Emulating Open-Collector operation with TTL 74LS138......

[John Robertson]

On 2022/10/06 7:50 a.m., John Larkin wrote:

On Wed, 5 Oct 2022 23:24:26 -0700, John Robertson <jrr@flippers.com
wrote:

Have a circuit that needs repair. Uses a TIP125 (NPN, Darlington)
emitter tied to Vbb (~20VDC). Collector to load, then to ground. 1K
resistor (R1) pullup on Base to Vbb, and a second 1KR (R2) to the TTL
controller.

That\'s not clear. Got a schematic?

Yes:

https://www.flippers.com/images/delet/74LS138_Lamp_Driver.png

Think going to a 7445 sub-board will be best way around this. It was the
designer who made an error on using the 138 and I am simply trying to
rescue 5 populated boards...blame the Scottish side of my family!

John :-#)#

Problem is of course, that R2 puts the ~20VDC to the TTL output gate on
a 74LS138.

Trying to solve this without a driver transistor. The circuit is for a
1ms ~20V strobe pulse repeated every ten ms.

I thought of putting a 10ufd cap in series with R2, but don\'t like
electrolytics as they fail after a few thousand hours.

Possible to use a 15V or so Zener Diode, but the Vbb is not regulated so
that won\'t work reliably.

Anyone have a single component in mind that will essentially emulate
(isolate) an Open-Collector output for the 138?

Thanks!

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.\"

 

[Phil Hobbs]

John Robertson wrote:

On 2022/10/06 4:41 a.m., Phil Hobbs wrote:
John Robertson wrote:
Have a circuit that needs repair. Uses a TIP125 (NPN, Darlington)
emitter tied to Vbb (~20VDC). Collector to load, then to ground. 1K
resistor (R1) pullup on Base to Vbb, and a second 1KR (R2) to the TTL
controller.

Problem is of course, that R2 puts the ~20VDC to the TTL output gate
on a 74LS138.

Trying to solve this without a driver transistor. The circuit is for
a 1ms ~20V strobe pulse repeated every ten ms.

I thought of putting a 10ufd cap in series with R2, but don\'t like
electrolytics as they fail after a few thousand hours.

Possible to use a 15V or so Zener Diode, but the Vbb is not regulated
so that won\'t work reliably.

Anyone have a single component in mind that will essentially emulate
(isolate) an Open-Collector output for the 138?

Thanks!

John :-#)#

Switch it to a 139 and use a 2N7002?

Cheers

Phil Hobbs


Well, this is to fix 5 x PCBs we are stuck with at the moment, next run
will fix the drives - either 7445s or 138 with P-Channel MOSFETs and
drivers.

Circuit I am trying to fix with fewest components - one x two legged
device preferred per 138 output:

https://www.flippers.com/images/delete/74LS138_Lamp_Driver.png

VLAMP is roughly 20VDC.

Thanks,

John :-#)#

The MOSFET cascode thing is pretty good actually, if you put a diode to
the supply or want to live dangerously and rely on the ESD protection to
discharge the gate. If it\'s a matter of using the boards or tossing
them, there\'s not much to lose by using barefoot FETs.

2N7000s are still around IIRC, and they\'re TO-92. There are
multiple-section devices such as the

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Lasse Langwadt Christensen]

torsdag den 6. oktober 2022 kl. 18.51.13 UTC+2 skrev Phil Hobbs:

John Robertson wrote:
On 2022/10/06 4:41 a.m., Phil Hobbs wrote:
John Robertson wrote:
Have a circuit that needs repair. Uses a TIP125 (NPN, Darlington)
emitter tied to Vbb (~20VDC). Collector to load, then to ground. 1K
resistor (R1) pullup on Base to Vbb, and a second 1KR (R2) to the TTL
controller.

Problem is of course, that R2 puts the ~20VDC to the TTL output gate
on a 74LS138.

Trying to solve this without a driver transistor. The circuit is for
a 1ms ~20V strobe pulse repeated every ten ms.

I thought of putting a 10ufd cap in series with R2, but don\'t like
electrolytics as they fail after a few thousand hours.

Possible to use a 15V or so Zener Diode, but the Vbb is not regulated
so that won\'t work reliably.

Anyone have a single component in mind that will essentially emulate
(isolate) an Open-Collector output for the 138?

Thanks!

John :-#)#

Switch it to a 139 and use a 2N7002?

Cheers

Phil Hobbs


Well, this is to fix 5 x PCBs we are stuck with at the moment, next run
will fix the drives - either 7445s or 138 with P-Channel MOSFETs and
drivers.

Circuit I am trying to fix with fewest components - one x two legged
device preferred per 138 output:

https://www.flippers.com/images/delete/74LS138_Lamp_Driver.png

VLAMP is roughly 20VDC.

Thanks,

John :-#)#
The MOSFET cascode thing is pretty good actually, if you put a diode to
the supply or want to live dangerously and rely on the ESD protection to
discharge the gate. If it\'s a matter of using the boards or tossing
them, there\'s not much to lose by using barefoot FETs.

\'138 is push-pull, though very wimpy high side

 

[Ricky]

On Thursday, October 6, 2022 at 12:44:21 PM UTC-4, John Robertson wrote:

On 2022/10/06 7:50 a.m., John Larkin wrote:
On Wed, 5 Oct 2022 23:24:26 -0700, John Robertson <j...@flippers.com
wrote:

Have a circuit that needs repair. Uses a TIP125 (NPN, Darlington)
emitter tied to Vbb (~20VDC). Collector to load, then to ground. 1K
resistor (R1) pullup on Base to Vbb, and a second 1KR (R2) to the TTL
controller.

That\'s not clear. Got a schematic?
Yes:

https://www.flippers.com/images/delet/74LS138_Lamp_Driver.png

Think going to a 7445 sub-board will be best way around this. It was the
designer who made an error on using the 138 and I am simply trying to
rescue 5 populated boards...blame the Scottish side of my family!
I still think you can make the capacitor idea work. It would need a cap and a pullup resistor for each output. Or you could use a Schottky diode to +5V instead of the resistor. That would make the recovery time shorter after a pulse and protect the 138 output.

--

Rick C.

++ Get 1,000 miles of free Supercharging
++ Tesla referral code - https://ts.la/richard11209

 

[Phil Hobbs]

Lasse Langwadt Christensen wrote:

torsdag den 6. oktober 2022 kl. 18.51.13 UTC+2 skrev Phil Hobbs:
John Robertson wrote:
On 2022/10/06 4:41 a.m., Phil Hobbs wrote:
John Robertson wrote:
Have a circuit that needs repair. Uses a TIP125 (NPN, Darlington)
emitter tied to Vbb (~20VDC). Collector to load, then to ground. 1K
resistor (R1) pullup on Base to Vbb, and a second 1KR (R2) to the TTL
controller.

Problem is of course, that R2 puts the ~20VDC to the TTL output gate
on a 74LS138.

Trying to solve this without a driver transistor. The circuit is for
a 1ms ~20V strobe pulse repeated every ten ms.

I thought of putting a 10ufd cap in series with R2, but don\'t like
electrolytics as they fail after a few thousand hours.

Possible to use a 15V or so Zener Diode, but the Vbb is not regulated
so that won\'t work reliably.

Anyone have a single component in mind that will essentially emulate
(isolate) an Open-Collector output for the 138?

Thanks!

John :-#)#

Switch it to a 139 and use a 2N7002?

Cheers

Phil Hobbs


Well, this is to fix 5 x PCBs we are stuck with at the moment, next run
will fix the drives - either 7445s or 138 with P-Channel MOSFETs and
drivers.

Circuit I am trying to fix with fewest components - one x two legged
device preferred per 138 output:

https://www.flippers.com/images/delete/74LS138_Lamp_Driver.png

VLAMP is roughly 20VDC.

Thanks,

John :-#)#
The MOSFET cascode thing is pretty good actually, if you put a diode to
the supply or want to live dangerously and rely on the ESD protection to
discharge the gate. If it\'s a matter of using the boards or tossing
them, there\'s not much to lose by using barefoot FETs.


\'138 is push-pull, though very wimpy high side

Sure, but anything above +4ish reverse-biases the upper output transistor.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[Phil Hobbs]

Lasse Langwadt Christensen wrote:

torsdag den 6. oktober 2022 kl. 20.04.09 UTC+2 skrev Phil Hobbs:
Lasse Langwadt Christensen wrote:
torsdag den 6. oktober 2022 kl. 18.51.13 UTC+2 skrev Phil Hobbs:
John Robertson wrote:
On 2022/10/06 4:41 a.m., Phil Hobbs wrote:
John Robertson wrote:
Have a circuit that needs repair. Uses a TIP125 (NPN, Darlington)
emitter tied to Vbb (~20VDC). Collector to load, then to ground. 1K
resistor (R1) pullup on Base to Vbb, and a second 1KR (R2) to the TTL
controller.

Problem is of course, that R2 puts the ~20VDC to the TTL output gate
on a 74LS138.

Trying to solve this without a driver transistor. The circuit is for
a 1ms ~20V strobe pulse repeated every ten ms.

I thought of putting a 10ufd cap in series with R2, but don\'t like
electrolytics as they fail after a few thousand hours.

Possible to use a 15V or so Zener Diode, but the Vbb is not regulated
so that won\'t work reliably.

Anyone have a single component in mind that will essentially emulate
(isolate) an Open-Collector output for the 138?

Thanks!

John :-#)#

Switch it to a 139 and use a 2N7002?

Cheers

Phil Hobbs


Well, this is to fix 5 x PCBs we are stuck with at the moment, next run
will fix the drives - either 7445s or 138 with P-Channel MOSFETs and
drivers.

Circuit I am trying to fix with fewest components - one x two legged
device preferred per 138 output:

https://www.flippers.com/images/delete/74LS138_Lamp_Driver.png

VLAMP is roughly 20VDC.

Thanks,

John :-#)#
The MOSFET cascode thing is pretty good actually, if you put a diode to
the supply or want to live dangerously and rely on the ESD protection to
discharge the gate. If it\'s a matter of using the boards or tossing
them, there\'s not much to lose by using barefoot FETs.


\'138 is push-pull, though very wimpy high side

Sure, but anything above +4ish reverse-biases the upper output transistor.
Cheers

with the upper transistor on does it ever get there?

Sure, it\'s an NPN emitter. If it were a NFET, it would be fine.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

 

[John Larkin]

On Thu, 6 Oct 2022 09:44:13 -0700, John Robertson <jrr@flippers.com>
wrote:

On 2022/10/06 7:50 a.m., John Larkin wrote:
On Wed, 5 Oct 2022 23:24:26 -0700, John Robertson <jrr@flippers.com
wrote:

Have a circuit that needs repair. Uses a TIP125 (NPN, Darlington)
emitter tied to Vbb (~20VDC). Collector to load, then to ground. 1K
resistor (R1) pullup on Base to Vbb, and a second 1KR (R2) to the TTL
controller.

That\'s not clear. Got a schematic?

Yes:

https://www.flippers.com/images/delet/74LS138_Lamp_Driver.png

404 error.

 

[John Robertson]

On 2022/10/06 12:41 p.m., John Larkin wrote:

On Thu, 6 Oct 2022 09:44:13 -0700, John Robertson <jrr@flippers.com
wrote:

On 2022/10/06 7:50 a.m., John Larkin wrote:
On Wed, 5 Oct 2022 23:24:26 -0700, John Robertson <jrr@flippers.com
wrote:

Have a circuit that needs repair. Uses a TIP125 (NPN, Darlington)
emitter tied to Vbb (~20VDC). Collector to load, then to ground. 1K
resistor (R1) pullup on Base to Vbb, and a second 1KR (R2) to the TTL
controller.

That\'s not clear. Got a schematic?

Yes:

https://www.flippers.com/images/delet/74LS138_Lamp_Driver.png


404 error.

sorry:

https://www.flippers.com/images/delete/74LS138_Lamp_Driver.png

Currently wire wrapping up a board to jam a 7445 in place of the 138.

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.\"

 

[Lasse Langwadt Christensen]

torsdag den 6. oktober 2022 kl. 18.35.18 UTC+2 skrev Ricky:

On Thursday, October 6, 2022 at 12:17:08 PM UTC-4, lang...@fonz.dk wrote:
torsdag den 6. oktober 2022 kl. 17.51.16 UTC+2 skrev Fred Bloggs:
On Thursday, October 6, 2022 at 11:37:46 AM UTC-4, lang...@fonz.dk wrote:
torsdag den 6. oktober 2022 kl. 17.29.21 UTC+2 skrev Fred Bloggs:
On Thursday, October 6, 2022 at 2:24:33 AM UTC-4, John Robertson wrote:
Have a circuit that needs repair. Uses a TIP125 (NPN, Darlington)
emitter tied to Vbb (~20VDC). Collector to load, then to ground. 1K
resistor (R1) pullup on Base to Vbb, and a second 1KR (R2) to the TTL
controller.

Problem is of course, that R2 puts the ~20VDC to the TTL output gate on
a 74LS138.

Trying to solve this without a driver transistor. The circuit is for a
1ms ~20V strobe pulse repeated every ten ms.

I thought of putting a 10ufd cap in series with R2, but don\'t like
electrolytics as they fail after a few thousand hours.

Possible to use a 15V or so Zener Diode, but the Vbb is not regulated so
that won\'t work reliably.

Anyone have a single component in mind that will essentially emulate
(isolate) an Open-Collector output for the 138?
The TIP125 is a complementary dual.
it is? https://www.onsemi.com/pdf/datasheet/tip120-d.pdf
Yeah it is. See Figure 1. All of that is in that TO-220 looking package.
and the three missing legs are where?

TIP120, TIP121, TIP122 (NPN); is the right drawing

TIP125, TIP126, TIP127 (PNP); is the left drawing
It would seem a combination of a high side PNP driver and a low side NPN level shifter would be a useful, single package device. Is there a reason why this is hard to make on a single die?

what package would you put it in? you\'d need more than 3 pins

I think the closest is arrays like UDN2981

 

[Fred Bloggs]

On Thursday, October 6, 2022 at 12:17:08 PM UTC-4, lang...@fonz.dk wrote:

torsdag den 6. oktober 2022 kl. 17.51.16 UTC+2 skrev Fred Bloggs:
On Thursday, October 6, 2022 at 11:37:46 AM UTC-4, lang...@fonz.dk wrote:
torsdag den 6. oktober 2022 kl. 17.29.21 UTC+2 skrev Fred Bloggs:
On Thursday, October 6, 2022 at 2:24:33 AM UTC-4, John Robertson wrote:
Have a circuit that needs repair. Uses a TIP125 (NPN, Darlington)
emitter tied to Vbb (~20VDC). Collector to load, then to ground. 1K
resistor (R1) pullup on Base to Vbb, and a second 1KR (R2) to the TTL
controller.

Problem is of course, that R2 puts the ~20VDC to the TTL output gate on
a 74LS138.

Trying to solve this without a driver transistor. The circuit is for a
1ms ~20V strobe pulse repeated every ten ms.

I thought of putting a 10ufd cap in series with R2, but don\'t like
electrolytics as they fail after a few thousand hours.

Possible to use a 15V or so Zener Diode, but the Vbb is not regulated so
that won\'t work reliably.

Anyone have a single component in mind that will essentially emulate
(isolate) an Open-Collector output for the 138?
The TIP125 is a complementary dual.
it is? https://www.onsemi.com/pdf/datasheet/tip120-d.pdf
Yeah it is. See Figure 1. All of that is in that TO-220 looking package.
and the three missing legs are where?

TIP120, TIP121, TIP122 (NPN); is the right drawing

TIP125, TIP126, TIP127 (PNP); is the left drawing

STM has the version with both.

 

[Lasse Langwadt Christensen]

fredag den 7. oktober 2022 kl. 02.34.10 UTC+2 skrev Fred Bloggs:

On Thursday, October 6, 2022 at 12:17:08 PM UTC-4, lang...@fonz.dk wrote:
torsdag den 6. oktober 2022 kl. 17.51.16 UTC+2 skrev Fred Bloggs:
On Thursday, October 6, 2022 at 11:37:46 AM UTC-4, lang...@fonz.dk wrote:
torsdag den 6. oktober 2022 kl. 17.29.21 UTC+2 skrev Fred Bloggs:
On Thursday, October 6, 2022 at 2:24:33 AM UTC-4, John Robertson wrote:
Have a circuit that needs repair. Uses a TIP125 (NPN, Darlington)
emitter tied to Vbb (~20VDC). Collector to load, then to ground. 1K
resistor (R1) pullup on Base to Vbb, and a second 1KR (R2) to the TTL
controller.

Problem is of course, that R2 puts the ~20VDC to the TTL output gate on
a 74LS138.

Trying to solve this without a driver transistor. The circuit is for a
1ms ~20V strobe pulse repeated every ten ms.

I thought of putting a 10ufd cap in series with R2, but don\'t like
electrolytics as they fail after a few thousand hours.

Possible to use a 15V or so Zener Diode, but the Vbb is not regulated so
that won\'t work reliably.

Anyone have a single component in mind that will essentially emulate
(isolate) an Open-Collector output for the 138?
The TIP125 is a complementary dual.
it is? https://www.onsemi.com/pdf/datasheet/tip120-d.pdf
Yeah it is. See Figure 1. All of that is in that TO-220 looking package.
and the three missing legs are where?

TIP120, TIP121, TIP122 (NPN); is the right drawing

TIP125, TIP126, TIP127 (PNP); is the left drawing
STM has the version with both.

in a three pin to-220 ?

 

[Fred Bloggs]

On Thursday, October 6, 2022 at 8:50:58 PM UTC-4, lang...@fonz.dk wrote:

fredag den 7. oktober 2022 kl. 02.34.10 UTC+2 skrev Fred Bloggs:
On Thursday, October 6, 2022 at 12:17:08 PM UTC-4, lang...@fonz.dk wrote:
torsdag den 6. oktober 2022 kl. 17.51.16 UTC+2 skrev Fred Bloggs:
On Thursday, October 6, 2022 at 11:37:46 AM UTC-4, lang...@fonz.dk wrote:
torsdag den 6. oktober 2022 kl. 17.29.21 UTC+2 skrev Fred Bloggs:
On Thursday, October 6, 2022 at 2:24:33 AM UTC-4, John Robertson wrote:
Have a circuit that needs repair. Uses a TIP125 (NPN, Darlington)
emitter tied to Vbb (~20VDC). Collector to load, then to ground. 1K
resistor (R1) pullup on Base to Vbb, and a second 1KR (R2) to the TTL
controller.

Problem is of course, that R2 puts the ~20VDC to the TTL output gate on
a 74LS138.

Trying to solve this without a driver transistor. The circuit is for a
1ms ~20V strobe pulse repeated every ten ms.

I thought of putting a 10ufd cap in series with R2, but don\'t like
electrolytics as they fail after a few thousand hours.

Possible to use a 15V or so Zener Diode, but the Vbb is not regulated so
that won\'t work reliably.

Anyone have a single component in mind that will essentially emulate
(isolate) an Open-Collector output for the 138?
The TIP125 is a complementary dual.
it is? https://www.onsemi.com/pdf/datasheet/tip120-d.pdf
Yeah it is. See Figure 1. All of that is in that TO-220 looking package.
and the three missing legs are where?

TIP120, TIP121, TIP122 (NPN); is the right drawing

TIP125, TIP126, TIP127 (PNP); is the left drawing
STM has the version with both.
in a three pin to-220 ?

They need 5 pins + the heat sink tab for 6 total. It should read \"dual complementary\":
I must have misread it- not finding it now. Anyway- the TIP125 is a PNP and the TIP120 is its NPN complement- both Darlington. The simplest thing is acquire some TIP120, configure as CE with E GND\'ed. Base directly to \'138 OUT ( input is 8k which is high for LS and VOL, MAX way below 1V to even begin to turn on Darlington), C to 6.8kR to junction TIP125 B||1k existing base bleed. Figure 2 of typical VBE at even 100mA IC is 1.25V which is about what you would expect- and well within reach LS VOH standard TTL totem pole with some Schottky speedups and self limiting-no protection needed at this small loading. That\'s two parts, the TIP120 and 6.8kR. This is low power, he can hot melt the one TIP on top of the other, no daughter board necessary. Famous next to last words: It will be just fine.
https://media.digikey.com/pdf/Data%20Sheets/ON%20Semiconductor%20PDFs/TIP120-22.pdf

Mouser has stock for $0.81 in qty <10.
Digikey has them for about same.
NTE TIP120- super widely available at any electronics store- sells for $1.00- that\'s the same as junk box part in hand.
https://www.nteinc.com/specs/original/TIP120.pdf

 

[Ricky]

On Thursday, October 6, 2022 at 7:45:49 PM UTC-4, lang...@fonz.dk wrote:

torsdag den 6. oktober 2022 kl. 18.35.18 UTC+2 skrev Ricky:
On Thursday, October 6, 2022 at 12:17:08 PM UTC-4, lang...@fonz.dk wrote:
torsdag den 6. oktober 2022 kl. 17.51.16 UTC+2 skrev Fred Bloggs:
On Thursday, October 6, 2022 at 11:37:46 AM UTC-4, lang...@fonz.dk wrote:
torsdag den 6. oktober 2022 kl. 17.29.21 UTC+2 skrev Fred Bloggs:
On Thursday, October 6, 2022 at 2:24:33 AM UTC-4, John Robertson wrote:
Have a circuit that needs repair. Uses a TIP125 (NPN, Darlington)
emitter tied to Vbb (~20VDC). Collector to load, then to ground. 1K
resistor (R1) pullup on Base to Vbb, and a second 1KR (R2) to the TTL
controller.

Problem is of course, that R2 puts the ~20VDC to the TTL output gate on
a 74LS138.

Trying to solve this without a driver transistor. The circuit is for a
1ms ~20V strobe pulse repeated every ten ms.

I thought of putting a 10ufd cap in series with R2, but don\'t like
electrolytics as they fail after a few thousand hours.

Possible to use a 15V or so Zener Diode, but the Vbb is not regulated so
that won\'t work reliably.

Anyone have a single component in mind that will essentially emulate
(isolate) an Open-Collector output for the 138?
The TIP125 is a complementary dual.
it is? https://www.onsemi.com/pdf/datasheet/tip120-d.pdf
Yeah it is. See Figure 1. All of that is in that TO-220 looking package.
and the three missing legs are where?

TIP120, TIP121, TIP122 (NPN); is the right drawing

TIP125, TIP126, TIP127 (PNP); is the left drawing
It would seem a combination of a high side PNP driver and a low side NPN level shifter would be a useful, single package device. Is there a reason why this is hard to make on a single die?

what package would you put it in? you\'d need more than 3 pins

I think the closest is arrays like UDN2981

I was thinking of a package like the TO220 that is currently in the design, but with an extra pin. I\'ve seen packages like that, some with five pins. I\'m pretty sure only four pins are needed, in, out, power, ground.

The UDN2981 package might be too limiting for the power dissipation.

--

Rick C.

--- Get 1,000 miles of free Supercharging
--- Tesla referral code - https://ts.la/richard11209

 

[Jasen Betts]

On 2022-10-06, John Robertson <jrr@flippers.com> wrote:

Have a circuit that needs repair. Uses a TIP125 (NPN, Darlington)
emitter tied to Vbb (~20VDC). Collector to load, then to ground. 1K
resistor (R1) pullup on Base to Vbb, and a second 1KR (R2) to the TTL
controller.

Problem is of course, that R2 puts the ~20VDC to the TTL output gate on
a 74LS138.

Trying to solve this without a driver transistor. The circuit is for a
1ms ~20V strobe pulse repeated every ten ms.

I thought of putting a 10ufd cap in series with R2, but don\'t like
electrolytics as they fail after a few thousand hours.

Possible to use a 15V or so Zener Diode, but the Vbb is not regulated so
that won\'t work reliably.

Anyone have a single component in mind that will essentially emulate
(isolate) an Open-Collector output for the 138?

Thanks!

John :-#)#

Yes,

Use a \"digital\" PNP transitor, base to VCC emitter to 138 output,
collector to the base resistor of the darlington

UNR221900L or similar.


--
Jasen.

 

[whit3rd]

On Thursday, October 6, 2022 at 9:30:55 PM UTC-7, Jasen Betts wrote:

On 2022-10-06, John Robertson <j...@flippers.com> wrote:
Have a circuit that needs repair. Uses a TIP125 (NPN, Darlington)
emitter tied to Vbb (~20VDC).

(actually, PNP Darlington)

Anyone have a single component in mind that will essentially emulate
(isolate) an Open-Collector output for the 138?

Use a \"digital\" PNP transitor, base to VCC emitter to 138 output,
collector to the base resistor of the darlington

UNR221900L or similar.

Um... the 138 going active low, you want that emitter to be of an NPN transistor,
not PNP.
For example, ROHM DTC124XE3 HZG
Although, with eight outputs to handle, a dual package might
be good, too.

 

[legg]

On Thu, 6 Oct 2022 07:35:27 -0700, John Robertson <jrr@flippers.com>
wrote:

On 2022/10/06 12:07 a.m., Lasse Langwadt Christensen wrote:
torsdag den 6. oktober 2022 kl. 08.24.33 UTC+2 skrev John Robertson:
Have a circuit that needs repair. Uses a TIP125 (NPN, Darlington)

PNP ..

emitter tied to Vbb (~20VDC). Collector to load, then to ground. 1K
resistor (R1) pullup on Base to Vbb, and a second 1KR (R2) to the TTL
controller.

Problem is of course, that R2 puts the ~20VDC to the TTL output gate on
a 74LS138.

Trying to solve this without a driver transistor. The circuit is for a
1ms ~20V strobe pulse repeated every ten ms.

I thought of putting a 10ufd cap in series with R2, but don\'t like
electrolytics as they fail after a few thousand hours.

Possible to use a 15V or so Zener Diode, but the Vbb is not regulated so
that won\'t work reliably.

Anyone have a single component in mind that will essentially emulate
(isolate) an Open-Collector output for the 138?


mosfet,
gate to 5V, source to 138, drain is your output

Can\'t see that working...this is switching around 20V.

Here is the circuit:
https://www.flippers.com/images/delete/74LS138_Lamp_Driver.png

What I want to do is add a single component in series with RC0/RC3/RC6\'s
1KR to enable the 138 to drive the PNP TIP125 and isolate the output of
the 138 when output is High. MOSFET could work, but switching surges
might be excessive. Leaning towards making small PCB to convert 74LS138
to original 7445 as the simplest solution. Space is a problem around the
transistors you see, no room for additional drive transistors as this is
finished PCB.

Thanks,

John :-#)#

Driving the fet source from the 138 will isolate the 138 from the
higher voltage. Fet gate at 138 VCC.

RL

 

[Fred Bloggs]

On Thursday, October 6, 2022 at 10:50:15 AM UTC-4, John Larkin wrote:

<snip gibberish>

That\'s not clear. Got a schematic?

He\'s wasting his time, there\'s nothing wrong with the original circuit.

He\'s just one of those people who, when they can\'t understand something, think something is wrong.
LS doesn\'t have ESD clamp diodes on its outputs, and, in fact, nothing is being avalanched by applying a very current limited 20V to its outputs. Maybe an insignificant elevated leakage is all that happens. The original circuit is well-designed and should be left as is.

 

[piglet]

On 06/10/2022 07:24, John Robertson wrote:

Have a circuit that needs repair. Uses a TIP125 (NPN, Darlington)
emitter tied to Vbb (~20VDC). Collector to load, then to ground. 1K
resistor (R1) pullup on Base to Vbb, and a second 1KR (R2) to the TTL
controller.

Problem is of course, that R2 puts the ~20VDC to the TTL output gate on
a 74LS138.

Trying to solve this without a driver transistor. The circuit is for a
1ms ~20V strobe pulse repeated every ten ms.

I thought of putting a 10ufd cap in series with R2, but don\'t like
electrolytics as they fail after a few thousand hours.

Possible to use a 15V or so Zener Diode, but the Vbb is not regulated so
that won\'t work reliably.

Anyone have a single component in mind that will essentially emulate
(isolate) an Open-Collector output for the 138?

Thanks!

John :-#)#

You can probably omit the R1 E-B resistor, 1k is rather low and all the
TIP125 I saw had on-chip E-B resistors to soak up and leakage currents.

The base drive resistor R2 is also very low value, 1k at 20V is 20mA
which while within 7445 spec is way above the 8mA Iol of 74LS parts. The
darlington has minimum current gain of 1000 so I am sure you can use
very much lower base drive.

If you get base drive to a couple hundred uA then a series 0.47uF cap
could solve the level shift problem. Otherwise you could try lifting the
base end of R2 and wiring an NPN in series - so collector of NPN drives
darlington base and emitter of NPN goes via R2 to the TTL output. The
buss all the bases together to the TTL 5Vcc. 4mA drive should be enough
for up to 4A of lamp loads. The resistor R2 in series with the TTL
output will also cushion the TTL output against the stored charge spikes
Phil Hobbs pointed out.

piglet

 

[John Walliker]

On Friday, 7 October 2022 at 18:25:51 UTC+1, Fred Bloggs wrote:

On Thursday, October 6, 2022 at 10:50:15 AM UTC-4, John Larkin wrote:

snip gibberish
That\'s not clear. Got a schematic?

He\'s wasting his time, there\'s nothing wrong with the original circuit.

He\'s just one of those people who, when they can\'t understand something, think something is wrong.
LS doesn\'t have ESD clamp diodes on its outputs, and, in fact, nothing is being avalanched by applying a very current limited 20V to its outputs. Maybe an insignificant elevated leakage is all that happens. The original circuit is well-designed and should be left as is.

I can see that the original circuit might work fine, but it does depend on parameters that
are not specified in the data sheet like the breakdown voltage of the output pull-down
transistor, so I wouldn\'t be comfortable calling it \"well designed\".

John

 

[Phil Hobbs]

John Walliker wrote:

On Friday, 7 October 2022 at 18:25:51 UTC+1, Fred Bloggs wrote:
On Thursday, October 6, 2022 at 10:50:15 AM UTC-4, John Larkin
wrote:

snip gibberish
That\'s not clear. Got a schematic?

He\'s wasting his time, there\'s nothing wrong with the original
circuit.

He\'s just one of those people who, when they can\'t understand
something, think something is wrong. LS doesn\'t have ESD clamp
diodes on its outputs, and, in fact, nothing is being avalanched by
applying a very current limited 20V to its outputs. Maybe an
insignificant elevated leakage is all that happens. The original
circuit is well-designed and should be left as is.

I can see that the original circuit might work fine, but it does
depend on parameters that are not specified in the data sheet like
the breakdown voltage of the output pull-down transistor, so I
wouldn\'t be comfortable calling it \"well designed\".

John

I\'d also be worried about zenering the B-E junction of the top transistor.

Probably fine for a hobby project, but not for a mission-critical
application such as a pinball table in a bar.

Cheers

Phil Hobbs

(Those wizards can cut up rough if their favourite table isn\'t working.)

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com