Switching 35V, 500 mA with digital output? 2

[llrelease]

Does anyone know of a relay or other component that will allow me to switch up to 500 mA at 30V using a 5V, 10mA control signal from a digital output?

Thanks!
LLRelease

 

[xnuke]

A quick search on Google yielded several relays. Here's the first one that popped up on the search as an example:

http://www.magnecraft.com/products/section2_25.pdf

xnuke

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[BrianG]

Although you can get miniature reed relays with coils that can be driven from a 5V logic signal, the reed contact itself won't usually manage more than about 200mA. You could use either a transistor buffer stage, or a logic buffer chip that can sink much more current, so that you could use a heavier duty relay.

The best plan it probably to use a power FET in conjunction with a transistor level shifter stage to interface between the logic output and the FET gate. N-channel FETs have the lowest on-resistance but would require a voltage rail above 30V for biasing the gate on. As I suspect you don't have any other rails then you will need to use a P-channel FET source follower circuit where you pull the gate towards logic ground.

 

[IRstuff]

Are you sinking or sourcing? That'll make a big difference in the circuit configuration.

If sourcing, then you'll need an open collector output with resistor load to the 30V. The open collector output drives p-channel MOSFET or pnp Darlington.

If sinking, then you can drive n-channel MOSFET or npn Darlington directly.

TTFN

 

[nbucska]

There are FETs which could be destroyed by 30V. Check the data.

You can find FETs which can be turned on with logic directly.

<nbucska@pcperipherals.com>

 

[felixc]

Depending if you want to switch the positive or negative side of your load the approach is different. I assume that you also do not need electrical isolation.

You will want to switch the negative side if the load does not have electrical outputs. The cost of implementation is then much lower.

To switch the negative side, look at the IRLD-110 from international Rectifier. It is a N-channel MOSFET that switches with a logic level, with 100 volts of tolerance at the drain.

To switch the positive side, an interesting part to look at is the IPS511G, from the same company. (Other manufacturers have equivalents, but if you type these numbers in Google you will find the specs easily.) It is called a high-side switch and is quite popular in the automotive industry. It is also driven by a logic level, and can switch at voltages to 35 volts.

 

[llrelease]

I am getting some very good feedback, but I realize that I need to provide a little more information about my application in order to get more specific information.

I need to control a DPDT mechanical relay to switch a 35 V, 500 mA constant current programmable supply between 2 different circuits (one circuit is a back up). I already have a DPDT mechanical relay that requires >115 mA for the coil. I think that I should keep the relay and simply control it with the digital output, but it will only suppy up to 15 mA.

Any additional advice about whether to use a transistor, Mosfet, or other device would be much appreciated.

Thanks!

 

[IRstuff]

n-channel MOSFET or npn transistor with diode across relay coil.

Just about any EE circuits textbook will have an example.

TTFN

 

[automatic2]

Use opto isolation on a darlington driver.

 

[nbucska]

If need schematics, we need FAX#

<nbucska@pcperipherals.com>

 

[biomedicalEng]

Hi,

Try Texas Instrument's ULN2003.

It can satisfy all of your requirements in a very low cost.

BR,

Kasra

email: Kasra_Ravanbakhsh@hotmail.com

 

[felixc]

Hi,
depending if you want to built with low or high volumes, and your circuit density, just a regular npn transistor like a PN2222 used as a switch will energize the coil, and a reversed diode to protect from back-emf as IRstuff stated.
I have a concern about the current sources that you want to switch. A current source that is disconnected from its load will see its output rise to saturation so you may have voltage spikes at your load when you switch between sources. Just keep an eye on that one.
felixc

 

[OptoMark]

An Opto SSR will do this job very simply. Look at NEC PS710A or PS7206.

 

[piedade]

DO the folowing circuit

_35v
|
coil
|
|
|/
+5v pulse-- resistor ---|
470ohm |\>
|
|
|
gnd

the transistor can be a BD135 or BD139 or other equivalent
use an 1N4007 DIODE in paralel with the coil cathode linked to the 35v power line and anode to the colector of the transistor.

 

[47gandalf]

I agree with biomedicalEng. I have used the ULN2003 (and other members of the family) since Sprague first introduced it. It has a number of advantages in this type of use.

Among them are: 1) low cost; 2) multiple manufacturers; 3) multiple channels in one package; 4) drive directly from a logic level output; 5) built-in diode to suppress the voltage spike when you turn off the relay coil.

If you want isolation I'd suggest pass the logic output through a PS2501 type opto coupler and then to the input pin of the ULN2003.

There are many solutions to this but the '2003 is a very good one.