Dual output DC/DC query...

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jonescg
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Dual output DC/DC query...

Post by jonescg »

Quick question to anyone knowledgeable...

I'm putting a simple ammeter and voltmeter (two DPMs from Altronics) in the dash of the race bike and it appears that some meters run on 12 V, and others 5 V.

I want to use an isolating power supply so the test wires aren't connected to the traction pack, but I found that when searching Element14 you check 'dual output' they always seem to mean -5V and +5V, not 5 V and 12 V.

So my question is: can I power the 12 V DPM from the -5V and +5V outputs, while the 5 V DPM can run from the 0 V and +5 V outputs?

Before you ask, I have considered searching for two DPMs which run on the same voltage, but I just happen to have a couple here.
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Dual output DC/DC query...

Post by Adverse Effects »

that sort of thing is used a lot in FPV RC flying craft

RMRC 5V/12V Dual Step Down Regulator

google search
Last edited by Adverse Effects on Sun, 03 Jan 2016, 10:58, edited 1 time in total.
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Dual output DC/DC query...

Post by evric »

Be careful with this... dc-dc converters need to be isolated types and you will need a separate one for each meter.
If your meter is powered by 12V use a 12V - 12V isolated converter and if your meter is to powered by 5V use a 12V - 5V isolated converter.
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Dual output DC/DC query...

Post by jonescg »

evric wrote: Be careful with this... dc-dc converters need to be isolated types and you will need a separate one for each meter.
If your meter is powered by 12V use a 12V - 12V isolated converter and if your meter is to powered by 5V use a 12V - 5V isolated converter.


Thanks Eric, I was always going to use isolated types.

So you're saying that I should not use the dual output isolating DC/DC converters to power my ammeter and voltmeter?

Alternatively I could use a single 12 V to 5 V isolating DC/DC converter, like this one: http://www.farnell.com/datasheets/1750785.pdf
And power both meters from the same isolated 5 V supply?
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Dual output DC/DC query...

Post by evric »

If the dual isolated dc/dc converter has a real 12V and a 5V output and both are isolated from each other on the output - that is OK.
You cannot power both meters from the same single DC-DC converter.The meters need to be isolated from each other.
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Dual output DC/DC query...

Post by jonescg »

Ahah - just drew the diagram up and see why you can't use the one supply:

Image

Two separate power supplies it is then.

Edit - Hang on, the high impedance between the voltage divider (now drawn correctly) should prevent any high potentials across the meters right?
Last edited by jonescg on Sun, 03 Jan 2016, 12:57, edited 1 time in total.
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Dual output DC/DC query...

Post by evric »

No, it's not OK. You are basically shorting out the 13K ohm resistor.
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Dual output DC/DC query...

Post by neilg »

Jonescg

I think you have the connections to the 13k resistor from the voltmeter reversed.
The negative of the voltmeter should be to the negative of the battery and the positive goes to the junction of the 13k and 1.3Meg resistor.
You then have the -ve of both meters going to the same point (-ve of the battery) and you can then use a common 12v isolated power supply.
If necessary you could then use a simple +12 to +5 volt regulator to power one meter.
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Dual output DC/DC query...

Post by jonescg »

Correct - the colours aren't helping :) I'll edit the image when I get a chance.
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Post by coulomb »

neilg wrote: You then have the -ve of both meters going to the same point (-ve of the battery) and you can then use a common 12v isolated power supply.

I still don't think it's a good idea. You'll likely get some shunt voltage affecting the reference for the voltmeter (after the voltage divider, so it's not insignificant). So there would likely be some contamination of the voltage reading when driving.

Moreover, it's difficult to check for this effect in the garage, since it's hard to safely make the motor draw significant current while stationary. You also need to know a lot about your meters to be sure that they work the way you think they do; tracking their circuit can be tricky.

An extra ~ $20 for the second converter seems like a good investment to me.
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