Printer Friendly Forum Posts - diode direction?

Link has diagram of circuit. My question is. Are the diode's in the correct direction to protect the ground inputs? Relay #1,#2 are triggered from a PCM (output ground). When both inputs lose the ground the relay's will sig the 3rd momentary relay thus causing a momentary ground output sig. Approx 1/2 sec. Just want to be shure not to leak + back to PCM.

https://photos.thedieselstop.com/showphoto.php?photo=32449&papass=&sort=1&thecat=998

Thanks

Bill

If what you want to do is protect your two ground signal inputs, you should place the diodes across the coil like you have in the third relay. Putting the diodes in series doesn't do much to limit the voltage spike that occurs when the ground signal tries to abruptly stop the flow of current through the coil.

This setup is going to be constantly drawing power through those first two relays on standby; is that what you wanted? Unless this is only going to be used while the car is running, it'll drain your battery in short order. You might consider using solid state devices to do the controlling; it'll be safer on your input signal and draw negligible power.

Yes motor running. I really do not need power relay's. The load is small. I would like to go solid state. Really most interested in the speed of transfer (ms,ns) and length of time for the momentary connection. How can i go electronic? I would have to drop to 5v dc for and IC.

Thanks Bill

Cannot edit post. The first 2 relay's, the input grounds are allway's changing (pcm control). When the combo of both lose the signal (ground) it then sets momentary output ground (3rd relay).

Thanks Bill

If you use a CD4011BCN (available at mouser.com for 45 cents last I checked), it'll run off the car's 12V supply, so you don't need to go to 5V.
It's a NAND gate, so you can just connect your two inputs to it, and whenever either one goes low, the output will go high, and you can use that signal output to drive your output relay pulse circuit.

Thoughts were to use JWD-170-7 as per link. Will check data on CD4011BCN.

https://www.mouser.com/search/refine.aspx?Ntt=655-JWD-107-7

Bill

Checked the CD4011BCN. Its has 100mw max. Have to figure how to wire due to i'am electronics dumb. (nand,nor,cmos,ect). My momentary load is a solenoid in a auto transmission (200ma). Also have to use trimable resistor to tune momentary time.

You would use the logic gate output to control a transistor that can drive the actual relay instead of doing it directly, so the power of the gate doesn't really matter.

You can use a BS170 transistor to drive your output, also available from mouser, for about a dime. It can be considered to be like a relay that is driven from a voltage input, and provides a switch to ground when it's on.

I'll sketch out a possible setup if you need it-

A sketch would be GREAT!

Thanks

Bill

If your input signals aren't clean and free from voltage spikes, you should add filtering to the inputs to protect the gate from damage. Otherwise you could connect directly to the inputs.

Tweak the pot value from midrange to adjust the momentary on-time up or down.
The gate and transistor are ESD sensitive, so be careful to avoid static buildup when handling them.

By the way, the three gates shown above are all on a single chip, so you don't need three chips to do this. Just connect the pin numbers shown for the individual gates- less than 75 cents in parts, not including your relay.

Thanks! I will give it a shot.

Bill

Dualsport

On C1 the cap. What voltage and type? Resistor type and %? Also do i need a voltage regulator or over protection device to protect from alternator (+/- 2vdc).

Thanks for all your help Bill

Pick a cap with double the expected voltage, above 30V should be fine. A standard aluminum electrolytic or tantalum would be okay for this.

Resistor type and tolerance is not at all critical, the two pull up resistors on the input may not even be needed if your input goes high on their own, rather than ground switched only.
You can change the cap value and potentiometer resistance as you need to change the on-time of the momentary pulse.

If your car is working properly, you shouldn't need a voltage regulator, but if it's prone to voltage spikes, you can add a zener diode and a series resistor to make sure the gate doesn't get too high a supply voltage.

I would like to add the diode and resistor for protection. This is for a diesel application. Its prone to spikes with glow plug on time. If you could show where to install and size.

Thanks you have been a major help.

Bill

Here's a modification adding extra clamps and limiting circuit that should protect the NAND gate from spikes.

Dualsport

Would i be able to replace the relay with a mosfet? I'am looking to drive a autotrans solenoid. It has a 30ohm coil. Trying to speed up the turn on time.

Bill

That would be about a half amp draw, so you should be able to drive it directly from the transistor. The continuous rating on the BS170 is 500mA which should be able to do it, especially since it'll only be a momentary pulse.
You'll definitely need to make sure the D1 suppression diode is in place to supress the large transient voltage spike coming back from your solenoid when it turns off, or else it'll destroy the MOSFET.
Basically the relay shown will be substituted with your solenoid.

Have to wonder though, would the switching time of a mechanical relay really make that much difference for the application? It's just a tiny fraction of a second; that would be a problem?

With the mosfet i can get into the na second reaction time. I have to look but i think a reed is milli second. How quick on (signal) and off time (momentary) is important. I'am trying to sync a tork converter lockup with a 4th gear shift in a auto trans. I have to let the PCM unlock the TC or it will set a code. In its current form i have a momentary switch to hold the ground during the shift command to hold it locked. Works great but sets a code. So i have to let it unlock then lock. When the 2 input grounds break that is when the TC unlocks.

If you were doing it manually before, it'd probably be a much bigger delay than a relay would add. I wouldn't think the bit of extra time would make a difference, but you can try it out and see if it works both ways.

dualsport

You are the MAN!!! I owe you a cold one. Assembled and bench tested. All seems good. Next is a test in the real world. Have high hopes.

Bill----Thanks for all your help.

Got my fingers crossed- hope it works out! posted_image

dualsport

It works great!!! I installed the board in a plastic box. This is going to be mounted outside under hood. Should/can i encapsulate/pot with epoxy? Just to avoid vibrations/ heat/water ect. I dont know if the 170 needs to be air cooled.

Thanks Bill

Cool beans posted_image

If you just seal up the assembly to keep water out it should be fine- if you wanted to you could encapsulate things, but it isn't really necessary if your solder connections are good. I'd leave it open for a while so it could be tweaked to change values if necessary.
The BS170 won't generate much heat since it's acting as a momentary switch and it shouldn't have problem with temperature if your plastic box isn't in danger of melting where you mounted it.