Tag Archive for voltmeter

ICL7660 feeding two meters

I mentioned back a few posts when discussing using a separate power supply for the meters, that I could use the ICL7660 IC to provide the -5V side of the power supply for the meters, rather than a cascade of negative voltage regulators stepping down from 24V. I’m not exactly sure why the 7107 requires a symmetrical supply, but it makes sense because it will have to perform negative voltage measurements as well as positive. No problem.

I conducted a few quick tests to see where the current is flowing and found some encouraging results. On the -5V line, I was only able to detect about 90µA of current flowing through it when it’s doing nothing. This is encouraging as it means I most likely can use one 7660 IC for two meters rather than shell out for another chip. I say most likely as I am having a hard time finding what the actual max sink current is for the 7660. If someone knows, please let me know. I figure I will just go ahead and build the ammeter (learning from my mistakes with the voltmeter) and attempt to power both from my single 7660 and see if it melts on me. They are replaceable to why not? Even if the sink current doubles (which is what I expect), it will still only be 180µA.

I also measured the +5V and 0V leads and found that the bulk of the current, no doubt to power my humungous 1″ LED displays, travels from +5V to 0V and the -5V rail is just to provide a reference for the meter. Displaying all zeros, it draws 56mA of current.

Soon, it will be time to test and build the control voltage board. I have a junk transformer somewhere on my bench and I will test it’s suitability tonight. If one pair of secondary leads can be rectified to a couple of volts above 24VDC then I’m in business. I know this one to have several secondaries so that even raises the possibility of having separate secondaries for 24V/12V/5V supplies which would be even better. Otherwise, I’ll have to go shopping which I enjoy anyway.

Another thing to do would be to measure the total current draw of 4 relays when they are closed, add the odd 120mA for both meters, and the tiny quiescent currents of the regulators. I expect all of this to fall under an Amp which should do nicely.

Voltmeter success!

Now that I’m back to where all my tools are, I had a few minutes to pop down and perform the changes I have been documenting.

After messing about with a few alligator leads, I determined that not only does IN LO need to be connected to analog common, but the voltmeter ground as well. When IN LO wasn’t grounded, I got an initial reading (with nothing connected to the input) way off zero, beyond the point the trimpot could calibrate it.

As the forum posts mentioned in my previous articles stated – the dotted connections. must both be connected for the thing to work properly. This means REF LO is connected to ANALOG COMMON which is connected to IN LO and then GROUND. Given the language in the datasheet, I would have that it was an either/or scenario, not both. Regardless, I am pleased it’s working.

The assembly process was a bit messy, I cleaned up a lot of solder blobs and accidental solder bridges. Unfortunately, I soldered/desoldered and overheated a couple of pads, removing them from the board, the result works but it’s messy. If it can survive a few knocks and keep working, good enough for now. I can always build another one.

The divider resistor values calculated in my previous post worked a charm. Rather than wasting money and time grabbing 1% resistors, I tried various combinations of 5% ones until I got very close to those values. I tried a number of test voltages from batteries and my soon-to-be-replaced power supply and noted that not only was the reading linear across a range of voltages, but along it’s scale ranges as well which is exactly what I was looking for. After calibrating to 100mV and further trimming it a hair to get it in line with my multimeter, I am pleased to say it seems accurate to better than 1% which is not only good enough for it’s intended purpose, but better than I expected.

I have earned myself a beer tonight!

Voltmeter Scaling

As a follow-up to my last post, I ran a bunch of figures on how to set the proper scale ranges for the voltmeter and ammeter for the power supply project. The numbers end up being really close to the forum post I stumbled upon a few days ago which is great. Using 100mV instead of 1V for my reference voltage, I was able to recalibrate it by a factor of 10 and solved my former problem where I would have had to use 1MΩ as my divider resistor and it equalling the input resistor. This way, I end up using more sane values and I’m pretty sure when I retrofit my voltmeter they will work out ok.

CircuitLab mock-ups for voltmeter/ammeter scale divider resistors

CircuitLab mock-ups for voltmeter/ammeter scale divider resistors

On the back of the envelope, it would make sense to use round numbered divider resistors to get even factors of 10, but as it turns out in analysis, those numbers end up being slightly off for some reason probably involving calculus.

Not wanting to waste my sunday morning trying to remember math I last used in high-school, I consulted CircuitLab (invaluable resource and well worth the subscription price) and mocked up some quicky input divider circuits for my voltmeter and ammeter. The results I will display here along with the schematic.

Voltmeter Divider

Scale Resistor Ideal Value Resistor Calculated Value
2V (0-1.999V) 1kΩ 1.001kΩ
20V (0-19.99V) 10kΩ 10.1kΩ
200V 100kΩ 111.1kΩ

Ammeter Shunt Resistors

Scale Shunt Resistor Value
2A (0-1.999A) 0.1Ω
20A (0-19.99A) 0.01Ω

So that solves that. All that is left is to build it up (may breadboard it first to avoid unnecessary soldering) and see if she floats. Obviously, I’m going to have a hell of a time finding the Voltmeter divider resistors in those odd formats so I will just play with series combinations until I get as close as possible to those values. It will save me hunting for expensive 1% resistors when I can just compensate with 5% resistors of which I have a ton.

For the ammeter, you will note the numbers are a bit off, this is due to the 4 significant digits in CircuitLab’s calculation which is absolutely fine and I can trim any error with the reference voltage.

For the ammeter (as well as the current sense for the limiter) I have some interesting ideas which may warrant it’s own post, coming soon.

Voltmeter

A power supply needs a display

In my second instalment of the power supply project, I’ve constructed the volts display. I’m kind of working back to front, as I’ve already been though much designing and I also built the AC-DC module which I will detail in a post to come soon.

What I’ve built is just a voltmeter and a display for it on a single piece of protoboard using the popular ICL7107 IC which is a voltmeter, ADC, and LED display driver in a monster of a 40-pin dip package from Intersil. This was quite a godsend as the power supply was already getting quite complicated and I wanted a simple, one-chip solution for the volts and amps display.

I had previously breadboarded it up and it works well. Actually, very little hassle considering what it’s actually doing. I did, however, run into problems with obtaining the IC briefly. I payed a visit to Creatron at Spadina and College (in Toronto) and they only had one in stock for $17! I stupidly bought it thinking “ok, it’s an expensive IC” but as soon as I got home, I looked it up online and found it for $2 something at Jameco. Bah! I was so pissed.

A couple of weeks later, I went to Supremetronic (for those who don’t know, it used to be on Queen Street West and moved to College St. then amalgamated with the Home Hardware. It now lives in the basement of the Home Hardware at College and Spadina) to grab some more parts. They not only had the ICL7107, but had it for $3 and change! So I bought four of them. Lesson learned: shop around.

I still like Creatron, especially for their nice protoboards, but it really does pay to shop around. Also, I have this horrible need to shop at brick and mortars for some reason. It’s more fun.

Back on topic.

So last night I actually go to solder the thing up. It was a bit fiddly to get it all to fit on two sides of one protoboard. Lots of ugly resistor forests which hide under the elevated 7-segment displays. I chose 1″ 7-segs just because I could and they look cool.

The font view, note the crappy soldering

The font view, note the crappy soldering

I modified the circuit to have a switchable range: 0-200V (199.9V), 0-20V (19.99V) and 0-2V (1.999). The last one I’ll use purely for calibrating it as the supply has a minimum voltage of 1.2V the last range is sort of useless for day to day use. I wired it via an 8pin pcb connector to a 2P3T band switch to change the range and set the decimal point. Lovely.

Rear view. Monster of a DIP

Rear view. Monster of a DIP

Took me about 8 hours of soldering, the last two of that finding and fixing cold or otherwise inadequate solder joints. I’m not the best at soldering as you can see, it’s a fine art I’m still developing skill in. When I first powered it on, it was intermittent and displayed junk characters. Fixing the joints stabilized it nicely. The oscillator is working fine, the auto-zero is auto-zeroing.

It’s not without it’s problems however. I did note that the calibration seemed off for the different ranges as well as it measured voltage non-linearly. A big problem and most perplexing. As with most things, the reasons and solutions are rather simple.

When test measuring, I made the beginners mistake of measuring it’s own supply. So when the calibration trimpot was set to a reference voltage of 1V, 5V read ok on the 200 and 20V range (with minor error) but 12V (again measured from the same supply) was horribly off on the 20V range. What’s more, the 2V range was off by a mile, displaying numbers about 1/2 of what they should be in the wrong range. Drove me mad.

I started reflowing joints and yanking out caps and resistors trying to find the problem. I realized only later that the problem wasn’t me or my build, but the schematic I was using. The must frustrating “trap for young players” (as Dave Jones from the EEVBlog would say) is that schematics you find online are frequently wrong, or the idea is right but whomever drew up the schematic forgot something or got a part value wrong. The error then gets multiplied as you modify the circuit to suit your application. Shit happens.

I used the following schematic which was deceptively simple and did (sort of) work on breadboard. Works reasonably well for the 200V and 20V range as presented provided the following:

  • you don’t be a smartass and replace the 1M input resistor with a 10k thinking that it will set the range to 2V (100k sets it to 20V)
  • you don’t measure the voltage of the same supply (or without a high impedance input and even then expect some error)

These two traps are something an experienced design engineer or seasoned hobbyist would know, but I’m just learning. After a couple hours swearing at the thing and bodging in new caps (unnecessarily) I realized that I can actually troubleshoot this. Going through it logically, the IC is working fine. The clock works, the displays work, the measurements are just off. The reference voltage reads 1V as it should so it MUST be the voltage divider on the input!

Looking at the schematic, and comparing it to the official data sheet, I notice a discrepancy. This guy (he isn’t mentioned on that page) modified the scale by changing the 1M input resister, rather than the resistor between the IN HI and IN LOW pins. This will work, after a fashion, but check what happens when you want a 2V scale. 200V scale is 1M, 20V is 100k, so I thought “duh i’ll put a 10k in there for 2V”. Logical right? Not really. The two resistors form a voltage divider which needs to scale the input by a factors of 10. So 1M/10k is 100:1, 100k/10k is 10:1, and 10k/10k is … 1:1! No wonder I was getting such an off reading, it was dividing the input by half instead of 10. Thus, it was half the proper reading and in the wrong range. Yes, I’m an idiot.

Fortunately, to correct the problem all I have to do is move a few resistors around and move a couple of jumper wires. Fortunately, I saw someone with a similar problem and a kind gent (goes by Ron H) posted a solution for correct values for the resistor that will yield the same ranges here. he even provided exact (I assume calculated) values to further reduce error. The new values are: 200V = 10.1k, 20V = 111k, 2V = infinite (open). I’ll put in the next closest values with 1% and/or series resistors to get close to that. Best part – it keeps the input high impedance (1MΩ) as any voltmeter should be!

I’ll post the results of my tinkering in a future (soon to come) post.