I have a 24V power supply, and I want to use it to power several devices (B1, B2, B3) which each require between 3 and 24 volts. When the devices are unplugged, everything looks right -- I can measure the voltage drop at B1, B2, B3 and I see the expected voltage. When I plug in a device, however, the circuit goes dead. For testing, I'm using a little 3V motor.
Did I design the circuit wrong? It has been 20 years since I studied electronics in high school, so I may have forgotten some of the basics. It is also possible that I wired it wrong. In any case, I'd appreciate some expert advice.
Well, your 3V motor might require 100mA. Do you know what it requires to run properly? At 100mA, all that would have to go through your
100k ohm pots to reach it. Assuming you measured 3V before hooking up the motor, that would suggest a pot setting close to the maximum.
100mA through a 100k ohm resistance is about 10,000 volts drop. So it just doesn't make sense. I don't know the actual current requirements of your motor, but you can see that it doesn't require much current in order to completely drop away all 24V at the pot and leave nothing for the motor.
Well those pots are going to have a quite high resistance most of the time, there's no way that circuit can power anything useful in this context. You can measure the voltage you want because a multimeter won't load the circuit, ie it looks like an open circuit, it doesn't need much current. The moment you connect a load, it will look like a short circuit compared to the pot and therefore all the voltage shows up across the pot, none for the load.
You might try using a simple BJT emitter follower, with a divider set at the base of the BJT and actually measure the emitter voltage with the motor load installed to adjust until it is 3V. Considering that you already have some parts and might want to minimize the changes, you could include your 14k as a parallel load like you already did while adjusting to set the voltage before attaching the motor. If you are stuck with the 100k pots, too, and given that you have 24V to work with you might prefer a Darlington arrangement (use two BJTs) to require a bit less base current and thus a bit less adjustment after adding the motor. Do you follow? You won't be able to supply exactly
24V with this, but you should be able to get within a volt or two of it, should you need that much.
One issue will be the voltage dropped by the BJT at some current level. If the 3V motor really does require something like 100mA, then the BJT (or Darlington arrangement) will need to drop away 21V. The power at 21V times 100mA is 2.1 watts continuous. That's probably going to really hurt with a small TO-92 package as seen for the cheaper 2N3904 or PN2222. So you may need to pick something in a larger package. (This is an area I'm not as familiar with, so I'll leave high quality specific recommendations to others.)
There are other considerations too and since you haven't disclosed a lot of detail about things, a proper design is 'difficult.' But given where you are at, right now, the above might be okay.
Yeah. It's not efficient. However, it's the "It has been 20 years since I studied electronics in high school, so I may have forgotten some of the basics," part that made me figure that incremental steps are better for now.
Thanks to everyone for the advice. I appreciate your effort to keep it simple.
If I understand correctly, here is what you guys are telling me: My design is wrong because the voltage at B1, B2, and B3 will change depending on the current required by the devices I plug in there. I might be able to get it to work with minor changes, but even in the best case it will be very inefficient. In order to redesign the circuit properly, I should research BJT emitter followers, Darlington arrangements, and SMPS regulators. I'll do that when I have time and see what I come up with.
Look into the LM317 adjustable voltage regulator. It will still be inefficient, but might be easier than building your own regulator from transistors.
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If you're considering the SMPS route (significantly more efficient, but also significantly more complex), the following datasheet should give you a reasonable idea of what's involved in using a switching regulator:
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A linear regulator such as the LM317 is a lot simpler to use, but very inefficient when the output is significantly lower than the input (e.g.
12.5% efficiency for a 24V->3V conversion). Apart from wasting energy, getting rid of the heat could require a sizable heatsink and possibly a fan.
There isn't much point in building a linear regulator from discrete components.
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