I am adapting the digital electronics lab assignments I teach so they can be done at home with inexpensive supplies rather than with the equipment in the hardware lab. I have breadboards, TTL chips, and LEDs and will want students to build full-adders out of AND, OR, and NOT gates, so I don't need much current. I am looking at MB102 power supply modules, most of which have USB inputs. I understand that I won't get 5V out with 5V input through the USB port. Will I get a high enough voltage to use TTL chips without flakiness? I am trying to avoid the expense of buying and shipping wall warts (but can do so if necessary).

To answer some questions, soldering is not an option. The unit is only on digital electronics, not analog. I'd like something quick and easy for students that just works, especially because it will be hard to debug circuits remotely.

  • \$\begingroup\$ Why not take 5V straight from USB? \$\endgroup\$ – TimWescott Aug 30 '20 at 23:14
  • \$\begingroup\$ @TimWescott How do I get it onto the breadboard? \$\endgroup\$ – Ellen Spertus Aug 30 '20 at 23:19
  • \$\begingroup\$ You may want to add the answers to all these questions to your question, as edits. Are you supplying parts, or just instructions on the web? Is something that requires a bit of soldering OK? What level of depth of electronics assembly is OK? Are you willing to make some PC boards? \$\endgroup\$ – TimWescott Aug 30 '20 at 23:26
  • \$\begingroup\$ The easiest thing to do if you can solder -- if a bit sporting in that it provides no protection to the USB power source -- is to get a cable, cut off the B end, strip the wires back, and solder a pigtail to the +5V wire and the ground wire. These will probably be red and black, respectively -- and you can tell by testing with a voltmeter. This is what I do to repurpose USB wall-warts as 5V supplies. \$\endgroup\$ – TimWescott Aug 30 '20 at 23:29
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    \$\begingroup\$ As it is stated, this is an off-topic "usage" question. To be on-topic, you would need to provide the internal circuitry of this "MB102" product. It's not very clear what you hope to gain by using that - seemingly what you really need is just a physical translation from a USB connector to breaboard pins. \$\endgroup\$ – Chris Stratton Aug 31 '20 at 0:32

Standard USB ports can supply up to 5.25V but could be as low as 4.6V, which is slightly out of spec for TTL but should still work. The total output current should not exceed 100mA, if the goal is to 'just plain work' with whatever the remote student has available. Check the datasheets of all of the devices in the circuit, and add up all of the maximum load currents.

  • \$\begingroup\$ Thank you. From what I've read, there should be a voltage drop through the MB102. I don't know how great that would be. \$\endgroup\$ – Ellen Spertus Aug 30 '20 at 23:52
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    \$\begingroup\$ If you want to try the MB102, why not get one and measure the 5V output under a little bit of load? If it measures 5V -- great! If not, at least you know. \$\endgroup\$ – TimWescott Aug 31 '20 at 1:26

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