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Old Jun 27, 2013, 12:50 AM
20M northeast of Hell Mi
liljimmy's Avatar
United States, MI, Howell
Joined Jul 2012
318 Posts
LM317t

Good day I am using a lm713t with fixed resisters to regulate voltage for a “hacked “ servo to spin a radar I have a few winch motors that I would like to build the same setup (at a deferent voltage )
But the draw amps is a little bit more than a servo motor can I run the lm317t in a parallel to reduces the amps pulled with just one ? 2 volts 1.75 stall amp on two motors the other is 3 vdc at 1.5 amp stall the chances of a stall would be very slight do to a ratcheting clutch in the gear sets . I would build 3 sets of this, one for each motor To keep things cooler? the lm317t is 1.5 amp max
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Old Jun 29, 2013, 06:34 AM
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Joined Jun 2011
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Hi,
Check your datasheet and I think you may find there is a current limit (often poorly controlled) and also a power disappation limit. The latter may be the limting facor especially if dropping down from say 12V. What you could find is that the junction temperature rises too far and the chip starts shutting down and limits the output voltage. If you drop from say 12V to 3V you have 9V across the regulator. Multiply this by the current draw (1A say) and you have 9Vx1A =9W. This would require a heatsink to keep the chip from shutting down.
A switching regulator (Buck type) maybe more suited. These are effectively low loss power convertors so Watts in is roughly Watts out and hence if you are dropping supply voltage you also multiply output current. If you wanted 3V out at 1A (3W) then it would draw around 1/4A at 12V input (=3W)
Switching regulators are a little more involved to make so you may want to find a ready made board that meets the purpose. They may also radiate EMI so do a range check with the unit running.

Cheers,
David
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Old Jun 29, 2013, 09:41 AM
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United States, NJ, Livingston
Joined Mar 2012
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What about adding a small pot to the circuit, so you can tune it?
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Old Jun 30, 2013, 02:31 PM
20M northeast of Hell Mi
liljimmy's Avatar
United States, MI, Howell
Joined Jul 2012
318 Posts
Input voltage is 6 volts. The one used for the radar spinner is dropped to 2.5vdc I ran the vr for about 2 hrs it was a little bit warm just above room temp without any heat sink attached. (Side note) The radar spins at 17.5 rpm I found out the sps10 rotates at 15 rpm
My plan is to run a pot for the other winches so I can vary the speed if needed for adjustment.
I did not know about the EMI that’s very good advice. I already lost control of the ship one time in the pool and did some damage to the bow doors. I had put the receiver too close to the ESC and I have some rare earth magnets under the flight deck to hold on the causeways. When I moved the rx under the superstructure that nasty problem went away
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Old Jul 02, 2013, 01:38 AM
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Joined Jun 2011
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Hi,
6V down to 2.5V is not too bad (Pwr=3.5V x motor current). Provided your gear train and shafts are free to move the motor drain at this low voltage should be very modest. If you have a multimeter set to about 100mA you can see what it is but I think it should be around 45mA if alignment is good. This would be a bit under 160mW which you would detect as just warm.
Just check the 317 datasheet - I believe there is a minimum current needed for the divider chain to set the output voltage. This helps to swamp unit to unit variations in bias current in the common terminal. This will mean a low value preset pot (220R - 470R and not something like 10k.

I only wish it was easier to get tiny stepper motors like those used in camera lenses. A stepper can be driven at really slow speeds (via suitable electronics) and has the advantage that there is no gear train to crunch if accidentaly stalled.

Good luck.
David
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Old Aug 06, 2013, 10:34 AM
20M northeast of Hell Mi
liljimmy's Avatar
United States, MI, Howell
Joined Jul 2012
318 Posts
Getting close to finalizing the voltage regulators for the lights and motors. I added the heat sinks to the ramp motor regulators (just in case). Then ran the motors with my finger on the winch drums to load them down they barely got warm after several minutes of running them
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Old Aug 12, 2013, 05:32 AM
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Blackpool, Lancs
Joined Feb 2006
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Slowing a DC motor to a set speed from a fixed voltage is just a matter of regulating the current. The voltage regulator used like this is effectively acting a resistor. Probably simpler to determine the current that the motor needs for the speed and just use a series resistor. The voltage regulator only comes into its own when either the supply varies or the load varies, or both.
To increase the current being regulated by a voltage regulator, just use the output voltage to drive a power transistor configured as an emitter follower. The transistor will almost certainly need a healthy heat sink/fan to lose unwanted heat.
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Old Aug 13, 2013, 03:37 AM
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Legot's Avatar
United States, AZ, Gilbert
Joined Nov 2009
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Even simpler than the above (ok, not really, but definitely a more stable option), would have been to just use the lm317 in current regulator mode.

Constant speed no matter the voltage as long as its above ~9v!
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Old Aug 13, 2013, 05:07 AM
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Quote:
Originally Posted by Legot View Post
Even simpler than the above (ok, not really, but definitely a more stable option), would have been to just use the lm317 in current regulator mode.

Constant speed no matter the voltage as long as its above ~9v!
You still have to keep within both the current carrying limits and the power dissipation. Also, as the poster said, he is starting with 6 volts.
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