Forums

Back emf when unplugging coil?

Started by George Herold July 10, 2019
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
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
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?
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.
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
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
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
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
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