Suitable replacement for 2SA1175 transistor? - Page 1
Forum section: electronics / beginners

author: [Chrille]
author_rank: Contributor
timestamp: June 08, 2022, 10:28:35 am
content: Hi,I am sourcing parts for a project. The BOM states that you should use 2SA1175 transistors. The problem is that these are no longer manufactured so I have to find a replacement component with similar properties. I tried using Mousers filtering functions but I was unable to find an exact match. What is the most important part of the specs that must be more or less 100% matching when looking for replacements?Datasheet for the 2SA1175 transistor:https://datasheetspdf.com/pdf/608459/NEC/2SA1175/1Thanks!

author: RES
author_rank: Regular Contributor
timestamp: June 08, 2022, 04:57:52 pm
content: NTE2362https://www.nteinc.com/specs/2300to2399/pdf/nte2361.pdf

author: edavid
author_rank: Super Contributor
timestamp: June 08, 2022, 05:32:02 pm
content: Quote from:  on June 08, 2022, 10:28:35 amIt's impossible to answer that without seeing the schematic.Quote from: RES on June 08, 2022, 04:57:52 pmGood suggestion, and they are in stock at tme.euA good current production substitute would be 2N5087/MMBT5087.

author: [Chrille]
author_rank: Contributor
timestamp: June 08, 2022, 08:50:37 pm
content: Quote from: edavid on June 08, 2022, 05:32:02 pmThanks for your input. Here is the schematics:https://trastero.speccy.org/cosas/JL/Harlequin/Documentacion/harlequin_rev_g.pdfI would prefer to find a suitable replacement from Mouser if possible since I am about to order some other components from them.

author: edavid
author_rank: Super Contributor
timestamp: June 08, 2022, 08:58:06 pm
content: Quote from:  on June 08, 2022, 08:50:37 pmNot too helpful since the transistors are just driving a connector. What is that connector used for?Are you using the circuit board layout? If so, you would want to find something with ECB pinout.

author: [Chrille]
author_rank: Contributor
timestamp: June 08, 2022, 09:20:43 pm
content: Quote from: edavid on June 08, 2022, 08:58:06 pmThe connector "J1" is one of two connectors used to connect a keyboard membrane used for input. The schematic is for a computer.

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: June 08, 2022, 10:48:44 pm
content: This looks like it's just a switching transistor to drive the keyboard matrix. Any general-purpose PNP should work, as long as the pinouts match (if using the original PCB). The 2N5087 suggested earlier has a different pinout, but NTE2362 will work. A BC557 would also work, at a lower cost. The higher the beta of the transistor, the lower the load placed on the address bus of the Z80.

author: edavid
author_rank: Super Contributor
timestamp: June 09, 2022, 05:02:03 am
content: Quote from: radar_macgyver on June 08, 2022, 10:48:44 pmDoesn't BC557 have the wrong pinout?(But OP hasn't said if he's going to use the original PCB layout, so maybe it doesn't matter.)

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: June 09, 2022, 05:12:20 am
content: You're right - sorry, late night posting!

author: [Chrille]
author_rank: Contributor
timestamp: June 09, 2022, 03:52:04 pm
content: I have been browsing Mouser and found two transistors from Onsemi that are somewhat similar in specs to the 2SA1175 that was originally specified as the one to use.The ones I have found are these:KSC1815YTAhttps://www.mouser.se/datasheet/2/308/1/KSC1815_D-2314448.pdfKSA1015YTAhttps://www.mouser.se/datasheet/2/308/1/KSA1015_D-2314383.pdfBoth of them have the same pinout as 2SA1175 and KSC1815 have the exact same "Collector−Base Voltage", "Collector−Emitter Voltage" and "Emitter−Base Voltage" but the "Collector Current" and "Base current" are not at all similar. How important is this?

author: edavid
author_rank: Super Contributor
timestamp: June 09, 2022, 04:29:15 pm
content: Quote from:  on June 09, 2022, 03:52:04 pmThat is NPN and won't work.QuoteThat is an OK choice.

author: [Chrille]
author_rank: Contributor
timestamp: June 09, 2022, 05:46:11 pm
content: Quote from: edavid on June 09, 2022, 04:29:15 pmSorry, I totally missed the NPN part of the first one. I was too busy checking out the other specs. Thanks for pointing it out.But when you say it's OK, do you mean "OK it will just about do" or like is it a really good replacement part?What is the importance of the "Collector−Base Voltage"? It's a 10V diff in the specs (60 vs 50). Is that the maximum voltage they can handle? The board itself is powered by a 9V PSU so it will never be anywhere near 50V

author: edavid
author_rank: Super Contributor
timestamp: June 09, 2022, 06:02:41 pm
content: Quote from:  on June 09, 2022, 05:46:11 pmI mean it will work fine, don't overthink it.QuoteIt's a breakdown voltage, and as you point out it's irrelevant since it's used with a 5V supply.

author: andy3055
author_rank: Super Contributor
timestamp: June 09, 2022, 07:21:42 pm
content: Quote from:  on June 08, 2022, 10:28:35 am

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: June 09, 2022, 08:26:06 pm
content: In this application, the transistors are being used as switches at a fairly low frequency (keyboard scanning is done at the kHz range, typically). This means the switching doesn't need to be too fast. Because it's a switching application, the linearity and gain stability (with Vce) is not important, as it would in, for example, an audio amplifier application. Pretty much any PNP transistor with a matching footprint will work here, don't sweat it.

author: [Chrille]
author_rank: Contributor
timestamp: June 10, 2022, 05:01:47 am
content: Quote from: andy3055 on June 09, 2022, 07:21:42 pmThanks that is very kind of you, but I will order the Onsemi KSA1015 from Mouser since they are in stock and I am already waiting for some other components that are on backorder. I will add the transistors to that order.

author: tepalia02
author_rank: Regular Contributor
timestamp: June 10, 2022, 06:36:39 am
content: Here are some choices:https://alltransistors.com/transistor.php?transistor=6995

author: [Chrille]
author_rank: Contributor
timestamp: June 14, 2022, 08:15:07 pm
content: Thanks for your help so far, guys!I am trying to figure out how the keyboard-part of this circuit works so I have been reading up a bit on how different transistors work, but I have a couple of questions.When used as a switch the PNP transistor gets the current to flow from the emitter to the collector when current flows out of the base, right?The way I see it VCC (5V) is connected through a resistor network "R13" (4.7k) and is then connected to the emitter of the transistors Q6...Q13 which in turn has the collector pins connected to GND. What it the use of this? Nothing happens after current passes through the transistor and out of the collector. It just goes to ground. It doesn't look like you use the current that passes through. Obviously I must be missing something.The base pin of the transistors seem to be connected to a bunch of ICs if I understand the schematics correctly (ROM, CPU, 74HC245 Octal bus transceiver)If you are not using the transistor as an amplifier or using the base as a switch to get current to pass from emitter to collector and use it afterwards then why not just pass the current directly to the ICs and keyboard connector right after it has passed through the resistor network?Please help me understand what is going on here

author: edavid
author_rank: Super Contributor
timestamp: June 14, 2022, 09:24:14 pm
content: The keyboard matrix is scanned using the CPU's address lines, which have weak drivers.The PNP transistors are used as emitter followers - they increase the strength of the scanning signals, without really changing the voltage.They also provide buffering, so when multiple keys are pressed at once, it doesn't short together address lines and disrupt the operation of the CPU.It looks like the pressed keys are sensed at the "KB" connector.

author: radar_macgyver
author_rank: Frequent Contributor
timestamp: June 14, 2022, 10:59:16 pm
content: The transistors are serving a couple of functions here. First, they act as a current amplifier. The ratio of current flowing out of the base to the current flowing into the emitter is the current gain (beta) of the transistor. Hence, to get 1 mA emitter current (5V/4.7k) one would only pull 1mA/100 = 0.01 mA from the address bus (A8-A15) of the Z80.The other function is to act as an 'open emitter' driver, to isolate the various keyboard scanning lines from each other. The keyboard in this case would be a matrix of switches between the pins on J1 and J2. Columns in the matrix are driven by address lines (via J1) and the rows are read through the state of the pins on J2 (via U6). The read is done when only one of the address lines A8-A15 is high. It can then infer which, if any, key is pressed. The trouble arises when multiple keys are pressed on adjacent columns of the matrix. Now you'd have two address lines connected together. If you had directly driven the key matrix with the address lines, this would be a problem. Using the transistors isolates the columns from each other, since the columns are either grounded (through the transistor) or open, they are not actively driven high.Here's a longer article talking about keyboard matrix scanning:https://www.baldengineer.com/arduino-keyboard-matrix-tutorial.htmlEDIT: sorry, looks like edavid covered this already while I was typing out my response!

author: [Chrille]
author_rank: Contributor
timestamp: June 15, 2022, 12:33:57 pm
content: edavid&radar_macgyver: Great explanations!With this in mind I have a question related to the original question for this post:If this transistor is used as a current amplifier and not just as a switch then perhaps the Onsemi KSA1015 that I was thinking of using is too weak? According to the datasheet it has a hFE of 70(min) to 400(max), no typical value specified, whereas the 2SA1175 originally used in the circuit has a specified hFE of 110(min) to 600(max) with a typical value of 200.Some quick calculations of the current draw from the Z80 using the formula you previously used:0,014 mA usingKSA1015(70 hFE)0,009 mA2SA1175, (110 hFE)That's a 55% increase in current draw. But perhaps this is still such a small current that it doesn't matter?So in the end the exact same current will be delivered through the keyboard membrane but the difference is how much is taken from the Z80 depending on the transistor being used, right?

author: Zero999
author_rank: Super Contributor
timestamp: June 15, 2022, 12:44:30 pm
content: Those currents are tiny and are just an approximation. They might be a little lower, or higher. Don't worry about it.

author: edavid
author_rank: Super Contributor
timestamp: June 15, 2022, 03:49:45 pm
content: Quote from:  on June 15, 2022, 12:33:57 pmYou missed something in the datasheet. You said you were considering the KSA1015YTA. On page 2 of the datasheet, the "Y" bin has a beta range of 120-240.However, a beta of 70 would certainly work.