# How to determine what resistor to use to reach the desired amperage?

Discussion in 'General Electronics Discussion' started by Glitchsteal, Dec 16, 2014.

1. ### Glitchsteal

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Dec 16, 2014
Hey all,

I'm pretty new to the electronics scene, so I'm also pretty bad at it and there are plenty of things to learn from you guys

Basically I'm hooking up a bunch of PC fans to cool down a bank of electronics equipment at the sound desk at our school. I'm going to use a 12VDC 1A transformer, then hook it up to a PCB, run it through some resistors to then power the fans, as they all run on 12V, just different aperages. I was wondering how I would calculate the value of the resistors I need to, for example, change the 12V 1A load to a 12V, 0.7A load for one of the pc fans? there are a bunch of other different bearings for the other fans so if you could explain how as well that would be great thanks

2. ### davennModerator

13,802
1,941
Sep 5, 2009
hi there
welcome to the forums

in the case of fans it doesn't matter. The fans will only draw the current they need from the 12V supply
connect the fans in parallel across the 12V DC supply.

just ensure that the total current drawn by ALL the fans adds up to less than 1A
else you will need a bigger PSU

cheers
Dave

2
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Dec 16, 2014
thanks!!!

4. ### Bluejets

4,673
984
Oct 5, 2014
No need for any resistors.
The motors will draw only what they need.
You will NOT however run "a bunch of these motors" as just this one is already 70% of the loading that your 1A power supply will handle.

5. ### Gryd3

4,098
875
Jun 25, 2014
Well... here's where things get interesting. You are missing some vital details that you could pick up in Physics, or an entry level electronics class.

Here is a very very very important relationship
Voltage = Current * Resistance

So... what happens when you connect a resistor in-line with something?
It does not simply lower the amperage and leave the voltage alone... it increases the resistance of the entire circuit, which will allow less current to flow but the kicker here is that there will be voltage lost across the resistor as well... So that 12V fan, may only be getting 7 or 8 V depending on the resistor you chose.
Here is something else to consider as well...
The fan will have a resistance, the power supply you have will provide a voltage. Using the relationship above, the fan will only allow so much current to flow through when used at that voltage... There is no need for a resistor here!
Here is what you may need to worry about though. It sounds as though you are using multiple fans.
If all fans are 12V fans, they must be connected in parallel so that each fan receives 12V. This also means that your power source must be able to supply the sum of the current of each fan.
This means that if each fan is rated at 12V and 120mA, then you will only be able to run 8 fans at MOST... and this leaves you with almost no headroom. (1A = 1000mA ... 1000mA / 120mA = 8)

6. ### davennModerator

13,802
1,941
Sep 5, 2009
no probs

for many things it doesn't matter, as I stated earlier
some things it does like LED's (Light Emitting Diodes) for them you do need current limiting resistors

Dave