Hi guys,

I would like to know if there is any simple circuit that could help me amplify current (keeping voltage the same at 24V). I have the output of an Op-Amp that gives out 24V but current is really low at some mA. I need to drive a relay at 24V, 1.4A. How do I achieve that 1.4A? I read about using ULN2004A Darlington array chip, but dont know how it works. Can someone help me with that or is there a way to just use Op-Amps to amplify current?

Thanks!

V

 

It would help a good deal if you post a schematic of your circuit as it is now, and explain as best you can what the function is currently, and what you would like for it to do.

ULN200x Darlington arrays are limited to about 350mA sink current per output, and about 700mA per package. You would need a different device.

 

thanks for the reply. I was able to quickly put a schematic with paint , not very good at that But I hope it conveys the needed information. If I cannot use Darlington array, can I use someother op-amp?

VeeJay

 

You could just use a logic level (5V gate drive) FET to directly drive the relay.

Connect a resistor (eg. 1K) from gate to source, source to 0V
Connect the switch between 5V and gate.
Connect the source to on side of the relay coil and the other to +24V.
Fit a 1N4007 (or any 1N400x) diode across the relay coil, cathode to +24V.

If you want to drive it from a logic signal rather than a switch, use a 10K resistor rather than a 1K.

 

thanks! I don't want to seem to be pushy, but if you could post a schematic, that would be awesome

 

OK, i am drawing the schematic myself. If I have question, I will ask again....

 

Hi are you sure the 1.4 A is the current needed to activate the relay? I think it may be the max current the relay switches can handle. Anyway you can not control the relay with an OPAMP so your work up to now is not wasted
Can you find some type indification on the relay

 

It is a Tyco Electronics K10 series relay. k10P-11D15-24. Hey, you may be right. That is the maximum current it may need. But it certainly needs more than 30mA that I am getting out of the Op_amp. I am using the Op-Amp to obtain 24V from 5V input from a logic switch.

VeeJay

 

Your relay coil is 650 ohm as far as I can see.

 

Coil current is going to be around 40mA actually.

http://www.tycoelectronics.com/commerce/DocumentDelivery/DDEController?Action=showdoc&DocId=Data+Sheet%7F1308242_K10%7F0303%7Fpdf%7FEnglish%7FENG_DS_1308242_K10_0303.pdf

 

Typically boost the op amp with a transistor, a fet or bipolar would work, perhaps a darlington. Google or any op amp reference should show you the way.

 

Oh guys, thanks for your help. I finally realized that 1.4A is the max. Now, the relay switches with the op-Amp. And now i understand how to read datasheets.

For another relay that operates on 6mA, I am trying to step this 37mA to 6mA using a pull down resistor.

Cheers!

VJ

 

If the input will only be 0V or 5V, you could drive a logic-level MOSFET directly, or a Darlington transistor if you don't mind losing a bit of voltage across the collector-emitter junction.

Here's a couple of examples:

Note that you MUST use a diode across the coil of the relay, or the reverse voltage may destroy the MOSFET or the Darlington.

The Darlington requires a current limiting resistor on the base.

The MOSFET needs a small resistor between the signal input and the gate. This is to help prevent the gate from "ringing" due to the parasitic inductance of the wiring and the capacitance of the gate.

The 10k pull-down resistor from the gate to the source terminal (in this case, same as ground) is a good idea. It will keep the MOSFET turned off in case the input signal is disconnected (high impedance).

 

Current Amplifier

 

@sgtwookie.

I ran simulations on the circuits you had provided. The relay does not switch when a 5V input is given.

V

 

@sgtwookie.

I ran simulations on the circuits you had provided. The relay does not switch when a 5V input is given.

Post your circuits.

 

here are the attached circuits.

V

 

Did it occur to you that I used a different Darlington transistor and a different MOSFET?

The MOSFET you used is a 2N7000. While it is not rated for logic-level operation, hobbyists frequently use it as such. However, it has a current limit of 200mA, which is quite a bit short of your requirement.

Similarly, the ZTX605 is not capable of sinking the required current.

Try using the components that I showed in my schematic.

If you wish to use different components, you can certainly do so; however they must be similar or exceed the specifications of the components that I used in order to work.

MOSFET: N-channel enhanced mode logic level, Vdss of 30 or more, Id of 3A or more.
Darlington: NPN, hFE>=1,000, Ic>=3A, Vce>=30v.

 

The datasheet for the relay shows that its coil current is only 37mA.

 

In that case, I can't offer an explanation why it doesn't work in simulation for our OP.

Perhaps the simulation won't run at all due to the floating nodes on the relay contacts?