I have an ECU A controlling a relay, in which when there is an activation command I can measure 12 V and 100 mA.

The project I'm working on is to replace that ECU A with an ECU B, but when I send my activation command to the relay, it is only able to draw 12 mA from ECU B due to internal limitations. The voltage I measure in that situation is 3 V, but ECU B has a voltage driving system connected to a 12 V supply.

My question is what kind of circuitry I can use to increase both the voltage and the current to reach the values I got with ECU A. I have access to a power supply of 12 V, which is the one being used for the voltage driving system. I imagine some sort of amplifier would be the necessary component, but then which type and how to determine the specs for it?

My background on electrical circuits is not very extensive, so please bear with me. I could really use your feedback. Thank you!

  • \$\begingroup\$ You want a better switch ( lower resistance) to switch Electronic Control Units? You need to be more specific on V,I of source and load and crossover time with break before make? (deadtime) \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jul 10 '18 at 14:09
  • \$\begingroup\$ I got completely lost with your comment. Can you clarify what additional information would be needed? The source (ECU B) has a voltage driving system being fed by a normal 12 V car battery. \$\endgroup\$ – Alegom Jul 10 '18 at 15:05
  • \$\begingroup\$ What input and output currents ? And purpose? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jul 10 '18 at 15:09
  • \$\begingroup\$ The current the relay needs is 100 mA, but ECU B is only able to provide around 12 mA. The purpose is to activate the relay itself. \$\endgroup\$ – Alegom Jul 10 '18 at 15:26
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    \$\begingroup\$ Possible duplicate of Arduino Uno with 12V pump: transistor or optocoupler+12V relay? \$\endgroup\$ – Dmitry Grigoryev Jul 12 '18 at 8:51

It sounds to me that with a limited ability to drive current to the relay you are currently using you have two options: -

  1. Choose a relay that takes less current (maybe an opto relay or SSR)
  2. Use a MOSFET (that can be driven from the IO line at low current) to connect the relay coil to 12 volts. Don't forget the flyback diode of course.

The 2nd option would require an N channel MOSFET interface like this: -

enter image description here

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  • \$\begingroup\$ Those are exactly my findings so far, good to have confirmation. Option 1 is not possible, I need to use that relay. The MOSFET is the way to go, but I never designed a circuit like this. Within the N channel MOSFETs, should I choose an enhancement or depletion type? Can you help me with getting the specs for the circuit? \$\endgroup\$ – Alegom Jul 10 '18 at 14:38
  • \$\begingroup\$ You would choose an enhancement type MOSFET of voltage rating at least 20 volts and maximum current rating greater than 100 mA would be sufficient. It needs to have an "on" resistance that is no more than a few ohms and there are thousands to choose from. I'd start by deciding on the package (surface mount SOT-23 or through hole TO-92) and work from that point. Plenty of distributors have search engines that are very useful. If you come across one that might be suitable just leave a comment containing the link to the device and I'll check it out. \$\endgroup\$ – Andy aka Jul 10 '18 at 14:51
  • \$\begingroup\$ Great, this is a good start! Voltage rating and maximum current done, but I'm not sure what you mean with an "on" resistance. What is its specific purpose? To be honest, I never heard of SOT-23 or TO-92. What would be the criteria to choose among those thousands? I don't have any more requirements for this system, except get it as cheap and safe as possible. Can you make a suggestion and tell me why you'd choose it? \$\endgroup\$ – Alegom Jul 10 '18 at 15:21
  • \$\begingroup\$ A MOSFET can be regarded as a contact closure when driven correctly at the gate. The "contact" when off will be nearly open circuit (passing only a few microamps) or, when on, it will be like a contact closing but not with zero resistance but with a few thousandths of an ohm to several ohms. No, I'm not going to make a suggestion. Do some background research and learn a little about MOSFETs. Requests for product suggestions are off-topic for a reason - nobody would learn anything. \$\endgroup\$ – Andy aka Jul 10 '18 at 15:33
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    \$\begingroup\$ As I said previously, If you come across one that might be suitable just leave a comment containing the link to the device and I'll check it out. Tony's suggestion is basically my suggestion except I have provided the fuller detail. Asking for suggestions is off-topic as I have said before - site rules: "Questions seeking recommendations for specific products or places to purchase them are off-topic as they are rarely useful to others and quickly obsolete." \$\endgroup\$ – Andy aka Jul 11 '18 at 14:24

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