Answers.Body: <p>The short answer is <strong>about 150mA</strong>.</p>
  
  <h1>Electrical Characteristics</h1>
  
  <p>From the limited testing that I've done, the component you pointed out appears to be a <a href="https://en.wikipedia.org/wiki/Thermistor#PTC_(Positive_temperature_coefficient)" rel="noreferrer">positive temperature coefficient thermistor</a>. Here's how Wikipedia describes their use as current limiters:</p>
  
  <blockquote>
    <p>When first connected to a voltage source, a large current
    corresponding to the low, cold, resistance flows, but as the
    thermistor self-heats, the current is reduced until a limiting current
    (and corresponding peak device temperature) is reached. The
    current-limiting effect can replace fuses.</p>
  </blockquote>
  
  <p>I only have a basic multimeter and a few fixed voltages handy at the moment, so I'm not able to derive the precise characteristics of this device. I was able to measure the following:</p>
  
  <ul>
  <li>Open circuit resistance is 4 ohms</li>
  <li>Short circuit current at 12V is ~50ma.</li>
  <li>Short circuit current at 10V is ~50ma.</li>
  <li>Short circuit current at 8.3V is ~60ma.</li>
  <li>Short circuit current at 5V is ~100ma.</li>
  <li>Short circuit current at 3.3V is ~120ma.</li>
  </ul>
  
  <p>The tolerances on these parts are pretty loose, but for mine the maximum it can supply consistently in series with a motor is about 150mA. If I keep current above this for long, the thermistor begins to heat and total current decreases due to the limiting.</p>
  
  <p>It might seem odd that the max constant current I was able to sustain is greater than the short circuit current, but this is due to the fact that this component is effectively limiting the amount of <em>power</em> it dissipates, and not the amount of <em>current</em> it passes. Power is the product of current and voltage, so because some voltage is dropped by the load, it is able to handle a current higher when loaded than it is able to when shorted.</p>
  
  <h1>Physical example</h1>
  
  <p>As a point of reference if milliamps aren't very familiar to you, in my testing I was able to cause the clutch gear (60c01) to slip indefinitely on 43362c01 with the thermistor in series.</p>
  
  <p><a href="https://i.sstatic.net/ychfi.png" rel="noreferrer"><img src="https://i.sstatic.net/ychfi.png" alt="60c01"></a> <a href="https://i.sstatic.net/WVLPB.png" rel="noreferrer"><img src="https://i.sstatic.net/WVLPB.png" alt="43362c01"></a></p>
  
  <p>This drew about 100mA for me. So, you can drive most of the lighter 9v parts just fine. The current limiting will kick in at about half the stalled current for most of the motors. Larger PF motors, and especially the RC buggy motor, will not be able to reach their full potential with the overload protection in place on this battery box.</p>
  
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2017-12-16T18:38:01.730Z
Answers.Text: Interesting. I'm a little bit confused about the 150mA thing though. Your listed short circuit currents and physical example seem to suggest it's less (~100mA?). Have I misunderstood something?
Answers.UserDisplayName: Alexander O'Mara
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2017-12-16T21:12:42.130Z
Answers.Text: I've added an explanation to the end of the electrical characteristics section. I'm fairly confident that this is due to the fact that the thermistor sees a lower voltage when there is a proper load in use.
Answers.UserDisplayName: jncraton
Answers.ContentLicense: CC BY-SA 3.0
Answers.CreationDate: 2017-12-16T14:47:44.237Z
Answers.LastActivityDate: 2017-12-16T21:08:27.537Z
Answers.LastEditDate: 2017-12-16T21:08:27.537Z
Answers.UserDisplayName: jncraton
Body: <p>Recently I disassembled a classic <a href="https://www.bricklink.com/v2/catalog/catalogitem.page?P=4760c01" rel="noreferrer">4760c01 Electric 9V Battery Box Small Complete Assembly</a> to see what was inside.</p>
  
  <p>Battery Box:</p>
  
  <p><a href="https://i.sstatic.net/GtC5i.png" rel="noreferrer"><img src="https://i.sstatic.net/GtC5i.png" alt="Battery Box"></a></p>
  
  <p>Disassembled:</p>
  
  <p><a href="https://i.sstatic.net/HZf5h.jpg" rel="noreferrer"><img src="https://i.sstatic.net/HZf5hm.jpg" alt="disassembled"></a></p>
  
  <p>In the <a href="https://www.bricklink.com/v2/catalog/catalogitem.page?P=4763" rel="noreferrer">switch component</a>, we find what appears to be a current limiter:</p>
  
  <p><a href="https://i.sstatic.net/G02hD.jpg" rel="noreferrer"><img src="https://i.sstatic.net/G02hDm.jpg" alt="switch component"></a></p>
  
  <p>It slides right out if you use a pin or something to push it out.</p>
  
  <p>Here is a closeup on it with one of the other pieces:</p>
  
  <p><a href="https://i.sstatic.net/lEInK.jpg" rel="noreferrer"><img src="https://i.sstatic.net/lEInKm.jpg" alt="current limiter"></a></p>
  
  <p>It's a small metallic looking disk with a black edge and some divots on the faces, and no identifying or informational markings.</p>
  
  <p>Based on usage experience, I <em>believe</em> it limits the current to less than what a 9V battery can actually output, but I don't know for sure or how much that limit is. I've search around, but wasn't able to find any information on the current limiters in the battery boxes before <a href="https://www.bricklink.com/v2/catalog/catalogitem.page?P=54950c01" rel="noreferrer">the one from 2006</a>.</p>
  
  <p>Does anyone know what the current limit of this old battery box is? Or know a way to discover what the limit is?</p>
  
Tags: <piece-information><electronics><9v><battery>
Title: Classic 9V battery box current limiter?
UserDisplayName: Alexander O'Mara
ThreadText: # Classic 9V battery box current limiter?
  
  (Asked by: username_0 on 2017-12-15)
  
  Recently I disassembled a classic [4760c01 Electric 9V Battery Box Small Complete Assembly](https://www.bricklink.com/v2/catalog/catalogitem.page?P=4760c01) to see what was inside.
  
  Battery Box:
  
  [![Battery Box](https://i.sstatic.net/GtC5i.png)](https://i.sstatic.net/GtC5i.png)
  
  Disassembled:
  
  [![disassembled](https://i.sstatic.net/HZf5hm.jpg)](https://i.sstatic.net/HZf5h.jpg)
  
  In the [switch component](https://www.bricklink.com/v2/catalog/catalogitem.page?P=4763), we find what appears to be a current limiter:
  
  [![switch component](https://i.sstatic.net/G02hDm.jpg)](https://i.sstatic.net/G02hD.jpg)
  
  It slides right out if you use a pin or something to push it out.
  
  Here is a closeup on it with one of the other pieces:
  
  [![current limiter](https://i.sstatic.net/lEInKm.jpg)](https://i.sstatic.net/lEInK.jpg)
  
  It's a small metallic looking disk with a black edge and some divots on the faces, and no identifying or informational markings.
  
  Based on usage experience, I *believe* it limits the current to less than what a 9V battery can actually output, but I don't know for sure or how much that limit is. I've search around, but wasn't able to find any information on the current limiters in the battery boxes before [the one from 2006](https://www.bricklink.com/v2/catalog/catalogitem.page?P=54950c01).
  
  Does anyone know what the current limit of this old battery box is? Or know a way to discover what the limit is?
  
  **Tags:** piece-information, electronics, 9v, battery
  
  
  ---
  
  ## 1 answers
  
  ### Answer by username_1 on 2017-12-16. Score: 8 | Accepted answer
  
  The short answer is **about 150mA**.
  
  # Electrical Characteristics
  
  From the limited testing that I've done, the component you pointed out appears to be a [positive temperature coefficient thermistor](https://en.wikipedia.org/wiki/Thermistor#PTC_(Positive_temperature_coefficient)). Here's how Wikipedia describes their use as current limiters:
  
  > When first connected to a voltage source, a large current
  > corresponding to the low, cold, resistance flows, but as the
  > thermistor self-heats, the current is reduced until a limiting current
  > (and corresponding peak device temperature) is reached. The
  > current-limiting effect can replace fuses.
  
  I only have a basic multimeter and a few fixed voltages handy at the moment, so I'm not able to derive the precise characteristics of this device. I was able to measure the following:
  
  * Open circuit resistance is 4 ohms
  * Short circuit current at 12V is ~50ma.
  * Short circuit current at 10V is ~50ma.
  * Short circuit current at 8.3V is ~60ma.
  * Short circuit current at 5V is ~100ma.
  * Short circuit current at 3.3V is ~120ma.
  
  The tolerances on these parts are pretty loose, but for mine the maximum it can supply consistently in series with a motor is about 150mA. If I keep current above this for long, the thermistor begins to heat and total current decreases due to the limiting.
  
  It might seem odd that the max constant current I was able to sustain is greater than the short circuit current, but this is due to the fact that this component is effectively limiting the amount of *power* it dissipates, and not the amount of *current* it passes. Power is the product of current and voltage, so because some voltage is dropped by the load, it is able to handle a current higher when loaded than it is able to when shorted.
  
  # Physical example
  
  As a point of reference if milliamps aren't very familiar to you, in my testing I was able to cause the clutch gear (60c01) to slip indefinitely on 43362c01 with the thermistor in series.
  
  [![60c01](https://i.sstatic.net/ychfi.png)](https://i.sstatic.net/ychfi.png) [![43362c01](https://i.sstatic.net/WVLPB.png)](https://i.sstatic.net/WVLPB.png)
  
  This drew about 100mA for me. So, you can drive most of the lighter 9v parts just fine. The current limiting will kick in at about half the stalled current for most of the motors. Larger PF motors, and especially the RC buggy motor, will not be able to reach their full potential with the overload protection in place on this battery box.
  
  > **Comments:**
  > **username_0**: Interesting. I'm a little bit confused about the 150mA thing though. Your listed short circuit currents and physical example seem to suggest it's less (~100mA?). Have I misunderstood something?  
  > **username_1**: I've added an explanation to the end of the electrical characteristics section. I'm fairly confident that this is due to the fact that the thermistor sees a lower voltage when there is a proper load in use.  
  
  ---
  
  