redwire

I agree 100% with Electropepper. Buy a commmercial programmer or build one from a kit. You will find support in either case if you have questions. The one shown looks old and who knows if it supports the chips you will eventually want to program. Do you plan to use AVRStudio for writing and compling your programs?

These actually look pretty good to me but I'm no expert.

Here is U2 at 12V at startup with 0.5 amp load. Second try was a bit shaky as it was just holding against pin 7 while trying to plug in the transformer.

Here are views for U1 at 12V 0.5 amp load

No, I don't have any beefy N or PFETs. I'll probably order some in a week.

I did some testing with the TIP141 and 5 amp load and nearly burned up the TIP because I didn't have a decent heat sink. I put the Tip on medium sized heat sink and noticed the temperature rise to about 125 C which was hotter than anything else on the board, even the output transistor heat sink. I wonder if I miscalculated something. I thought operating more or less like a switch would not generate that kind of heat (except at start up). I was thinking that the voltage drop when fully on would be about 1.2V at 5 amps or 6 watts. I'm definitely burning more than 6 watts. The other thing I noticed is that the turn-on delay was nearly triple or about 3 seconds.

No, let me do some thinking on what I could use to load it up. I do have a headlamp that is 12V and about 4.5A. Perhaps I can find a small electric heater.

A 34Ah battery? Was it for a riding lawn mower? If you are going to use the power supply at its limits for long periods of time you may want to consider upgrading to a couple of 2N5886 or MJ11016 output transistors mounted on a large heat sink with a small fan blowing across it. A 2N3055 with a case temperature of 125 C is only good for about 50 W. A 2N5886 at 125 C is good for about 82 W. For a 30v, 3 amp power supply operating at very low voltage and high current, the output transistor may consume 115 W. Two 2N3055's are borderline depending on the cooling.

Yes, that's it. What are you using to create your pics, Eagle?

Ok I've gotten something that seems to be working fairly well I have a 39k connect to the high side, then a 2.2k connected to the 39k then a 47uf connected between the 2.2k and ground. Then I have 4K7 resistor connected at the voltage divider between the 39K and 2.2k. The other end of the 4k7 is connected to the Base of the TIP141. Start up is nice and shutdown occurs nearly instantly . The first picture is startup the second is shutdown. The blue line is connected to the main cap (10,000uf) and the yellow is the output. Test was taken with about a 0.5amp bulb.

Attached is a picture of the charge on the main cap vs output to a 12.4 V setting under a 0.5 Amp load. The turn on is a flatter curve and slower but I don't know what I saw yesterday when I thought I had a 1 sec delay. I must have misread the time period. I agree that the turn on after the cap charges is the ultimate goal and using a mosfet has many advantages.

After some experimenting, I'm not thrilled with the soft start results. I don't know if adding all those parts are worth the benefit.

Concur, I suggested some caps earlier in the thread.

Yes that's it. I want to try with a low value resistor and no resistor where you show the 18K reisistor to see if the delay remains approximately the same. For the test I am using a 10,000uf main cap. Glad you got the part.

Do you have a suggestion on a different output transistor or opamp? EDIT: This combination exists on the other PS project but I don't recall anyone mentioning a problem with the tLE2141 getting fried.

Hold off on populating the soft start. I started testing last night and the arrangement and values will likely need to be tweaked. As drawn, the PS kicked on at the same time as the main cap -no good. I then rearranged the set up by connecting the 47uf cap directly to ground and used a 39k resistor connected between the collector and cap. I then used a 18k resistor to the base of the TIP141. Timing seemed pretty good (about 1 sec delay to reach 12V) with reasonable ramp up on the scope. I did not test it with a load. After running through the numbers (39k and 18K resistors) and the data sheet for the TIP, I am concerned that the base current of approximately 0.7mA to the TIP may be a little low under a 3-5 Amp load. I may need to change up the resistors to increase the base current to the TIP without shortening the delay period. Otherwise I may need to increase the value of the cap. Will keep you posted.

Yep, your right about the component values.

I also sketched this a few days ago up but your Eagle skills are much better. Please check on the following items: 1. The LM358N has a range of 3V-32V which is out side of the needed operating range of the voltage sense amplifier. Suggest using a TLE2141. 2. The current sense amplifier has no connected rails. In this mode the high would be 0 to 1 V, the LED would always be on, and Q3 might never conduct. I suggest a TLE2141 with a cap between the output (pin 6) and Pin 2. 3. I think R9 should be 2K2 instead of 2K7. 4. Is R13 47 ohms? 5. RSense is 0.22ohms Suggestions for discussion: Changes I intend to implement- 1. Use a 15V Zener to feed a 7812 linear voltage regulator instead of the fet and zener combo used to set up a reference voltage. Using a 15V zener would reduce the rail to rail voltage on the current opamp as well as the voltage to the linear voltage regulator. 2. C3 is correctly listed as 100uf but this seems high. 3. The unnamed main filter cap (4,700uf) seems small for 3A. 4. A small cap between the output of the voltage control opamp and inverting pin may be helpful.

Luisitou, liquibyte has posted a parts list with part numbers to assist you in ordering. Unless you are using parts from your lab.

No, I have been focused on the shut down issues. I'm confident a soft start can take care of the inititial startup spike. I just sent you a TIP 141 today. It will likely be next week before you get it. It is estimated that at full output (3 amps), the TIP will be using about 3.6 watts. So if it is mounted to some metal or a small heatsink it should be ok. A MOSFET may be a better solution but for testing purposes the TIP should work.

From memory I think it looks like the inverse of the first one. Spike right before it ramps up.

Yes it is with Q1, R13, R14 and D10. The picture is when I unplug the transformer and the voltage is decreasing.

I installed the diode between the emitter and base of the output transistor. It seems to have helped but I still seem to be getting a spike right (first picture) before the voltage drops when I pull the plug on the transformer. The second picture is an expanded view of the first picture. I wonder if it is from the transformer when the field collapses.

I don't know how you could see a transient spike without a trigger and screen capture. It happens so fast. I often have to change the trigger voltage, type of edge trigger, and capture rate because the first shot doesn't always catch it. If the event happens early, the scope will capture the initial spike but not the actual drop/increase in voltage at the output because it used all of the available memory, so I will do a second shot at a much slower rate. I am still learning the features on the caputure end.

liquibyte, did you have the Q1 clamp transistor installed? Did you have D10 and R15 installed? Have you tested at 12V with the current setting at about half way? On a couple of my tests I have also noticed what appears to be a one point spike both postive and negative early in the the decaying voltage signal.