Obviously the 7805 is powered from the external DC supply. In some cases, there is provision to increase its output voltage to about 6V for use when verifying. When using the external power input, a conventional 7805 or similar voltage regulator is used to provide the +5V supply. This is due to a combination of factors including a limited amount of power being available from the USB input, possibly poor design and possibly poor quality components. This is especially true for the older technology 21V and 25V EPROM chips. Unfortunately quite a lot of inexpensive Willem (and other USB powered) EPROM/EEPROM programmers can’t manage to maintain the required voltage at the current demanded by the EPROM. To get the 12.5V, 13V, 21V or 25V programming voltage, a DC-DC converter is used. So EPROMs and EEPROMs that require a 6V VCC supply when verifying will only be run at whatever the USB supply can deliver (nominally 5V). When using USB power, the +5V supply is directly derived from this. When using USB power, the lead should be short and a good quality type to minimise voltage loss (volt drop across the connectors and cable losses). Hence there are various versions and variations of Willem programmers in the wild. Hopefully that's true, as that seems like a good deal.The Willem Programmer is a design that has been implemented and manufactured by various companies. Now I see that it can in the 3x/4x series. When I was purchasing years ago, the Willem couldn't do 21v/25v chips (without power hacks, adaptors, etc.). Yeah the interface is old DOS-feel to it, but it simply works. I have a PocketProgrammer2 and it reliably writes to all of my 25xx and 27xxx chips including TMS ones without problem. They have a unique feature to test 74LSxxx chips out (if you can trust the results), but the software is horribly designed, and clunky UI and Chinenglish bad translations (if translated at all). Trust me, I've used these programmers, and I have rarely had 12V's programm successfully, and I think only one 21V ever work right, and no 25V's. They may claim they can (they read fine) but don't have enough juice, unless perhaps you use a 1 inch USB cable and somehow crank the power up on the bus. They do not have enough power to handle 12+ volt chips. They are only powered via 500ma USB cable.
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January 2023
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