Vespian

I dont quite know how to make a schematic, so i'll do my best to spell out the contents of the circuit.

The NE555 is setup to deliver a 10:1 Mark/Space Ratio to the base of an 2N3055HV transistor at 42 Kilohertz. I have the output of the 555 limited through a 10K resistor. The 3055 is driven off its own supply (its not pulling current through the 555), and the circuit shares a common ground. The input voltage to the circuit is between 12 and 16 volts (its going in a car). The 555 is working. I am getting voltage out of the 555. I can also light an LED by shorting the ouput of the resistor limited pin 3(output) and pin1(GND). (the multimeter read 11v... but i think the fact that its not a steady voltage might be messing with the multimeter and giving a short reading -unless there is a voltage drop within the 555). THe problem is... i cant get transistor to activate. I've looked at the data sheets for the 3055 and i know the minimum required base voltage is 1.5 volts. I havent been able to find anything for this so called high voltage version. I have verified that i have indeed connected the output of the 555 to the base of the TO-3 transistor. Is the frequency i selected too fast for this transistor? Am i not supplying it with the needed voltage? Should i use a different transistor? I am going to use this to drive an electrolysis cell. I dont want any comments on the fact that i am putting a browns gas generator in my car. That is not my question. Leave that science to me. (I have read some of the other posts dealing with electrolysis... they got waaaaaay off topic) Any suggestions/insights would be very helpful at this point.

Thanx in advance guys... (and gals)

~Vespian

eblc1388

Try replacing the 10K resistor between NE555 and base of 2N3055HV by 47 ohm resistor as a start.

This is good for about 2Amp. If you want to switch more than a few amperes, then additional transistor drive state is required.

__________________
L.Chung

Vespian

Just curious... what will replacing the 10K resistor do... other than allow more current to flow through the 555? Will that increase the voltage to the base of the 3055? Is that what my problem is? Also... i thought the 3055 was rated to 15A continuous?

k7elp60

That is true of the 2N3055 having a 15A continuous collector current but the hfe is minimum of 20 which means when the IC= 15A the base current is about 750mA. The 10k will on give the base about 1.2mA.

__________________
The great thing about electronics is unlimited ways to do the job. The only limit is one\'s imagination. I generally think my way is best.
Show me a different way. I have an open mind.

crutschow

The 3055 is a bipolar transistor which requires base current to turn on. It's minimum current gain is 20 thus you just divide the desired output current by 20 get the base current you need. And if you use the transistor as a switch where a low on-resistance is desired, you probably should use a gain of no more then 10. Thus to switch 15A you would need 1.5A of base current.

In your application it may be better to use a power MOSFET which is voltage controlled and is more readily driven by a low current driver such as the 555. Of course large power FETs have a high gate capacitance which limits the frequency response if the gate driver has a high impedance, so you must take that into account.

Vespian

I dont think i can safely draw that amount of current through the 555 without damaging it. So that means i have to run a smaller transistor off of the 555... and use that to run the 3055. Any suggestions on a transistor thats rated to 800 mA? That wont pull more than say.. 6 milliamps on the base?

Vespian

Or am i going to need to step up the current through a series of darlington pairs?

Krumlink

a 555 can only drive 200ma max before it explodes. Try a NPN and a PNP tied together across the output of the 555.

__________________
MechTronics

World of Warcraft is a disease

Vespian

Alright.. i got it. I tied the output of the 555 through a MPS2222A transistor. The transistor is rated to 600 mA, and with an hFE of 50 only draws 12 mA max. Well within the 555's tolerance. Granted this will allow for only 12 Amps through the 3055, it should be enough to power the cell... for now. Perhaps later i'll step this up. All i know is that i got it working... and its thanks to you guys.. Cuz i didn't know a whole lot about transistors till i read your posts. Say... do i need to get a heatsink for the 3055? At a 2 amp draw this thing got hot on me pretty quick.....

audioguru

I think you guys need to look at the datasheets:
1) The 2N3055 doesn't have any spec's above 10A.
It needs a base current of 3.3A to saturate poorly with a max saturation voltage loss of 3V when its collector current is only 10A.
2) The minimum current gain (hFE) of a 2N3055 is only 5 when its collector current is only 10A and it has 4V across collector to emitter.
3) The 2N2222 has a max saturation voltage loss of 1.6V when its collector current is 500mA and its base current is 50mA.
Its hFE is measured when it has 10V from collector to emitter.

So if the resistor from the 555 to the 2N2222 is 220 ohms for a base current of 50mA, the base current of the 2N3055 will be a minimum of about 500mA and its collector current will be a minimum of about 5A.

__________________
Uncle $crooge

Vespian

i'm allowing the 2N2222 to be driven by the 555 timer without a resistor. This should work out ok as long as i dont draw excessive current. in which case i've wired in a 10A fuse on the Vcc source. Btw.. i got all the data i was working off of from this datasheet: http://www.ortodoxism.ro/datasheets/...onics/4079.pdf

States that max collector current is 15A. Max base current is 7A.
States that it does indeed require 3.3A to achieve a 10A collector draw, with a voltage drop of 3V
States that the 3055 has an hFE of 20 with a 4A collector current, and 4 volts across collector to emitter
(but what you said is true about it being 5 when collector current is 10A)

Either way it means i will need to step up the current to the base of the 3055 in order to achieve the desired current draw capabilities.

3 amps is nothing to gawk at....

blueroomelectronics

Why not use the resistor, it's a penny part?

__________________
Bill
Smart Kits build Smart People
http://www.blueroomelectronics.com

audioguru

The 555 will smoke and burn when it drives the "dead short" of the base of a transistor without a current-limiting resistor. The transistor also might smoke and burn.

The max allowed output current of a 555 is about 200mA but it will try to drive much more current into a short circuit then it will get much too hot.

You want the current-limiting resistor to get hot, not the 555.

__________________
Uncle $crooge

Vespian

i am going to need one... now. i didn't put one on before because i assumed the max amount of current the base of 2N2222 would draw was well under the current draw the 555 was rated for. But now i know better. With an hFE of 5, at 10A.... that means the 3055 will draw (or try to) 3 amps of power from the 2N2222. Its not rated for this amount of current. It will burn up... whether it will burn itself out before taking out my 555, i dont know. Being as the current draw should be around 150mA at 3A collector draw (which it will never acheive) from the 2N2222, the chip should survive long enough for the transistor to fail.

I think i'll simply do another step up.. run the 2N2222 to another transistor rated to 3A... and run that to the 3055.

The circuit actually works under a small load (i've tested up to 2A). Where i will run into problems is if i attempt to run a load close to 10A. That's when the stuff will be driven past tolerance.

blueroomelectronics

Don't forget to mount the 3055 on a nice heatsink. 10A is a pretty heavy load. The 2N2222A is only 800ma, why not use a darlington like a TIP120 or the like.

__________________
Bill
Smart Kits build Smart People
http://www.blueroomelectronics.com