Nekmech Posted August 25, 2006 Report Share Posted August 25, 2006 Hi,I'm new to the Forum so be kind please.I have a battery charger I built from a schematic I found a long time ago. Basically it's a transformer, bridge, filter caps, LM350K regulator circuit with an op-amp stage and current sense resistor.It works fine charging my lead-acid batteries.I wanted to add a DC panel meter to watch the charge progress, so I took a really cheap far-east type multimeter and hacked it. Instead of the battery I added to it a simple bridge, filter caps and a 7809 regulator. The multimeter ac source is coming from the same ac transformer which powers the charger.I jumpered the required pads on the multimeter to lock it into the correct mode and range I want.The DC meter works fine as long as I'm measuring anything besides the charger voltage.As soon as I measure the output voltage of the charger, the meter fluctuates all over the place.I was hoping to overcome the "floating ground" problem by taking the meters dc power feed from the transfomer, but it doesn't work right.Can I get some advice?Thanks. Quote Link to comment Share on other sites More sharing options...
audioguru Posted August 25, 2006 Report Share Posted August 25, 2006 Most electronic meters need a power supply that is completely separate from what they are measuring. A digital multimeter has its own battery, doesn't it? Quote Link to comment Share on other sites More sharing options...
Nekmech Posted August 25, 2006 Author Report Share Posted August 25, 2006 Usually it does, but as I said, I hacked up a cheap one and am trying to supply it's power from the chargers transformer instead of from a 9v battery. Quote Link to comment Share on other sites More sharing options...
audioguru Posted August 25, 2006 Report Share Posted August 25, 2006 I hacked up a cheap one and am trying to supply it's power from the chargers transformer instead of Quote Link to comment Share on other sites More sharing options...
Nekmech Posted August 25, 2006 Author Report Share Posted August 25, 2006 Thanks for trying to help audioguru.The meter and charger don't share a common supply ground.The only part they have in common is the ac transformer secondary.How is this done in a tradtional power supply with digital readout? Quote Link to comment Share on other sites More sharing options...
audioguru Posted August 25, 2006 Report Share Posted August 25, 2006 The meter and charger don't share a common supply ground.The only part they have in common is the ac transformer secondary.Then they aren't completely separate.How is this done in a tradtional power supply with digital readout?With a separate winding on the transformer to power the meters.Or their circuit is designed for reading only a positive voltage then the negative input is the same as its 0V supply. Quote Link to comment Share on other sites More sharing options...
ante Posted August 25, 2006 Report Share Posted August 25, 2006 If you got some space on top of the transformers winding you can add an extra winding here. Use any thin insulated wire and start with say 10 turns then measure what voltage these 10 turns gives you, the rest is mathematics a rectifier and a few more parts.Another possible way is to build a small galvanic insulated DC-DC converter. Quote Link to comment Share on other sites More sharing options...
Nekmech Posted August 25, 2006 Author Report Share Posted August 25, 2006 With a separate winding on the transformer to power the meters.Or their circuit is designed for reading only a positive voltage then the negative input is the same as its 0V supply.Ah. Now I get it. Thanks. Quote Link to comment Share on other sites More sharing options...
Nekmech Posted August 25, 2006 Author Report Share Posted August 25, 2006 If you got some space on top of the transformers winding you can add an extra winding here. Use any thin insulated wire and start with say 10 turns then measure what voltage these 10 turns gives you, the rest is mathematics a rectifier and a few more parts.I looked at my transformer and I don't think I'll be able to do that although I like the suggestion.I'll try to remember it.Another possible way is to build a small galvanic insulated DC-DC converter.This sounds interesting.I've never heard of a galvanic DC-DC converter.I'll look it up and see if it's something I can implement.It's either that or buying a new transformer.Thanks for the help. Quote Link to comment Share on other sites More sharing options...
indulis Posted August 25, 2006 Report Share Posted August 25, 2006 It would be worth while for "meter users" to read a few of these App's note's...http://store.cd4power.com/cgi-bin/cd4power.storefront/44ef513100171934271d0c9f89420625/Catalog/1120 Quote Link to comment Share on other sites More sharing options...
Nekmech Posted August 25, 2006 Author Report Share Posted August 25, 2006 It would be worth while for "meter users" to read a few of these App's note's...http://store.cd4power.com/cgi-bin/cd4power.storefront/44ef513100171934271d0c9f89420625/Catalog/1120Thanks. Will do. Quote Link to comment Share on other sites More sharing options...
Nekmech Posted September 1, 2006 Author Report Share Posted September 1, 2006 Just finished adding a 12v 500ma transformer to my charger to power the DC meter.Seemed to be the easiest solution. Quote Link to comment Share on other sites More sharing options...
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