Continue to Site

Welcome to

Welcome to our site! is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

i want to drive a small relay from PLD device.

Not open for further replies.


Full Member level 3
Jun 20, 2001
Reaction score
Trophy points
Activity points

i want to drive a small relay from PLD device.

is it good to do it directly if PLD has enought current, or is it better to use transistor or on otthe ULN drivers ?

hello friend,

don´t drive the relay directly... it can damage your circuit.... use a BC337 (500mA) in a common emmiter configuration with a 10K resistor in the base and the relay coil in series with the collector.DON´T forget the protection diode (1n400X) and optionally you can use a capacitor also. Remember that the CE config. inverts your signal!!!!
another option, and I recomend depending on the number or relays to drive, is ULN2003. It´s neat and clean and already have the protection diodes and the base resistor just plug and play.
there´re good tips at A*R*O*M*A*T site.

good luck,



It is best to use a transistor or ULN driver. Even if a pin can drive it direclty, you might still get noise problems


u can also use a small FET instead of a bipolar transistor. But as mentioned before: NEVER forget the protection diode antiparallel to the coil in order to avoid damage of the transistor/fet and the PLD. Those ULN-devices are great but if u want to drive only one relais a transistor might be easier and requires less space.
One good small fet is the BS170.


in my design,i drive 3 relays.i use a transistor 2sc9013 to drive a small is good for a PLD or CPU io.I use a MOSFET to drive a big relay derectly from a cpu io,it is easy,but must a logic level mosfet,and do not forget a pull down resistor in gate.
and never forget the protect diode.

I don't think there's a problem controlling your relay directly by you PLD, if the pin can provide enought current. Since every PLD, micro,... has fet controlled outputs, or open collector outputs. ( if you decided to do in this manner don't forget to put antiparalel (reversed) diode)).

Like many people say, if you are going to be more professional or simply you want to be more safety, use a transistor to drive solenoide. It doesn´t matter the type of transistor, i mean PNP NPN FET, MOSFET.... But be sure, you'll need more parts in you design.


Not open for further replies.

Part and Inventory Search

Welcome to