Dummy Development

Now that I’ve got my Nixie clock all ready to be housed in a box, and lacking said box, I figured I’d motor ahead and try to get something else going. Next on the list is the Dummy Load, which has sat on a breadboard gathering dust for 3 years now. Funny thing is, I had figured it ready for assembly, but always good to check it out and see if improvements can be made.

To my surprise and delight, my design is working just fine.

Re-assembly

The breadboard was all a mess and coated thickly in dust so I thought I’d pull it all apart and clean it up, then re-assemble to make sure I didn’t leave it with any dangerous connections or if something got knocked around etc. I decided this time to include the dinky panel meter I bought three years ago for this project. I forgot what an abject piece of shit it is.

I hate this thing

I hate this thing

I reviewed the very bad chinglish instructions and had to grumble at them. Not only is it poorly, and vaguely written, but what it suggests doing is rather irritating. In order to change the range on the inputs, it suggests desoldering a little surface mount jumper, and a smd resistor next to it, and whacking in some rather odd value metal film resistors. What a pain in the neck!

Yeah this is a pain

Yeah this is a pain

I figured sod it, I’ll just build a 10:1 divider on the input. I chose my values as 10kΩ for the ballast and 1.112kΩ for the shunt just roughly. It works okay… except that 1.112kΩ loads the circuit unacceptably. I did try 100kΩ and 11.12kΩ but that just weakened the signal too much so the meter couldn’t read it.

I did have the idea of using an opamp to divide the input by ten, thinking it a more elegant solution but it ended up being a bit worse, resistor-wise. Would need a 1kΩ resistor (easy) and a 900Ω one (hard). I’d also have to get those values fairly precise so in the end I just went back to the resistor divider, loading be damned.

If I wanted to, I could put an opamp on the input configured as a voltage follower to buffer the input before the voltage divider. The more I think about this, the more it begins to make sense and I may include that… but not on the dummy load board. The connections to the panel meter are conveniently a standard header, so I could make a daughter board to go over it and provide space for my divider resistors as well as an opamp… maybe.

Or I could just sod the whole thing and get a 4.5 digit panel meter which would be sufficient to read the whole range of possible inputs without switching ranges.

Quality construction

Looks like whomever assembled this unit left his or her signature on it, in the form of hair. There’s a strand stuck to the board, and another stuck to the potting that goes over the gooped chip-on-board. Classic, I can see the quality LOL! I had taken a photo, but as usual, it’s awful and you can’t see the hair, you’ll have to trust me on that.

Onward, testing the load

After being satisfied enough, I assembled the meter and the dummy load together on one board to see how it all worked. Apart from me forgetting one crucial thing, it worked fine. The thing I forgot was the LM358 loves a lot of headroom! the threshold voltage for the gate is between 3-4V and I found the output of the 358 stopped, quizzically at about 3.5V. This resulted in the MOSFET not being driven all the way on, and my current sink could only sink about 30mA. Not exactly useful! Then I had a memory of it needing more headroom so I changed its power supply pin to before the 78L05 and then it worked like a dream.

Shown here the complete circuit, pulling 1A from a crappy old car battery charger

Shown here the complete circuit, pulling 1A from a crappy old car battery charger

I am very pleased with this design, it works quite well and smoothly. Those Dale 5W 1% resistors I found at A1 Parts were really a win. This one shunt will work just fine. If I choose, I can use a more precise one down the road, but honestly I couldn’t be much happier with it. Obviously everything will need to be trimmed for accuracy and tested under a variety of conditions.

I did a power test for about a half hour, trying out various loads, was able to do them all without a hiccup, up to it’s design maximum of just over 3A. I had my thermocouple on the MOSFET to check temperatures and found the rather large heatsink I gave it works fairly well, though I still will want a fan in the case. It is rated to run up to 175°C which is quite insane, I do not intend to get it any warmer than 70°C if I can help it. The datasheet doesn’t indicate anything awful happening until much higher temperatures than I intend to run it at. I pushed it up to just shy of 60°C today with no issues.

A new design, a new build

As I hinted in my last post, the Nixie clock project made me really sick of jumper wires and protoboards. I also had concerns that the small traces on the protoboards wouldn’t be able to handle the high currents I want to run through some of them. Soldering more wire on those paths is a solution, just rather an inelegant one. I think it’s time to kick it up a notch and make something properly.

EasyEDA

In watching my usual bunch of electronics videos on youtube, I kept running into mention and use of EasyEDA. As it turns out, it solves two of my problems with one go! One being that I can’t afford my CircuitLab subscription (which is still a shame, I loved the easy visual simulation) and need a way to make schematics and test them. The other issue was how to make proper PCBs for my projects.

The schematic part is easy enough, and after making an account I was quite quickly whacking together schematics no worries. What’s even cooler is that it will take it right from there to making the PCB layout. In addition, they have a partnership with JCLPCB in China and you can upload the gerber files directly from there and order your boards…. for like $2 for 10 boards! Within limits of course, like standard green solder mask, no crazy stuff, 100mm maximum boards for that price. Still perfect.

Here’s what I’ve done so far for the Dummy Load project:

The "production" schematic. The project hasn't changed from before, but the off-board components are replaced with headers/connectors

The “production” schematic. The project hasn’t changed from before, but the off-board components are replaced with headers/connectors


My first PCB layout! EasyEDA is living up to its name for sure

My first PCB layout! EasyEDA is living up to its name for sure

Nice huh? When I’m satisfied that I’ve got this thing thoroughly debugged and triple checked, I’m going to order the boards and see how this goes. I think it’s going tobe awesome, I’m excited!

Update

I realized I ought to have checked the PCB mounting holes. As I thought they were a bit small at 80mil. An M3 standoff and screw is about 125mil, plus clearance for the panhead/nut. So a bit of widening and tweaking will be needed. I also realize I have no provision for a fan so far. I’ll have to sort out how I do that as well. Likely just keep it going full blast all the time, but I need a good place to tap that off from. I don’t really want to add another 12V regulator…

The next day…

So I whacked together a quick addition and tried using it with a junk 12V fan I had laying about. Seems to work just fine. The fan’s addition to the total load seems to be well within the limits of the power supply and there was no wackiness in terms of the dummy load’s operation. If anything, I may just add a bit more capacitance to the power input. I did notice a 2V drop on the power pin of the opamp, but it’s still way more overhead then it needs so no worries. In terms of power supply, I was humming and hawing over what sort of power supply circuit I would need. I can’t run a 12V fan off of unregulated 16V. Well I could but its lifetime would be dramatically reduced. The quicky circuit I made was just a 12V zener diode with 33Ω of resistance. While this worked fine, it was a bit dicky and I’m not sure I like it. Me and zener voltage regulators don’t really get along apparently! Problem is I’d need a 33-47Ω resistor in there, and it would need to be at least 0.5W (1W for safety margin). I just don’t have one in that value, I tried using 10x 330Ω 1/4W ones in parallel and that’s a bit of a mess. The zener itself would dissipate 1.08W (80mA load) which is just 80mW higher than the zener I have is rated for. It did get warm, though not hot. I really don’t want to chance melting the zener since the fan is going to be on full-tilt whenever the unit is on. I think the easiest solution is to just use an LM7812 which I have several of. Really love the LM78xx regulators, so very useful.

The next step is to figure out the impact, if any, to the PCB I want to print, shown above. I’m trying to decide whether or not to add the 7812 and its caps to the board, and a connector for a fan. Part of me really wants to to avoid having a little protoboard by the fan. I’ll decide later tonight. Incidentally, I enlarged the board a bit and made proper mounting holes for the PCB as mentioned above.

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