Panel mount meter not reading correct values - Page 1
Forum section: electronics / beginners

author: PotatoBox
author_rank: Regular Contributor
timestamp: June 24, 2020, 01:56:39 am
content: I just built my lm317 based power supply but my voltmeter that I’m using is way off with its readings. It shows voltages as little as 0.5 volts to 5 volts off from the actual value. I’m reading the outputs from my multimeter and the power supply is outputting the voltage I want. I have it setup so that theres the lm317 which is the output voltage for the bench power supply as well as an lm7805 tapped off from the transformer after rectification and the filter caps to power the meter. I have it kinda setup like this on the bottom picture (independent supply).Could it just be a faulty meter?

author: eblc1388
author_rank: Frequent Contributor
timestamp: June 24, 2020, 03:23:42 am
content: [ Specified attachment is not available ]Quote from: PotatoBox on June 24, 2020, 01:56:39 amThat's the problem. An independent supply must be truly independent, without any electrical conductivity to the remaining circuit. By powering a 7805 from the main rectifier/smoothing capacitor, that's not independent.Your connection choice should be the top picture instead without the thin black wire. The "Thin red line" should be connected to the main smoothing capacitor before the LM317 instead of output. If you worry about the high voltage supply to the meter then may be you can reduce the voltage by putting in series a 0.5W zener diode to the thin red line, say 15V to 32V.

author: PotatoBox
author_rank: Regular Contributor
timestamp: June 30, 2020, 02:54:05 am
content: Quote from: eblc1388 on June 24, 2020, 03:23:42 amWhich zener diode would you recommend?Also how will a zener help? isn't the voltage drop only 0.7v across it anyways?The output of the transformer is 37v without any load and the panel meter's max power supply is 30v so i definitely need to drop the voltage in some way.

author: WattsThat
author_rank: Frequent Contributor
timestamp: June 30, 2020, 04:42:02 am
content: A zener diode isn’t going to fix anything, its a non-starter.If the output of your LM317 power supply never goes below 4 volts or above 30 volts, wire it per the top diagram.If the output of your LM317 power supply goes outside that 4-30 volt range, as is typical for adjustable bench supplies, you need a second supply that is isolated from the source you're trying to measure. With a second supply, wire per the lower/bottom diagram.On an adjustable supply where I used one of those meters, I repurposed a Nokia 5 volt cell phone charger purchased at my local thrift store for a $1. Crack it out of the plastic case and mount however you want/need to. I mounted mine using heavy solid wire soldered to the mains connections.

author: eblc1388
author_rank: Frequent Contributor
timestamp: July 01, 2020, 12:58:27 am
content: Quote from: PotatoBox on June 30, 2020, 02:54:05 amIt will drop 0.7V only if the zener diode is connected like an ordinary diode, with current flowing in the direction of the diode. Zener diode, however, will breakdown and conduct if it is connected in "reverse" configuration. Under this condition, it will drop roughly the rated zener voltage across itself.Use an 20V to 24V 0.5W~1W zener diode. Wires it in series and note the correct polarity. The diode will drop roughly 20V to 24V, reducing the supply voltage to your meter module. See result of simulation below:

author: eblc1388
author_rank: Frequent Contributor
timestamp: July 01, 2020, 01:04:10 am
content: Quote from: WattsThat on June 30, 2020, 04:42:02 amWhen you put out a comment like that, you should at least explain why the method will not work so others, including myself, can learn from the mistake.

author: xavier60
author_rank: Super Contributor
timestamp: July 01, 2020, 10:22:13 am
content: Quote from: PotatoBox on June 24, 2020, 01:56:39 amI can't make sense of the problem the way you are describing it.The way you say it's powered sounds fine to me. No need to use the thin black if you have the thick black is connected.Draw a diagram showing how you have wired it.DSN-VC288 from some suppliers do have tracking errors where if it's calibrated at say 5V, it might then read 31.0 when 30V is input. Not a huge error but unnecessary due to botched firmware changes.

author: tunk
author_rank: Super Contributor
timestamp: July 01, 2020, 10:30:20 am
content: To test if it works you could power it with a 9V battery,and then measure e.g. 1.5, 9 and 12V batteries.