Change in current

Change in current

When I used the adaptor of 12 volt 3 amp, then the current takes the load approx ##2.5 ampere## but when I used the adaptor of 12 volt 5amp then the current takes the load approx ##0.36amp##.what was the reason behind it? WhatsApp Image 2020-07-15 at 5.50.40 PM


Great @Farhan, :slight_smile:

Check if I understand you correctly.
You say with 12V 3A Adapter, the load draws 2.5 A.
And with 12V 5A adapter, it draws 0.36A.

There could be multiple things that you need to clarify before we get at the reason. :thinking:

  1. What is the type of load? It looks like a motor in the picture.
  2. How did you measure the current drawn by the load? There could be some issue with the measurement itself?
  3. What other indicators did the load show when connected to the power supply?



Keeping the load constant

P = VA
P = 2.6×12
= 31.2w

If the current drawn is 0.36 amp then the voltage must be different

V= P/A
= 31.2/0.36
= 86.6 v


Notes from Chatshaala (16/07/2020)
Rating of the Motor: 12V 2.6A.

Observations made by Farhan:

  1. 12V 1A Adapter — Motor was not working (current not checked?)
  2. 12V 2A Adapter — Motor was slowly running without load (was just vibrating. How much current?). Not running with a load.
  3. 12V 3-4A Adapter — Motor was working perfectly, 2.5 A (with load) Sounding good :smiley:
  4. 12V 5A (SMPS) Adapter — Motor was working perfectly. initial current 1 A and steady-state 0.36A (with load) This appears fishy!

Measured using multimeter (MAS 803L).

Jude: Motor is an inductive load. What is the kind of power supply? Switch Mode PS may not be able to work very well! But if we use a transformer-based supply, and attain gradually the require speed. Measurements on SMPS needs to be made using a scope and multimeter may not give reliable measurements. What @Farhan might be measuring is a back emf.

Ashish: What is kind of adapter 12V 3-4A? Farhan: It is also SMPS.

We need to unpack the underlying mechanisms.

Farhan: How do you identify what kind of load (resistive, capacitive or inductive)? For e.g. router. The size of this could be very small?

Buck and boost circuits: How do they work? If I use a MOSFET at the output,

Action Plan: Check what happens when resistance, capacitance and inductor are in series and parallel.
Explore Capacitive adapters?? Inrush current and Surge currents with respect to motors and what ?? What is the response with other motors?


SMPS have a cycle by cycle current limit. Consequently one needs an inrush current limiter to start correctly. Also one requires a quenching circuit to keep the motor back emf from flowing back into the PSU. Often such back EMF will cause other circuitry powered by the supply to malfunction.

In the case of equipment like routers the inductors on board are operating at gigahertz frequencies. Consequently inductors in routers are tiny, with values of a few nano henries.


Also, @jtd, did you say something about inductive adapters, the other day?

@Farhan @Ashish_Pardeshi @sandysat and I were wondering if there is something like that, the next day!


I said driving inductive loads like motors from SMPS always poses problems.


An inductor, also called a coil, choke, or reactor, is a passive two-terminal electrical component that stores energy in a magnetic field when electric current flows through it. … As a result, inductors oppose any changes in current through them.
The current cannot change instantaneously in an inductor because for this to occur, the current would need to change by a finite amount in zero time which would result in the rate of current change being infinite, di/dt = ∞, making the induced emf infinite as well and infinite voltages do no exist.