Home-brewed Power Supply  - Page 1
Forum section: electronics / projects

author: UnkleBidii
author_rank: Contributor
timestamp: November 12, 2017, 12:00:58 am
content: Hey guys, I've been working on a power supply built from a step down transformer, full bridge, and 317 as well as 7812-7805 on different rails.. I currently have a 1A analog meter display. However, I also want to incorporate a smaller display, say a 100mA full scale or 500mA. But I don't want to burn out the smaller meter in case amperage over shoots. Would I be able to incorporate both meters?Thanks

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 12, 2017, 01:44:13 am
content: Note that you want the meter(s) upstream of the regulator, so their voltage drop does not affect the output voltage. (This reduces the regulator's headroom slightly, so expect a corresponding reduction in maximum output voltage setting before ripple pops in. This does introduce a small error due to the regulator's bias current, but that's a few mA, easily calibrated out with the meter zero screw.)The smaller meter will have some maximum DC current rating. You can clamp its voltage drop somewhere around that point, with a diode. This isn't the best approach, because diodes aren't abrupt, they begin to conduct at lower voltages -- this causes a small reduction (nonlinearity, compression) in the meter scale towards the top end. You need to weigh how much compression is acceptable, versus what maximum voltage and current the meter should be exposed to.Or, make an LM3914 circuit where the ten output LEDs are the tenths place (0 to 0.9A), and the remainder (0 to 99.999...mA) is displayed on the 100mA meter. That strictly bounds the small meter's range (within the 0-1A range, that is). Boy, that'd be exciting to watch, ramping up and down...Tim

author: UnkleBidii
author_rank: Contributor
timestamp: November 12, 2017, 05:25:33 am
content: Meter should be place before the regulators?.. After the full bridge? Not quite clear what you mean by clamp.. Sounds beyond my scope at the moment..😁

author: Kleinstein
author_rank: Super Contributor
timestamp: November 12, 2017, 09:37:24 am
content: Clamping the voltage drop on the meters will limit the current that flow through the meter - extra current will flow through the camping circuit instead. In practice this is just having a diode in parallel to the meter. The typical voltage drop at fill scale an analog meters is somewhere in the 50 mV to 500 mV range. So a diode will no conduct very much at this low current range. A diode can limit the voltage drop and thus the current through the meter to something like 400 mV to 800 mV, depending on the diode used. How much the meter can stand, depends, but chances are something like 5 times full scale are OK.The current meter before the regulator is a good idea. Especially the LM317 has a constant current consumption and thus would only add a small offset that can be adjusted. So the meter would be between the large filter capacitor and the regulator with a small capacitor at it's input.

author: UnkleBidii
author_rank: Contributor
timestamp: November 13, 2017, 12:42:23 am
content: What's the reasoning for putting the meter before the regulator? 5 times a 50mA meter is about 250mA..that would be a safe range? Right now I just have a 1A meter at the negative load side.. After the regulator..

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 13, 2017, 12:54:33 am
content: Measure the output voltage, as you vary load current. Measure it before (at the regulator to ground) and after the meter. You'll see the difference.Tim

author: UnkleBidii
author_rank: Contributor
timestamp: November 14, 2017, 03:02:35 am
content: It's dropping voltage!

author: grifftech
author_rank: Frequent Contributor
timestamp: November 14, 2017, 09:53:46 pm
content: Quote from: Kleinstein on November 12, 2017, 09:37:24 amcamping circuit

author: Circlotron
author_rank: Super Contributor
timestamp: November 14, 2017, 10:04:52 pm
content: Quote from: UnkleBidii on November 12, 2017, 12:00:58 amYou could potentially add a voltmeter.