Current Monitoring w/ LEMs

Question for you all…We are trying to monitor our drive motor currents using LEM hall effect transducers. We are a rookie team and are following guidance from a white paper written by Steve Bunning, a mentor for Team 620.

The paper mentions powering the sensors from Pin 1 on the analog input to the robot controller. This pin is +5 v for “Gyro.” The other 5v pins do not have sufficient drive to power the sensors. The analog input connector only has one pin for +5v for Gyro.

My question is how would we power multiple current sensors if there is only one pin that provides enough power for the sensors? Options that come to mind are to try the other 5v pins and see if they work, gang two wires off of pin1, or build some sort of custom circuit.

If anyone has had to deal with this, I’d appreciate any advice. Many thanks.

-Chip

*Originally posted by Chipawa *
**Question for you all…We are trying to monitor our drive motor currents using LEM hall effect transducers. We are a rookie team and are following guidance from a white paper written by Steve Bunning, a mentor for Team 620.

The paper mentions powering the sensors from Pin 1 on the analog input to the robot controller. This pin is +5 v for “Gyro.” The other 5v pins do not have sufficient drive to power the sensors. The analog input connector only has one pin for +5v for Gyro.

My question is how would we power multiple current sensors if there is only one pin that provides enough power for the sensors? Options that come to mind are to try the other 5v pins and see if they work, gang two wires off of pin1, or build some sort of custom circuit.

If anyone has had to deal with this, I’d appreciate any advice. Many thanks.

-Chip **
If you are going to exceed the current capacity of Pin1, you’ll need to create a new +5V supply from the +12V Buss fuse panel.

IMO the easiest way is to create a 1A +5V supply off of the +12V is with a 7805 style three terminal voltage regulator. (Search for “7805” at http://www.digikey.com, then select “voltage regulators”, to see numerous brand options. A typical number is the Fairchild LM7805CT.) They are easy to use, and are internally current limited.

Buffer the +12V from the output of a “custom circuit” 20A breaker by first passing it through a 1A diode (such as a 1N4001 - 1N4004), then to a 100uf 25V capacitor to ground. This isolates you from motor noise. Now use the app note circuit to create your +5V at 1A supply.
Example - see: http://rocky.digikey.com/scripts/ProductInfo.dll?Site=US&V=261&M=LM7805CT )
Use the most basic “fixed voltage regulator” circuit on page 21, the chip itself and the pair of capacitors.

Notes:

  1. Those small filter capacitors (.1uf - .33uf) on BOTH the input and output of the regulator to ground on this chip are important. They prevent high frequency oscillation. Even though you are using a 100uF capacitor, ALSO include the other small input cap.

  2. If you choose a 1.5A version of the regulator instead of a 1A version, replace the 1N4001 blocking diode with one rated at least 1.5A as well.

<edit>
3) Radio Shack carries this part as 276-1770.

  1. You WILL need a heat sink for this item. Radio Shack also has the “TO-220 heat sink” and “heat sink grease” for it (and the small nut and bolt to tie them together in their hardware packs :slight_smile: )…
    </edit>

Good luck!

  • Keith

As far as I know, you’re supposed to power any custom circuitry with a single self-resetting 20Amp breaker, which supplies +12VDC. With that, you can use a voltage regulator (page 467 of the Digikey catalog) to regulate the +12VDC down to +5VDC, and supply +5VDC to whatever components need it, including the current sensing LEMs you’re talking about.

Otherwise, you could just solder multiple connectors / wires to pin 1 on the analog input port, or to gang them, as you say, which would supply +5VDC to all connected components (if they’re in parallel), but would be limited by the amount of current pin 1 can put out.

After checking out the RC manual, I see that pin 1 of the analog input port is fused by F3 on the RC. F3 is a 1.25Amp slow-blow fuse, and should be quite capable of supplying enough power for your needs (I tend to doubt that what you’re thinking of doing would draw more than 1.25Amps), so perhaps ganging them all off of one pin is the best bet.

I checked a spec sheet on the current transducer. The drive current required for one sensor would be less than 60 mA for my application. I would think that powering both sensors off of pin 1 would work so long as I’m not breaking any FIRST rules by doing so.

and if I can avoid custom circuits…:smiley:

-Chip

*Originally posted by Chipawa *
**I checked a spec sheet on the current transducer. The drive current required for one sensor would be less than 60 mA for my application. I would think that powering both sensors off of pin 1 would work so long as I’m not breaking any FIRST rules by doing so.

and if I can avoid custom circuits…:smiley:

-Chip **
You’ll be fine then running a BUNCH of them off of the gyro power pin. :slight_smile:

BTW, though they don’t feel like it, the LEMs ARE considered “custom circuit components”, so be careful the total cost for them doesn’t exceed the $200 limit. If I remember correctly, they’re about $25 each, so you can have something like 7 or 8 max if you do NOTHING else.
(BTW, If you wish to use say 8 of them, but it would take you slightly over budget, and buying 10 gives you a price break, consider buying 10. Then you have a couple “spares”, and can take advantage of the “prorating” rule on quantity purchases to maybe sneak that 8th one in… :smiley: )

Also, be careful where you run high current wiring to insure they’re not interfered with electromagnetically, especially if you’re using a BUNCH of them. As you can see in the original CDF app note using them, the two parallel “current splitting” wires are kept quite far from the LEM body. The BUD case was also helping to keep OTHER wiring physically away from it.

  • Keith

The spec sheets from Systron list the max operating current for the gyro at 20mA. Did the gyro from previous kits have a higher operating current? I’m wondering why they have a > 1A fused output for the gyro.

Pin1 of the Analog Input port is intended to power Gyros only.

Other 5V pins on the Analog port are intended to power potentiometers only.

I think rule C29 is pretty clear on the subject of power supplied to custom circuits.

To power any custom circuit, the rule says you MUST use 12V supplied through a 20A breaker. So if your circuit needs 5V, you’ll have to provide the power conditioning yourself. The post by kmcclary earlier in this thread provides good instructions on how to do this.

I think one of the intents of the rule is to eliminate the need for FIRST officials to do a safety analysis of each and every custom circuit – instead they just need to make sure that each custom circuit draws its power from a 20A breaker.