some expensive RF transistors from over-voltage.
They\'re fed from a linear supply which is notorious for shorting its
pass transistors and putting 42V across these devices instead of the
required 28.5V. Until such time as I get around to finding a better
PSU, I thought the simplest expedient would be to rig up a
straightforward crowbar circuit based on sensing the voltage on the
wiper of a preset resistor in a divider chain between the 28.5V and
GND. Would this be okay for votage sensing? I don\'t really want to get
involved with op amps and precision voltage references unless it\'s
really necessary (but if it is, then so be it as these trannies aren\'t
cheap and are becoming hard to come by).


how much current do they need and how much over voltage can they handle?

12A. Vceo max is 35V.

 

[Eddy Lee]

On Sunday, July 23, 2023 at 6:21:16 AM UTC-7, Cursitor Doom wrote:

On Sun, 23 Jul 2023 04:35:29 -0700 (PDT), Lasse Langwadt Christensen
lang...@fonz.dk> wrote:

søndag den 23. juli 2023 kl. 11.39.55 UTC+2 skrev Cursitor Doom:
Gentlemen,

I need to protect some expensive RF transistors from over-voltage.
They\'re fed from a linear supply which is notorious for shorting its
pass transistors and putting 42V across these devices instead of the
required 28.5V. Until such time as I get around to finding a better
PSU, I thought the simplest expedient would be to rig up a
straightforward crowbar circuit based on sensing the voltage on the
wiper of a preset resistor in a divider chain between the 28.5V and
GND. Would this be okay for votage sensing? I don\'t really want to get
involved with op amps and precision voltage references unless it\'s
really necessary (but if it is, then so be it as these trannies aren\'t
cheap and are becoming hard to come by).

.
how much current do they need and how much over voltage can they handle?
12A. Vceo max is 35V.

I would put in some heavy duty zeners and fuses. OV almost always kills zeners and shorting them and taking out the fuses. Having bunch of them in parallel can make sure at least one of them is shorted.

I am using plenty of 12V 1A zeners and 3A/5A fuses in my project.

 

[Lasse Langwadt Christensen]

søndag den 23. juli 2023 kl. 15.21.16 UTC+2 skrev Cursitor Doom:

On Sun, 23 Jul 2023 04:35:29 -0700 (PDT), Lasse Langwadt Christensen
lang...@fonz.dk> wrote:

søndag den 23. juli 2023 kl. 11.39.55 UTC+2 skrev Cursitor Doom:
Gentlemen,

I need to protect some expensive RF transistors from over-voltage.
They\'re fed from a linear supply which is notorious for shorting its
pass transistors and putting 42V across these devices instead of the
required 28.5V. Until such time as I get around to finding a better
PSU, I thought the simplest expedient would be to rig up a
straightforward crowbar circuit based on sensing the voltage on the
wiper of a preset resistor in a divider chain between the 28.5V and
GND. Would this be okay for votage sensing? I don\'t really want to get
involved with op amps and precision voltage references unless it\'s
really necessary (but if it is, then so be it as these trannies aren\'t
cheap and are becoming hard to come by).


how much current do they need and how much over voltage can they handle?
12A. Vceo max is 35V.

https://www.m0spn.co.uk/2016/10/24/over-voltage-protection/

not overly accurate but you have +5V to work with
might want a zener for the short time it takes the relay to open

 

[Fred Bloggs]

On Sunday, July 23, 2023 at 5:39:55 AM UTC-4, Cursitor Doom wrote:

Gentlemen,

I need to protect some expensive RF transistors from over-voltage.
They\'re fed from a linear supply which is notorious for shorting its
pass transistors and putting 42V across these devices instead of the
required 28.5V. Until such time as I get around to finding a better
PSU, I thought the simplest expedient would be to rig up a
straightforward crowbar circuit based on sensing the voltage on the
wiper of a preset resistor in a divider chain between the 28.5V and
GND. Would this be okay for votage sensing? I don\'t really want to get
involved with op amps and precision voltage references unless it\'s
really necessary (but if it is, then so be it as these trannies aren\'t
cheap and are becoming hard to come by).

CD.

Assuming this is off-the-line linear with transformer stepdown, crowbar the line via an optotrigger on the load side of the fuse. Select triac with I^2T in excess of the fuse. Your mains wiring should provide adequate current limiting. You\'ll need to strobe your optotrigger with an adjustable threshold comparator monitoring the 28.5V line. For starters, triggering at 7.5% up or 30.6V should be safe from both supply drift, transients, and underprotection.

Crowbaring the line allows you to use lower energy clamps because all they\'ll be doing is taking the discharge from 42V input filter capacitors of the supply. The fairly massive overcurrent of the crowbar should open a reasonably sized fuse in under a millisecond is all I can predict. If you use something like this gem in series:

https://www.vishay.com/docs/73010/sup90p06-09l.pdf

you can just switch off the 28.5V feed line.

 

[Fred Bloggs]

On Sunday, July 23, 2023 at 10:49:12 AM UTC-4, Lasse Langwadt Christensen wrote:

søndag den 23. juli 2023 kl. 15.21.16 UTC+2 skrev Cursitor Doom:
On Sun, 23 Jul 2023 04:35:29 -0700 (PDT), Lasse Langwadt Christensen
lang...@fonz.dk> wrote:

søndag den 23. juli 2023 kl. 11.39.55 UTC+2 skrev Cursitor Doom:
Gentlemen,

I need to protect some expensive RF transistors from over-voltage.
They\'re fed from a linear supply which is notorious for shorting its
pass transistors and putting 42V across these devices instead of the
required 28.5V. Until such time as I get around to finding a better
PSU, I thought the simplest expedient would be to rig up a
straightforward crowbar circuit based on sensing the voltage on the
wiper of a preset resistor in a divider chain between the 28.5V and
GND. Would this be okay for votage sensing? I don\'t really want to get
involved with op amps and precision voltage references unless it\'s
really necessary (but if it is, then so be it as these trannies aren\'t
cheap and are becoming hard to come by).


how much current do they need and how much over voltage can they handle?
12A. Vceo max is 35V.
https://www.m0spn.co.uk/2016/10/24/over-voltage-protection/

not overly accurate but you have +5V to work with
might want a zener for the short time it takes the relay to open

Play circuit for pre-teen hobbyists...

 

[Cursitor Doom]

On Sun, 23 Jul 2023 07:49:06 -0700 (PDT), Lasse Langwadt Christensen
<langwadt@fonz.dk> wrote:

søndag den 23. juli 2023 kl. 15.21.16 UTC+2 skrev Cursitor Doom:
On Sun, 23 Jul 2023 04:35:29 -0700 (PDT), Lasse Langwadt Christensen
lang...@fonz.dk> wrote:

søndag den 23. juli 2023 kl. 11.39.55 UTC+2 skrev Cursitor Doom:
Gentlemen,

I need to protect some expensive RF transistors from over-voltage.
They\'re fed from a linear supply which is notorious for shorting its
pass transistors and putting 42V across these devices instead of the
required 28.5V. Until such time as I get around to finding a better
PSU, I thought the simplest expedient would be to rig up a
straightforward crowbar circuit based on sensing the voltage on the
wiper of a preset resistor in a divider chain between the 28.5V and
GND. Would this be okay for votage sensing? I don\'t really want to get
involved with op amps and precision voltage references unless it\'s
really necessary (but if it is, then so be it as these trannies aren\'t
cheap and are becoming hard to come by).


how much current do they need and how much over voltage can they handle?
12A. Vceo max is 35V.

https://www.m0spn.co.uk/2016/10/24/over-voltage-protection/

not overly accurate but you have +5V to work with
might want a zener for the short time it takes the relay to open

Sorry, I wasn\'t particularly clear at all in my initial message. I
won\'t be using a relay. If an over-voltage condition arises, I plan to
short the supply out with a MOSFET and blow a F12.5A fuse to protect
the downstream circuitry. I think a relay would be too slow in this
app.

 

[Cursitor Doom]

On Sun, 23 Jul 2023 08:13:31 -0700 (PDT), Fred Bloggs
<bloggs.fredbloggs.fred@gmail.com> wrote:

On Sunday, July 23, 2023 at 5:39:55?AM UTC-4, Cursitor Doom wrote:
Gentlemen,

I need to protect some expensive RF transistors from over-voltage.
They\'re fed from a linear supply which is notorious for shorting its
pass transistors and putting 42V across these devices instead of the
required 28.5V. Until such time as I get around to finding a better
PSU, I thought the simplest expedient would be to rig up a
straightforward crowbar circuit based on sensing the voltage on the
wiper of a preset resistor in a divider chain between the 28.5V and
GND. Would this be okay for votage sensing? I don\'t really want to get
involved with op amps and precision voltage references unless it\'s
really necessary (but if it is, then so be it as these trannies aren\'t
cheap and are becoming hard to come by).

CD.

Assuming this is off-the-line linear with transformer stepdown, crowbar the line via an optotrigger on the load side of the fuse. Select triac with I^2T in excess of the fuse. Your mains wiring should provide adequate current limiting. You\'ll need to strobe your optotrigger with an adjustable threshold comparator monitoring the 28.5V line. For starters, triggering at 7.5% up or 30.6V should be safe from both supply drift, transients, and underprotection.

Crowbaring the line allows you to use lower energy clamps because all they\'ll be doing is taking the discharge from 42V input filter capacitors of the supply. The fairly massive overcurrent of the crowbar should open a reasonably sized fuse in under a millisecond is all I can predict. If you use something like this gem in series:

https://www.vishay.com/docs/73010/sup90p06-09l.pdf

you can just switch off the 28.5V feed line.

That\'s all very fine, Fred, but as I said, I\'m looking for a simple
expedient here. It\'s only a temporary measure until I manage to source
a better PSU (Kenwood shame on you!)

 

[TTman]

On 23/07/2023 10:39, Cursitor Doom wrote:

Gentlemen,

I need to protect some expensive RF transistors from over-voltage.
They\'re fed from a linear supply which is notorious for shorting its
pass transistors and putting 42V across these devices instead of the
required 28.5V. Until such time as I get around to finding a better
PSU, I thought the simplest expedient would be to rig up a
straightforward crowbar circuit based on sensing the voltage on the
wiper of a preset resistor in a divider chain between the 28.5V and
GND. Would this be okay for votage sensing? I don\'t really want to get
involved with op amps and precision voltage references unless it\'s
really necessary (but if it is, then so be it as these trannies aren\'t
cheap and are becoming hard to come by).

CD.
Transorb ???

--
This email has been checked for viruses by Avast antivirus software.
www.avast.com

 

[Ralph Mowery]

In article <dp4qbi5nihq1fokl0fb2jun9n4e3s0a5kr@4ax.com>,
jlarkin@highlandSNIPMEtechnology.com says...

A giant SCR crowbar would be easy. A zener and an SCR. And a fuse
maybe.

I vote for that all so for a quick fix. Simple and no adjustment.

 

[John Larkin]

On Sun, 23 Jul 2023 15:20:10 -0400, Ralph Mowery
<rmowery42@charter.net> wrote:

In article <dp4qbi5nihq1fokl0fb2jun9n4e3s0a5kr@4ax.com>,
jlarkin@highlandSNIPMEtechnology.com says...

A giant SCR crowbar would be easy. A zener and an SCR. And a fuse
maybe.




I vote for that all so for a quick fix. Simple and no adjustment.

Soft triggering an SCR, from a zener, is a potential failure mode, but
the SCR would fail shorted, which is better than blowing out the RF
stuff.

 

[Phil Allison]

Cursitor Doom wrote:
------------------------------------

how much current do they need and how much over voltage can they handle?

12A. Vceo max is 35V.

** The Vceo rating applies when the base and emitter terminals are left open circuit.
However, the maker\'s published \" Vcer \" rating is 60V and applies when a low resistance exists between base and emitter ( < 100ohms) and is usually the same as the Vcb rating.
This does not change things much because in RF power output operation the max Vce on the transistor is likely to approach or even exceed 2 times the DC supply.

....... Phil

 

[Cursitor Doom]

On Sun, 23 Jul 2023 16:52:29 -0700, John Larkin
<jlarkin@highlandSNIPMEtechnology.com> wrote:

On Sun, 23 Jul 2023 15:20:10 -0400, Ralph Mowery
rmowery42@charter.net> wrote:

In article <dp4qbi5nihq1fokl0fb2jun9n4e3s0a5kr@4ax.com>,
jlarkin@highlandSNIPMEtechnology.com says...

A giant SCR crowbar would be easy. A zener and an SCR. And a fuse
maybe.




I vote for that all so for a quick fix. Simple and no adjustment.

Soft triggering an SCR, from a zener, is a potential failure mode, but
the SCR would fail shorted, which is better than blowing out the RF
stuff.

Well, by a strange coincidence I only recently bought a load of diacs
which breakover at 32V. I could pick the lowest breakover device out
of the bunch and maybe shave a volt or so off of that and that sorts
out the soft-trigger problem as well.

 

[Eddy Lee]

On Monday, July 24, 2023 at 1:23:00 AM UTC-7, Cursitor Doom wrote:

On Sun, 23 Jul 2023 16:52:29 -0700, John Larkin
jla...@highlandSNIPMEtechnology.com> wrote:

On Sun, 23 Jul 2023 15:20:10 -0400, Ralph Mowery
rmow...@charter.net> wrote:

In article <dp4qbi5nihq1fokl0...@4ax.com>,
jla...@highlandSNIPMEtechnology.com says...

A giant SCR crowbar would be easy. A zener and an SCR. And a fuse
maybe.




I vote for that all so for a quick fix. Simple and no adjustment.

Soft triggering an SCR, from a zener, is a potential failure mode, but
the SCR would fail shorted, which is better than blowing out the RF
stuff.
Well, by a strange coincidence I only recently bought a load of diacs
which breakover at 32V. I could pick the lowest breakover device out
of the bunch and maybe shave a volt or so off of that and that sorts
out the soft-trigger problem as well.

By the same strange coincidence, i brought a load of 13V zeners. They are too high to protect Lithium batteries, since I really want 12.5V. Two in series would clamp it at 26V. Zener is the same as Diacs without the negative cycle that you don\'t really need. There are many voltages to choose from as well. 12V and 13V are very common, but not 12.5V, unfortunately.