T&L Publications

As said by others, a schematic of what you have now is a must for trouble shooting.

The symptoms suggest that Oxford is using the 3055 as a high side driver (load from emitter to common). With a single supply the transistor base can not get enough current to saturate. Result - low current and transistor gets hot. An N channel FET would have same problem, not enough gate voltage to turn on.

Solution - move 3055 to low side, or use a PNP transistor or P channel FET on high side.

Whether decision is high side or low side, another transistor (like 2N2222), and 2 or 3 resistor are needed. For high side with PNP or P MOSFET the 555 turns on the 2N2222 which pulls base or gate of power transistor/MOSFET to common.

Minimum gain of TIP3055 is 20. So to use 3055 on low side the 2N2222 is connected as Darlington with 3055 to reduce current load on 555.

Resistors placed and sized to limit currents to just enough to ensure satuation.

I would go with low side if wiring of heaters allows, but decision is yours Oxford.

Cheers,

Bigglez wrote:We're trying to ween the OP away from bipolar hogs (like '3055).

Only showing it was a TIP, not a 2N. (Yes, still bipolar.) Design as you wish. Actually, as the OP prefers.

CeaSaR

I have the IRF510 MOSFET on the board with 12V battery power and when pulsed by the 555 it drives the led on and off with full range adjustment beautifully.
I will hook up the load later. I gota go out.

Thanks for the support,

Roger

I just realized that I have no idea how to send a bit map drawing to this site.
FAQ 's didn't help.
Someone please send a drawing to help me understand how to do it.

Roger

I do not have experience on this site to attach / send a bit map drawing.

Any help on this topic would be appreciated.

Roger

[/list]

Oxford wrote:I do not have experience on this site to attach / send a bit map drawing.

Any help on this topic would be appreciated.

Convert it to JPG, use a third party to host the
image. Use the BBCode tags for IMG to embed
it to your post. (Or use the BBCode tags for
URL to link the image to your post).

If you are still stuck PM me for assistance.

I think a GIF or PNG would be better. I always use one of those for scematics.

an IRF510 will work ok but it's Rds is around 400 mOhm which means it will drop 800 mV at 2A and consume 1.6 Watts. It will run pretty hot - 120-130C. I'd definitely put it on a heat sink. If you can get a MOSFET with a much lower Rds, you can avoid a heat sink.

Oxford wrote:I do not have experience on this site to attach / send a bit map drawing.

Any help on this topic would be appreciated.

Roger

[/list]

You dont see "add image to post" under the box you type in?
Click that then browse your computer for the file and upload.

Al,

I think he needs help selecting a third party server too! And I can't help with that because I have my own domain name and server, and don't use a free third party server.

This has been all been explained to newbies in the past, maybe sticky thread is needed.

Cheers,

Oxford wrote:I do not have experience on this site to attach / send a bit map drawing.

Here's the OP's diagram:

Much of what has been suggested is sinking in now.

Transistor biasing has never been need in my other very simple efforts, but it surely is needed now. I just printed out two good sources.
I will cool it until I get a grasp on biasing as away to think about using transistors effectively.

Roger

I have the LM555CN pulsing a 2n2222 transistor with proper biasing. The transistor output can be placed on the base of a TIP120 Darlington or on the gate of an IRF510 power MOSFET Transistor and each will drive an LED. Neither of the last two items will turn on fully because I cannot observe the 2.12 amp load expected when I hook up 5 ohms of heating elements. ( yes the 12 volt power supply will run the heating elements at the expected amperage)
Can someone give me some heavy hints on fully turning on the last two items.
Roger

Oxford wrote: Neither of the last two items will turn on fully because I cannot observe the 2.12 amp load expected when I hook up 5 ohms of heating elements. ( yes the 12 volt power supply will run the heating elements at the expected amperage)

You can do some simple testing with just the FET,
an LED and the supply (plus two resistors).

Connect the battery (or supply) +ve to the LED anode,
connect the LED cathode to a 1k0 resistor, connect the
other end of the resistor to the -ve of the battery, the
LED will light up. (If not check that it is not in backwards).
If the LED doesn't light up replace it with a fresh one.

Now remove the resistor from battery -ve, and connect
it to the FET drain (D). Connect the FET source (S) to the
-ve of the battery.

Firstly, return the FET gate to ground (gate and source
now shorted) and the LED should not light. If it does
your FET is damaged and must be replaced with a
fresh one.

Secondly, connect the FET gate to another 1k0 resistor,
and the other side of that resistor to the battery +ve.
and the LED should light fully. If it doesn't the FET is
damaged and must be replaced with a fresh one.

Once you have this working with the LED, place the heater(s)
across the LED and first resistor and again place the
FET gate at battery +ve. The heater will draw current,
the FET should not get hot, but may get warm to the
touch. If the FET gets very hot, remove the battery.

If you have a DMM handy make and record some
voltages and report them here.

For these voltages measure from battery -ve to battery
+ve, then across the FET (drain to source) and finally
across the heater.

Do you need a schematic for this testing set up?

Firstly, return the FET gate to ground (gate and source
now shorted) and the LED should not light. If it does
your FET is damaged and must be replaced with a
fresh one.

It lights at this point and must be replaced. I'll report back later.

Roger