Electronics > Projects, Designs, and Technical Stuff
Noob question isolating homemade programmer
<< < (3/6) > >>
Fixed_Until_Broken:
Makes sense on the USB Isolation will be updating the design to include that. What rating should I be looking at for the diodes?

None of this for while it is connected to a car battery its all done on a work bench with a dc power supply.(dc power supply is off while programming but I want to to be able to leave it plugged in while test boot some of the eeproms I am still working out different parts of the bins on them so I make lots of changes) I just want to put some protection into the design in case someone else tries to make this and does something stupid I dont want them to damage their computer, Arduino or instrument cluster.
Ian.M:
The clamping diodes are a compromise - too high a current rating means a large junction area and too much capacitance for fast data signals.   Therefore, worst case, if the user connects one of the data lines to +14V from the bench PSU, the clamping diode may not survive, but as long as the energy the polyfuse lets through isn't enough to blow the failed diode open, there should be no damage other than to the diode.  I'd start by looking at BAT54S series Schottky diode pairs, which can withstand 600mA non repetitive peak If for no more than 10ms and see if that's enough for a reliable trip of a 50mA polyfuse.  If not, that should at least give you some idea how beefy a diode you actually need.

The line drivers should be powered from the switched supply at the drain of the P-MOSFET, and that supply should be protected against overvoltage by a crowbar circuit with a 5.5V trip point that turns off the P-MOSFET and grounds the switched supply via a beefy N-MOSFET, leaving the polyfuses in the switched Vcc lines and the data lines to hopefully isolate them before damage occurs to the programmer or the target.

If your programmer had a higher voltage power source than the 5V USB Vbus supply, you'd have better options as the switched Vcc supply could come from a LDO with a current limit, an external Adj pin and an enable pin, eliminating the need for a P-MOSFET and permitting target power to be current limited at source, and allowing a Schottky diode to protect it against being back-fed from a powered target without excessive voltage drop.  You'd still need the crowbar circuit and polyfuses to provide protection against misconnection to a powered target.
rstofer:
How about jumper headers on the various pins?  If you have to modify the PCB, jumpers take less design effort than MOSFETs and such.  It probably isn't sufficient to just isolate the power and ground, I would expect that the signals need to be isolated as well.
Ian.M:
Yes.  If  one needs the capability to run a powered target without disconnecting the programmer, the signal output lines will need to be isolated when not actively programming or reading the target memory.  As long as the line drivers (buffers) are powered by target Vcc, it should be sufficient to tristate their outputs.  The extra loading of input lines to the Arduino should have negligible effect on the target unless the clamping diodes have excessive junction capacitance.


I presume the O.P. wants to to avoid 'brute force'ing it with a bunch of reed relays or even a manual multipole switch!
TomS_:
If all you're doing is powering an EEPROM device, you could potentially just use tri-state buffers, something like a 74HCT245 which operates from 5V and so will output 5V, but who's inputs will work down to 3V3. The buffer should be able to supply enough power for an EEPROM, and also act to isolate the signals when not in use (I presume that by isolate you really mean to have your programmer go HiZ).

But I suspect any EEPROM that is on a board is going to share a power rail with much more, and that all may exceed the current supply ability of the buffer.

So perhaps as people have suggested, a P type transistor of sorts to supply power, and then you could use a tri-state buffer to isolate all of the signal lines when you don't want to be talking to the EEPROM.
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod