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Back emf when unplugging coil?

Started by George Herold July 10, 2019
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
How the hell does a coil UNPLUGGED hurt anything else ? 
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.
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.
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."
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.
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.