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

[John Robertson]

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 :-#)#

--
(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. 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

 

[whit3rd]

On Thursday, October 6, 2022 at 12:07:59 AM UTC-7, lang...@fonz.dk 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. ...

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

So, a cascode-like second transistor atop the OC output. Good idea.

Can also use bipolar NPN, with the base through a resistor to +5V; has
slightly lower pull-down.

 

[Phil Hobbs]

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

--
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

 

[Ricky]

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?

Thanks!

If your Vbb is always within 30V, you can replace the 74LS138 with a 7445, although not the same pinout. If the load current is no more than 80 mA you could also dump the TIP125. But since the TIP125 is rated for amps, I expect 80 mA is not enough. The 7445 is not easy to find, however Jamesco claims to have them. I assume DIP is ok?

https://www.jameco.com/z/7445-Major-Brands-IC-7445-BCD-to-Decimal-Decoder-Driver_50403.html

--

Rick C.

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

 

[Phil Hobbs]

whit3rd wrote:

On Thursday, October 6, 2022 at 12:07:59 AM UTC-7, lang...@fonz.dk 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. ...

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

So, a cascode-like second transistor atop the OC output. Good idea.

Can also use bipolar NPN, with the base through a resistor to +5V; has
slightly lower pull-down.

Using a BJT in a \"collapsible cascode\" like that is liable to produce a
nasty ~1 us spike of nearly the full supply voltage on the OC output,
due to charge storage in the cascode transistor.

And the capacitances of the MOSFET can do the same, except not as badly.

Either way, a Schottky diode from the \'138 output to +5 would be prudent
to avoid stressing the ESD structure on the chip.

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

 

[Ricky]

On Thursday, October 6, 2022 at 3:07:59 AM UTC-4, lang...@fonz.dk 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.

A cap won\'t do what you want. It will charge up and the 138 won\'t be able to drive the TIP125. Unless... you add biasing so the 138 output was at say, 10V while open. The cap would charge up to 10V while idle. When the 138 pulls low, the cap will provide the current to drive the TIP125 for 10 ms.. I think this would need about a 10 uF cap which is available in ceramic. It depends on the base drive current of the TIP125, but that should be pretty minimal because of the high gain.

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

This will isolate the voltage, but with no current gain. Since the TIP125 will drive amps and is in a TO220 case, I\'m thinking the current gain is required.

--

Rick C.

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

 

[John Robertson]

On 2022/10/06 6:39 a.m., Ricky wrote:

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?

Thanks!

If your Vbb is always within 30V, you can replace the 74LS138 with a 7445, although not the same pinout. If the load current is no more than 80 mA you could also dump the TIP125. But since the TIP125 is rated for amps, I expect 80 mA is not enough. The 7445 is not easy to find, however Jamesco claims to have them. I assume DIP is ok?

https://www.jameco.com/z/7445-Major-Brands-IC-7445-BCD-to-Decimal-Decoder-Driver_50403.html

7445 was in the original design, the board I\'m dealing with was
redesigned and installed a 138 instead of a 45 - I am trying to save the
boards by modifying the drive to the PNP TIP125s.

I am considering simply making a small 74138 to 7445 PCB - might be the
easiest.

Next board will have 7445s. I have reasonable source of 7445, the number
needed is under 100 for the entire run of the boards AFAIK. Digikey has
around 1500 @ $4 each, and there are other sources for less.

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.\"

 

[Ricky]

On Thursday, October 6, 2022 at 9:56:50 AM UTC-4, Ricky wrote:

On Thursday, October 6, 2022 at 3:07:59 AM UTC-4, lang...@fonz.dk 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.
A cap won\'t do what you want. It will charge up and the 138 won\'t be able to drive the TIP125. Unless... you add biasing so the 138 output was at say, 10V while open. The cap would charge up to 10V while idle. When the 138 pulls low, the cap will provide the current to drive the TIP125 for 10 ms. I think this would need about a 10 uF cap which is available in ceramic. It depends on the base drive current of the TIP125, but that should be pretty minimal because of the high gain.

Sorry, for some reason, I thought the \'138 was an open collector part. I don\'t follow what the original circuit was supposed to be. I\'m thinking you are designing a circuit from scratch??? The \'138 could never have been used to drive the TIP125 in this arrangement.

To drive a high current load from the 20V supply, you either need a higher voltage output than the \'138 offers, or you need to use transistor as a low side switch, or you need two transistors to provide the voltage shifting.

The 7445 will work if you are happy with using such an outdated part.

--

Rick C.

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

 

[John Robertson]

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 :-#)#
--
(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.\"

 

[John Robertson]

On 2022/10/06 7:27 a.m., Ricky wrote:

On Thursday, October 6, 2022 at 9:56:50 AM UTC-4, Ricky wrote:
On Thursday, October 6, 2022 at 3:07:59 AM UTC-4, lang...@fonz.dk 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.
A cap won\'t do what you want. It will charge up and the 138 won\'t be able to drive the TIP125. Unless... you add biasing so the 138 output was at say, 10V while open. The cap would charge up to 10V while idle. When the 138 pulls low, the cap will provide the current to drive the TIP125 for 10 ms. I think this would need about a 10 uF cap which is available in ceramic. It depends on the base drive current of the TIP125, but that should be pretty minimal because of the high gain.

Sorry, for some reason, I thought the \'138 was an open collector part. I don\'t follow what the original circuit was supposed to be. I\'m thinking you are designing a circuit from scratch??? The \'138 could never have been used to drive the TIP125 in this arrangement.

To drive a high current load from the 20V supply, you either need a higher voltage output than the \'138 offers, or you need to use transistor as a low side switch, or you need two transistors to provide the voltage shifting.

The 7445 will work if you are happy with using such an outdated part.

Repairing a new PCB design...and yes, the circuit originally used a
7445. Kinda stuck for this PCB run - next revision will use the 7445 (or
MOSFETs) in place of the 138s...

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.\"

 

[John Robertson]

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 :-#)#
--
(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.\"

 

[John Larkin]

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?

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 :-#)#

 

[Ricky]

On Thursday, October 6, 2022 at 10:23:03 AM UTC-4, John Robertson wrote:

On 2022/10/06 6:39 a.m., Ricky wrote:
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?

Thanks!

If your Vbb is always within 30V, you can replace the 74LS138 with a 7445, although not the same pinout. If the load current is no more than 80 mA you could also dump the TIP125. But since the TIP125 is rated for amps, I expect 80 mA is not enough. The 7445 is not easy to find, however Jamesco claims to have them. I assume DIP is ok?

https://www.jameco.com/z/7445-Major-Brands-IC-7445-BCD-to-Decimal-Decoder-Driver_50403.html

7445 was in the original design, the board I\'m dealing with was
redesigned and installed a 138 instead of a 45 - I am trying to save the
boards by modifying the drive to the PNP TIP125s.

Any idea how the \'138 board was supposed to work? Was it intended to drive something different than the TIP125?

I am considering simply making a small 74138 to 7445 PCB - might be the
easiest.

Next board will have 7445s. I have reasonable source of 7445, the number
needed is under 100 for the entire run of the boards AFAIK. Digikey has
around 1500 @ $4 each, and there are other sources for less.

You might be able to make the capacitor drive work. It would need a few 10s of uF which you can get in a ceramic cap. Since the \'138 drives a 5V range and the TIP125 requires a 2.8V swing at R2, you should be able to make it work. It will probably require pullup resistors on the \'138 outputs to restore the proper voltage on the cap when not driving, maybe 1k. The circuit will power up driving the TIP125 on until the cap is charged up. Is that a problem?

This would be sensitive to voltage transients on the Vlamp rail. Fast voltage transients will trip the TIP125 on, but if they are just powering lamps, it likely won\'t be visible.

--

Rick C.

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

 

[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. From your description, they\'re using the PNP to drive the grounded load, and the NPN is unused. Use the NPN to pull down the base of the NPN., and use the LS138 to drive the base of the NPN.. Are you sure they\'re not doing that already??? Check the pinout of the TIP125 schematic and your board layout. If it\'s connected the way it should be and you\'re seeing 20V on the 138 output, something is blown, and you need to replace of course.

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.\"

 

[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?

Should be use the NPN to pull down the base of the PNP- you\'ll need current limiting there. Check your schematics!

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.\"

 

[Lasse Langwadt Christensen]

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

 

[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.

 

[Lasse Langwadt Christensen]

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

 

[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?

--

Rick C.

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