Transistor Equivalents in larger packages for prototyping; what to look for - Page 1
Forum section: electronics / beginners

author: jbee
author_rank: Newbie
timestamp: April 18, 2020, 02:03:28 am
content: I'm looking to build a NiMH battery charger circuit using a DS2712, and am using the schematics from the DS2711/DS2712K dev kit as a baseline, or really as the final circuit. The datasheet for the devkit is here on the last page:https://datasheets.maximintegrated.com/en/ds/DS2711K-DS2712K.pdfI'm only interested in charging one cell, and it's going to be via a USB connection, so 100mA is the target charge current as of now (I'll look into USB negotiation, and the 500mA+ options later on after I get something working). I'm therefore going to use the "on board regulation" option and intend on copying the regulation circuit from their schematics.The thing is, as a beginner, I would like to prototype *everything* on those little white protoboards. That way, I know everything works before I design a PCB and ship it off to get made. I have a whole kit of SSOP to DIP adapters, which also work for the DS2712 itself as well, however, there are a couple components on the DS2711/DS2712K dev board that are too small, package-wise, to prototype on a protoboard. The transistors, for example, such as the ZXM62P02E6 MOSFET and the FCX718 are SOT-89 and SOT-23 respectively. They're so small I don't even know if I can work with them on a final PCB, let alone prototype with them.So I guess my actual question is, what would be good through-hole equivalents for those two transistors, and more importantly, what am I looking for in both the BJT and MOSFET case for equivalent parts so I can search on Digikey with a more friendly package filter? For the BJT, I have a small general kit and it looks like the S8550 is a close replacement FCX718 because the Vceo and Vcbo are similar, as well as the Ic (which I assume is the more important part). For the MOSFET, I'm not as sure. I assume Vdss and Id are important parameters, but what about Rds? When I search on Digikey, for example, with the Id and Vdss filters with a through-hole package, I get the following MOSTFET:https://www.onsemi.com/pub/Collateral/NDP6020P-D.PDF.However, Rds is 50mOhms instead of the 200mOhms of the ZXM62P02E6 part. Is that okay? Is trying to get a through hole equivalent a bad idea? Anything I can learn from this is a good thing, I think.

author: WattsThat
author_rank: Frequent Contributor
timestamp: April 18, 2020, 02:38:42 am
content: Don’t do that, use the final parts of design. You can purchase sot-23 and 89 prototype board adapters that you solder the parts to and they terminate in 0.1/2.54 header pin spacings. SOT-23 by hand isn’t difficult at all, -89’s even more so. Embrace surface mount. Learn it, live it. It’s the only packaging you’ll find in low voltage/current mosfets.Beware trying to put low ohmic value shunts for current monitoring on a breadboard, that isn’t going to work, period. Those things where designed for digital logic, not power or anything more than a few MHz. They have their place but don’t become overly reliant on them, many times soldered together prototypes are the better way to develop.eBay is full of adapters for cheap from China with some on Tindie from a US seller, Azzy’s Electronics if you cannot wait on China shipping.

author: jbee
author_rank: Newbie
timestamp: April 18, 2020, 03:24:16 am
content: Okay, all fair points, and it makes sense to me. These itsy bitsy parts probably shouldn't be sitting on top of all this metal under the breadboard.QuoteShort of CAD'ing, and waiting for China to ship PCBs, what is the best way, or most acceptable way to prototype small parts/designs like this? I'm sure extremely seasoned professionals could CAD a board in a couple days that works out of the gate, but I just can't risk that, personally. Something more like perfboards with those adapters you mentioned?

author: perdrix
author_rank: Frequent Contributor
timestamp: April 18, 2020, 04:04:29 am
content: I suggest Jim Williams' standard approach - just use a piece of copper clad (single sided), using the copper as ground plane and "dead bug" construction.David

author: WattsThat
author_rank: Frequent Contributor
timestamp: April 18, 2020, 04:05:53 am
content: Yes, perfboards for the one-offs. Have a look at the Tindie seller I mentioned (no connection, I just like his products), he sells a series of decent quality perfboards, some generic, some Arduino centric.I’m not someone who can bang out a board in an hour on whatever EDA package so its still old school for me.

author: Whales
author_rank: Super Contributor
timestamp: April 18, 2020, 06:54:16 am
content: Don't be afraid of making messy prototypes. Wires everywhere, blobs of ugly solder. Just get stuff to work, prettiness can wait for the PCB or later.I like to use a copper foil technique. EDIT: sot23 is easy, place a square of copper down and then cut 3 small lines out of it with a knife. I'll try to put some photos up later.https://halestrom.net/darksleep/blog/039_coppertape/

author: rstofer
author_rank: Super Contributor
timestamp: April 18, 2020, 02:55:58 pm
content: That copper tape method is quite interesting. Copper tape is all over eBay so no particular problem but what is the yellow tape?

author: not1xor1
author_rank: Frequent Contributor
timestamp: April 18, 2020, 06:22:09 pm
content: Quote from: rstofer on April 18, 2020, 02:55:58 pmkapton tape I guess... what else?

author: Whales
author_rank: Super Contributor
timestamp: April 19, 2020, 12:22:34 am
content: Yes, double-check the pictures. A lot of sellers call kapton/polyamide tape as "copper tape" or "conductive tape". Polyamide is definitely not conductive!

author: james_s
author_rank: Super Contributor
timestamp: April 19, 2020, 12:39:33 am
content: I solder SOT parts on protoboard, either tacking the pins to the holes in perfboard or stick the part upside down on blank copper clad and solder thin wires to the pins. It takes a steady hand but it really isn't all that hard.