CVT controller

[pyrogaz]

We're hoping to test the cvt system in a couple of weeks. We have a 24v actuator which draws a maximum of 1.8A at stall but under most conditions will be no more than 0.75A, it's speed means it will only be run in 0.25 second bursts following a comparison carried out every 2 seconds.

We prefer the idea of using two DPDT relays to provide bi-directional control of the actuator, does anyone know if a picaxe will drive these relays http://www.rapidonline.com/netalogue/specs/60-0400e.pdf The spec sheet says the pickup voltage is 75% of the rated voltage so 3.75V for the 5V unit and power consumption of the coil seems to be in the correct range for a 28A board with Darlington driver output, power rating is sufficient for the actuator but is there anything in the spec I'm overlooking?

 

[fernando_g]

"does anyone know if a picaxe will drive these relays?"
"5VDC @ 102ma"

Not directly. You'll need a buffer transistor. Most general purpose NPN jobs will be fine.
Also do not forget the freewheeling diode and decoupling caps.

Lastly, I don't know if the following consideration will apply to your project or not, but here it goes....some posters have reported in the past that the PICAXE resets or behaves erratically whenever driving an inductive load. Most of the erratic behavior has been traced back to improper layout and/or grounding techniques. So double check for this condition.

 

[pyrogaz]

The Darlington output on the 28A board is the ULN2803A which I thought was capable of 500mA per output and has internal diodes for inductive loads.

 

[BeanieBots]

The darlington output should be OK with those relays.
(as fernando_g correctly states, not direct from a PICAXE output).

The only other point to look for in the spec of the darlington is the volt-drop.
Darlingtons drop more than conventional transistors. Typically twice as much as a regular semiconductor junction. (typ ~1.2v).

Even at worst case, this would still leave 3.8v for the relay which is within 75% (just) so I would not anticipate any issues with pulling in.

 

[pyrogaz]

I just measured 4.07V across the output, that's with the three AA battery box as supply, would it be best if we could use one of the traction batteries through a voltage regulator to give a 5v supply?

 

[MartinM57]

maybe...how long will your AA battery box last at the duty ratio that the relay is on for (eg. 10 seconds every minute - but I've not read the thread in detail) at the spec'd 102mA current draw? Maybe you could do a test...

 

[BeanieBots]

I just measured 4.07V across the output, that's with the three AA battery box as supply, would it be best if we could use one of the traction batteries through a voltage regulator to give a 5v supply?

4.07v volts across what output and under what load conditions?
Using the traction battery could certainly have advantges but could also have dissadvantages.

What voltage is the traction battery?
What other loads are there on the 5v supply?

 

[pyrogaz]

4.07V was one of the Darlington outputs through a cluster of large LED's drawing around 90mA. Traction is two 12v lead acid so have the option of 12 or 24V. The 5V supply runs only the picaxe and motor control relays.

 

[BeanieBots]

In which case I'd use a 5v regulator to supply the PICAXE and use the darlington to switch 12v relays or even 24v relays if you can find them.
5v relays from a 12v source very inefficient and not the most sensible choice.
Using batteries will be expensive and can be eliminated by using 12/24v for the coils.
Batteries will last MUCH longer (if used at all) for the logic if no relay current has to be sourced by them.

 

[papaof2]

maybe...how long will your AA battery box last at the duty ratio that the relay is on for (eg. 10 seconds every minute - but I've not read the thread in detail) at the spec'd 102mA current draw? Maybe you could do a test...

Duracell has the data sheets for their various batteries online: http://www.duracell.com/oem/productdata/default.asp You can see what the expected life is under various loads.

The MN1500 (alkaline AA) has a life of about 25 hours (to 1.1 volts) under a continous 100ma load.

John