What would be the advantage? I don't see it. I want to keep the parts count is low, distributing and dumping heat is the biggest problem. Everything in this unit, including the high amperage resistors, is going to get hot. Fortunately most of the use for this will be of relatively short duration, though I can see the possibility of using this for a long term burn in. I may need to add a wall wart power in as an alternate.

The 10A switch is part of the current measurement as well as the range select.

 

The advantage is having a smaller switch. Just shorting the base of Q1 to ground will turn Q3 off, and the current is way smaller than the 10A the main switch would have to handle.

 

You're going to have to show me. This is a range select switch. It shorts between the current control resistors in a very specific sequence. As a side benefit it also brings out the current measurement.

 

I meant the main switch, didn´t notice the new one for range.

 

The main switch only pulls a few ma. I was referring to the range select switch.

 

Don't think I've see that configuration before. I'll keep it in mind.

I'll look for a power transistor for Q2, the point about heating is a good one. Looks like I'm going to need a bigger case, toggle switches rated for 10A aren't too small either. If this gets as hot as I suspect worst case I may need more fans too.

Thanks for the input.

One advantage of the configuration I suggested is that the thermally induced Vbe shift in Q3 will have very little effect on the output current as the transistor heats up. Q1's temperature will change very little, so you shouldn't have to deal with thermal settling, which otherwise might take several seconds. Or maybe not.

 

I'll try it and see. I keep going through the math and then not writing it down (and then forgetting it). The 0.1Ω at 10A will dissipate 10 watts, the 1Ω at 1A will also be a watt, and the 10Ω at 100ma is 0.1W. The transistor will have a impressive amount of dissipation to contend with.

 

There should be several liquid nitrogen vendors in the DFW area.

 

OK, next generation. Is this what you were talking about Ron?

I foresee the need for more fans around the resistors.

 

OK, next generation. Is this what you were talking about Ron?

I foresee the need for more fans around the resistors.

Yep, that's it. If you think about it, Q2/Q3 are a sziklai pair, and Q1/(Q2/Q3) are another sziklai pair.

 

Bill,

I suggest a build-out resistor between the wiper of the pot and the base of the 2N2222. If built as shown, the roll-off frequency of the modulating signal input will shift radically between when the wiper of the pot is near ground and when it is mid-range (5 uf and 0 ohms to 5 uf and 5k ohms).

While on this part of the circuit, your modulating signal source will probably be referenced to ground, in that case, you would only need C2, which should never be reverse biased.

Is Q2 still and 2N2907? If so, you will only be able to get several tens of milliamps before running into power dissipation problems. At high currents, a 2N3055 will have and hfe of about 10, meaning that you would need about an amp of base current to pull 10 amps through the collector. I suggest a much larger transistor to drive the 2N3055; preferably one that can be mounted on its own heatsink.

 

When I came up with the concept the modulation input was a major afterthought. It would be where you would feed a function generator (probably a square wave) into the box, and it would offer a decent dynamic test of the UUT. Thing about a true square wave, it swings both ways, as in a true AC signal, so I adjusted the cap accordingly.

The input should never exceed 1.0VDC, and should usually be significantly less. On that basis I could remove one of the caps.

I understand what you are saying about the roll off, but the diode should offer some resistance, and like I said, it is not a major function. I'll stick a BNC connector off to side just in case I want to do it. Basically I'll measure the current with a oscope on the current outputs, and see how flat the power supply/voltage regulator is.

I'd mentioned replacing the 2N2907A earlier, with something a bit more robust. A lot of the final design will be in how I lay out the heat sinks. People talk about linear regulators being space heaters, I suspect this is going to be their mama.

I'll make a very crude PCB for the transistors and power resistors, and try to figure out how to use a 3rd fan to keep the resistors cool. I'll try to document what I've done as I do it, since as a piece of test equipment this odd ball may be useful for other people.

 

I didn't think I was all that interested in this thread until the last page, when I finally realized I could really use on of these. I've been experimenting with a novel alternator design and that involves testing it under a series of loads at any given rpm. I have a rack of resistors (all sharing the same ground) that I've been using, just attaching the alternator briefly to successively larger loads (lower ohms). I measure the ∆V and move to the next.

This device would require me to measure both ∆V and the amperage (shunt voltage), but that would be OK. Turning the dial to vary the load, and not having to worry about my little resistors burning up, would be nice.

But even better, I've recently set up for computer data acquisition and I think your device could be automated. If I read your schematic correctly, and I haven't studied it yet, instead of sending a square wave I could actually send a programmed voltage, with feedback based on what voltages appear at the shunt. So instead of me moving a wire from resistor to resistor, I could have the DAQ make a sweep over a few seconds and collect a nice full curve of the alternator performance.

What do you think, would control by an intermediate voltage - instead of a square wave - work to vary the load on the power supply under test?

 

Bill,
I sent you some MJD32's, which are basically SMD versions of TIP32's. You might substitute those for the 2N2907's.

Of course, you'll need to provide good heat sinking.

I made a typo in the packing list; they have a max Ic of 3A, not 30A. Hfe (min) = 25 when Ic=1A

 

I'm not sure what you think the square wave is for, but it has very little to do with the control of this device. It is a constant current source that is an electronic load. The base of Q1 is bias from 0.6 to 1.6VDC, this is the controller of this device.

The LED is equivalent to a zener, it is a constant voltage source. R1 is adjusted to set the max voltage to match the range you want, which is why it is a calibration tweak.

The modulation input is used to make the load vary between two arbitrary setpoints, say 1 and 5 amps (it can be anything). A good regulator or power supply will not vary it's voltage much between these two values. There is also the fact that this change is fairly fast, so it is what I call a dynamic test, as opposed to a static test.

A static test is where you would vary the current between two point and measuring the voltage out between the two points. A regulator may slowly correct itself and you would never know this was happening. With the dynamic test you could see it.

Basically a lot of measurements will be done with an oscilloscope due to their speed. It is a must for any dynamic measurements, but a regular DVM will work for simple static measurements.

 

Bill,
I sent you some MJD32's, which are basically SMD versions of TIP32's. You might substitute those for the 2N2907's.

I made a typo in the packing list; they have a max Ic of 3A, not 30A. Hfe (min) = 25 when Ic=1A

Thanks Wookie, I'll hunt them down. The box is right next to me. Even though the project hasn't changed much, it seems to be evolving.

 

OK. There are also some MJD117's, which are Ic=2A max Darlingtons, but you'd probably be better off with the MJD32's. Vce will vary too much over Ic and temp with the Darlingtons.

 

The modulation input is used to make the load vary between two arbitrary setpoints, say 1 and 5 amps (it can be anything).

Got it. I may head to the drawing board, though, now that you've inspired me. Having my new DAQ device - which includes outputs - enables a whole new range of thinking.

 

OK, I'm picking this thread back up. Here is the latest schematic. The reason is I will need it for this project, Power Supply Circuits.

The test selects are just that, they will be tweaked to make the measurements accurate. I have most of the PoP (Pile of Parts). From here on the design is mostly mechanical.

Starting values:

R8 = No Resistor
R9 = 9Ω
R10 = 90Ω

 

Bill,
I don't know if it's of any use to you, but this guy built a dynamic load using op-amps.
It looks like he's a bit of a whizz with spice modelling.... might be some helpful info there.

HTH Steve.