projects:tek222bat
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projects:tek222bat [2016-05-30 06:33] robert [Gotchas and Design Decisions] |
projects:tek222bat [2020-05-31 11:32] (current) robert [Tektronix 222 Battery Pack] |
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| ===== Tektronix 222 Battery Pack ===== | ===== Tektronix 222 Battery Pack ===== | ||
| - | For those of you interested in a multimedia audio-visual experience, there is a little overview video of the battery pack of youtube [[https://youtu.be/LJ2VS3aohV0|here]]. | + | For those of you interested in a multimedia audio-visual experience, there is a little overview video of the battery pack on youtube [[https://youtu.be/LJ2VS3aohV0|here]]. |
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| + | If you would like to buy a ready-made replacement battery unit based on this design, N0DY Electronics is building and selling them [[https://www.n0dy.com/product/tekcharger/|here]]. | ||
| ==== Introduction ==== | ==== Introduction ==== | ||
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| ==== Overview ==== | ==== Overview ==== | ||
| - | Starting out, I new that I had to (or wanted to) build a circuit that wraps around one or more lithium cells and makes them behave like the original 8V SLA battery as far as the scope is concerned. The way I chose to achieve this is shown in the following diagram: | + | Starting out, I knew that I had to (or wanted to) build a circuit that wraps around one or more lithium cells and makes them behave like the original 8V SLA battery as far as the scope is concerned. The way I chose to achieve this is shown in the following diagram: |
| {{ :projects:scope-bat.png?direct&400 |}} | {{ :projects:scope-bat.png?direct&400 |}} | ||
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| Series cells are a fair bit more complex: A multi-cell charger is required, as well as a cell balancing circuit. In addition, all cells need to be present when they are placed in series. With the general idea being to be as flexible as possible when it comes to cells, a series arrangement is not ideal from that point. | Series cells are a fair bit more complex: A multi-cell charger is required, as well as a cell balancing circuit. In addition, all cells need to be present when they are placed in series. With the general idea being to be as flexible as possible when it comes to cells, a series arrangement is not ideal from that point. | ||
| - | However, connecting cells in series increases to input voltage of the converter, potentially to over the required 8V for three in series. This would be desirable for the following reason: Internally, the oscilloscope has a bunch of switch-mode converters, some of which draw quite substantial current on power-up. If the battery voltage is below the required 8V, that a multiple of that current is drawn from the batteries. In addition, if a boost converter is used, that by itself will have a fir inrush current, so on power-on the current that needs to be supplied by the batteries and the converter may be substantial. | + | However, connecting cells in series increases to input voltage of the converter, potentially to over the required 8V for three in series. This would be desirable for the following reason: Internally, the oscilloscope has a bunch of switch-mode converters, some of which draw quite substantial current on power-up. If the battery voltage is below the required 8V, a multiple of that current is drawn from the batteries. In addition, if a boost converter is used, that by itself will have a fair inrush current, so on power-on the current that needs to be supplied by the batteries and the converter may be substantial. |
| On the other hand, if the battery voltage exceeds 8V, the inrush current drawn from the batteries is a fraction of that drawn by the scope. Furthermore, buck converters are lighter themselves in terms of inrush current, so the overall situation would be better. | On the other hand, if the battery voltage exceeds 8V, the inrush current drawn from the batteries is a fraction of that drawn by the scope. Furthermore, buck converters are lighter themselves in terms of inrush current, so the overall situation would be better. | ||
| - | Trading off the charger complexity against the converter topology, I decided to stick with parallel cells. The simplicity of managing and charging a single cell in my opinion outweighs keeping the inrush currents low. Li-Ion cells will be quite happy to provide large peak currents anyway, and massively oversizing storage capacitors will help the keep down the peak current that the boost converter needs to supply. | + | Trading off the charger complexity against the converter topology, I decided to stick with parallel cells. The simplicity of managing and charging a single cell in my opinion outweighs keeping the inrush currents low. Li-Ion cells will be quite happy to provide large peak currents anyway, and massively oversizing storage capacitors will help to keep down the peak current that the boost converter needs to supply. |
| - | The choice of cells was largely based on mechanical constraints. The scope can just about accommodate a holder for 3 18650 cells, which are amongst the most common cylindrical Li-Ion cells. This makes them cheap and hopefully easily available in the future, which really is the main requirement I have. Furthermore even lower capacity types will easily match the capacity of the original SLA battery, which is another important requirement taken care of. | + | The choice of cells was largely based on mechanical constraints. The scope can just about accommodate a holder for 3 18650 cells, which are among the most common cylindrical Li-Ion cells. This makes them cheap and hopefully easily available in the future, which really is the main requirement I have. Furthermore even lower capacity types will easily match the capacity of the original SLA battery, which is another important requirement taken care of. |
| === Quiescent Power Consumption === | === Quiescent Power Consumption === | ||
| - | In order to behave like the expected lead acid battery, the circuit must provide 8V all the time - even when the oscilloscope is turned off. It is therefore important that the quiescent power consumption is kept low so that the battery pack can stay in the scope for a reasonable amount of time without having to recharge. However, it turns out that it may not be necessary to squeeze out the last microamp, because when powered off, the scope itself will draw a healthy 300µA, which translates into >600µA from the battery. | + | In order to behave like the expected lead acid battery, the circuit must provide 8V all the time - even when the oscilloscope is turned off. It is therefore important that the quiescent power consumption is kept low so that the battery pack can stay in the scope for a reasonable amount of time without having to recharge. However, it turns out that it may not be necessary to squeeze out every last microamp, because when powered off, the scope itself will draw a healthy 300µA, which translates into >600µA from the battery. |
| Low power consumption was mainly achieved by component choice. This covers both basic quiescent current figures, as well as operating modes. For example, the LTC1872 boost converter only operates in short burst at very light loads, which improves overall efficiency compared to parts that may have lower quiescent current, but operate continuously. | Low power consumption was mainly achieved by component choice. This covers both basic quiescent current figures, as well as operating modes. For example, the LTC1872 boost converter only operates in short burst at very light loads, which improves overall efficiency compared to parts that may have lower quiescent current, but operate continuously. | ||
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| You can download the design files from here: | You can download the design files from here: | ||
| - | * Schematic PDF {{:projects:scope-bat-sch0.pdf?linkonly}} | + | * Schematic PDF {{:projects:scope-bat1.pdf?linkonly}} |
| - | * Bill of Materials {{:projects:scope-bat-bom.csv?linkonly}} | + | * Bill of Materials {{:projects:scope-bat1.csv?linkonly}} |
| - | * PCB Gerbers {{:projects:scope-bat-gerbers0.zip?linkonly}} | + | * PCB Gerbers {{:projects:scope-bat1.zip?linkonly}} |
| - | * Schematic and PCB layout (DipTrace 2.9) {{:projects:scope-bat-design0.zip?linkonly}} | + | * Schematic and PCB layout (DipTrace 3.1) {{:projects:scope-bat1-design.zip?linkonly}} |
| + | * DXF files for acrylic case parts {{:projects:bat-acrylic.dxf?linkonly}} | ||
| - | **IMPORTANT:** This schematic reflects the board design, and does not incorporate the required bodges. If you just build the circuit from this schematic, it will not work. Please see this updated schematic for a circuit that incorporates the changes. | + | The above files are for Revision 1 of the board, which has been built a number of times and proved to be robust. |
| - | + | ||
| - | If you decide to build the circuit from the revision 0 board / schematic, you need to apply the following fixes: | + | |
| - | * Add a 1MΩ pull-up resistor on OUTA (pin 1) to Vs (pin 5) of the LT6700. | + | |
| - | * Add a 4.7µF ceramic capacitor in parallel with the decoupling capacitor of the LT6700. | + | |
| - | * Add a 10MΩ feedback resistor between OUTA (pin 1) and INA, INB (pins 3 and 4) of the LT6700. This increases the hysteresis voltage. | + | |
| - | + | ||
| - | There are a few other changes that I'd like to make to the PCB, but I think it's unlikely that I'll ever do another run. | + | |
| {{ :projects:bat-switcher-top.jpg?direct&400 |}} | {{ :projects:bat-switcher-top.jpg?direct&400 |}} | ||
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| The datasheet does mention that the reference is sensitive to noise, but I didn't expect it to have that kind of effect. Adding a 4.7μF decoupling cap on the comparator in parallel to the existing 100nF solved that issue. If I were to re-spin the board, I'd probably add an RC filter on the supply line, just to be on the safe side. | The datasheet does mention that the reference is sensitive to noise, but I didn't expect it to have that kind of effect. Adding a 4.7μF decoupling cap on the comparator in parallel to the existing 100nF solved that issue. If I were to re-spin the board, I'd probably add an RC filter on the supply line, just to be on the safe side. | ||
| - | |||
| - | ==== Free Boards! ==== | ||
| - | Thanks to obtaining [[http://dirtypcbs.com/|dirt cheap PCBs]] from China, I have quite a few bare boards spare which I'm probably never going to use. If you'd like to build your own board, please just contact me and I'll send you one for the cost of postage. | ||
projects/tek222bat.1464600799.txt.gz · Last modified: 2016-05-30 06:33 by robert
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