Back emf when unplugging coil?

[George Herold]

Hi all, I've got a coil driver 3 amps max. coils are ~10 ohms and I'd have
to look up the inductance... big open air coil.
So a user had a problem and it looks like the pass transistor blew.
(2N6059... I did this design ~15-20 years ago.) The circuit is a
simple opamp voltage to current converter. And has never had problems
before (that I know of.)* So the tech at the school who is replacing the
transistor was speculating that maybe the students unplugged the coil while
it was under power and the back emf blew out the pass transistor. Playing
around on the bench I see very little back emf.. a few volts at most.
I do see some sparks when hot plugging the coil in and out (with 36 V supply
voltage.) But I think this is just arcs from the power supply... it looks
to be the same size spark when plugging in or out.

Anyway I'm not sure what happened, and is there something obvious
I might not be thinking about?

George H.


*One can always worry that it failed on someone and then they never asked
for help, but just stopped using it.

 

On Wednesday, 10 July 2019 21:16:10 UTC+1, George Herold wrote:

Hi all, I've got a coil driver 3 amps max. coils are ~10 ohms and I'd have
to look up the inductance... big open air coil.
So a user had a problem and it looks like the pass transistor blew.
(2N6059... I did this design ~15-20 years ago.) The circuit is a
simple opamp voltage to current converter. And has never had problems
before (that I know of.)* So the tech at the school who is replacing the
transistor was speculating that maybe the students unplugged the coil while
it was under power and the back emf blew out the pass transistor. Playing
around on the bench I see very little back emf.. a few volts at most.
I do see some sparks when hot plugging the coil in and out (with 36 V supply
voltage.) But I think this is just arcs from the power supply... it looks
to be the same size spark when plugging in or out.

Anyway I'm not sure what happened, and is there something obvious
I might not be thinking about?

George H.


*One can always worry that it failed on someone and then they never asked
for help, but just stopped using it.

We don't know your circuit.
I could make a wild finger in the air guess, but it probably won't be what's going on.
Why not...
Tr switches off softly enough not to cause excessive kickback spikes
Coil is unplugged while tr on.
Coil sparks to connector, but by this time tr is fully off. Result: >1kV on tr.

Hopefully you've got enough protection in place though, and it's just a random part failure.


NT

 

[whit3rd]

On Wednesday, July 10, 2019 at 1:16:10 PM UTC-7, George Herold wrote:

Hi all, I've got a coil driver 3 amps max. coils are ~10 ohms and I'd have
to look up the inductance... big open air coil.
So a user had a problem and it looks like the pass transistor blew.

... speculating that maybe the students unplugged the coil while
it was under power and the back emf blew out the pass transistor. Playing
around on the bench I see very little back emf.. a few volts at most.

The stray capacitance on the coil terminals, and variations in 'bounce' times,
make for a lot of non-repeatability here. Wiring a tiny MOV on the coil-side
connector would be an easy fix, if it WERE possible to store enough
coil energy to make a transistor go into runaway.

More likely, an RF source and accidental resonance might put inconvenient
AC into the transistor. Were there transmitters nearby, and can you
grid-dip the assembly to find its resonances?

 

[John Larkin]

On Wed, 10 Jul 2019 13:16:06 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

Hi all, I've got a coil driver 3 amps max. coils are ~10 ohms and I'd have
to look up the inductance... big open air coil.
So a user had a problem and it looks like the pass transistor blew.
(2N6059... I did this design ~15-20 years ago.) The circuit is a
simple opamp voltage to current converter. And has never had problems
before (that I know of.)* So the tech at the school who is replacing the
transistor was speculating that maybe the students unplugged the coil while
it was under power and the back emf blew out the pass transistor. Playing
around on the bench I see very little back emf.. a few volts at most.
I do see some sparks when hot plugging the coil in and out (with 36 V supply
voltage.) But I think this is just arcs from the power supply... it looks
to be the same size spark when plugging in or out.

Anyway I'm not sure what happened, and is there something obvious
I might not be thinking about?

George H.


*One can always worry that it failed on someone and then they never asked
for help, but just stopped using it.

Does the transistor get very hot? It could dissipate about 30 watts.




--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[George Herold]

On Wednesday, July 10, 2019 at 4:34:30 PM UTC-4, tabb...@gmail.com wrote:

On Wednesday, 10 July 2019 21:16:10 UTC+1, George Herold wrote:
Hi all, I've got a coil driver 3 amps max. coils are ~10 ohms and I'd have
to look up the inductance... big open air coil.
So a user had a problem and it looks like the pass transistor blew.
(2N6059... I did this design ~15-20 years ago.) The circuit is a
simple opamp voltage to current converter. And has never had problems
before (that I know of.)* So the tech at the school who is replacing the
transistor was speculating that maybe the students unplugged the coil while
it was under power and the back emf blew out the pass transistor. Playing
around on the bench I see very little back emf.. a few volts at most.
I do see some sparks when hot plugging the coil in and out (with 36 V supply
voltage.) But I think this is just arcs from the power supply... it looks
to be the same size spark when plugging in or out.

Anyway I'm not sure what happened, and is there something obvious
I might not be thinking about?

George H.


*One can always worry that it failed on someone and then they never asked
for help, but just stopped using it.

We don't know your circuit.
I could make a wild finger in the air guess, but it probably won't be what's going on.
Why not...
Tr switches off softly enough not to cause excessive kickback spikes
Coil is unplugged while tr on.
Coil sparks to connector, but by this time tr is fully off. Result: >1kV on tr.

Hopefully you've got enough protection in place though, and it's just a random part failure.


NT

Right... opamp through 5k ohm feeds base, npn darlington,
collector to V+ emitter to coil (and 10 ohm series C.. good size
alum. electro, in parallel. Zobel compensation) then 0.5 ohm I_sense resistor to ground.
There's zero compensation on the feedback network...
I was much younger, electronically then. seems to work fine,
good enough is often good enough.

George H.

 

[George Herold]

On Wednesday, July 10, 2019 at 5:51:24 PM UTC-4, John Larkin wrote:

On Wed, 10 Jul 2019 13:16:06 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

Hi all, I've got a coil driver 3 amps max. coils are ~10 ohms and I'd have
to look up the inductance... big open air coil.
So a user had a problem and it looks like the pass transistor blew.
(2N6059... I did this design ~15-20 years ago.) The circuit is a
simple opamp voltage to current converter. And has never had problems
before (that I know of.)* So the tech at the school who is replacing the
transistor was speculating that maybe the students unplugged the coil while
it was under power and the back emf blew out the pass transistor. Playing
around on the bench I see very little back emf.. a few volts at most.
I do see some sparks when hot plugging the coil in and out (with 36 V supply
voltage.) But I think this is just arcs from the power supply... it looks
to be the same size spark when plugging in or out.

Anyway I'm not sure what happened, and is there something obvious
I might not be thinking about?

George H.


*One can always worry that it failed on someone and then they never asked
for help, but just stopped using it.

Does the transistor get very hot? It could dissipate about 30 watts.

That is my guess. I didn't tell you* that for over ~1 amp I've got
a switch that lets you plug in an external DC supply in the back.
We use to supply a 36V 3A Kenwood... went away,
so now a 60V 3A Volteq.. I'll ask the tech and prof what PS
they have in the lab.

Thermally I haven't tested it... Tomorrow,
(in the voice of foghorn leghorn)
"Fortunately, I still have product on the shelf that
can be used for testing."

George H.

*Phil A. please don't yell for me not telling everything up front.

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[John Larkin]

On Wed, 10 Jul 2019 16:36:44 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

On Wednesday, July 10, 2019 at 5:51:24 PM UTC-4, John Larkin wrote:
On Wed, 10 Jul 2019 13:16:06 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

Hi all, I've got a coil driver 3 amps max. coils are ~10 ohms and I'd have
to look up the inductance... big open air coil.
So a user had a problem and it looks like the pass transistor blew.
(2N6059... I did this design ~15-20 years ago.) The circuit is a
simple opamp voltage to current converter. And has never had problems
before (that I know of.)* So the tech at the school who is replacing the
transistor was speculating that maybe the students unplugged the coil while
it was under power and the back emf blew out the pass transistor. Playing
around on the bench I see very little back emf.. a few volts at most.
I do see some sparks when hot plugging the coil in and out (with 36 V supply
voltage.) But I think this is just arcs from the power supply... it looks
to be the same size spark when plugging in or out.

Anyway I'm not sure what happened, and is there something obvious
I might not be thinking about?

George H.


*One can always worry that it failed on someone and then they never asked
for help, but just stopped using it.

Does the transistor get very hot? It could dissipate about 30 watts.


That is my guess. I didn't tell you* that for over ~1 amp I've got
a switch that lets you plug in an external DC supply in the back.
We use to supply a 36V 3A Kenwood... went away,
so now a 60V 3A Volteq.. I'll ask the tech and prof what PS
they have in the lab.

Thermally I haven't tested it... Tomorrow,
(in the voice of foghorn leghorn)
"Fortunately, I still have product on the shelf that
can be used for testing."

George H.

*Phil A. please don't yell for me not telling everything up front.

What's the heat sinking like on the transistor?

Post a pic!

A 60V supply and a 10 ohm load peaks at about 90 watts in the
transistor.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[George Herold]

On Wednesday, July 10, 2019 at 7:51:14 PM UTC-4, John Larkin wrote:

On Wed, 10 Jul 2019 16:36:44 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Wednesday, July 10, 2019 at 5:51:24 PM UTC-4, John Larkin wrote:
On Wed, 10 Jul 2019 13:16:06 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

Hi all, I've got a coil driver 3 amps max. coils are ~10 ohms and I'd have
to look up the inductance... big open air coil.
So a user had a problem and it looks like the pass transistor blew.
(2N6059... I did this design ~15-20 years ago.) The circuit is a
simple opamp voltage to current converter. And has never had problems
before (that I know of.)* So the tech at the school who is replacing the
transistor was speculating that maybe the students unplugged the coil while
it was under power and the back emf blew out the pass transistor. Playing
around on the bench I see very little back emf.. a few volts at most.
I do see some sparks when hot plugging the coil in and out (with 36 V supply
voltage.) But I think this is just arcs from the power supply... it looks
to be the same size spark when plugging in or out.

Anyway I'm not sure what happened, and is there something obvious
I might not be thinking about?

George H.


*One can always worry that it failed on someone and then they never asked
for help, but just stopped using it.

Does the transistor get very hot? It could dissipate about 30 watts.


That is my guess. I didn't tell you* that for over ~1 amp I've got
a switch that lets you plug in an external DC supply in the back.
We use to supply a 36V 3A Kenwood... went away,
so now a 60V 3A Volteq.. I'll ask the tech and prof what PS
they have in the lab.

Thermally I haven't tested it... Tomorrow,
(in the voice of foghorn leghorn)
"Fortunately, I still have product on the shelf that
can be used for testing."

George H.

*Phil A. please don't yell for me not telling everything up front.

What's the heat sinking like on the transistor?

Post a pic!

A 60V supply and a 10 ohm load peaks at about 90 watts in the
transistor.

Right! there's an Al heat sink onto a big brass back panel,
(with holes in the top) and inside a fan pushing air out the bottom.

With the 36V supply I think I got the whole thing up to 60-70C.
(worst case)

With a 60 V supply that's going to be ~3-4 times! that's looking bad.
Maybe there will be burn marks where the brass back plate
attaches to the wooden box. :^)
It could be something else,

George H.

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[George Herold]

On Wednesday, July 10, 2019 at 7:51:14 PM UTC-4, John Larkin wrote:

On Wed, 10 Jul 2019 16:36:44 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Wednesday, July 10, 2019 at 5:51:24 PM UTC-4, John Larkin wrote:
On Wed, 10 Jul 2019 13:16:06 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

Hi all, I've got a coil driver 3 amps max. coils are ~10 ohms and I'd have
to look up the inductance... big open air coil.
So a user had a problem and it looks like the pass transistor blew.
(2N6059... I did this design ~15-20 years ago.) The circuit is a
simple opamp voltage to current converter. And has never had problems
before (that I know of.)* So the tech at the school who is replacing the
transistor was speculating that maybe the students unplugged the coil while
it was under power and the back emf blew out the pass transistor. Playing
around on the bench I see very little back emf.. a few volts at most.
I do see some sparks when hot plugging the coil in and out (with 36 V supply
voltage.) But I think this is just arcs from the power supply... it looks
to be the same size spark when plugging in or out.

Anyway I'm not sure what happened, and is there something obvious
I might not be thinking about?

George H.


*One can always worry that it failed on someone and then they never asked
for help, but just stopped using it.

Does the transistor get very hot? It could dissipate about 30 watts.


That is my guess. I didn't tell you* that for over ~1 amp I've got
a switch that lets you plug in an external DC supply in the back.
We use to supply a 36V 3A Kenwood... went away,
so now a 60V 3A Volteq.. I'll ask the tech and prof what PS
they have in the lab.

Thermally I haven't tested it... Tomorrow,
(in the voice of foghorn leghorn)
"Fortunately, I still have product on the shelf that
can be used for testing."

George H.

*Phil A. please don't yell for me not telling everything up front.

What's the heat sinking like on the transistor?

Post a pic!

I can't now,
looks to be a 641A wakefield HS
https://www.westfloridacomponents.com/G523APF06/TO3+Heatsink+641A+Metal+Cased+Semiconductors+Wakefield.html

onto a brass plate, ~0.10" thick, 12" x 8"

The fan mostly cools stuff inside and I don't think,
does much to the back panel and heat sink.

GH

A 60V supply and a 10 ohm load peaks at about 90 watts in the
transistor.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[George Herold]

On Wednesday, July 10, 2019 at 8:50:14 PM UTC-4, John Larkin wrote:

On Wed, 10 Jul 2019 17:36:49 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Wednesday, July 10, 2019 at 7:51:14 PM UTC-4, John Larkin wrote:
On Wed, 10 Jul 2019 16:36:44 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Wednesday, July 10, 2019 at 5:51:24 PM UTC-4, John Larkin wrote:
On Wed, 10 Jul 2019 13:16:06 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

Hi all, I've got a coil driver 3 amps max. coils are ~10 ohms and I'd have
to look up the inductance... big open air coil.
So a user had a problem and it looks like the pass transistor blew.
(2N6059... I did this design ~15-20 years ago.) The circuit is a
simple opamp voltage to current converter. And has never had problems
before (that I know of.)* So the tech at the school who is replacing the
transistor was speculating that maybe the students unplugged the coil while
it was under power and the back emf blew out the pass transistor. Playing
around on the bench I see very little back emf.. a few volts at most.
I do see some sparks when hot plugging the coil in and out (with 36 V supply
voltage.) But I think this is just arcs from the power supply... it looks
to be the same size spark when plugging in or out.

Anyway I'm not sure what happened, and is there something obvious
I might not be thinking about?

George H.


*One can always worry that it failed on someone and then they never asked
for help, but just stopped using it.

Does the transistor get very hot? It could dissipate about 30 watts.


That is my guess. I didn't tell you* that for over ~1 amp I've got
a switch that lets you plug in an external DC supply in the back.
We use to supply a 36V 3A Kenwood... went away,
so now a 60V 3A Volteq.. I'll ask the tech and prof what PS
they have in the lab.

Thermally I haven't tested it... Tomorrow,
(in the voice of foghorn leghorn)
"Fortunately, I still have product on the shelf that
can be used for testing."

George H.

*Phil A. please don't yell for me not telling everything up front.

What's the heat sinking like on the transistor?

Post a pic!
I can't now,
looks to be a 641A wakefield HS
https://www.westfloridacomponents.com/G523APF06/TO3+Heatsink+641A+Metal+Cased+Semiconductors+Wakefield.html

onto a brass plate, ~0.10" thick, 12" x 8"

The fan mostly cools stuff inside and I don't think,
does much to the back panel and heat sink.

GH

You (or actually, your customer) are likely frying the transistor.

That heat skin is a toy. An infinite sheet of 1/8" thick aluminum will
be about 2 K/W, so a thinner non-infinite sheet of brass will be
worse, wild guess 5-ish. 90 watts * 5 K/W = 450C above ambient.

Yeah, all the extra voltage of the new PS is across my poor
transistor. I'll fry some tomorrow. I'll stick a thermal couple
on the top of the to-3, post numbers.
If things didn't keep changing....

George H.

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[John Larkin]

On Wed, 10 Jul 2019 17:36:49 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

On Wednesday, July 10, 2019 at 7:51:14 PM UTC-4, John Larkin wrote:
On Wed, 10 Jul 2019 16:36:44 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Wednesday, July 10, 2019 at 5:51:24 PM UTC-4, John Larkin wrote:
On Wed, 10 Jul 2019 13:16:06 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

Hi all, I've got a coil driver 3 amps max. coils are ~10 ohms and I'd have
to look up the inductance... big open air coil.
So a user had a problem and it looks like the pass transistor blew.
(2N6059... I did this design ~15-20 years ago.) The circuit is a
simple opamp voltage to current converter. And has never had problems
before (that I know of.)* So the tech at the school who is replacing the
transistor was speculating that maybe the students unplugged the coil while
it was under power and the back emf blew out the pass transistor. Playing
around on the bench I see very little back emf.. a few volts at most.
I do see some sparks when hot plugging the coil in and out (with 36 V supply
voltage.) But I think this is just arcs from the power supply... it looks
to be the same size spark when plugging in or out.

Anyway I'm not sure what happened, and is there something obvious
I might not be thinking about?

George H.


*One can always worry that it failed on someone and then they never asked
for help, but just stopped using it.

Does the transistor get very hot? It could dissipate about 30 watts.


That is my guess. I didn't tell you* that for over ~1 amp I've got
a switch that lets you plug in an external DC supply in the back.
We use to supply a 36V 3A Kenwood... went away,
so now a 60V 3A Volteq.. I'll ask the tech and prof what PS
they have in the lab.

Thermally I haven't tested it... Tomorrow,
(in the voice of foghorn leghorn)
"Fortunately, I still have product on the shelf that
can be used for testing."

George H.

*Phil A. please don't yell for me not telling everything up front.

What's the heat sinking like on the transistor?

Post a pic!
I can't now,
looks to be a 641A wakefield HS
https://www.westfloridacomponents.com/G523APF06/TO3+Heatsink+641A+Metal+Cased+Semiconductors+Wakefield.html

onto a brass plate, ~0.10" thick, 12" x 8"

The fan mostly cools stuff inside and I don't think,
does much to the back panel and heat sink.

GH

You (or actually, your customer) are likely frying the transistor.

That heat skin is a toy. An infinite sheet of 1/8" thick aluminum will
be about 2 K/W, so a thinner non-infinite sheet of brass will be
worse, wild guess 5-ish. 90 watts * 5 K/W = 450C above ambient.


--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

[Tim Williams]

No, the EMF is generated across the unplugged part, which appears across the
source plus the arc just described. In fact, the source thinks there is
less voltage across the load, because the impedance is rising.

Could be any number of things, vulnerable design (1-2 decades ago, who knows
right?..), ESD, mains surge, fat fingers shorting out transistors, ...??

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

"George Herold" <gherold@teachspin.com> wrote in message
news:29e507c8-62e9-4b76-88fb-bc7e7a5eb5ff@googlegroups.com...

Hi all, I've got a coil driver 3 amps max. coils are ~10 ohms and I'd have
to look up the inductance... big open air coil.
So a user had a problem and it looks like the pass transistor blew.
(2N6059... I did this design ~15-20 years ago.) The circuit is a
simple opamp voltage to current converter. And has never had problems
before (that I know of.)* So the tech at the school who is replacing the
transistor was speculating that maybe the students unplugged the coil
while
it was under power and the back emf blew out the pass transistor. Playing
around on the bench I see very little back emf.. a few volts at most.
I do see some sparks when hot plugging the coil in and out (with 36 V
supply
voltage.) But I think this is just arcs from the power supply... it looks
to be the same size spark when plugging in or out.

Anyway I'm not sure what happened, and is there something obvious
I might not be thinking about?

George H.


*One can always worry that it failed on someone and then they never asked
for help, but just stopped using it.

 

How the hell does a coil UNPLUGGED hurt anything else ?

 

[John Robertson]

On 2019/07/10 6:28 p.m., George Herold wrote:

On Wednesday, July 10, 2019 at 8:50:14 PM UTC-4, John Larkin wrote:
On Wed, 10 Jul 2019 17:36:49 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Wednesday, July 10, 2019 at 7:51:14 PM UTC-4, John Larkin wrote:
On Wed, 10 Jul 2019 16:36:44 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Wednesday, July 10, 2019 at 5:51:24 PM UTC-4, John Larkin wrote:
On Wed, 10 Jul 2019 13:16:06 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

Hi all, I've got a coil driver 3 amps max. coils are ~10 ohms and I'd have
to look up the inductance... big open air coil.
So a user had a problem and it looks like the pass transistor blew.
(2N6059... I did this design ~15-20 years ago.) The circuit is a
simple opamp voltage to current converter. And has never had problems
before (that I know of.)* So the tech at the school who is replacing the
transistor was speculating that maybe the students unplugged the coil while
it was under power and the back emf blew out the pass transistor. Playing
around on the bench I see very little back emf.. a few volts at most.
I do see some sparks when hot plugging the coil in and out (with 36 V supply
voltage.) But I think this is just arcs from the power supply... it looks
to be the same size spark when plugging in or out.

Anyway I'm not sure what happened, and is there something obvious
I might not be thinking about?

George H.


*One can always worry that it failed on someone and then they never asked
for help, but just stopped using it.

Does the transistor get very hot? It could dissipate about 30 watts.


That is my guess. I didn't tell you* that for over ~1 amp I've got
a switch that lets you plug in an external DC supply in the back.
We use to supply a 36V 3A Kenwood... went away,
so now a 60V 3A Volteq.. I'll ask the tech and prof what PS
they have in the lab.

Thermally I haven't tested it... Tomorrow,
(in the voice of foghorn leghorn)
"Fortunately, I still have product on the shelf that
can be used for testing."

George H.

*Phil A. please don't yell for me not telling everything up front.

What's the heat sinking like on the transistor?

Post a pic!
I can't now,
looks to be a 641A wakefield HS
https://www.westfloridacomponents.com/G523APF06/TO3+Heatsink+641A+Metal+Cased+Semiconductors+Wakefield.html

onto a brass plate, ~0.10" thick, 12" x 8"

The fan mostly cools stuff inside and I don't think,
does much to the back panel and heat sink.

GH

You (or actually, your customer) are likely frying the transistor.

That heat skin is a toy. An infinite sheet of 1/8" thick aluminum will
be about 2 K/W, so a thinner non-infinite sheet of brass will be
worse, wild guess 5-ish. 90 watts * 5 K/W = 450C above ambient.

Yeah, all the extra voltage of the new PS is across my poor
transistor. I'll fry some tomorrow. I'll stick a thermal couple
on the top of the to-3, post numbers.
If things didn't keep changing....

George H.

What is the Duty Cycle of the coil? If momentary then the transistor
should be just fine with a wee bit of thought - 2N6059s were used for
Gottlieb pinball Pop Bumper driver boards for over a decade and they
lasted until something else failed and took the transistor along with
it. These were triggering coils of perhaps 5 ohms across 24VDC to
ground. The pulse would perhaps be 500ms. The coils always had back EMF
diodes installed.

These pop bumper driver boards had NO heat sink installed. Very short
duty cycle even during play...

I expect that much of the risk to the transistor could be removed by
simply adding a diode across the Emitter and Collector of the transistor
to protect it against momentary back EMF. If the coil was unplugged
while powered then contact noise would be enough to send a few hundred
volts of back EMF through the transistor and as a 2N6059 is only rated
around 100PIV. There is a back diode in the 2N6059, but putting an
external 1N4004 (or higher PIV) would probably protect it even better.
Another diode across the coil would be worth looking into as well. Can't
have too many when dealing with back-EMF!

John :-#)#
--
(Please post followups or tech inquiries to the USENET newsgroup)
John's Jukes Ltd.
MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
(604)872-5757 (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."

 

On Thursday, 11 July 2019 01:50:14 UTC+1, John Larkin wrote:

You (or actually, your customer) are likely frying the transistor.

That heat skin is a toy. An infinite sheet of 1/8" thick aluminum will
be about 2 K/W, so a thinner non-infinite sheet of brass will be
worse, wild guess 5-ish. 90 watts * 5 K/W = 450C above ambient.

You're saying it like it's a bad thing


NT

 

On Thursday, 11 July 2019 02:49:23 UTC+1, jurb...@gmail.com wrote:
> How the hell does a coil UNPLUGGED hurt anything else ?

Put your hand on one & try it. You will regret it.

 

[George Herold]

On Thursday, July 11, 2019 at 2:48:25 AM UTC-4, John Robertson wrote:

On 2019/07/10 6:28 p.m., George Herold wrote:
On Wednesday, July 10, 2019 at 8:50:14 PM UTC-4, John Larkin wrote:
On Wed, 10 Jul 2019 17:36:49 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Wednesday, July 10, 2019 at 7:51:14 PM UTC-4, John Larkin wrote:
On Wed, 10 Jul 2019 16:36:44 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Wednesday, July 10, 2019 at 5:51:24 PM UTC-4, John Larkin wrote:
On Wed, 10 Jul 2019 13:16:06 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

Hi all, I've got a coil driver 3 amps max. coils are ~10 ohms and I'd have
to look up the inductance... big open air coil.
So a user had a problem and it looks like the pass transistor blew.
(2N6059... I did this design ~15-20 years ago.) The circuit is a
simple opamp voltage to current converter. And has never had problems
before (that I know of.)* So the tech at the school who is replacing the
transistor was speculating that maybe the students unplugged the coil while
it was under power and the back emf blew out the pass transistor. Playing
around on the bench I see very little back emf.. a few volts at most.
I do see some sparks when hot plugging the coil in and out (with 36 V supply
voltage.) But I think this is just arcs from the power supply... it looks
to be the same size spark when plugging in or out.

Anyway I'm not sure what happened, and is there something obvious
I might not be thinking about?

George H.


*One can always worry that it failed on someone and then they never asked
for help, but just stopped using it.

Does the transistor get very hot? It could dissipate about 30 watts.


That is my guess. I didn't tell you* that for over ~1 amp I've got
a switch that lets you plug in an external DC supply in the back.
We use to supply a 36V 3A Kenwood... went away,
so now a 60V 3A Volteq.. I'll ask the tech and prof what PS
they have in the lab.

Thermally I haven't tested it... Tomorrow,
(in the voice of foghorn leghorn)
"Fortunately, I still have product on the shelf that
can be used for testing."

George H.

*Phil A. please don't yell for me not telling everything up front.

What's the heat sinking like on the transistor?

Post a pic!
I can't now,
looks to be a 641A wakefield HS
https://www.westfloridacomponents.com/G523APF06/TO3+Heatsink+641A+Metal+Cased+Semiconductors+Wakefield.html

onto a brass plate, ~0.10" thick, 12" x 8"

The fan mostly cools stuff inside and I don't think,
does much to the back panel and heat sink.

GH

You (or actually, your customer) are likely frying the transistor.

That heat skin is a toy. An infinite sheet of 1/8" thick aluminum will
be about 2 K/W, so a thinner non-infinite sheet of brass will be
worse, wild guess 5-ish. 90 watts * 5 K/W = 450C above ambient.

Yeah, all the extra voltage of the new PS is across my poor
transistor. I'll fry some tomorrow. I'll stick a thermal couple
on the top of the to-3, post numbers.
If things didn't keep changing....

George H.


What is the Duty Cycle of the coil? If momentary then the transistor
should be just fine with a wee bit of thought - 2N6059s were used for
Gottlieb pinball Pop Bumper driver boards for over a decade and they
lasted until something else failed and took the transistor along with
it. These were triggering coils of perhaps 5 ohms across 24VDC to
ground. The pulse would perhaps be 500ms. The coils always had back EMF
diodes installed.

The coils are run CW. I was thinking of a diode across the output...
(but if the problem is over heating a diode doesn't help. :^)

George H.

These pop bumper driver boards had NO heat sink installed. Very short
duty cycle even during play...

I expect that much of the risk to the transistor could be removed by
simply adding a diode across the Emitter and Collector of the transistor
to protect it against momentary back EMF. If the coil was unplugged
while powered then contact noise would be enough to send a few hundred
volts of back EMF through the transistor and as a 2N6059 is only rated
around 100PIV. There is a back diode in the 2N6059, but putting an
external 1N4004 (or higher PIV) would probably protect it even better.
Another diode across the coil would be worth looking into as well. Can't
have too many when dealing with back-EMF!

John :-#)#
--
(Please post followups or tech inquiries to the USENET newsgroup)
John's Jukes Ltd.
MOVED to #7 - 3979 Marine Way, Burnaby, BC, Canada V5J 5E3
(604)872-5757 (Pinballs, Jukes, Video Games)
www.flippers.com
"Old pinballers never die, they just flip out."

 

[George Herold]

On Wednesday, July 10, 2019 at 9:28:12 PM UTC-4, George Herold wrote:

On Wednesday, July 10, 2019 at 8:50:14 PM UTC-4, John Larkin wrote:
On Wed, 10 Jul 2019 17:36:49 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Wednesday, July 10, 2019 at 7:51:14 PM UTC-4, John Larkin wrote:
On Wed, 10 Jul 2019 16:36:44 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

On Wednesday, July 10, 2019 at 5:51:24 PM UTC-4, John Larkin wrote:
On Wed, 10 Jul 2019 13:16:06 -0700 (PDT), George Herold
gherold@teachspin.com> wrote:

Hi all, I've got a coil driver 3 amps max. coils are ~10 ohms and I'd have
to look up the inductance... big open air coil.
So a user had a problem and it looks like the pass transistor blew.
(2N6059... I did this design ~15-20 years ago.) The circuit is a
simple opamp voltage to current converter. And has never had problems
before (that I know of.)* So the tech at the school who is replacing the
transistor was speculating that maybe the students unplugged the coil while
it was under power and the back emf blew out the pass transistor. Playing
around on the bench I see very little back emf.. a few volts at most.
I do see some sparks when hot plugging the coil in and out (with 36 V supply
voltage.) But I think this is just arcs from the power supply... it looks
to be the same size spark when plugging in or out.

Anyway I'm not sure what happened, and is there something obvious
I might not be thinking about?

George H.


*One can always worry that it failed on someone and then they never asked
for help, but just stopped using it.

Does the transistor get very hot? It could dissipate about 30 watts.


That is my guess. I didn't tell you* that for over ~1 amp I've got
a switch that lets you plug in an external DC supply in the back.
We use to supply a 36V 3A Kenwood... went away,
so now a 60V 3A Volteq.. I'll ask the tech and prof what PS
they have in the lab.

Thermally I haven't tested it... Tomorrow,
(in the voice of foghorn leghorn)
"Fortunately, I still have product on the shelf that
can be used for testing."

George H.

*Phil A. please don't yell for me not telling everything up front.

What's the heat sinking like on the transistor?

Post a pic!
I can't now,
looks to be a 641A wakefield HS
https://www.westfloridacomponents.com/G523APF06/TO3+Heatsink+641A+Metal+Cased+Semiconductors+Wakefield.html

onto a brass plate, ~0.10" thick, 12" x 8"

The fan mostly cools stuff inside and I don't think,
does much to the back panel and heat sink.

GH

You (or actually, your customer) are likely frying the transistor.

That heat skin is a toy. An infinite sheet of 1/8" thick aluminum will
be about 2 K/W, so a thinner non-infinite sheet of brass will be
worse, wild guess 5-ish. 90 watts * 5 K/W = 450C above ambient.

Yeah, all the extra voltage of the new PS is across my poor
transistor. I'll fry some tomorrow. I'll stick a thermal couple
on the top of the to-3, post numbers.
If things didn't keep changing....

George H.

Hi all, Welp there is more than just thermal stress that is a problem.
I put a thermal couple on the transistor case.

V_supply I_coil V_coil Temp case
30 V 1.1 A 12.3 51.5 C
40 V 1.78 A 20 V 68.7 C
50 V 2.16 A 25 V 89.9 C

I then turned up the voltage ~52 V and it broke.

Here's the schematic
https://www.dropbox.com/s/1t7qeq4bw6ab9ob/3A%20cur-src.JPG?dl=0

Well duh! The poor LT1013 can only take a 44 V supply. But I powered
down, replaced the transistor and it works again. So it's more complicated
than just the opamp breaking. The opamp got flaky and that broke the transistor?

Anyway there's not much to do but put out a warning to users not to exceed
40 V on the input. (There's a warning, IN BOLD, in the manual saying as
much, but no one reads the manual.) Maybe I can send out stickers to
put on the back panel?

George H.

--

John Larkin Highland Technology, Inc
picosecond timing precision measurement

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

 

Here's the schematic
https://www.dropbox.com/s/1t7qeq4bw6ab9ob/3A%20cur-src.JPG?dl=0

I don't want to be too presumptive here but it seems like that thing would not be all that efficient. Unless you are playing some cool tricks with that coil I see too much across it. Like ten ohms and a 330uF ? I think that's alot. I can only assume you had reasons for ding it.

 

[George Herold]

On Thursday, July 11, 2019 at 8:10:11 PM UTC-4, jurb...@gmail.com wrote:

Here's the schematic
https://www.dropbox.com/s/1t7qeq4bw6ab9ob/3A%20cur-src.JPG?dl=0

I don't want to be too presumptive here but it seems like that thing would not be all that efficient. Unless you are playing some cool tricks with that coil I see too much across it. Like ten ohms and a 330uF ? I think that's alot. I can only assume you had reasons for ding it.

Oh that's the 'Zobel' network, makes the inductor look like a resistor
to the drive circuit. We can estimate the inductance from that,
(equal time constants) RC=L/R; L = C*R^2 (.33mF*100 ohm^2) = 33 mH
Someone asked about the inductance up stream...
it's probably a bit more than that.

The RHS turns an led on when the supply falls out of compliance.
(measures c-e voltage.)

George H.