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Using Arduino => General Electronics => Topic started by: eboudreau26 on Feb 22, 2019, 09:09 pm

Title: Issue with 7809 1.5A Linear Voltage Regulator
Post by: eboudreau26 on Feb 22, 2019, 09:09 pm
Hello,

I've been using a voltage regulator (which has a heat sink) to try and convert a 12V power supply into 9V to power a few motors. I need a little over 1A of current. However, the current seems to be maxing out at about 0.6A when I use the voltage regulator. The motors with the same circuiting work fine when I use a 9V supply with a max of about 1.2A and don't use the voltage regulator. 

Does anyone know what the issue could be here? This is my first time using a voltage regulator.

Thanks
Title: Re: Issue with 7809 1.5A Linear Voltage Regulator
Post by: wolframore on Feb 22, 2019, 09:37 pm
please post schematics. Is the regulator getting hot?
Title: Re: Issue with 7809 1.5A Linear Voltage Regulator
Post by: DVDdoug on Feb 22, 2019, 11:33 pm
The 1.5A rating doesn't tell the whole story...  The heat/power dissipation usually limits the amount of current you can get.  

The good news is that the 79xx regulators are thermally protected so they don't permanently burn-up and die when they get too hot.   They just shut-down, or partially shut-down.  

It's easy to calculate power dissipation - You're dropping 3V across the regulator at 1.2 Amps, so that's 3V x 1.2A = 3.6 Watts.

What's NOT always easy is calculating the thermal characteristics of the heatsink.   You may need a bigger heatsink, or heatsink grease, or a fan (or cooler environment for your heatsink), etc.

Or, one (not very elegant) solution is a separate voltage regulator for each motor.

The best solution is usually a switching (aka "switchmode") regulator.    Switching regulators are more complicated so it usually best to buy a complete regulator board, but they are nearly 100% efficient so they don't have the same heat problems.      

The efficiency also means that you can get 1.2A out of the regulator with less  than 1.2A in (when the voltage is stepped-down).
Title: Re: Issue with 7809 1.5A Linear Voltage Regulator
Post by: FantomT on Feb 23, 2019, 12:39 am
Voltage regulator short-current protected, limiting output is not gonna to work well with a motor, as it''s need x10 times current at start-up - known as stall current.
Title: Re: Issue with 7809 1.5A Linear Voltage Regulator
Post by: eboudreau26 on Feb 23, 2019, 06:37 pm
Thanks for the replies, and yes, the voltage regulator is getting very hot. I'll look into getting a bigger heatsink and/or putting it in a cool environment, and if that doesn't give me the required current output then I'll look into a switching regulator.

I'm also extremely new to this; I'm a mechanical engineering student working on one of my first real Arduino projects. So, the less complicated the better :P.
Title: Re: Issue with 7809 1.5A Linear Voltage Regulator
Post by: eboudreau26 on Feb 23, 2019, 07:25 pm
What do you guys think about me buying a 9V output 2.5A buck converter? Again, the extent of my knowledge on these things is basically just a few articles and youtube videos. Does it connect it in a similar way, and I'd just simply connect the supply 12V and the ground to the input and ground of my buck converter, and the output and ground would connect to the rest of my circuit, giving me 9 volts and hopefully enough current to run the motors? 
Title: Re: Issue with 7809 1.5A Linear Voltage Regulator
Post by: MarkT on Feb 23, 2019, 10:23 pm
Hello,

I've been using a voltage regulator (which has a heat sink) to try and convert a 12V power supply into 9V to power a few motors. I need a little over 1A of current. However, the current seems to be maxing out at about 0.6A when I use the voltage regulator. The motors with the same circuiting work fine when I use a 9V supply with a max of about 1.2A and don't use the voltage regulator. 

Does anyone know what the issue could be here? This is my first time using a voltage regulator.

Thanks
Much better to feed 12V directly to the motors, and use PWM to set the speed, these days we use switching
to regulator power to loads like motors, not the series resistance of a regulator - much less waste
power and heat.
Title: Re: Issue with 7809 1.5A Linear Voltage Regulator
Post by: DrAzzy on Feb 24, 2019, 01:16 am
Switching regulator to generate the necessary 9v is an appropriate solution.
Title: Re: Issue with 7809 1.5A Linear Voltage Regulator
Post by: TomGeorge on Feb 24, 2019, 07:31 am
Hi,
Can you post a picture of your project please, so we can see your component layout?
Can you please post a copy of your circuit, in CAD or a picture of a hand drawn circuit in jpg, png?

Have you got the bypassing capacitors that the spec sheet recommends on the 7809?
(http://forum.arduino.cc/index.php?action=dlattach;topic=599411.0;attach=296277)
What is the 12V supply?

When you have this problem, what is the voltage from the 12V supply?

Thanks.. Tom... :)
Title: Re: Issue with 7809 1.5A Linear Voltage Regulator
Post by: elvon_blunden on Feb 24, 2019, 08:05 am
Have you got the bypassing capacitors that the spec sheet recommends on the 7809?
Please indulge me with a question slightly OT.... if the 78xx spec sheets recommend certain caps, why don't they just build them in? (Implication of not doing so is that there are cases where no / different caps are better?)

Title: Re: Issue with 7809 1.5A Linear Voltage Regulator
Post by: Smajdalf on Feb 24, 2019, 08:17 am
Please indulge me with a question slightly OT.... if the 78xx spec sheets recommend certain caps, why don't they just build them in? (Implication of not doing so is that there are cases where no / different caps are better?)
I think there are two reasons:
1) Different application may need different caps.
2) Integrating a cap large enough is more expensive than adding an external one. (I think this is more important here.)
Title: Re: Issue with 7809 1.5A Linear Voltage Regulator
Post by: TomGeorge on Feb 24, 2019, 08:18 am
Please indulge me with a question slightly OT.... if the 78xx spec sheets recommend certain caps, why don't they just build them in? (Implication of not doing so is that there are cases where no / different caps are better?)
It would be difficult to do at a substrate level, and yes values may have to be tailored for different conditions.
Without at least this minimum you can get the regulator oscillating or in, I suspect this case, poor regulation to changes in load current.
Tom.... :)
PS. A valid and not an OT question.. :)
Title: Re: Issue with 7809 1.5A Linear Voltage Regulator
Post by: elvon_blunden on Feb 24, 2019, 08:26 am
Thanks TomGeorge and Smajdalf

Title: Re: Issue with 7809 1.5A Linear Voltage Regulator
Post by: eboudreau26 on Feb 25, 2019, 06:13 pm
Ok, this is a fritzing schematic I made a while back, but it should give you all a good idea of what I'm trying to do. I have MG996R Servo motors and 12V stepper motors. So the voltage and current specifications for the steppers and servos aren't the same, but I tested it with a laboratory DC power supply and I can get all of the motors to run at a good speed when I'm giving them 9V and about 1.2 A max. Basically I'm trying to find the cheapest and easiest way to run all of these motors at a decent speed, since I'm very low on time, knowledge, and (especially) funds.
Title: Re: Issue with 7809 1.5A Linear Voltage Regulator
Post by: TomGeorge on Feb 25, 2019, 09:23 pm
Hi,
OPs fritzy;
(http://forum.arduino.cc/index.php?action=dlattach;topic=599411.0;attach=296495)
For a start you are going to need more than 2A to drive that lot, what do you expect the current to be when they are driving a load?

Can you post links to specs on the three steppers and to the stepper driver modules you are using?
If your layout is exactly like that, then I'm surprised that it is performing at all, the protoboards are not rated for much current, especially three steppers.

If you are supplying the 5V from the servo end, then try supplying  it from the UNO end.
All the servo and stepper current will be causing a volt-drop in the wiring that will possibly be putting noise on the supply that eventually gets to the UNO, causing it to reset.

Try putting 100uF or 1000uF capacitors on the supply rails as well as 0.1uF caps.

Currently you have everything daisy chained together power supply wise.
Really each servo and stepper and UNO should have its own power wires going back to a central pair of connections.

What are you measuring the current with and do you measure the 5V voltage when you project exhibits these problems?

I hope when you connect 9V to the motors to test, you disconnect the feed to the 5V pin of the UNO?

Can you post a picture of your project so we can see your component layout.

When you used the LM7809, did you put the recommended capacitors around it?

Tom... :)
Title: Re: Issue with 7809 1.5A Linear Voltage Regulator
Post by: eboudreau26 on Feb 26, 2019, 03:34 pm
Hi Tom,

I'm viewing the voltage and current readings on the laboratory power supply. It's at 9V, and it jumps up to about 1.2A when the servo motors run. Sorry, I guess this isn't the exact layout, everything is actually connected to a single larger and better protoboard. I ran all of these motors with an Arduino program fine, and I was running it for several minutes.

The servo motors require more current than the stepper motors, seemingly. The stepper motors are each connected to an A4998 driver as shown in the image, which is why I have the logic voltage and ground connected to the Arduino. The A4998 drivers also have a current limiting knob. The power supply voltage and ground are not connected to the Arduino in any way.
Title: Re: Issue with 7809 1.5A Linear Voltage Regulator
Post by: eboudreau26 on Feb 26, 2019, 06:39 pm
I guess I should have also mentioned that I'm never running more than two motors at the same time with my programming code... It's always 1 or 2 motors at the time. And I didn't use capacitors with my 7809... whoops again. But I'm getting a buck converter from one of my professors to test out, so hopefully I can get it to work.
Title: Re: Issue with 7809 1.5A Linear Voltage Regulator
Post by: Idahowalker on Feb 26, 2019, 07:33 pm
(https://www.eleccircuit.com/wp-content/uploads/2007/05/increasing-regulator-current-by-78xx-mj2955.jpg)

I have, in the past, used a design similar to the image. I have added in a diode to the ground pin of the 7805 to make up for the Vdrop of the transistor junction.. The diode will need to handle 1A. The transistor should be rated a bit higher then the Imax as well as the components feeding the transistor.