RFID Reader - Minimum voltage affects read range

nick412

Just thought it is worth noting that the read range of tags (white plastic cards) seems to be severely affected if anything around the minimum voltage is supplied.

I have a reader that works just fine on the workbench connected to a 5v voltage regulator, however when I put it in a production environment (install it), the reader is connected via solid core telephone cable to a power source 3m away.· The voltage reaching the reader is reduced to around 4.6v (0.1v above the recommended minimum according to the spec sheet) however at 4.6v the read range is only around 1cm.· When connected directly to the power supply, I get around 4-5cm.

Desy2820

I realize that this isn't part of the observation you were making, but I'd consider moving the regulator as close to the RFID board as possible.· Send the unregulated power over your wires and put the regulator right next to the reader.· I'd imagine that there could be momentary dips in the supply voltage which would put your voltage below the minimum.

If you don't have them already, I'd also add some capacitors right at the reader across the positive/negative rails to help stabilize the incoming power.· Maybe 100 uF to start with?

nick412

Thanks for the suggestion. That’s what I ended up doing (12v directly to the box housing the reader, with a 5v regulator and 100uf capacitor across the regulators output, connected to the reader) and I am now getting a workable read range. Hopefully the heat generated by the regulator in a small sealed box does not cause any of its own problems.

Desy2820

If the heat does become a problem and your box is metal, you could mount the regulator to the box with heatsink compund and a mica insulator and use the enclosure to help dissipate the heat.

You could also try lowering the input voltage, so the regulator isn't dumping 7V.· I think around 7.5, even 9V would help lower the heat generated.··If you·use an LDO or·low dropout regulator, you could even use 6V incoming power.

Hope this helps!

willthiswork89

what is the read range of those things?

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nick412

The box is plastic, I might try using a 7/7.5v power supply instead - the reason for using the 12v was that I already have one in the location powering other electronics.

I am getting 5cm or so read range through the box.

LoopyByteloose

If you are using a standard 7805 regulator, you can easily preregulate down to 9volts before putting it onto the cable.
That would eliminate quite a bit of the heat at the 7805 because it is proportionate to the voltage that requires reduction.

As Desy2820 mentions, with the 7805, you need about 7.0volts minimum input and being as close as possilbe to that is ideal.
If you used a 2940-5.0 [noparse][[/noparse]low drop out regulator], you could get down to 6.0volts.

Please tell us some range estimates because there have been people stuck with bad results before, but no one consider that low voltage was a contributing factor.

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nick412

I purchased some LM2940CT voltage regulators, and hooked them up (one after another) to a multimeter.

I consistently got odd readings, such as:

Regulated PSU Input (v) - LM2940CT Output (v)
7.5 - 6.3
9 - 7.1
12 - 8.8

I thought they were faulty, however after adding a 100uf electrolytic capacitor across the output I got the expected result ie 5v on the output. The 7805 regulator works without the capacitors, and I thought they were just for stabilisation, but it seems the 2940 requires an output capacitor for proper operation.

Mike Green

Different regulators have different requirements for stable operation. Many of the LDO (low dropout) regulators require a large value (like 100uF) capacitor across the output. Some regulators require a minimum input capacitor. When in doubt, look at the datasheet.

LoopyByteloose

The low dropout LM2940CT 5.0 requires at least 22mf. Assuming that all regulators are the same will get you into trouble. The 7805 was specifically designed to work without capacitors, but 1mf on the output won't hurt. Some will suffer damage if more than 10mf is put on the output unless there is a protection diode.

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nick412

Kramer said...
Please tell us some range estimates because there have been people stuck with bad results before, but no one consider that low voltage was a contributing factor.

To get the 5cm read range, the voltage regulator is on the same PCB - although it is separated by around 7cm of wire (for both ground and VCC) running from the regulator to the RFID readers 4 pin socket.