Using Relays or FET's to turn on extra capacitance


I have a situation where I need to add some extra Capacitance that needs to sometimes be added to a RC discharge circuit. There is 48 volts on the rails with extra capacitance controlled currently with two relays. Relays need current that drains the system. they are two small 12 volt relays but they still use current. would it be practical to use Higher current FET’s instead of relays? I could control them with one of the Arduino processors. My biggest fear is that the FETs will have a small amount of resistance making the capacitors drain slower and hosing the system up. Any thoughts??

Circuit is attached



Can you connect the FETs between cap & Gnd, vs between 48V & Cap? Then you can use Arduino output to directly control a Low Rds, Logic Level, N-channel MOSFET to do the swithing. Off-state resistance is very high, and low Rds, down in the low tens of milliohms, will look very much like a relay. For example ~100nA leakage from Drain to Source at 48V, 125degreeC, Figure 6 of datasheet, can you live with that?

If it has to go between 48V & cap, then a Low Rds P-channel MOSFET, you will need another transistor in front of it under Arduino control, such as NPN with emitter to Gnd and collector to pull the gate low, and a resistor from the gate to 48V to turn it off.

There are a couple of potential problem with using mosfets to switch the capacitors:

  1. In order for the mosfet not to affect the discharge, it must have a sufficiently low Rds(on) at the discharge current. So you need to know what the peak discharge current is, then you can look for a suitable mosfet.

  2. A mosfet has a body diode, which is in parallel with the rest of the mosfet. So you can't use a single mosfet to control both charge and discharge. Maybe you don't mind if the extra capacitance always charges, but you want to control whether it is allowed to discharge? If so, then you can use a mosfet to determine whether or not the capacitor is switched in with you discharge it, and allow the capacitor to charge through the body diode. You could achieve this by connecting a P-channel power mosfet between the negative side of the capacitor and ground, source to ground and drain to capacitor. To connect the capacitor in the circuit for discharge, you will need to drive the gate negative with respect to ground. if the discharge is short, you can do this quite easily using a capacitor and a diode.

Here are a couple of other possibilities:

  1. Use latching relays.

  2. If you only need to control the discharge, you could charge all the capacitors through diodes. Then you need only power the relays up just before and during the discharge, which is presumable quite a short period of time, so the current drain may be less important. This assumes you have enough notice of the start of the discharge to turn the relays on in time.

Here was the part I was planning on using


So try that one connecting to Gnd then.

CrossRoads: So try that one connecting to Gnd then.

Doing that (i.e. mosfet source to ground, mosfet drain to negative side of capacitor) will mean you can prevent the capacitor from charging (by holding the gate low), but not from discharging (through the body diode) if it is already charged. But this may not matter to you, if you know whether you want to use the capacitor or not before you charge it. You still haven't said what the discharge current will be, however the body diode is good for at least 30A (at which it has a maximum voltage drop of 1.5V), and probably a lot more for short pulses.