1. Not old. Vintage. :)

The 'Cool' Tangerine Microdrive for ORIC1 & ATMOS

Discussion in 'SIG: 8-Bit Hardware' started by Graham, Jun 21, 2020.

  1. by Graham

    Graham Deckhand

    Blog Posts:
    For some internal Photo's including stripping the 3" Drive unit to clean the Head properly please see this Media Library https://atari-owner.com/club/media/albums/oric-microdrive.4/

    I'm sure if you are an ORIC or ATMOS owner, that you are aware they along with the Micro drive run a 'bit' warm, there have been a few thoughts as to why, one which is fairly commonly mentioned is that the original power transformers were the same as those used in Europe being 220V rated and not the 240V seen in the UK. the difference is very close to 10% and to be honest doesn’t really account for the very high DC voltages I've seen output from the Oric’s Micro drive PSU (I’ll add I don’t yet own either but I have been working on a Microdrive unit to alleviate the temperatures it gets too)

    The power brick supplies 21-22V, to be regulated to 12V and 15-16V for the 5V Rail the same approx 15V is sent to the computer itself, hence why they run so hot as approx 2/3's of the power is converted to Heat.

    These high voltages are then fed to the Micro drive unit and the 12V & 5V levels required are regulated within the Micro drive case itself, the additional voltage causes the Micro drive to warm up and many have said this is the cause of the 3" drives being so unreliable.

    Using commonly available linear voltage regulators, for both the Oric and the Micro drive means they will have to dissipate the heat they produce.

    We can work out how much with a simple bit of math. Power = Volts X Amps.

    Input voltage 15V x 1.6 A (the current draw) gives 24W
    Voltage required is 5V so 5V x 1.6A = 8W
    The excessive voltage supplied causes the regulator to dissipate 16W as wasted heat.

    The 12V rail is similar however as 12V is only used very intermittently for the motor in the floppy drive it’s current pull is very low unless the drive is in constant use, so although it wastes heat the effect is very small in comparison.
    22 x 1A = 22W 12 x 1A = 12W 10 W however it’s usually a current draw of about 150mA

    22x 0.15 = 3.3W 12V x0.15 = 1.8W so about 1.5W continuous heat , but when the motor is spinning a disc, 10W

    Modern Switch mode power supplies and regulators are very much more efficient, and using a high switching speed also reduces the amount of smoothing capacitance required.

    The replacement for the 5V regulator (original LM323K in a TO3 case) able to source 3A, was purchased from the USA, and has been purposely designed for the arcade gaming industry as a direct replacement.

    It’s able to be used at its fully rated current load of 3A without the use of any heat sink

    The 12V regulator is based on a common 7812 type device a LM340T12 in a TO220 package, I used the same switching regulator as the 1088XLD uses and again high speed switching internally means no additional smoothing capacitors are required.

    Both items are direct replacements for the originals, and are the two items mounted on the rear heat sink of the Micro drive.

    On the Transformer ‘Brick’ although I did shorten some leads from this and remove the additional one for a second Micro drive. I also changed the wiring to the LED on this as internally was likely to short across one of the bridge rectifiers within.

    The two Items I’ve used are

    5V EzSbc.com LM323K replacement

    12V Texas Instruments TPSM84212EAB

    I'd like to Thank Andy Barr for loaning me his prized possession to create this Post.
    Vyper68 likes this.

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