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Registered Member #190
Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567
I just got some photocouplers to play with.
Not having used these before, I wanted to make sure I was not going to destroy them. I have a 600v isolated voltage I want to measure on a 15v system. If I am reading the datasheet correctly, it says the typical current on the input is 20ma. I was going to put a 30k resistor in series with the voltage to achieve this current (Rd on page 8). I can then scale Rload as needed using Vcc = 15v for reading the output voltage. Is this reasonable?
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Wouldn't it be more usual to convert the voltage to be measured into a pulse train by means of a divider network and V-F converter, and transmit that across the coupler?
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
If you look at Fig.6 Current Transfer Ratio vs. Forward Current you will see that CTR varies with current so you will get almost a square-law response. One method is as above, V to F
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
Such optocouplers are not designed to transfer an analog signal so the the voltage to frequency suggestion is very sensible if you want accuracy and long term stability.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The ones Sulaiman suggested are designed to transfer analog signals with high linearity. But they need an op-amp drive circuit on the high voltage side, and that needs isolated power. (As would a V-F converter.)
A regular optocoupler won't work, because the CTR varies so widely.
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photocoupler
