Meter Selection (Coil/Iron) for High Crest Factor Waveform - Page 1
Forum section: electronics / beginners

author: killingtime
author_rank: Regular Contributor
timestamp: July 23, 2022, 11:57:13 am
content: Hello,I'm restoring an old 12V car battery charger. It's a simple design. Transformer -> FWB Rectifier -> DC Ammeter -> Output Terminals.No capacitors, which is one of the reasons it still works after 40 years. 3KG transformer tested to about 20A RMS with a scope and a resistive load.The meter needs to be replaced, it's reading way off.Looking at the current output while charging, I can see it's not a sine wave. The output only conducts when the voltage exceeds the battery voltage, so that's somewhere near the middle of each crest peak. Looks like this:https://images2.imgbox.com/8a/39/HOUeTm5v_o.pngThe current crest factor isn't fixed either. It's lowest when the battery voltage is lowest and highest when it's fully charged.Q: What current meter would be best in this scenario? A DC moving coil ammeter (and shunt) on the DC side, or a moving iron ammeter on the LV AC side?I know coil meters measure the average of a waveform, and iron meters measure the RMS.Is this true for all crest factor values?I read on a spec sheet that the accuracy class of the meter was for a defined crest factor.Thanks.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 23, 2022, 12:23:36 pm
content: The moving iron meter reads the RMS even for a non sine waveform. However with a high crest factor the accurcy can suffer a little. Changes are it would still be good enough for most cases.The scale is usually nonlinear (e.g. square law), which can be a slight problem to read small currents. The meter is usually only needed as a very crude indication, e.g. to see if the charging is slowing down or the battery bad (does not take current even though the battery is weak when loaded).The probably better alternative is a DC meter on the DC side to read the average current. For the carge going to the battery is average current times the time.The RMS current is what is relevant to loading of the cables and transformer - e.g. to see when you are overloading it.There are usually no capacitors in the simple chargers. They may however have an inductor to get an additional possibly adjustable series impedance to limit the current when the battery voltage is low. In some cases the transformer is made with extra series inductance. The inductor would also provide some power factor and crest factor improvement.

author: killingtime
author_rank: Regular Contributor
timestamp: July 23, 2022, 04:54:50 pm
content: Hi Kleinstein,This is the AC meter I was going to use.https://images2.imgbox.com/74/4b/iAdQkEea_o.jpgIt has one advantage over the DC meter, and that's an overload scale, which will probably be relevant as there is no OCP on this supply. You can overload the transformer to about 30 amps, and after a minute or so a bimetalic circuit breaker opens which auto closes when it cools down.RMS amps it is then.Thanks for the detailed reply.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 23, 2022, 04:58:20 pm
content: The scale looks resonable, though maybe missing a few details at the low end.