didital ammeter

[F.C. RACING]

Hi i'am new to this board and only have basic grasp electronics i am building a power supply to run slotcar motors on and would like to include digital amp meter the local electronics place has didgital panel meter that require 5 volts to run they show of to hook it up for volts but not amps is there a circuit i can use to make it read amps.
Thanks

 

[erosennin]

There is... but it's not very ahhh, "legit".

It also depends on how many amps you anticipate the slot cars will draw.
You can wire a 1R 10W resistor on the positive power rail, and measure the voltage across it.
V = I*R
I = V/R as R = 1, V = I.
You are able to set the range of current you wish to measure by using 10R, 100R, 1k, 10k, 100k etc. etc.

The problem with this can be, the resistor may dissipate more than 10W and thus fry itself. Make sure you check to see what the maximum current may be, and apply the formula P=I^2*R, to see how much power the resistor will be dissipating at maximum current.

I stress that you take care in ensuring that you choose the right value resistor with the correct power rating, and make sure the resistor you choose wont drop more than 5V (Or whatever the rating of your panel meter is), otherwise you may cook your panel meter as well.

 

[kchriste]

It really depends on how much current you want to measure and the sensitivity of the meter. Below is an example circuit of how to enable the digital meter to measure both amps and volts via a switch. R2 is the current sense resistor and should be as low a value as practically possible to limit the power it must dissipate. 0.1hm: would be a good value for a 2amp scale since it would only drop 0.2V at 2 amps and dissipate 0.4watt. R1, R3 & R4 and the OpAmp amplify the voltage across R2 and reference it to ground so the digital meter can display the amperage. The values of these components depend on the meter and the current/voltages you intend to use. The meter, via a regulator, is powered from the left side, or input, of the circuit.

 

[F.C. RACING]

Amp Draw

Thanks for the quick ansawers guys not sure i understand it totally yet
but slotcar motors can be power hungry little suckers the power supply is meant to break in new or rebuild motors and they can draw anything from 3 to 12 amps on start up and run at as high 8-9 amps at 4 volts.

 

[erosennin]

Hooley Dooley... 12A... I thought they were pissy slot cars, right...
In that case... neither mine nor kchristie's methods would be appropriate, as 12A through 1R is 144W and 12A through 0.1R is 14.4W...
I think the best option would be to buy an el-cheapo multimeter that is capable of measuring 20A and hardwire it into the circuit.

Your location says Rockhampton, so I'm guessing that your outlet is Dick Smith/Tandy... They don't have much... Your nearest Jaycar is Townsville which is completely cactus... Hmmmm...

I'll think about it... lol... the others might have a solution... you could make a current clamp, but that's just overkill...

 

[nickelflippr]

There are some cheap Chinese digital amp meters on Ebay. Look up 50A, or 100A shunt meter. Thats going to be one hefty power supply, good luck.

If you decide to build your own, one of those high amp, 50mv shunts will be your starting point.

 

[F.C. RACING]

amp meter

Erosenin rockhampton is correct Dick Smith is no help at all there is a jaycar dealer here now but it's part of a computer store they have lots of electronics components but no real tech advise is avaliable I have been used digiat multimeter to read amps/volts. i was going to use analog meters just thought the digital would save on space and be easyier to read.
The amp draw is real and this is only on the power supply on the track and under full load they can peak at 40 amps

 

[erosennin]

Dick Smith is no help at all... but no real tech advise is avaliable

Yep, that's how it is, all of the people who work at those places have no idea what they're talking about, and it's getting worse year by year.

under full load they can peak at 40 amps

Well, you're definately going to need to make/get a current clamp/shunt.

 

[stevez]

Does your digital panel meter have a fairly low range for voltage - say 200 millivolts (mv)?

A shunt of 0.01 ohm would provide 200 mv voltage drop at 20 amps. You could make your own shunt - an example about 2.5 ft of 16 ga copper wire (wind in coil, won't matter at DC) - or about 4 ft of 14 ga. You'd use very little with 18 or 20 ga but it might get too hot. The nice thing about 200 mv and a 0.01 shunt is that the meter will read "200 mv" with 20 amps so you can insert the decimal point in your mind as you read it.

 

[F.C. RACING]

ammeter

the one i have been looking at has a number of different settings from 200mv to 500vdc funny you should mention making a shunt from winding wire i had the same thought myself. because in high power slotracing we use a thing called a choke to slow down the current it's not like a true choke but it is usually made up of coils of winding wire in 2ft lenghts with relays so you can dial in the amount you want 2ft or 10ft.
I was going to go with the 20 volt setting but most available shunts seem to be made for 200mv range.
if i got with 20 volt setting will it be 0.10 ohm

 

[eblc1388]

Ready made current shunts are accurate with different ambient temperatures, not that expensive and need no calibration.

Buy one unless you have a good constant current source to actually calibrate an home made one made from thick cable.

 

[F.C. RACING]

still need help

My power supply is coming along niceley hooked up digital panel to read in the 20 volt scale it uses 5volt common ground i can't work out the shunt value to read to 20 amps. And am i correct that the shunt goes accross the vin +and the ground wires.

 

[nickelflippr]

Whoa, hold it right there, not that type of shunt. There can be high and low side shunts. An example of a high side current sense ic would a Max4080, or maybe a Zetex ZXCT1021E5TA. The Max4080 shows where your Rsense (i.e. DC shunt) goes, and a table of typical component values.