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Old 08 August 2019, 14:23   #1
jbilander
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Using a 3V lithium coin cell with an old A501 expansion memory?

Hi all,

Question regarding putting a coin cell battery holder on an old A501 expansion memory.

I want to use a 3V lithium coin cell instead of a 3.6V rechargeable NiCd barell battery. I'm rescuing a battery damaged A501. For obvious reason I need to stop the recharging current to flow to the non-rechargeable coin cell, is it safe to replace the 470 Ohm resistor at R913 with a diode? (e.g. 1N4148 DO-35 100V with Anode connected to the battery positive terminal, or maybe a germanium type will probably give a bit longer battery lifetime because of the lower voltage drop). However, looking at the schematics on pcbexplorer the RTC chip (Oki M6242B) seems to be powered from the +12V rail (pin 55) and from what I can see reading the datasheet for the Oki M6242B RTC it has 2-6V as Vdd input range. Is R911 (470 Ohm) and R913 (470 Ohm) acting as a voltage divider here bringing it down to just below 6V together with a small voltage drop in D912 or what is going on here? Can someone with a little more electronics knowledge please shed some light for me, thanks
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Old 08 August 2019, 14:31   #2
Hewitson
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Without looking at the schematics, I'm not sure. If you're worried about the voltage drop, I'd use a Schottky diode.

Last edited by Hewitson; 08 August 2019 at 14:36.
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Old 08 August 2019, 15:00   #3
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Not so worried about the coin cell voltage drop, just a matter of choosing a suitable diode (I happen to have a couple of 1N4148 at home), more worried that removing the 470 Ohm resistor at R913 and replacing it with a diode will make too high of a voltage to the RTC chip.
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Old 08 August 2019, 17:26   #4
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You'll be fine. With such low currents involved, you'll find that the resistor has very little effect on the supply voltage, and besides, the chip runs on 5V when the machine is powered up, so the 3V from the battery won't be a problem. Generally they specify something like 2-6V as you say.

The resistor's there to limit the charging current of the battery, and is not needed if you replace it with a non-rechargeable battery. Some A4000s were fitted with CR2032s, and this is exactly what they do - replace the charging resistor with a 1N4148. I've done that mod on other 4000s, as well as A500 Pluses and A501s, so it'll be fine. You could also use a 1N914 as per the older machines, but both are used on different Amiga models with similar circuits.

Edit: A little more detail. The power supply is indeed 12V, but it's dropped by the other 470 ohm resistor and one of the other diodes to somewhere around 5.5V, which is what is supplied to the chip and used to charge the battery when the machine is powered up.

Last edited by Daedalus; 08 August 2019 at 17:32.
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Old 08 August 2019, 18:37   #5
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Many thanks Daedalus for the information. I will go ahead now and do this little mod. I was just a little concerned setting the voltage divider out of play but if this is a well known working mod that you have made several times without any problems, then I have no worries any longer
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Old 08 August 2019, 18:43   #6
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Has anyone tried using the rechargeable Lithium coins (for example, ML2032) instead of adding a diode?
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Old 08 August 2019, 22:40   #7
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Quote:
Originally Posted by jbilander View Post
I was just a little concerned setting the voltage divider out of play but if this is a well known working mod that you have made several times without any problems, then I have no worries any longer
The trick with the circuit is that it's not actually a potential divider - at least, not in the conventional sense. The power supply for the chip is clamped by the diode that connects to the +5V supply (D911). Since the drop across a semiconductor is more or less constant, typically 0.6V (for simplicity I'm ignoring other small factors)), it means that the voltage at the anode must be 5V+0.6V = 5.6V, meaning the resistor is dropping the rest of the potential from 12V, or 6.4V. So, the chip and the charging circuit actually get around 5V because another 0.6V is dropped by D912.

Quote:
Originally Posted by th4t1guy View Post
Has anyone tried using the rechargeable Lithium coins (for example, ML2032) instead of adding a diode?
Some accelerators use these as standard, however they have a charging circuit specially adapted for them. The ML2032 requires a different charging voltage to the NiCds, so it's important that the circuit is modified to compensate. I believe some people have just replaced them directly, but the datasheets say the charge voltage should never exceed 3.3V, and the standard Amiga circuit provides around 5V. I'm generally a fan of treating lithium batteries with a healthy respect, so I would make sure you drop it to a safe level, perhaps using a zener diode.
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Old 09 August 2019, 00:59   #8
davideo
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Quote:
Originally Posted by th4t1guy View Post
Has anyone tried using the rechargeable Lithium coins (for example, ML2032) instead of adding a diode?
Have a look at this old thread where I found one solution

HTML Code:
http:\\eab.abime.net/showthread.php?t=35291

Last edited by davideo; 09 August 2019 at 01:02. Reason: Trying to fix link !!
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Old 09 August 2019, 01:05   #9
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There's also something like this as a drop-in replacement.
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Old 09 August 2019, 14:05   #10
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Many thanks again Daedalus. That was very educational, I learned something today I had completely disregarded the D911 but now it all makes sense. Indeed a nice trick to clamp it that way using a diode to +5V.

I did this mod yesterday using a 1N4148 and also measured the voltages. I got:

before R911: 11.94V
before D912 (after R911): 5.77V
RTC Vdd: 5.21V (after D912, Machine Powered ON)
RTC Vdd: 2.78V (Machine Powered OFF, RTC only powered by battery)

Looks all good so far!!!
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