Problem with Shunt Regulator

[VinceB]

But at last!!

I connected the following string to the Fet's GATE: regular diode, a 10 K resistor and  1 uF capacitor. The end is grounded.  The capacitor gets charged at the max GATE value, which is 12.4 V, not the 8V I was reading (which is the mean value)!!

One problem though: I checked Digi-Key's website and the IRFZ40 is now obsolete. What would a good replacement be??

Thanks a lot everybody

[VinceB]

A few more questions...

I know the 555 does not tolerate supply past 16 V so there could be an issue...

But instead of adding a voltage regulator, could one just add a 15 V Zener diode between the 555 supply and ground? Maybe throw a resistor in there, I'm not sure if it's needed...

In your schematic, I noticed you used power connectors (screw-type) to connect the dumpload and controller. Do you have a part number for this item?? I'm looking for something that is PC mount type.

What is your dump load's resisance ?

Finally, if you made this circuit on a PC board, what trace width did you use to route the power FETs ?? I'm assuming they must be pretty large !!

[dinges]

Good point on the zener & resistor; it might have been better if I had supplied the power to the NE555 via a 15V zenerdiode (kathode of zener to + line of NE555; anode to gnd; kathode of zener via a resistor to positive power supply line). That might have given some extra voltage to drive the gate of the FET a little harder.

Just about all my components are scrounged, i.e. taken from old pieces of electronics. I don't have a part.nr of the terminal block but in my IRC gallery you can see a few pictures of the PCB:

http://www.anotherpower.com/gallery/dinges/dumpload_controller_homepower_PCB_component_side

For a dumpload I used a H4 car headlight bulb (12V, 55/60W) when testing. It draws about 4.5A and the dumpload controller can withstand it without problem, yet I've limited its rating to a maximum of 2A. It's intended to be used with a small PV panel I still have to mount. For a dumpload I'd think I'd use some smaller car lights (12V @ 1A or so?) in parallel, in case one bulb should fail. I know others dislike incandescent bulbs for dumploads since they fail easily, but I like the optical indication of dumping they give. (even though there's a small LED in my version of the HP controller that indicates dumping)

I hardly ever etch PCB boards; since all my projects are prototypes I prefer working with perf board (vero board). The limiting factor for my dumploadcapability are the wires that connect to the FET; I do not want to load them with more than 2A continuously.

http://www.anotherpower.com/gallery/dinges/dumpload_controller_homepower_finished

The cooling fin in the picture above is ridiculously oversized for this application.

The picture below shows how the bottom of my PCBs usually look like (for another dumploadcontroller, but you get the idea)

http://www.anotherpower.com/gallery/dinges/RossW_dumpload_controller_copper_finished

As I said in another post, my version of the controller is only meant for a small system. In case you want to dump 40A or more of current, different construction methods (esp. w.r.t. wiring the FET) may be needed.

More pictures of the dumploadcontroller and of other projects can be seen by clicking on the link that appears at the top of this reply.

Regards,

[ghurd]

IRFZ40 has been obsolete for a long time. Try 42 or 44.

Sounds like the cap will make the fet stay linear for longer.

[VinceB]

Hey guys,

I started reflecting about PWM duty cycle and the RMS dump load value...

In Chris Greacen's design, the 555 is operated in monostable mode. Everytime the input is triggered LOW, there's a pulse of a fixed width coming out of the ouput. If a the end of the timing sequence the input is still LOW, the pulse still goes on, as long as the input doesn't go high. It is operated in inverse logic.

This makes it perfect for our shunt loads, because the whole dump load is being used during the time period, which could be compared to 100% duty cycle.

But, in my desire to reduce the number of parts, I decided to try a TL494 PWM shunt controller. I used OperaHouse's design. The problem so far is, the maximum duty cycle seems to be 50%. When it's operated in push-pull fashion, the maximum DC goes down to 25%. Suppose you have a 500W genny and a 500W dump load. If your genny is producing full power, your controller will only dump 25% of 500W if it's operated in push-pull mode, and your batteries will burst.

So I am starting to reconsider the 555 design, which can produce a 100% duty cycle, over other PWM IC's that simply can't.

Any comments, or additionnal reflections???

Thanks

Vincent

[commanda]

TL494, pin 13 is the output control. Connect this to ground for single-ended output. Then you should get close to 100% duty cycle.

Amanda