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 The 7445 will work if you are happy with using such an outdated part.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."
Emulating Open-Collector operation with TTL 74LS138...
Started by ●October 6, 2022
Reply by ●October 6, 20222022-10-06
Reply by ●October 6, 20222022-10-06
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."
Reply by ●October 6, 20222022-10-06
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 :-#)#
Reply by ●October 6, 20222022-10-06
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
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?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."
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?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."
Reply by ●October 6, 20222022-10-06
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
Reply by ●October 6, 20222022-10-06
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.pdfYeah it is. See Figure 1. All of that is in that TO-220 looking package.
Reply by ●October 6, 20222022-10-06
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
Reply by ●October 6, 20222022-10-06
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 drawingIt 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. +- Get 1,000 miles of free Supercharging +- Tesla referral code - https://ts.la/richard11209
