Tuesday, October 11, 2011

Customer support-via email between me and a customer from Florida (USA)

Hello there Mr. Humphrey!
This is Gill from Coconut Creek, Florida (USA).
I just bought your book a few moments ago, and I’m enjoying reading it.

On chapter two you mention, that inductors allow low frequency to pass, and make it very hard for high frequencies to pass; and that capacitors allow high frequencies to pass but make very hard for low frequencies to pass.

So the EMI circuit has the inductor in series with the AC line, where the low frequency for the power supply will pass.

But what I'm a little lost is that the capacitors which oppose to LOW frequency are also present in this picture, and you mentioned that the low frequency will be allowed through the capacitors, or am I missing something here?

Good Question Gill let me explain a bit about these two very important component found in E.M.I circuit of all S.M.P.S.

Fact: Capacitors allow high frequencies to pass and make it very hard for low frequencies to pass.

Conclusion: on the circuit given in your book Chap 2, looking at the e.m.i circuit you will notice that the capacitor is placed in parallel to the AC line-this means the capacitor two pin are actually across the AC line (say one leg on the live and the other leg on the neutral) now when the high frequencies travel along the line they will find a capacitor which allow only the high frequency to PASS…pass to where? Pass to the Ground (neutral) hence effectively shorting them to the ground.

Again on the same line we have low frequencies (50 Hz or 60 Hz). When these low frequencies reach the capacitor which is in parallel, the capacitor will not pass them to the ground and therefore they will proceed with the power line.

On their way to the bridge rectifier they find an inductor which is in series with the supply (in series mean both pins of the inductor are sitting on the same line) two pins on the live and two pins on the neutral.

Because inductor will pass low frequency, this means the 50 Hz or 60 Hz will pass through the inductor comfortably. Now some of the high frequencies which the capacitor did not take the ground will try to pass the inductor but luckily the inductor is very harsh to the high frequencies and they will have no choice but to follow the capacitor which is waiting for them eagerly to short them to the ground.

Remember this is with regard to Power supply, there are other circuit with require the high frequency to pass and block low frequency like the DC. Therefore in this case the capacitor will be placed in series. A good example is with audio circuit output, you will find a capacitor there which is in series and therefore will only allow the audio signal (A.c) to pass and block any dc and hence you will receive a clean audio output from the speakers.

At the other hand I am troubleshooting a Toshiba TV model TN50X81, which has sound but no pictures. Have you seen this problem before? I'd appreciate your help with this TV.
May God bless you richly!

Thanks for the blessing Mr. Gill, on this TV with sound ok but no picture. We can approach this problem in two different ways.

1. Is there something on the screen? If there is something on the screen like blue background then when can assume the power supply is okay, horizontal drive circuit okay, vertical circuit  is ok, fly back ok, screen voltage is ok, focus and heater are all okay and therefore not considered for any troubleshooting.
2. There is nothing on the screen...if this is the case you need to ask these questions. Is there high Voltage (Hv) I mean can you hear CHAA!! When you power on the TV...if you hear this sound it mean HV is present and therefore the fly back is getting energized.
So where is the picture then? This could point to RGB supply voltage is HIGH…yes I mean HIGH, or the screen voltage is low. Also if there is no heater voltage the screen will be blank.