Dropping voltage

[bazjones1711]

Evening all

As can been seen in the pic i am currently building a working overhead crane , will be using  a 24 volt geared motor reclaimed from an old phone card dispenser ! this is linked to a 4ft threaded bar which will have a captive nut to make the longitudinal travel , the problem is that not having a 18 - 24 volt transformer , as running the motor of 12 volts the travel is to slow , i have another transformer from an unknown electrical item that has two outputs one at 16 volts ( which still runs a bit to slow) whereas  the other at 32 volts is to fast ! so is there a way to reduce the 32 volt output to around 20 volts which i think will be just about the right speed ,  ie; with capacitors or the like ?

Hopefully some one may have the answer  regards Baz

 

Edited July 22, 2018 by bazjones1711

[Ray H]

Do you know what current the motor is drawing?

 

It may be possible to use a suitably rated resistor to drop the voltage across to the motor

[bazjones1711]

This is the motor I will be using , as I'm not that electronically minded would not know what current it is drawing

[cliff park]

One of the difficulties of using a resistor is that the current drawn by the motor (and hence any volt drop across a fixed resistor) will vary, quite considerably with load. If the load is reasonably constant the best way would be to measure the current, and use that value to calculate a suitable resistor. I suspect it will be low value, a few ohms, but will need to have a large power rating. A much better way would be, as I think you suspected, to have a supply of the correct voltage to start with. You could add a variable voltage supply ( ie a good old fashioned DC train controller ) in series with your 12 volt supply, giving you a voltage variable between 12 and 24 approximately. You could then measure the voltage which gives you the speed you want, then 'organise' a supply of that voltage.

[meil]

Evening all

As can been seen in the pic i am currently building a working overhead crane , will be using  a 24 volt geared motor reclaimed from an old phone card dispenser ! this is linked to a 4ft threaded bar which will have a captive nut to make the longitudinal travel , the problem is that not having a 18 - 24 volt transformer , as running the motor of 12 volts the travel is to slow , i have another transformer from an unknown electrical item that has two outputs one at 16 volts ( which still runs a bit to slow) whereas  the other at 32 volts is to fast ! so is there a way to reduce the 32 volt output to around 20 volts which i think will be just about the right speed ,  ie; with capacitors or the like ?

Hopefully some one may have the answer  regards Baz

 

IMG_20180721_183944.jpg

So if I have understood you correctly you are feeding AC current into a DC permanent magnet motor? Does it still work?

 

That's probably one of the quickest ways to destroy a DC motor.

[John ks]

You could use a buck boost DC-DC converter.  The ones shown have inputs from about 6V to 30V DC & output from near 0V to 30V DC

One of these should work from either your 12V or 16V but not your 32V DC power supplies

 

Look here or here for a couple of examples

 

John

 

 

 

[davetheroad]

A variable voltage regulator should do the job, use it to boost say 12V DC to 20V DC. Try Pololu types, I use them extensively in my battery powered trains

[Gordon H]

An alternative to the resistor idea is to use a string of series connected diodes which will give a better defined voltage drop (~0.8V each) which won't vary so much with load. Use two such strings connected back-to-back to allow both polarities to work. A similar effect could be gained by using two reverse biased zener diodes in the same fashion, but strings of multiple conventional diodes would distribute the power dissipation better.

[Ray H]

An alternative to the resistor idea is to use a string of series connected diodes which will give a better defined voltage drop (~0.8V each) which won't vary so much with load. Use two such strings connected back-to-back to allow both polarities to work. A similar effect could be gained by using two reverse biased zener diodes in the same fashion, but strings of multiple conventional diodes would distribute the power dissipation better.

 

It would also have the advantage if the diodes are arranged appropriately of allowing am AC transformer to power a DC motor would it not?

 

We have used a string of diodes on the output from one transformer to provide us with the lower voltage that we require for some of our club's layout circuitry.

[sharris]

So if I have understood you correctly you are feeding AC current into a DC permanent magnet motor? Does it still work?

 

That's probably one of the quickest ways to destroy a DC motor.

 

I'm assuming that if the motor's running too fast rather than just buzzing that there must be some rectification happening somewhere. If so, then an LM317 variable voltage regulator with a couple of resistors and capacitors should do the job - just do a google image search for LM317 and you'll find plenty of circuit diagrams.

 

If you prefer to go the 'string of diodes' route, to get from 32 down to 20 volts you'll need about 18 diodes.  

Edited July 23, 2018 by sharris

[Crosland]

Try an old laptop power supply. They are DC and generally more than 12V and less than 24V.

 

When you say "transformer", what exactly are you referring to? As has been said, you should not be feeding a DC motor with 50 Hz AC from a transformer. 

 

If you mean and old DC controller or a power supply does it have half or full wave rectification? Any smoothig? Any voltage regulation?

 

If it's DC then you can divide the supply with a couple of electrolytic capacitors of suitable spec, but I would not bother and would do the job properly.

Edited July 23, 2018 by Crosland

[sharris]

Try an old laptop power supply. They are DC and generally more than 12V and less than 24V.

.

 

18.5V - 19.5V is quite common for Dell, HP and Acer - which sounds about right for this application. 

[bazjones1711]

thank you all for your feedback , browsing Ebay think this may solve my problem , probably need a 300 RPM one then i can control the speed hopefully , i will be looking for around 200- 250 RPM so wiring this into my track controller  should  make things easier 

https://pulsar.ebay.co.uk/plsr/clk/0/SADS/9?pld=%7B%22mecs%22%3A%22273106614581567ac6d9194b477d99fc1cef3ae236cc%22%2C%22enc%22%3A%22AQACAAACoGG2MFvPesRrmkAcNxl36F%2FFn5wG2zu4t1WklJAowyhhQ53CcsRoaiCLObhkK65vXTFdAPRog4GSlbJOmtno7mrX9n9mMRaYfCrW9SoxJQzuzg84JxJNnFGnhIEiKqKpz4QY1iQqcfBVVIEBGi%2B2zOpeA2o3UaYsZlX10JUIbBgKA7Ib2AFe4ppQgjP6KHaKXE0%2BTU%2FI7ETUrKEMmCciMvm8hxZ63XH7w4Uql7NcGXophwU2gI%2Fzdwq3Uxray2K6lJUHnLPG%2FFcRL8Xs%2F5lM8PjjcB3mzyi%2FOkEWmo4wxgGZTUoDxQ3Y4fJOM4ntATLNGh98WRtL5zb1KB6%2BS1gQD1ShvQuRY7P0zoVtxZGgfJCk1NAvzDeJdnsLv6xUZiv1uQGOLcqE6Ls7g%2BTzryfE31SGsS2ocfyyqi3QUJtRXhxpflKmVrieKu64A4KEnSo3OmtiZXo%2FYulbAXJITaxWMPPyGtFWYnVUojAVSQPE5KwHH32zvqo5%2FTHA1%2F6qO%2FQAN2lI%2FtkeVCDpl5g5JjWVDDM1L%2B0oA1vwicLnTZMq0lC49kzPBTK4HH2K2KkVzfDRlj8g%2BTgKe8xf%2FC4hQAFDqj6bCl7ncRhrxoL8vwLYDtVHmNFVDZKFb3HZH1eJUawlULcnxHwgkzUYiGlMMzOwdGM9%2FiEV1jPJz1tvIokoGQiSDbe24BD%2F0Tc%2Fpf%2Ba6X6XBRwtD7NiicnwiSb4mVAgMVUb44z0Cwia%2B4IBQURtlwDK0bGUCw7%2FQxqNkVQt7r0oyXVXURI1YwZ1rMFOGfI0RfIiAI%2Br7m3U3X3DOSx%2Ft6dSpbbc4Hpk5rnL1fF6CQTqF3ToODsi5p4zxfLmByqRR5YlXCIiLibJAj3%2BawoJhNsLTHEkruu52tZdIraZlDxaJA%3D%3D%22%7Dgards

 

Regards Baz

Edited July 23, 2018 by bazjones1711

[Suzie]

You could use a buck boost DC-DC converter.  The ones shown have inputs from about 6V to 30V DC & output from near 0V to 30V DC

One of these should work from either your 12V or 16V but not your 32V DC power supplies

 

Look here or here for a couple of examples

 

John

 

These are ideal, I have used the first example. Just adjust the voltage with a screwdriver until you are happy with the speed. They can be powered from 12V DC and should give 20V DC no problem.

[DCB]

My way to drop DC voltage  is to put a bunch of diodes in series, (all the same way round obviously).   The voltage drop across diodes is pretty much constant as opposed to using resistors where the voltage changes dramatically with load.    Some folk say diodes droop voltage by 1.2 volts but in my experience its nearer 0.7.     4 Amp diodes should be about right for your application. Shouldn't be more than 10 p a throw so the whole thing should be less than a quid.

 

I use 1N400 diodes strung around a 12 position rotary switch for a rudimentary speed controller on a 12 volt output, 2 diodes per speed 10 speeds so 20 diodes.  They cost me less than 50 p plus £1.50 for the rotary switch and 50p for the DPDT reversing switch which makes a very cheap controller.