help with PWM and volt/amp meter

[Macman]

I built a slotcar test bench test cars at different voltage. I used a PWM with a pot attached to adjust the speed of the motors it works great by it's self but when I added a digital volt/amp meter in between it no longer adjusts speed, its full on and the volt meter don't work right. can the two be used together and if so how do I wire them. bellow is a pic of how I have it wired. any help would be great

wire.jpg (93.71 KiB) Viewed 35677 times

[KMoffett]

Can you post a link to the PWM circuit you are using? And to the V/A meter you added?

Ken

[Macman]

got both on e-bay


http://www.ebay.com/itm/331046378386?_trksid=p2060353.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

http://www.ebay.com/itm/231577774826?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

[KMoffett]

If you look at the wiring diagram in the eBay link you see that the power supply to the load (POWER) and the power supply to the meter (DC4-30V) are not the same. This a very common arrange for this type of meter. The meter supply must be isolated, like a separate battery, from the supply to your load.

Ken

[Macman]

wow Ken thanks!!! I isolated dc4-30V power by using a 9V battery on a breadboard copy of my original project and it worked fine. I will apply this to my original project tomorrow and see if its the fix I need, but I think it's going to work. Thanks again Ken

[Macman]

I used a 9V battery to supply the power to light the volt/amp meter but it still don't work right. im concluding that you can not measure volts of a PWM because even when I put my hand held meter on it the volts don't read right. I am guessing that maybe its because the output is no steady but a pulse...any sugestions

[KMoffett]

PWM output is either full voltage or zero. As is the current flow. The relative on/off time is the variable. A meter with a long enough time constant relative to the PWM frequency it will read the Average voltage. Why do you want to read the output voltage and current of the PWM signal?

Ken

[Macman]

its for slotcars here raced competitively on a commercial track. the cars are powered by a small 12V motors that can draw up to 5 amps. the purpose of the PWM is to allow variable volts to run the motors for testing, adjusting and breaking in new and rebuilt motors. most racers will build a motor and then need to break in the brushes by running it at 3 to 5 volts for 30 minutes. they can run them at different speeds and check RPM to see motor condition or if it needs replaced

[KMoffett]

I'm not familiar with slot car systems. Are the speed controls to the tracks PWM? Curious if brush break-in with 3-5V "DC" would be the same as 12VPWM at 25% to 42% duty cycle.

Ken

[Macman]

entry level controllers are a resistor type

untitled (2).png (56.14 KiB) Viewed 35474 times

experienced racers use more expensive circuitry

images.jpg (11.09 KiB) Viewed 35474 times

[KMoffett]

I would think that running a these motors on say 12v at 50% PWM would not be the same as running them on 6VDC. The voltage to the motor is going from zero to 12v and back to zero many times a second, not sitting at a continuous reduced voltage. Especially if you are evaluating them with PWM yet running them on DC with resistance controllers. I'm going to bow out on this because I can't offer anymore knowledgeable help.

Ken

[Macman]

well back to the drawing board..is there any way to achieve the variable motor speed without use of a PWM?

[pebe]

There is an alternative way of doing it. You already have a 12V DC supply so you could feed that into a buck converter such as this one:

http://www.ebay.co.uk/itm/5-30V-to-0-8- ... 3a8900d308

You would then be able to adjust the output voltage to anywhere between 0.8V and about 10V. The maximum current of that one is 5A, though several models are available on Ebay.

A buck converter does for DC what a transformer does for AC, ie. changes the input voltage without changing the power used (I use a buck boost converter to give an adjustable 0.8V to 30V power supply). Let's know if you decide to go that route, and I will give you a diagram showing how to wire up the meters.

[Macman]

I have a buck converter supplying the 12 V to the PWM I guess I could just remove the PWM and then try to wire a POT to the BC... it has a small screw type POT but I need one you can turn by hand.

[Macman]

I have 12V coming out of a buck converter the pos going to the right outer prong of a 2000 OHM POT then coming out of the middle prong and to the motor the neg from the buck converter to the motor and I get no power to motor until POT is turned all the way, very little variance in motor speed and POT get real hot... any help?????