Hi all
I have converted a petrol car to electric and am having trouble with monitoring meters
I have tried several ways of measuring amps with no success. Currently NPI I am using one of the traction power cables as a shunt with some potentiometers to fine tune it once i get it working. The first effort I could not get the meter to stabilise. It flickered all over the range. I was then advised by jaycar I need to have a common earth ie - power supply (5v) connected to the - 144v. This appeared to work briefly until some sparks jumped out of the meter and I could'nt get them back in. the end of that meter. So the whole amp meter thing has been on hold.
I have just installed a 200 volt meter and although it lights up it only indicates a single 1 in the left display. I fear there is a similar problem with it to that of the amp meter.
some specs
Traction pack 144v dc
volt meter is chineese ebay job requires 5-15 vdc
I am able to get readings on my multimeter of the signals coming from shunt and the voltage so i feel as though the problem is with the way I have the meters hooked up.
Thank for any ideas in advance
Dedly
Ps not too technical please my knowledge about elec stuff is very basic.




r

 

Here is the problem. Your shunt. You need a shunt of known value of reistance and hope it doesn't change when it gets hot. Now that you know what the resistance is it is easy to devise a meter that will work. If you know the resistance and you hook up your voltmeter to the shunt, the voltmeter set to volts, because that is what you want to read is volts. Now using Ohms law I=E/R where I= Amps and E =volts and R=Resistance. so whatever volts you read divide by the resistance gives you the amperage. Since the voltage is small your new amperage circuit will need to beef up the voltage a bit in order to work the guage. meters have different ratings, like 50ma for full scale or 1 volt for full scale etc. You will need to know that in order to design for the meter you want to use. First thing do you know what the shunt resistance is?

 

Thanks for the feed back YOF
I am not concerned about the accuracy of the meter at this stage as I have some pots hooked up for calibrating later.
I just want to get a reading on the meters
My multimeter gives me good millivolt readings from the shunt for the amp meter and I get an accurate volt read for the voltage. I think the problem is with the power supplies to the meters as I mention both readings are good at the meters prior to connecting them to the meters.
dedly

 

I think the problem is with the power supplies to the meters as I mention both readings are good at the meters prior to connecting them to the meters.
dedly

You are exactly right on that issue.

There are different configurations for the power supply to these digital meters. As you have just found out, failure to understand how to connect them up often led to destruction.

However, it is a bit difficult in your case as you are working with DC and it is difficult to provide an isolated 5~15V supply to these meters. The simplest way is to use DC-DC converter to provide those power supply.

Many mobile phone chargers(not the heavy ones) are of switching type and will probably work(never tried myself) with DC 140V and worth a try.

Remember to always use an 1A fuse in series with the DC 140V during test.

 

A Digital meter will never show the correct reading if it is powered from the line which it is monitoring. DC-DC converter is the only solution. Its cheap and simple. Showing a 1 on the display says its out of range. Mostly due to power supply problems.

 

dont forget to add bypass capacitors along the line to keep the crud from going through the line, starting at the shunt and use shielded cable.

 

Thanks all for your comments
just an hour ago, before I read them I turned my latest meter into an unpleasant smelling non glowing group of electronic parts.

Firstly in response to EBLC
Yes I proved that I do need to learn more about hooking them up. I should clarify a couple of thing.s I am not powering the meters from the 144 v. I have a 12 volt system in the car as well as the 144. This consists of a 12v lead acid charged by a solar panel.
The 12 volt system is earthed to the body of the car however the 144 volt is not earthed to the car.
Thank you I will definitely invest in some fuse holders and 1 amp fuses for the future.
By the way the first meter I blew up required 5 volts. I used a 7805 voltage regulator with two capacitors. It also got very hot and failed.
If it is of any assistance I recollect that both times things blew was on either heavy acceleration or deceleration of the car as they both worked for a short while prior and during driving the car.

Secondly Thanks JJ alukkus
As i said above it was not powered from the same source I was measuring. However It may be a good Idea to try the DC DC converter. Will this surely clear up any interference? What kind of converter should I look for? We have "Jacar Electronics" here with lots of different things like converters and kits that can be made.

Thirdly Thanks YOF
I do not understand exactly what you mean.
Are you saying I should put capacitors in both positive and the negative wires coming from the shunt to the meter? If so what size capacitors?
Should I also put capacitors in both the wires coming from the 144 volt I am trying to measure?
Shielded cable?
Is this like using the centre wire in coaxial cable? And that I should use it on each of the cables that I am measuring with?

Once again thank you all for you help.
regards David

 

Use a DC-DC converter which has input voltage same as your main line and output about 12v or 5v as necessary by your meters. Dont supply higher voltages thinking you can regulate it down as necessary. It will dissipate a lot of power as heat. Also take care of the amperage required.

Regulators like 7805 will blow up if you draw heavy current (>1A for 78XX) for long time and use it on very high voltages. 3-5 volts greater than what a regulator is rated for is the best input for any regulator. Also use large heatsinks if you are planning to push it to limits.

 

about the caps, I am sorry for not explaining, in circuits with long runs of wires and other reasons, to get rid of noise on the line for instance a capacitor from say the shunt, each side of the shunt to ground. (where the measuring is derived from)I am guessing the exact size but should work fine is a .1 UF at 400 volt capacity to be safe. Not expensive. another note:If you use shielded cable thats helps keep external noise out of your measuring line. Shield part needs to be grounded. Noise gets in the line and screws up the readings because it is not pure for it to read, so it goes all over the place trying to focus on one voltage. even if you decide to go with other ways mentioned, doesn't hurt to keep the signal clean. I just thought of something else that might help that I have done in the past, is to place an electrolytic capacitor (different style than just mentioned) across the meter terminals. have to experiment with size, guessing about 1uf (not .1) to about 10 uf. What happens is it stops the meters from quickly bouncing and making dramatic changes and causes them to average out and move smootly and slower reaction. If you get a cap too big on there, the movement if using analog will seem to just hang there until the caps discharges. You could still put one across the digital as well at the voltge inputs to stop the quick flucuations. It actually will average out the input if the input jumps around a lot.

 

Thanks JJ
I have bought a Keno DC DC converter. I hook it up to 12 volt but can only get it down to 16 volts using the potentiometer they sold to me.
I will order another couple of meters. I guess there is no rush it will take another week or so before I get another meter here from China.
I expect those meters would draw much less than an amp if they were functioning correctly. So I am still puzzled why the 7805 blew up.

 

I expect those meters would draw much less than an amp if they were functioning correctly. So I am still puzzled why the 7805 blew up.

You 7805 blew up because there are large voltage difference between the two voltage systems, the 12V and the -140V.

Its fine when you power up the meter from 12V alone. The moment you connects the meter inputs to the target system(i.e. 140V), you have effectively bridged or cross-connected the two systems. This would result in large voltage difference and/or current flow.

In case you are not aware, you would need a total of TWO DC-DC converters, to power voltage and current meter, to be absolutely on the safe side.

 

Hi, those little LCD panel meters sold by Jaycar and Altronics have a flaw in the datasheet.

You need to connect the Vref out and Vref in pins together, that's pins 3 and 4.

Then the meter will display the proper result for signals 0mV to 200mV.

If you don't join those Vref pins the display will show "1 " or some weird high value that slowly floats downward over time. I was working with one a few weeks back.

 

Thanks for continued interest.
I am still puzzled as to why I can leave my multimeter hooked up to the voltage or monitoring the amps for hours and days under various driving conditions and this does not damage the multimeter.
I have ordered two more meters CX102 i think, the ones that run on 9 v
My plan is to run them off a small 9 v rechargble battery for the time being to see how that works.
I will heed all advice and place fuses and capacitors in the lines and gert some shielded cable.
Any thoughts on that?

the RB
I did join the 2 negitives and used some caps as advised by Jaycar specs. This worked at one point and did get the reading ok but the meter did still fail after short use.

 

I must have missed something somewhere. do you have the diagram to the electrical? I would hate to see you hook up a bunch of unnecessary stuff if not needed. m I know I made the suggestion but I would feel better If I looked over the diagram and I will go back and read the thread more closely to see what meters you are using. A dvm works great but whatever other meter you hook up doesn't? How long does the run of wires have to be from the shunt to the meters? This cant be too hard to do. I can see your frustration, that's why I would like to have another look. Something is odd about the whole thing. In my mind I might be seeing a pic of something of what i think it should be, and I do take things for granted sometimes. I can tell you why your 7805 blew up if you tell me which line you hooked it to, the 12 volt line or the 144 volt line. Also you need a dropping resistor even if you use 12 volts. also the current you need to run your stuff might have been more than it can handle. Another reason is if you dont have a heat sink on it it will fry too, even at the rated current. need to read the specs to see what the temp is on it and current for typical operation. If you need more current regulated you can use a pass transistor attached to the regulator.

 

For Dedlycow - you missed the point of my post. I've used that same LCD a few weeks back. The Jaycar specs fail to mention that you must connect the Vref pins together. I'm not talking about connecting the grounds to what you are measuring or connecting the LCD grounds etc together and to the decimal point select pin.

If you don't join Vref out and Vref in pins together (and not to ground) you get the fault where it seems to measure ok for a while THEN will only read a 1 in the left digit, which is the symptom you describe.

You didn't blow up the LCD, you just need to connect pins 3 and 4 and it will work normally again.

 

It is safer to measure current by measuring a small voltage across your shunt resistor, much less likely to blow the meter. I think the other advice here is right on. In particular digital meter really seem to have trouble with noise on the line. Good filtering can reduce that. It is easier to filter in a voltage mode than a current mode, esp using only caps and resistors.

 

hello helpers
I have been away for awhile
I have made a small circuit. it consists of a 9v rechargable battery controlled
by a relay switched on and off by the ignition switch. there is a 1 amp fuse in the 9volt circuit to protect the meters from the power supply. I expect I should be able to run both meters from this circuit. does this sound correct?
With regard to YOF's comment about the Capacitors. The 144 volt is not grounded so I am presuming that I put one 01UF 400v capacitor on each wire leaving the shunt with the other end of the capacitor connected to the other 144 v cable.
and the 1or 10 uf across the 2 inputs to the meter from the shunt.
try using the cables I already have installed if the signal is still not clear I will re install sheilded cabls from the sources to the meters.
Of couse this is for the amp meter I still have not received the new volt meter.
any comment are greatly apreciated
and thank RB
I dont have those meters any more but Ill remember what you have said for this one.
Even though the meter is from abifferent place it appears identicle to the jaycar ones.
regards david