Precautions to maintain before using a PC oscilloscope

In every stage of designing a circuit you need an oscilloscope to see whether you are getting the desired output signal or not, and if you are reverse engineering a circuit then oscilloscope is very much essential tool for that. A wide variety of oscilloscopes are available in the market and they are very expensive so most of the hobbyist choose to buy a PC oscilloscope. After some research I bought a cheap one for my basic experiments it’s a Hantek 6022BE, it has 20MHz of bandwidth and 48MSa/s of real time sampling rate. But before using it we have to cope with some problems that can blow this device in seconds.

Here is a awesome video by Dave from EEVBlog on this purpose.

Many of us use a computer SMPS as power supply personally I also use, but when you are probing a circuit with your PC oscilloscope which is powered by the SMPS you need to do a small and simple hardware hack in the power supply. At first there are two sections present in the SMPS, high voltage(HV) side and low voltage(LV) side. HV side and LV side are isolated either by isolation transformer or by opto-couplers.

But in most cases the ground of the LV side is connected with the earthing of the HV side and this is quite dangerous so our hack will be to isolate ground form earthing in HV side.

Now, we have to test the continuity to be sure that ground and earthing of LV side and HV side are connected. Probe the multimeter in the continuity mode if there is no continuity then the HV and LV side is isolated otherwise we have to connect the earth wire of the LV side to the HV side or the circuit

The metallic casing of the SMPS is earthed, so the screw mounts of the LV side needs to be isolated form the LV side circuitry by isolation tape I’ve used thick double sided tape.

From these pictures we can see that the earthing is attached only with the HV side screw terminal which is isolating the the earth and ground and after the hack in working condition a 5V output is tested.

I added a buck converter circuit to +12V output so that I can use it as a variable voltage output between 0-12 volts also drilled the casing and attached banana connectors to make the supply more portable.

Hacked! power supply from old SMPS

Written by : Subhadeep Biswas

 

Hacking the Switched Mode Power Supply

Hacking & something more on the SMPS

The SMPS or the Switched Mode power Supply generally used to drive our Personal Computers. It can also be used to drive other loads also so it is a versatile power supply.

Why SMPS is used in Personal Computers?

In the motherboard of a Personal Computer different sections needs different operating voltages those are +3.3V, +5V, -5V, +12V, -12V DC. The SMPS converts & switches 230V AC supply to those voltages respectively in a very short time so it minimizes the wastage of energy thus the efficiency increases. An ideal SMPS dissipates no power.

Stages of AC to DC conversion & Switching done in a SMPS:

 

Stages of conversion

Hacking the SMPS & using it as your own power supply

Using the Switched Mode Power Supply as a workbench power supply is not so difficult. Everyone can use it. Starting from beginner level, if you Plug in the SMPS and you will find it is not running. Well that not means that the device is damaged.

Running the SMPS

Step 1: Go for the ATX Power connector. It is the connector having 20 or 24 pins. 

ATX Power connector

Step 2: There is only a green wire(PS_ON). Now short PS_ON with any of the Black wire(COM/Ground) of the SMPS and Plug it again to AC. Now if the SMPS is working the fan will start rotating. Now you are able to use it as your power supply.

 

There is only one green wire(PS_ON) short it with and of the black wire(COM/Ground)

ATX Pin out

Step 3: Now check the voltages of the different pins using a multimeter. 

Reading 1

Reading 2

Reading 3

Step 4: Now use the SMPS as a power supply. I’ve use an old one as my workbench power supply. Here is a simple LED test circuit.

 

Test Circuit

 After this if the SMPS is not starting then you have to attach a dummy load to prevent that. In the casing of the SMPS you will find a chart which describes in which voltage level the current is maximum. If it is 5V then attach a 5Ω 10W resistor between one red wire(5V output) and black wire (ground).

Written by : Subhadeep Biswas