loading

Micro sstc trouble

I am trying to build the micro sstc by steve ward. My circuit  behaves like there is a short somewhere, but I checked  all the connections.
The mosfet isn't connected yet and the 555 timer is getting hot, any ideeas?

Picture of Micro sstc trouble
sort by: active | newest | oldest
-max-2 years ago

Also, I would look to replace those 1N60 diodes with zener diodes to clamp the voltage. The 1N60 I think is a germanium diode, germanium diodes are ancient technology, and cannot deal with a lot of current, and I think they have high leakage too. I would use 12V zener diodes to prevent the voltage from getting too positive or negative, that is the purpose of those diodes, anyways. It is common to use diodes to clamp the voltages to prevent them from overloading the op amp inputs. I would also use a resistor between the antanna and the input to the op amp to avoid too much current going into it, so the voltage of the antanna can get really high without damage to the rest of the circuit.

VictorH87 -max-9 months ago

so i need use 12v zener diode for it work

iceng -max-2 years ago

Those diodes are low forward shunting Schottky barrier DO-35 germanium diodes.

They protect the input to the TC4420 mosfet gate driver by clamping any antenna voltage over 12.3vdc and below -0.3vdc...

theVader75 (author)  iceng2 years ago

I am using 1N4148

I would use zener diodes for clamping that node instead, since the 1N4148 is a small signal diode, and not designed to be used for reverse breakdown. It breaks down @ 75V, and even a small amount of current will probably kill it. I do not know what the maximum voltage to that input is, but 75V above the 12V rail to the input of that amplifier seems like a really bad idea. Might as well not even bother using any input protection then!

iceng -max-2 years ago

You are thinking the diodes are regulators When in fact they are fast ESD protection for downstream circuitry.

http://www.vishay.com/docs/81857/1n4148.pdf

If you want to protect the LM7812 with a Zener you may want to put it in series with the input to guarantee that the regulator never sees over 35VDC

Breakdown beyond 15 volts is unimportant because any voltage outside 12.8 to -0.8 volts is redirected to the supply rails through the forward conduction of the diodes !

This Means The TC4420 Input Pin Will Never See A Voltage Greater Then 12.8 VDC Or Less Then -0.8 VDC.

The 1N60 Schottky barrier DO-35 germanium diodes do a better job because they protect the input to the TC4420 mosfet gate driver by clamping any antenna to the lower voltage of 12.3vdc and below -0.3vdc..

The picture below illustrates built in protection in existing CMOS logic..

BTW that resistor is used to slow down very fast rising ESD (Electro Static Discharge) transients like a finger touching that antenna so that the diode PN unction has time to shift lattice holes and electrons and establish current flow shunting energy to the supply rails and avoid HV puncture through the gate insulation of a logic transistor.

ESDiodes.gif
-max- iceng2 years ago

Oh, OK, I see what's going on. I thought that the diodes are being reverse biased to clamp the voltage to some values, like TVS diodes. that isn't necessary since under those overload conditions, the diodes are forward biased.

In that case, I would prefer 1N4007 diodes.

Those are what I design into protect delicate gates and they work as well.

-max-2 years ago

You Do realise that this circuit is exceeding the absolute maximum ratings of the 7812 voltage regulator, right? I just noticed that, since the input is from a 50V transformer, and that is probably RMS voltage, the peek voltage could be as high as 70.7V (RMS sine times sqrt(2))!!!! (the 78xx series of linear regs is only 35V, according to this datasheet: https://www.fairchildsemi.com/datasheets/LM/LM7812...)

Anyway, there might still be enough margin for it to work, I am sure the creator of the schematic made has made that circuit himself and had no problems with the 7812 like that, who knows. Even if the regulator could hadle that voltage, it will be dissipating LOTs to power for even small amounts of current! So check your voltage supply rails to make sure you did not cook your regulator, and all the electronics onboard.

theVader75 (author)  -max-2 years ago

the supply voltage is 24v 2a

Oh, I thought the schematic shows 50V, I was looking at the bridge rectifier rating, not the transmormer rating, I just noticed that looking at it now! :)

I thought that was a bit crazy high! Still though, 25 VAC rms * sqrt(2) = 35.35V. There is no margin in that whatsoever. A small power surge to the transformer input, and that regulator has a good change to die, get damaged, or at least fall out of spec on the output, possibly causing other failures. Then you'll need to consider the power dissipation with such a large Drop across it, too. Tack on a big heatsink to stay safe, and the reg should be able to handle a short circuit well enough with all the protection circuitry integrated into them.

Always check your supply rails

theVader75 (author)  steveastrouk2 years ago

I'm thinking that the 555 is fried somehow and it behaves like a short

Your 555 is likely dead. Earlier on today I have been messing around with a powerful 555 driver thrown together using an SE555 variant, on a breadboard, producing close to 4cm ZVS-like arcs, with a striking distance of 2cm It did not differ much from the standard config, but I did have some large HV ceramic caps bridging between the voltage rails and the connection between the flyback and IRF250, with a modified Xbox 360 power supply (shorted out all the safety regulation chips to get MAX power output >:) In my name, my style lol!)

Anyway I have had weird things happen to the 555 when other parts like when the MOSFET fails, after the 555, as rugged as it is, will also pop. Sometimes nothing more than just it, for no apparent reason. These things do not really like the HV circuits. In the past, I would buy 10 or more at a time for prototyping because they have a tendency to die. After replacing that regulator with a separate 12V power supply, and correcting any errors in the wiring, you should be good to go.

iceng2 years ago

Your SSTC is using resonance and the
point where you ground the 555 and MAKES a lot of difference in actual voltage going to your 555....

Yes it sounds dead... dont try another until your wiring is redone to avoid ground fault current flow.

Perhaps you have a picture of Steve's Micro wiring and follow it closely.