So this is a weird one... I am working on a device that has two different things, and one needs 4.5v and one needs 1.5v.
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@rasterweb a consideration would be whether there is any substantial difference in current draw between the two voltage uses. If the one providing 1.5V is depleted quicker, it would drop the total more quickly than otherwise.
Another approach would be a resistor voltage divider perhaps, if the current draw is low?
@ottaross I did think about a voltage drop via resistor but it seemed like dropping from 4.5 to 1.5 might waste quite a bit of energy... (But I am not an expert on these things!)
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So this is a weird one... I am working on a device that has two different things, and one needs 4.5v and one needs 1.5v.
Is there any reason I could not just solder a wire in place to just get power from one cell and also use the existing wire to provide 4.5v elsewhere?
I know this isn't the best way to do it, but it's a quick hack to make something work... I can't find an issue with it under that condition. Can you?
@rasterweb if your item can work for months on 1.5v then there is likely no problem at all with this solution. What is the current draw for the 1.5v and 4.5v sides of your project?
And yes, your instinct about a dropping resistor or divider is correct, it's inaccurate and wasteful of energy.
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@float13 For this situation we'd probably be looking at using fresh alkaline batteries each use, though maybe rotation them after each use would also be an option?
@rasterweb @float13 The 1.5V cell will always discharge first in this configuration. When it reaches the point where it is fully flat, if the device passes any current at all from the two remaining 4.5V cells, they will start charging the 1.5V cell in reverse. AFAIK this sort of thing causes alkaline cells to leak, because even small currents can lead to electrolysis and internal corrosion over time.
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@rasterweb @float13 The 1.5V cell will always discharge first in this configuration. When it reaches the point where it is fully flat, if the device passes any current at all from the two remaining 4.5V cells, they will start charging the 1.5V cell in reverse. AFAIK this sort of thing causes alkaline cells to leak, because even small currents can lead to electrolysis and internal corrosion over time.
@rasterweb @float13 Personally, I’d just put a 4.5V to 1.5V low I_q buck converter there. Over the last decade or two, these things got really small, cheap and efficient.
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@rasterweb @float13 The 1.5V cell will always discharge first in this configuration. When it reaches the point where it is fully flat, if the device passes any current at all from the two remaining 4.5V cells, they will start charging the 1.5V cell in reverse. AFAIK this sort of thing causes alkaline cells to leak, because even small currents can lead to electrolysis and internal corrosion over time.
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@rasterweb @float13 yeah, as long as you can be confident that the 4.5V rail won’t leak more than a few nanoamps in that state.
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@rasterweb @float13 Personally, I’d just put a 4.5V to 1.5V low I_q buck converter there. Over the last decade or two, these things got really small, cheap and efficient.
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@rasterweb @float13 3.2V maybe cuts it a bit close with the discharge voltages of three series alkaline cells if the 4.5V doesn’t need a lot of current. Depending on the load on the 1.5V rail, I’d mostly look for low quiescent current, not necessarily efficiency first as at low output current, I_q is the limiting factor to efficiency, and the efficiency rating is usually given at substantial current.
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@rasterweb @float13 3.2V maybe cuts it a bit close with the discharge voltages of three series alkaline cells if the 4.5V doesn’t need a lot of current. Depending on the load on the 1.5V rail, I’d mostly look for low quiescent current, not necessarily efficiency first as at low output current, I_q is the limiting factor to efficiency, and the efficiency rating is usually given at substantial current.
@rasterweb @float13 Oh, I just clicked through to the link. The chips on these modules are a fairly ancient design, and are always knockoffs. I would expect them to be pretty bad in quiescent current
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@rasterweb @float13 Oh, I just clicked through to the link. The chips on these modules are a fairly ancient design, and are always knockoffs. I would expect them to be pretty bad in quiescent current
@rasterweb spark fun and similar places probably have something that would work here, using a more modern brand name chip
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@rasterweb spark fun and similar places probably have something that would work here, using a more modern brand name chip
@rasterweb If you’re okay with 1.8V output voltage, this thing here has a chip that claims 90 microamp quiescent current (>100k hours on an AA cell) and doesn’t cost too much: https://www.sparkfun.com/sparkfun-buck-regulator-breakout-1-8v-ap3429a.html
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@ottaross I did think about a voltage drop via resistor but it seemed like dropping from 4.5 to 1.5 might waste quite a bit of energy... (But I am not an expert on these things!)
@rasterweb yeah could bleed off a bit of energy depending on how much current needed.
If not needed urgently, and you had time to source a couple of cheap efficient regulators, that's prob the best path, but of course more parts, complexity.
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So this is a weird one... I am working on a device that has two different things, and one needs 4.5v and one needs 1.5v.
Is there any reason I could not just solder a wire in place to just get power from one cell and also use the existing wire to provide 4.5v elsewhere?
I know this isn't the best way to do it, but it's a quick hack to make something work... I can't find an issue with it under that condition. Can you?
@rasterweb the problem with these type of multi-tap situations is that when you drain the bottom cell faster, the other two cells can continue to provide current and reverse charge the first cell beyond zero and cause a leaky mess.
The ideal solution is to find a little DC-DC module that steps the 4.5V down to 1.5V.
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So this is a weird one... I am working on a device that has two different things, and one needs 4.5v and one needs 1.5v.
Is there any reason I could not just solder a wire in place to just get power from one cell and also use the existing wire to provide 4.5v elsewhere?
I know this isn't the best way to do it, but it's a quick hack to make something work... I can't find an issue with it under that condition. Can you?
@rasterweb as long as they're isolated I don't see why it wouldn't work.
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