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12v inverting isolator

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Oznog

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I mentioned before that I need to supply 5v@<10mA to a current shunt and ADC which may be located on the positive side of a high voltage battery (maybe 250v). The ADC output will then pass through an optoisolator.

I have +12v versus ground readily available.

The 5v for the ADC cannot be generated by a linear reg. Both the voltage gap, inefficiency, and heat generation are too high.
I came up with the idea of putting a lithium battery on it, but it will always be drawing a few mA and will not last a very long time.
I evaluated switching reg configurations. They're looking a bit too complicated, especially since I was thinking of making it support both high and low side shunts so I would need 2 types of switching regs here.

I had some ideas of using a capacitive charge pump to charge a cap off of 12v, disconnect it, and reconnect it to the ADC/shunt board. The isolation is a bit tough here and I don't really like the idea.

I think the best idea is the 12v board- which reads the isolated signals off the ADC- could power a small isolation transformer. I think that's the best idea. Now I have only a small amount of board space and watch my component costs, so I'm not interested in using a 12v-to-120vac inverter and running a 120vac-to-5vdc transformer. And we need only a few mA. I think the best thing is to give it around a a 1:1 ratio, rectify it on the other side (1.4v FWB loss), and run it though a reg (0.5v drop if it's an LDO reg).

Perhaps I can find a small toroid here. Should I find a driver chip or just one of those two bipolar transistor oscillators? If it's a toroid, won't its low inductance require it to oscillate at fairly high freq if we can't replace some of the pulse width with dead time? Otherwise I think the current may get pretty high. I guess I don't have anything against higher freq but I'll need to use fast diodes for the FWB.
 
Gonzo bassackwards, I'm having trouble following you. Can you post a block diagram?
 
Ron H said:
Gonzo bassackwards, I'm having trouble following you. Can you post a block diagram?

Think electric car. We have a huge series stack of batteries to make 250v. There is a 12v battery for running accessories and it shares a neg gnd with the big storage battery. I need to get 5v to the ADC/shunt on top of the 250v battery. So I'd need either a 245v point or a 255v point, either one is fine.
 
Will **broken link removed** work for you? It transfers the current sense output to GND reference, so you wouldn't have to float the A/D.
Stupid question: Why can't you do low side sensing?
 
Think electric car. We have a huge series stack of batteries to make 250v. There is a 12v battery for running accessories and it shares a neg gnd with the big storage battery. I need to get 5v to the ADC/shunt on top of the 250v battery. So I'd need either a 245v point or a 255v point, either one is fine.

One way is not to use a current shut on the high side but instead use one of those hall-type current sensing devices. You now ground reference the output of this device so no level shifting and isolation will be required.

If the above suggestion is out of the question, I'd employ a buck switch mode regulator to generate 5V from the 12V supply. I can now put an extra winding in the coil to generate the isolated 5V supply for the ADC.
 

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I have used 600 ohm to 600 ohm telephone line transformers on a number of occasions to build isolated inverters. The junction of the 27k resistor and the CT is the + supply.
The attached circuit uses a 2N3904 transistor in a simple oscillator.
Hope this helps.
 

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Ron H said:
Will **broken link removed** work for you? It transfers the current sense output to GND reference, so you wouldn't have to float the A/D.
Stupid question: Why can't you do low side sensing?

That's an interesting solution! Won't really meet my needs though. I'm dealing with extremely low offsets and bidirectional current flow. They do have a neat principle I hadn't thought of before though.

Low side sensing, well I can but would like to avoid this being a requirement. It can mess with your versatility. Failing to isolate the shunt can also leave it very liable to destruction if the ground connection is disconnected.

motion, I've dealt with the best Halls and the offset error of even the closed loop devices can be a problem for precision apps. The readily available current range won't do it either. They don't handle surges well- exceeding their capacity with even a brief surge is bad for the device, I think it was it causes magnetization that messes up the offset further.
 
Have you thought of something as simple as this?
Diode D1 is to protect M2 from transient current through C1 when the battery is connected. The substrate diode might be sturdy enough to handle it.
M1 and M2 could be some sort of monolithic driver. Or, they could be bipolars.
 

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k7elp60 said:
I have used 600 ohm to 600 ohm telephone line transformers on a number of occasions to build isolated inverters. The junction of the 27k resistor and the CT is the + supply.
The attached circuit uses a 2N3904 transistor in a simple oscillator.
Hope this helps.

I kinda like this one. Is it really kosher to use a telephone transformer like this? This might go into a marketable product- is there anything funky here I should worry about?
 
It really makes things MUCH simpler monitoring the current in the low side, and makes the device work with any number of batteries.

It's also very easy to do, without a current monitoring resistor!.

The bottom end of the battery connects to chassis using a wire or strap of some kind?, this has resistance, and you can use an opamp to amplify the voltage drop across it, giving a positive or negative output voltage, depending on the battery charging or discharging.

By making the opamp gain variable, you can easily adjust it for different chassis strap resistances.
 
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