Trouble with a 7805 regulator.

Ok I am trying to power a servo motor with a regulator dropping 9vs to 5. I have checked the sketch and plugged the servo directly into the 5v from the arduino and know that it is working fine. However, when I try to wire it to the regulator I am maybe seeing the servo move a small amount and stop. I have tried hooking it up with many schematics, turned the regulator around to make sure I had the right pins for the input and output, and have even tried using many different caps across the regulators input and outputs. I am not sure what I am missing here. Do the caps have to be exactly calculated? Am I not supposed to run them from the positives from both input and output to ground? I thought I was following the schematics correctly and I have made sure all the ground leads go back to the same point... I am lost on this one.

Assuming the servo takes 800mA, your power dissipation will be:

(9-5) X 0.8 = 3.2 watts
That is quite a bit, are you using a heatsink?
I am assuming that you are using a TO-220 package.

See attachment.

reg_7805.jpg

I am lost on this one.

Me too. sounds like you have just randomly connected your 7805 and possibly damaged it. Below is one way to setup a 7805 regulator for use with a standard servo.

edit: If you are trying to use a 9v "transistor radio" battery to power a servo, that probably won't work well.

zoomkat:

I am lost on this one.

Me too. sounds like you have just randomly connected your 7805 and possibly damaged it. Below is one way to setup a 7805 regulator for use with a standard servo.

I had not seen a schematic using a diode for this yet. The schematic I was trying was something like this. The only difference was a .1 cap used on the input (9v) and a 22 uf cap used on the output (5v)

7805datasheet.gif

Riva:
Is the ground from the 7805 connected to ground on the arduino?

The ground from the arduino is on the breadboard rail as is the ground from both power supplies and the regulator ( the caps also run to the same ground.)

All linear regulators are capable of becoming unstable (oscillating) at certain loads, so it is always advisable to use an electrolytic and ceramic cap on both the input and outputs.
It also helps with regulation.
The electrolytics are for low frequency but are not very effective at higher frequencies hence the ceramics in parallel.
It's also worth mentioning that the electrolytic on the output of the regulator must not be too high or you risk damage to your regulator due to current dumping at switch off and surge current at switch on.

UnoDueTre:
All linear regulators are capable of becoming unstable (oscillating) at certain loads, so it is always advisable to use an electrolytic and ceramic cap on both the input and outputs.
It also helps with regulation.
The electrolytics are for low frequency but are not very effective at higher frequencies hence the ceramics in parallel.
It's also worth mentioning that the electrolytic on the output of the regulator must not be too high or you risk damage to your regulator due to current dumping at switch off and surge current at switch on.

I didnt try using any electrolytic caps however, I did try a 100nf ceramic cap on both the input and output at one point. Maybe this is the problem?

At least if it is nothing more than damage to the regulator I can just pick up another one tomorrow. I was just unsure about the wiring or the needed caps

Forgot to mention, place the caps as close as possible to the regulators input and outputs.
Look carefully at the schematic I posted before.
Notice how all the cap grounds and regulator ground go to one point?
Then you can run it (using as thick a wire as possible - within reason) to the common ground of the rest of the circuit.

UnoDueTre:
Forgot to mention, place the caps as close as possible to the regulators input and outputs.

I had them on the very next opening by the input and output and running to the ground coming out of the regulator so they were as close as they could be XD

OK, try it with the schematic I posted.
If it still does not work then suspect the regulator.

Will do

Don't know if this topic is dead since this morning but I went out and bought a micrometer to test my regulator and I have discovered the problem here. I need 4.8 volts min to power this servo. After everything is connected I am getting an output of 4.6 from my 5v regulator soooo how do i get the extra little umphh to run this thing?

Buy a power transistor and a heatsink.... (then look up a pass transistor circuit)

Or use a switching regulator (easier, more efficient. ..)

I have plenty of pnp transitors lying around but no heat syncs. I guess a transistor is the only option to getting any more juice out of the thing? Is it normal for a 5v regulator to be regulating at 4.6v lol?

4.6V from a 5V regulator?
Either that is a very poor quality regulator or you don’t have enough voltage on the input.
You can always “bump” up the voltage a bit by placing a diode on the ground pin of the regulator as long as the heatsink is not connected to ground.

See below.

EDIT:

Are you sure your servo is not drawing too much current (in excess of 1A) and overloading the regulator?
Did you measure the 4.6V with or without the servo connected?

reg_7805_2.jpg

UnoDueTre:
4.6V from a 5V regulator?
Either that is a very poor quality regulator or you don't have enough voltage on the input.
You can always "bump" up the voltage a bit by placing a diode on the ground pin of the regulator as long as the heatsink is not connected to ground.

See below.

EDIT:

Are you sure your servo is not drawing too much current (in excess of 1A) and overloading the regulator?
Did you measure the 4.6V with or without the servo connected?

I've only ever got 4.6~4.7v from all my 0805 regulators....

You could use a voltage divider (resistor & potentiometer) to tweak it if you need an accurate voltage.

Most of the more well known manufactures have better specs than that or make different grades of regulators, for example Fairchild has a "CT" and "ACT" range where the 7805ACT is quoted to be between 4.9 and 5.1 volts.
The ones from ST micro are quoted to be between 4.8 and 5.1 Volts.
The best ones seem to be made by Ti which I always try and use and normally get between 4.95 and 5.05 Volts.

Hi, measure the voltage going into the regulator and can you tell us what you are using as a 9V supply.
Also the circuit posted by zoomcat with the diode in the lead will do the job of supplying 5.7V or close enough to get the servo running.

Tom.... :slight_smile:

KvotheComplex:
Don't know if this topic is dead since this morning but I went out and bought a micrometer to test my regulator

A micrometer would be useful to check how large the regulator is, but a multimeter will check the voltages. :smiley:

You need at least a couple of volts more to feed the regulator, than you expect to get out of it. If you are using it only to power the servo (but likely feeding the same input voltage to "Vin" on the Arduino to use its internal regulator) then you can "jack up" the voltage an extra two volts by putting an LED in the reference terminal as illustrated above for plain silicon diodes, or use one to three of the silicon diodes in series to increase by 0.65V per diode.

We really need to know the voltage you are feeding into the regulator - particularly when the servo is connected and perhaps misbehaving. Something better than a "914" battery. A 780x regulator should generally be capable of supplying a small servo quite well - better indeed than 5V from the Arduino supplied through the USB connection, so this problem is quite curious. If too much current were being drawn, you would know as it would get very hot. For continuous operation, you should have a heatsink on it anyway; you should not need a pass transistor for 1 Amp (maximum).

You definitely need the 0.1µF bypass capacitors on the input and output of the regulator to ground (and they go to ground even if you have the series diode(s) in the reference terminal). You should not need a larger capacitor on the output, not because it will cause damage as such (and it will definitely not cause damage from inrush current as by definition, the regulator always limits this), but because the regulator is - a regulator and that is its job!

Paul__B:
You should not need a larger capacitor on the output, not because it will cause damage as such (and it will definitely not cause damage from inrush current as by definition, the regulator always limits this), but because the regulator is - a regulator and that is its job!

According to Power management | TI.com

The chances of this happening in a commercial environment? Pretty slim.
The chances of this happening in a hobbyist environment, experimenting on breadboards? Pretty good.
So why take a chance?