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
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"Old pinballers never die, they just flip out."
Emulating Open-Collector operation with TTL 74LS138...
Started by ●October 6, 2022
Reply by ●October 6, 20222022-10-06
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
Reply by ●October 6, 20222022-10-06
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 outputSo, 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.
Reply by ●October 6, 20222022-10-06
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
Reply by ●October 6, 20222022-10-06
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
Reply by ●October 6, 20222022-10-06
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
Reply by ●October 6, 20222022-10-06
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 outputThis 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
Reply by ●October 6, 20222022-10-06
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."
Reply by ●October 6, 20222022-10-06
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
Reply by ●October 6, 20222022-10-06
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 outputCan'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."
