psu->pwm and meter

[kooma]

I'm making an 2output adjustable psu with smps,pwm-module and led meter;
http://www.ebay.com/itm/12V-40V-10A-Pulse-Width-Modulator-PWM-DC-Motor-Speed-Control-Switch-Controller-/400586595552?pt=LH_DefaultDomain_0&hash=item5d44d25ce0
http://www.ebay.com/itm/DC-0V-99-9V-3-wire-Mini-Blue-LED-Panel-Meter-Digital-Voltmeter-/121335977252?pt=LH_DefaultDomain_0&hash=item1c40301524

and I'm having a problem with pwm's floating ground and my meters:
led meter has gnd,vcc and measurement wires, i had an idea to get power&ground for the meter from my smps and
the measurement from my pwm-output, but as they have seperate grounds I get just weird readings..
connecting pwm and smps grounds messes that pwm module output..

Is there any other way than adding another transformer/smps to power my meter and connect it's ground to pwm ground?

 

[JohnRoberts]

There should be no problem with connecting the grounds. The meter may be confused by noise from the PS.

JR

 

[kooma]

If I connect the grounds voltage adjustment isn't working..
I'm beginning to wonder could that pwm-module have it's silk screens wrong etc.
edit. output polarity is ok. so this is just weird pwm-module

when I adjust the voltage
the difference between grounds gets smaller..
(ie. when my output is adjusted to minimum 2.5V on pwm out, the voltage difference between grounds is something like 12v,
when I raise the voltage towards the max (16v) the difference gets smaller..)

difference on the pwm-module(+) and the smps(+) stays within 0.5V etc.

 

[PRR]

Some LCD meters can "not" take power from the rails they measure. The common-mode voltage does not go all the way to the rails. They are meant "only" for floating battery power.

 

[kooma]

Thank you for comments!
this meter seems to work when I take ground from the pwm out- and connect vcc&measurement to pwm out+
(but meter needs some volts before it works..)

I'm thinking that this pwm-module has some weird things going on,
don't you usually adjust the positive rail? as here only "ground" seems to vary.