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| How to use the same RELAY on 12V & 24V vehicles |
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| soldar:
--- Quote from: Zero999 on August 10, 2019, 06:11:38 pm --- Interesting point. I don't know whether the load would be constant or not. I would have thought that a turntable would have more friction when the stylus is on the outside and less when it's in the middle. --- End quote --- Yes, correct. And yet the load is so small and the force of the motor so multiplied by the speed reduction that the motor won't even notice it and the difference in speed will be negligible. And the same thing happens with the cassette spindle. The difference in motor load between the tape at the beginning and the tape at the end is negligible and the motor will not even notice it. You would need extremely precise instruments to measure it. The speed stabilization does not have to be 100% perfect, just good enough that a listener cannot hear it. Even if between the beginning and the end of a tape the speed would slowly change by a very small factor nobody would even notice it. I bet there were greater differences in speed between players. So, I say that with constant load, as is the case, the circuit maintains the motor speed. I don't think I have any cassette players any more but if I did I would not mind doing some tests. (My apologies to the OP for the hijack.) |
| Zero999:
The cassette players I've looked at have a motor with an integrated speed controller, which uses an IC to sense the motor back-EMF and keep it constant. |
| james_s:
--- Quote from: Zero999 on August 10, 2019, 06:11:38 pm --- --- Quote from: soldar on August 10, 2019, 01:09:31 pm --- --- Quote from: Zero999 on August 10, 2019, 12:52:44 pm --- He is right. Delivering a constant current to a permanent magnet DC motor will not result in a constant speed, but a constant torque. Voltage regulation is required to achieve a constant speed. Even if you're wrong about the original application of the circuit, you're right that it is suitable for enabling a 12V relay to work with either 12V or 24V. --- End quote --- Would constant torque with a constant load result in constant speed? Most or all turntables I have had worked with shaded pole motors which will slow down with increasing load but the variability of the load was so insignificant that it was considered constant. I think it would be the same with the capstan of a cassette player. The load would be pretty much constant . --- End quote --- Interesting point. I don't know whether the load would be constant or not. I would have thought that a turntable would have more friction when the stylus is on the outside and less when it's in the middle. --- End quote --- The load on a tape deck motor is not constant at all, it requires a fair bit of torque to drive, especially when you have things like fast-forward and rewind, some tapes have a lot more friction than others. The torque required to drive a turntable though is negligible, tracking force on the stylus is on the order of a few grams, if you can measure a difference in torque required depending on the position of the stylus then you must be hogging out a trench in the record. |
| JDW:
Seems like I should have titled my opening post “... & tape decks” :palm: |
| james_s:
Your question was answered already wasn't it? It's not unusual at all for threads to wander into other topics once the question has been answered. |
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