An EPROM memory can be erased with a UV-C type lamp, which emits radiation around the 2537 Å (Angstrom) or 254nm, at a distance of about 2.5 cm from the memory. The radiation reaches the memory cells through a transparent quartz window located on top of it.
To erase an EPROM it is necessary that the amount of radiation received by it is around 15 W/cm ^ 2 for one second. The actual erasing time is usually about 20 minutes because the lamps used usually have powers around 12 mw/cm ² (12 MW x 20 x 60 s = 14.4 W of power supplied). This time also depends on the manufacturer of the memory you want to delete. At this time all its bits are put to 1.
It is important to avoid overexposure of radiation time to eproms; That is to say, the luminous power supplied to the memory, because it produces a premature ageing of the same ones.
Because the solar radiation and even the artificial light from fluorescent tubes erases the memory slowly (from one week to several months), it is necessary to cover the window with an opaque label that avoids it, once they are recorded.
It should be clarified that an EPROM cannot be erased partially or selectively; Hence, as small as the eventual modification to be made in its content, it must inevitably be deleted and reprogrammed in its entirety.
Once programmed, an EPROM can be erased only by exposure to strong ultraviolet light. This is because light photons excite loselectrones cells causing them to unload. EPROMs are easily recognized by a transparent window in the top of the encapsulation, through which you can see the silicon chip and which supports ultraviolet light during erasing.
As the window quartz is expensive to fabricate, the OTP (one-time programmable) chips were introduced. The only difference with the EPROM is the absence of the quartz window, so it cannot be erased. The OTP versions are manufactured to replace both normal eproms and eproms included in some micro-controllers. The latter were progressively replaced by eproms (for the manufacture of small quantities where the cost is not important) and by flash memory (in the most used).
A programmed EPROM retains its data for ten or twenty years, and you can read an unlimited number of times. To avoid accidental sunlight erasure, the erase window must remain covered. The old bios of the personal computers were frequently EPROM and the delete window was usually covered by a label that contained the BIOS producer’s name, its revision and a copyright warning.
An EPROM eraser is an optically opaque case, with a C type UV light source, which is also used to sterilize surgical instruments and/or as a germicide.
To erase the EPROMs, you cannot use the “black UV ” light (which is commonly used to check banknotes, tickets, etc.), which emit in the UV-A region (365 nm). The only light that works is the UV-C, (254 nm), which emits “dangerous light” or “germicidal” (Kills Germs). “Dangerous light ” because prolonged exposure can cause long-term cataracts and skin damage; However, a brief exposure, about 5 seconds continuous on the skin, should not cause more than a mild dryness, so it is necessary to take all precautions to avoid these problems. Since this type of UV-C light is in the sunlight, if an EPROM is left directly under it, in some days or weeks it will be erased; So it is necessary to protect the eproms once they have been programmed.
A white light fluorescent tube has a phosphor cover inside the glass. The UV light of mercury excites phosphorus, which re-radiates energy in the visible range. UV lamps for EPROM or germicidal drafts directly use mercury vapor light. The glass must be made of quartz, instead of ordinary glass, to prevent the glass from absorbing most of the UV rays. Quartz is more transparent in the UV lengths of Mercury.
They could also be erased if exposed to the electric welding light (electrode), with the risk that a spark will burn the chip, because the EPROM should be approached about 10 or 15 cm to receive sufficient radiation to erase them. In theory, they can also be erased with X-rays, “by using the EPROM graphs “, the erasing time will depend on the calibration/emission of the X-ray equipment used.