• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

DPDT switch function using MOSFET's: how to?

Thread starter #21
If you change to a CMOS 555 (lmc555), the output swings much closer to both rails, assuring a complete turn-off of the three FETs it drives.

Add a pull down resistor from the Q7 drain to GND to assure turn-off of the two FETs it drives.

ak
Thks! And what do you think about the feasibility of linking the two bridges together as proposed?
 

crutschow

Well-Known Member
Most Helpful Member
#22
Why did you request that I change the circuit to allow two signals so that direction could be separately energized, and then use a single signal with an inverter? :confused:
My first circuit would do what you want without the inverter.
 
Thread starter #23
Why did you request that I change the circuit to allow two signals so that direction could be separately energized, and then use a single signal with an inverter? :confused:
My first circuit would do what you want without the inverter.
Euh? I have no idea how that would be possible?

EDIT: now I see! Your first circuit, the inverter function is done by M2.

EDIT2: so the drain of M3 can be connected to the drain of its duplicate if I want two bridges driven by one signal as shown in post #19?
 
Last edited:
Thread starter #24
Why did you request that I change the circuit to allow two signals so that direction could be separately energized, and then use a single signal with an inverter? :confused:
My first circuit would do what you want without the inverter.
When constructing this circuit, M1 stays in conductive mode. Exchanged mosfet's, same result?
 

crutschow

Well-Known Member
Most Helpful Member
#25
so the drain of M3 can be connected to the drain of its duplicate if I want two bridges driven by one signal as shown in post #19?
In my first circuit you just connect the the two IN1 inputs together to drive two bridges at the same time.
When constructing this circuit, M1 stays in conductive mode. Exchanged mosfet's, same result?
Then it would appear the MOSFET is not an N-channel, or you have it wired with the source and drain interchanged.
 
Thread starter #26
(...)
Then it would appear the MOSFET is not an N-channel, or you have it wired with the source and drain interchanged.
Both M1 and M2 are N-channel, the same as in your second circuit. When powering up, M2 conducts once, then switches over to M2 which stays in conductive mode (frequency is about 1Hz). Source and drain are correctly wired, I checked and rechecked and..
I have no clue at the moment; your circuit is very logical, yet here it does not work.
 

crutschow

Well-Known Member
Most Helpful Member
#27
Post your circuit diagram exactly as you have it.
 

crutschow

Well-Known Member
Most Helpful Member
#29
If that's exactly how you tested it, it has no load.
It needs a load to work properly.
 

crutschow

Well-Known Member
Most Helpful Member
#31
My simulation works okay with that load.
Try connecting another 1kΩ resistor directly across the bridge output.
 
Thread starter #33
My simulation works okay with that load.
Try connecting another 1kΩ resistor directly across the bridge output.
I put a scope on it: M2 stays conductive, the drain never goes high, the gate does alternate high and low (and M2 gets hot as hell).
I don't understand this..
 

dknguyen

Well-Known Member
#34
I put a scope on it: M2 stays conductive, the drain never goes high, the gate does alternate high and low (and M2 gets hot as hell).
I don't understand this..
To which transistor are you referring to when you say M2?

Which gate are you measuring that alternates? Q9? If so, that's not a relaible indicator of what's going on with the other gates.

Looks to me like the gates of Q11, Q10, and Q8 just go high impedance when you stop driving the gate of Q9 and it opens. That leaves the gate capacitors on the other three MOSFETs isolated retaining their charge which leaves them on. It appears to me that once you turn Q9 it will cause Q10 on, Q11 off, and Q12 off. And then it will stay that way even after you shut off Q9.

MOSFETs don't just turn off when you disconnect their gate. You actually have to discharge the gate capacitor. You can actually test MOSFETs out of circuit this way by using the multi-meter measurement currents to charge up the gate-source cap and then moving the probes to other pins to test if the source-drain is conducting or not.

Is that what you're seeing? I'm behind on this thread and only skimmed it.
 
Last edited:

dknguyen

Well-Known Member
#36
I decided the check the 555 output. On the scope you can see some strange oscillations every low to high transition, it gets worse at lower frequencies. Is this normal behavior? I thought a 555 would give clean square wave output?
I assume this is a separate question from the transistor never turning off...

Is it actually worse? Or are you just reading on a different time scale in each graph? Is the frequency of the ringing about the same between both except for the lower frequency it lasts a lot longer?

Because that doesn't look like ringing to me. It looks more like your 555 can't decide between thresholds or something like that.

Exactly what kind of 555 timer are you using?
 
Thread starter #37
I assume this is a separate question from the transistor never turning off...
As a result of M2 never turning on I decided to look at what comes at the gates of M2 and its opposite N-channel mosfet.
Is it actually worse? Or are you just reading on a different time scale in each graph? Is the frequency of the ringing about the same between both except for the lower frequency it lasts a lot longer?

Because that doesn't look like ringing to me. It looks more like your 555 can't decide between thresholds or something like that.
At lower frequencies there are more oscillations before the pulse settles to high.
 

dknguyen

Well-Known Member
#40
I just noticed that your 555 timer is not decoupled. Add a decoupling capacitor across the power pins. You might also want to do the same for the H-bridge itself.

If those don't work, then add a low value resistor (100 ohms or less, maybe even 10) in series with the gate of Q9. I feel like they should not be required though and your problem is something else.
 
Last edited:

Latest threads

EE World Online Articles

Loading

 
Top