Need an adjustable +- 8 mA current source 2

[EdHenrie]

Hi,
I'm looking for a power control circuit (supply) to drive a -8mA to +8mA electro-hydraulic servo. Previously, I've used a power supply set to limit current to 8mA, with a reversing switch to reverse polarity, but this seems so Flintstones. I've been looking for a way to have a single dial analog control to sweep from -8 to 0 to +8mA but having no luck.

The biggest problem so far seems to be the 1000 Ohms resistance of the servo. I tried using a signal conditioner configured to 0-20 mA, controlled to 8mA, and connected to the servo to in reverse polarity give -8mA, then use another signal conditioner configured to 0-20 mA and controlled in the 0-16 mA range via a pot and connected in straight polarity, with the thought that the 2nd supply would cancel out the 1st as I swept its control from 0 to 8 mA, thereby creating a -8 to 0 mA control, then as I swept it further to 16 mA, would get net 8 mA in the + direction. Turns out they require an output resistance <600 ohms, so that didn't work. The 2nd unit won't drive more than 15 mA through 1000 ohms, and when I paralleled them together (with a dummy 1K ohm load), neither would produce more than half the current they had when connected alone.

So, I searched for online resources for bipolar current sources, but all of them I found were limited to output resistance well less than my 1K Ohm.

I ordered a couple of cheap LM317 based regulator boards off ebay and plan to experiment with them configured as constant current sources to see if I can do something like above (except successfully), but using one as a constant 8mA supply driving the servo in the (-) direction, then introduce the second one to drive current in the opposite direction through 16 mA for a net -8 to +8 mA sweep. I may alternately have to use them as constant voltage sources, (one being static, one being swept) as that may be key to achieving success. Not sure yet. I may end up using one as CC, one as CV swept.

Can anybody point me to a very simple circuit I can build, or an off-the-shelf solution where I can sweep a dial to get a continuous sweep of -8 to 8mA through a 1KOhm load?
Thanks,
Ed

Edit: Forgot to mention I'm a Mech Eng, with some advanced Technician level Electrical/Electronics knowledge (for an M.E.) (Anti-Tank Missile Systems Tech, USMC, late '80's)

 

[ScottyUK]

The limiting factor is drive voltage so a device capable of 20mA into 800[&Omega;] requires 16V to drive the loop; you could equally drive 10mA into 1.6k[&Omega;] with the same device.

This example will give you a bipolar ±20mA output into 800[&Omega;] so it should be capable of ±16mA into 1000[&Omega;]. The input can be voltage or current - if you have stable 24V power available then a pot across the supply rails is the obvious answer, or if the supply varies use a resistor plus a zener to generate a stable reference voltage and use that as the input to the pot. How you set the scaling up is up to you: a DC input of 0 to +24V or 0 to +6.2V corresponding to ±8mA output should be completely possible.

 

[OperaHouse]

I looked around ebay for something simple. It appears hydraulic servo valves have been abandoned. If I had to order something to modify it would probably be a stereo headphone amplifier board. To build outright I would use a LM324 Quad op amp. This could be as simple as a unity gain amplifier from a ten turn 10K pot connected to =- 8V. There would be enough to add a 100Hz dither to prevent valve sticking. Is this going to be purely manual or is there another requirement in the future.

 

[zeusfaber]

The classic (in every sense of the word) drive circuit for these things is this:

You just need to watch the heat developed by TR3, TR6, R16 and R42.

A.

 

[OperaHouse]

That looks scary. I hesitate to post this because all the requirements are not known. This is simple and could be done with a single 9V battery. Two op amps are still available in the package for a dither oscillator and input buffer/level changer. One driver goes positive and the other goes negative providing almost full supply voltage to the valve coil. This outputs a variable

voltage. Current is a function of voltage and resistance. The two resistors on the positive input of B are both 100K to set the reference at half the supply.

 

[EdHenrie]

ScottyUK,
Thanks for the info. I have one on order with a control panel. They should be here tomorrow.

OperaHouse,
Thanks for the reply. No, they haven't been abandoned, still in frequent use in Aerospace applications, as well as others. I will save your 2nd post and build that circuit as a back-up plan if the item ScottyUK referenced doesn't work. Plus, it's simplicity makes it seem very flexible in application, and perhaps a great reliability factor. The unusual aspect of this particular servo is the +/- 8mA. Most I've dealt with use 0-10mA, 0-100mA, or something similar. But given that this application is a steering function, it makes sense to have the center = 0mA.

zeusfaber,
Thanks for the reply. That circuit is a bit too complicated for my patience level, but thanks for posting.

 

[benta]

This will do it:

http://www.tubecad.com/2011/10/28/V-to-I Amplifier 2.png

You'll need to power the opamp from at least +12 and -12 V supplies.

Instead of the 0.2 ohm resistor, choose 125 ohms. That will give you +/-8 mA from a +/-1 V input. Setting up a potmeter to deliver +/-1 V shouldn't be hard.
Choose an opamp with sufficient current capability.

Cheers,

Benta.

Caveat: if one side of your load is connected to neutral/ground, this circuit will NOT work. If your load is floating everything is fine.