SSTC - what capacitors use in halfbridge? / Tesla Coils / Forums

Forums

4hv.org :: Forums :: Tesla Coils

« Previous topic | Next topic »

SSTC - what capacitors use in halfbridge?

Move Thread

LAN_403

dingo27

Thu May 22 2008, 08:12PM

Registered Member #890 Joined: Tue Jul 10 2007, 10:06PM
Location: Slovakia
Posts: 180

Hi, i was making TC (2200 secondary, 8 primary) and the driver was only from invertors 74hc14e (texas instruments...) One half of chip was inverted to another , and it was connected to both ends primary, and at center primary was 17 ohm resistor to main ground or UCC. it was driven from antenna feedback

Well... it seems be ok schematics and i was able to get 1mm spark, but able to light neon bulb (3 of 6 parts...) and, it was really ok, but i wanted little bigger sparks.

So i found IRFS630A (the only i had 2 of them) and did GDT and driven that the mosfets... (it has 200V 0.4 ohm 6,5A) well... a lightbulb could light, but only one part, but strange thing was that all TC parts were connected to mosfets part - supplied with another power supply, onlÃ´y common thing was GDT. I did a mistake and connected inverted voltage to mosfets ( -5, that shouldnt be bad...) but the strange thing was, that TC was not poweret by mosfets at all!!! I saw that when i disconnected the mosfets power supply , but the lightbulb had still light. The question is - how the smart current must be to power another circuit powered only by GDT and turned off power supply?

Ok, since i think i destroyed both mosfets and 74hc14e, i will buy another. How can i drive mosfets with TTL signals (i did "skycraper" from 5 74hcs.... - putted one to another and another) so if was able to give 20mA(1 output)x6 invertorsx5chips=600mA (well half of that, because i was using one half inverted to other).

Can i drive GDT with only 600mA ? or should i get there some really fast transistor and drive it with that? Which one i should use?
I cant use mosfet driver, because here it is unknown part ... ( i dunno why...

)
i also cant get power TTL circuits (also unknown here...)

Resonant frequency is somwhere about 500KHz down... (i dont have ttl generator either)

Sorry for such a long post, i hope someone will read it - for those brave ones - THANKS !!!

//edit: i am just adding schematics here... it was on solderless board

Sulaiman

Thu May 22 2008, 09:13PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I guess that the most 'famous' hence common mosfet would be IRF510.
It has a very low gate charge so is easily driven by any logic ic.
(I'd probably go for an open-collector TTL output with a pull-up resistor to (9 to 12) volts for the gate.)
There are many newer/better transistors but I think IRF510 is widely available and not expensive.
The IRF510 is also used by amateur radio constructors for 5 to 10 watts PA for the HF bands,
I just bought 10 off eBay for moderate power hf experimenting myself.

dingo27

Thu May 22 2008, 09:28PM

Registered Member #890 Joined: Tue Jul 10 2007, 10:06PM
Location: Slovakia
Posts: 180

As i understand, i should use that irf510 as GDT driver.

Ok, but an idea kicked me - what if i only will be switching coil... not 2 mosfets, but only 1 turning primary on/off from Vcc??? should that work?

anyway, since i think i will ruin my electronic supplier (today i was buying that 5 dead 74hc14's , and he was looking for that nearly minute) i'll try to buy another 10 74hc14's and that irf510 and see what i can do.

(wish me luck :)

//i found out that it was not dead...

ShawnLG

Fri May 23 2008, 01:35AM

Registered Member #286 Joined: Mon Mar 06 2006, 04:52AM
Location:
Posts: 399

The 74hc14 can not source enouff current to run IRF640s gate capacitance. You would need a stronger inverter in between the 74hc14 and the MOSFETs.

I have used 2N7000s to drive my MOSFETs for my micro DRSSTC. They worked but not purfectly.

dingo27

Fri May 23 2008, 11:12AM

Registered Member #890 Joined: Tue Jul 10 2007, 10:06PM
Location: Slovakia
Posts: 180

but i dont have one, but 5 of them, so it can give 600mA...

today i was againg able to rebuild it, and sparks was 2mm long - running from 9.5 V , but sparks was only to ground.

I am still working on better schematics, as it has great potential.
(i was not able to sleep well last night

)

photos A.S.A.P

GeordieBoy

Fri May 23 2008, 12:20PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

The IRF510 is indeed a good device for HF linear and switching amplifiers, or as a driver for a larger PA stage. It can be used well into the tens of MHz with properly matched gate drive.

dingo27

Fri May 23 2008, 08:56PM

Registered Member #890 Joined: Tue Jul 10 2007, 10:06PM
Location: Slovakia
Posts: 180

ok, i let GDT for another times, i connected all invertors together, so it gives real 600mA (i think) and i am powering that mosfet IRF510. They only had one, so i cant do half/full bridge...

sparks are really good, running perhaps somewhere about 16V (i know, not lot) they are probably 8mm long, they are not doing RF burns, but still i cant get breakout... I am using toroid from beer aluminium bottom (as seen on avatar) ... and i have 4 screws holding primary and secondary together. Can i use them? as i know the are magnetic, and one screw is holding toroid on it.

I cant imagine resonant frequency, no scope, but i think somewhere about 350-400kHz. I stack 3 (!!!) PC power supplies to get 72V, can i drive it through mosfet? I think i use some choke (inductor), i have a lot of them...

Some photos of previous design... ( i will run notebook as i got them there...) A.S.A.P

transistor is not hot, i think somewhere around 40-50Â°C (cooled by fan)

one moment... i have 74l05 stabilisator to power TTL circuits (the tiny one... TO92 ?) . So to the mosfent cant get that 600mA, am i right?

------------------------------------------ ---------------------------
as i promised, there are pictures>>>> (that is not the final version with mosfet... only for imagination...)

dingo27

Sun May 25 2008, 09:12PM

Registered Member #890 Joined: Tue Jul 10 2007, 10:06PM
Location: Slovakia
Posts: 180

ok, TC is working but i have strange issue with that... since i burned irf510 (my first burned mosfet :) i am using IRFS630A, it was working good - 7mm sparks at 17V Power supply (it is not much powerful...) but then i tried half bridge, but little bit different - idea was :

(+) --------------------------------
           l                       l
           =                       =
           l                       l
           --------UUUUUU-----------
           D      tc primary       D
        G                       G
           S                       S
           l                       l
(-) --------------------------------

Hope you understand :) GDS is main mosfets, and i did not have anything to get GDT working, so i tried to use this schematics...

It did not worked /well there was i little hope/ but since then i get strange working of TC with ONE any-mosfet. I tried several mosfets to make sure it is not bad, but mosfets are good, same power supply, everything seems to be OK, but TC is once has resonant fraquency, once not, and it does randomly, and i dunno from what.
I am using same schematics as in first schematics, but without GDT and signal going RIGHT to mosfets.
I can unly light a small ... lightbulb... it has not connected 2 wires in glass bank , i dont know what it is called in ENG...

... ok now i really dont understand... but i am using twisted pair of wire to connect tc, once it was doing errors when running, so i untwisted it... now it was doing errors so i twisted it again (and all wires from both power supplies...) and it was working. I think i destroyed another mosfet, but now it was working as i imagine... But that IRF510 had a better performance that IRFS630A, it is still working...
i am sure it has something to do with "phase move" or how it is called, but i dont have oscilloscope (like that commercial - no redbull, no wings :) so i really dont know...

//ok, i will know now

I made halfbridge with IRF530, and it has waaaay too low output voltage and power. I am driving GDT with 2 mosfets (and they are driven by 1A invertor circuit), i use there faster diodes to turn off internal diode in mosfet. GDT is ticking, but then i get it to IRF530, and it is working, but terribly. As i found, IRF530 has 670pF input, and that is not too high compared to 1500pF mosfets.

What is better for mosfet > high peak current at low voltage or High voltage and low current when driving gate?

I was able to do better work with just one mosfet that with all of them

and thought halfbridge will increase output sparks length...

I think that the error is in halfbridge caps - now i am using 10nF foil capacitors (i think too low) should i use electrolythics ?

btw that mosfet i thought that i burned wan not burned

but i burned another one and that is burned really

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob