# Limit DC voltage draw

Discussion in 'General Electronics Discussion' started by SamH1, Jul 21, 2014.

1. ### SamH1

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Jul 21, 2014
Hello,

Quick question: is there some way I can limit how much voltage something draws on a power source? For example, can I limit a 24 volt motor to only draw 12 volts from a power source?

The issue I'm having is a motor that takes 12-24 volts tries to suck 24 volts out of a 12-volt power supply. The power supply is reporting "power trouble" as the motor is trying to get 24 volts- something the power supply can't provide. The "fuseless protection monitoring algorithm" then cuts off power. So... what can I do about this?

Thank you all so much for any help you can give me!
If I posted this in the wrong forum section or something, I apologize in advance!

Sam

2. ### davennModerator

13,833
1,950
Sep 5, 2009
Hi Sam
welcome to the forums

I think what you really mean is that it requires more current than what your power supply can handle

IF the motor really can run off 12V, as you stated. Then it will do so
BUT the current it requires will be more than what it requires at 24V to produce the same rotation speed and Torque power

IF you really only want it to run at 12V then you need a 12V supply capable of the required current
look at its label, the current should be stated, else google the datasheet for the motor model

Else you will need a higher voltage PSU, but still one that can deliver the required current

cheers
Dave

3. ### Gryd3

4,098
875
Jun 25, 2014
Please provide more details regarding the power supply you are using and the motor.
We are looking for ratings. Input voltage and amperage (current) for the motor, and output voltage and amperage for the power supply. Model numbers help as well.

Typically a device will 'pull' current from a device, not voltage. So supplying it with 12V should in theory run the motor at a slower speed. If the motor tries to draw too much current (Amps) then the power supply will either:
A- Dip and supply less voltage
B- Internally trip and shut off
C- Burn up and never turn back on.

Always be cautious when supplying energy to a device that it was not intentionally designed to do so. If in doubt, ask questions first. It may save you time and money

4. ### SamH1

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Jul 21, 2014
It's an electric strike. It is used for locking & unlocking doors. An internal motor (I think) does the unlocking.

So basically, when 12 (or 24) volts is sent to the strike, it unlocks. Even a 9 volt battery will make it unlock.

This strike will take up to 24 volts... so what I think is happening is that this strike is trying to get 24 volts from the power supply. The power supply senses the strike trying to get 24 volts and cuts power before anything gets damaged.

So I just need to somehow prevent the strike from being able to try to get 24 volts from the power supply.

5. ### KrisBlueNZSadly passed away in 2015

8,393
1,271
Nov 28, 2011
Hi Sam and welcome to Electronics Point

"Draw voltage" isn't a meaningful thing to say. Voltage is a quantity that's provided; it is measured between two points. Current is drawn by the device that is being supplied (this device is called a load).

A load is normally designed to operate from a particular voltage. If you don't provide enough voltage to it, usually it will not work properly; sometimes it won't work at all. If you provide too much voltage to it, many kinds of loads will be damaged.

Your solenoid (electromagnet) cannot "try to get" 24V from the power supply; even if that was a meaningful concept, the power supply can't tell what voltage the load wants, and even if it could, it couldn't (wouldn't) respond to that knowledge. You have to make sure your power supply is matched to your load.

That sounds like the load is drawing too much current from the power supply. If the solenoid is designed to run from 24V and you're only feeding 12V into it, it will draw about half the expected current. Assuming the power supply is rated to supply that amount of current, it won't be damaged, and it shouldn't cut out; all that will happen is that the solenoid will not pull in (or only pull in weakly) because the voltage and current are both half of what they should be.

Make sure you have the power supply connected with the right polarity. Some solenoids include a diode across them, for suppression of "back EMF". If you connect power the wrong way to them, the diode will conduct, and effectively short out the power supply, drawing too much current. That would make the power supply cut out.

Otherwise, tell us as much as possible about the striker solenoid and the power supply. Manufacturers, part numbers, links to data sheets.

6. ### Gryd3

4,098
875
Jun 25, 2014
Take a look at the replies above. A device will not 'pull' voltage from something.
From the sounds of it, what is happening, is the motor is trying to 'pull' too many Amps or mA from the power supply which is why it is failing.
You have two options:
A - Get a new 12V Supply (if that is what you wish to use) but make sure it is rated to supply enough Amps, or mA for your motor. (Ideal solution)
B - You can reduce the current draw of the motor by putting a resistor inline with it... this is far from ideal, as the resistor will need to be rated based on the current draw of the motor (which we don't know yet) and the result may be a motor that is far too weak to do anything.

For sake of argument, I will over simplify and tell you that voltage is how 'fast' the electricity is going, and current (Amps, mA, etc.) is how wide the pipe is. A device will not pull voltage faster than it is provided, but a device may have a wider path than the supply can deal with... this causes the supply to overextend trying to fill this wide path, which leads to failures mentioned above.

7. ### shumifan50

573
56
Jan 16, 2014
You could try and put a fairly large electrolytic capacitor in parallel to the battery. Solenoids tend to draw a lot of current for a short period of time otherwise they heat up and burn out. A large enough cap might provide enough current for long enough to handle lock/unlock.

8. ### KrisBlueNZSadly passed away in 2015

8,393
1,271
Nov 28, 2011
There's no point trying this. The solenoid or motor is designed to run at a certain voltage. The power supply must provide that voltage, and must be able to provide the current that the solenoid or motor needs. Dropping the voltage, using a series resistor, will reduce the current drawn from the power supply, but it will also reduce the voltage available to the solenoid or motor, so it will fail to work properly. Assuming the power supply voltage matches the rated voltage of the solenoid or motor.
That description doesn't match any analogy that I know of, and just confuses the issue. In the hydraulic analogy, voltage = head or pressure and current = flow. See https://en.wikipedia.org/wiki/Hydraulic_analogy.
Again that doesn't match the hydraulic analogy. A wide pipe can take a large or a small flow; it doesn't demand a high flow.

9. ### Gryd3

4,098
875
Jun 25, 2014
I'll watch my use of the analogies. I didn't know there was a wiki page specifically for it

10. ### davennModerator

13,833
1,950
Sep 5, 2009

It would have been really helpful to have told us all that in the first post. It would have saved all the wasted time talking about motors

AGAIN
as I said in my first post .... ( now edited to remove motor references)
IF the strike can really can run off 12V, as you stated. Then it will do so
BUT the current it requires will be more than what it requires at 24V to produce the same strike action

IF you really only want it to run at 12V then you need a 12V supply capable of the required current
look at its label, the current should be stated, else google the datasheet

Its NOT trying to get 24V from anything OK

what sort of power supply are you trying to run it off ?
tell us the DC voltage and current rating of it

cheers
Dave