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 drawingSTM has the version with both.
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
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 ?
Reply by ●October 6, 20222022-10-06
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
Reply by ●October 7, 20222022-10-07
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 UDN2981I 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
Reply by ●October 7, 20222022-10-07
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.
Reply by ●October 7, 20222022-10-07
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.
Reply by ●October 7, 20222022-10-07
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
Reply by ●October 7, 20222022-10-07
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.
Reply by ●October 7, 20222022-10-07
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
Reply by ●October 7, 20222022-10-07
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
