Power Supply switching

[marco666]

Hi,

I am not sure whether this is the correct forum to post this question but I think it may be a problem which may be common and someone may be able to help.
I am currently using a TTi TSX3510P DC power supply to drive a simple electro-magnet. One end of the windings goes to the positive and one to the negative of the supply (obviously) which allows me to ramp the current up through lab view to +10 amps crteating a magnetic field.The problem is I would now like to drive the current to -10 amps, the simple solution is to obviously switch the wires around on the supply but I wish to perform an automatic sweep utilising lab view from -10 to +10 amps. I am sure that there is circuitry out there to perform this operation. If anyone knows where and how to utilise this circuitry with labview I would be much obliged.

Thanks

Regards

Marc

[LV_Pro]

Not being familiar with the supply you are using I have to ask a few questions. Does the supply "go negative" or is it limited to 0 to max? How do you control it, to allow you to ramp it? Let us know to help you.
And a caution, when you have a field on a coil and abruptly disconnect the current source the collapsing field will induce a reverse current, producing a sometimes very large voltage. This is how the traditional automotive ignition system works. It is also the reason that you will see a diode across the coil on many relays, to protect the circuitry that control the relay.

P.M.

[Henrik_Volkers]

Two ideas:
1. use ready to buy equipment (muxer) with 10A! switching capability (SCXI??) that you can easily control with LabVIEW
2. (ab)use ONE RS232 hardware handshake line (DTR) to control 4 opto mosfets (solid state relay with optocoupler)
DTR line state can be accessed by VISA proberty node. DTR to GRND will be +/-10-12V(or so)
(not with a Laptop, use USB/Serial) and can drive some mA (enough for two LED in serie).
Connect two optos serial and the two pairs antiparallel. like that always only two of them are active.
As a current limiter for the LEDs one Resistor should be added (range depends on current of optos)
Or use a standard optocoupler and external power to drive a commercial pole changing relay

However additional protection circuitry should be added to protect whatever relay or switch if you want to or have the possibility to (e.g. always) change the current in a inductor. (You mentioned a current sweep, so you already plan to switch the leads at I = 0 A, but you never know)

[marco666]

Hi,

The supply is limited from +0 to +10 amps no negative, I am currently ramping up the current while leaving the voltage fixed at 32v. The current is ramped up using a basic array, and timer (through lab view) to determine how long a particular current is applied. I am at present controlling the power supply device using GPIB. The work i am carrying out involves applying fields to steel samples to determine BH curve characteristics.
The system I am trying to construct will involve applying a current say -10amps and using a hall sensor element in position under the electromagnet to output a reading of field strength into labview. I would then want to switch to -8 amps and repeat the test and so on upto 10 amps. You may think otherwise but beleive I will need 1 analogue input channel ( for the hall element to input to labview) and 2 digital channels to control the switching circuit from + to -, does this sound correct??. I am aware that this is an NI site but all NI data acquisition cards seem very high spec, could you recomend a cheap version about £100 which could accomadate my simple requirements???.

Thanks for your time any comments/ suggestions would be appreciated.

Regards

Marc

[marco666]

Hi

Thanks for the help your first solution sounds simple how much is this switching equipment likely to cost?? Do you have any manufacturer details.??

Thanks again

[LV_Pro]

You might want to check into NI's USB based DAQ. Since you are ramping using GPIB to control the supply I'm presuming that it is changing relatively slowly, which would allow the USB DAQ. I don't know what their price is in the UK, they start out around $245US with both analog and digital. You would also need a relay with: contacts rated at the maximum current expected, which should also be powered by a separate power supply and have its own driver circuitry (DAQ cards generally don't supply sufficient current to run relays directly). A US company, ELK products, makes a relay card with the appropriate circuitry, for about $20 US. I know that one model, ELK-924, has contacts rated to 7A @ 30VDC, I don't know if that is "hot switched" or just what they are rated for. I think hot switched as they have a separate 10A @125VAC rating. You would need a small 12V @~1A power supply to provide it with power (a "Wall Wart" supply should be fine), but it only requires the control signal to supply 1.2mA, with most DAQ digital I/O capabilities. You would need to be careful not to switch the relay while the electromagnets full current was flowing, the field collapse would generate a large voltage/current across the relay's contacts. The normal method of putting a reverse diode across the windings wouldn't work with your required polarity reversals.

Hope this helps,


P.M.

[Henrik_Volkers]

The USB way seems to be the best way for you..
have a look at
http://sine.ni.com/nips/cds/view/p/lang/en/nid/14604
NI USB-6008
12-Bit, 10 kS/s Multifunction Data Acquisition for USB

it will be £95 +17 for Accessories

additional parts
pole changing relay (if you spend a little more you can get a industrial one including protectiv circutry)
one transistor, two resistors, one diode (to protect the transistor against spikes from the relay coil) to drive the relay
small (wall) powersupply to drive the relay

to protect the contacts of your relays you can use a RC combination or two antiserial Z-diodes (>40V)

with the additional input channels you can monitor the coil temperatur