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Adjusting voltage down
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Adjusting voltage down

by rhartl on Wed Jun 17, 2020 8:06 pm

I have a device that runs optimally at 4.5 volts and a power supply that runs at 5.1 volts. I switch it on and off with a MOSFET transistor. How should I reduce the voltage so it will run optimally. Thanks.

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Re: Adjusting voltage down

by adafruit_support_bill on Wed Jun 17, 2020 8:47 pm

You could add a diode in series. Typical silicon diodes have a forward voltage drop of about 0.7v. Just make sure that the diode you use is rated for at least the amount of current drawn by your device.

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Re: Adjusting voltage down

by rhartl on Thu Jun 18, 2020 10:34 pm

So that didn’t work. Even a chain of series connected 1N4004’s did not drop the voltage below 5 volts. I have tried a voltage divider and that cuts down the voltage but also limits the current enough to hamper the devices operation. Any other suggestions?

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Re: Adjusting voltage down

by adafruit_support_bill on Fri Jun 19, 2020 5:52 am

What is the load you are trying to power?
Even a chain of series connected 1N4004’s did not drop the voltage below 5 volts.

Do you have the load connected? If there is no current flow, there will be no voltage drop. The forward voltage drop through a 1n4004 at 100mA is about 0.6v and goes up from there.
https://cdn-shop.adafruit.com/datasheets/1N4001-D.PDF

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Re: Adjusting voltage down

by rhartl on Fri Jun 19, 2020 2:35 pm

It is a small battery powered toy. It consists of an electric motor and a disc attached to the motor that spins around and there are 5 leds on the edge of the disc. It runs on 3 AAA batteries ordinarily and draws about 180 to 200 ma. I am trying to power 4 of them with a 5 volt supply that will handle 10 amps. The disc will spin much more rapidly at 5 volts so I’d like to keep the voltage down where the batteries are 4.5 volts. Testing with my power supply I get pretty satisfactory performance between 4.1 and 4.5 volts drawing 180 to 200 ma.

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Re: Adjusting voltage down

by adafruit_support_bill on Fri Jun 19, 2020 2:41 pm

At those current levels, the diode drop for a 1n4004 should be in the 0.6-0.7v range which would be just about right.

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Re: Adjusting voltage down

by rhartl on Sat Jun 20, 2020 1:20 pm

So this is probably a dumb question, but does it matter if the diode is on the + or - side of the supply as long as it is oriented correctly? And is the drop in voltage a dynamic thing such that if the voltage is measured with a meter which does not draw much current not as much of a drop will be measured?

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Re: Adjusting voltage down

by adafruit_support_bill on Sat Jun 20, 2020 1:39 pm

does it matter if the diode is on the + or - side of the supply as long as it is oriented correctly?

It doesn't matter if it is on the high side or the low side. The voltage drop will be the same.

And is the drop in voltage a dynamic thing such that if the voltage is measured with a meter which does not draw much current not as much of a drop will be measured?

Yes. It increases non-linearly with current as shown in figure 1 of the datasheet: https://cdn-shop.adafruit.com/datasheets/1N4001-D.PDF
With minimal current flow, the forward voltage drop will approach zero.

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