Predict this Toggle Circuits behavior

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I_Forgot

I_Forgot

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Joined 2005
cpemma said:


But learning to program a PIC in order to simplify a circuit is like suggesting the audio amplifier builder should learn to play the piano or read music notation to further his hobby.

Far better to understand how the dumb circuit works IMO. :)

Any relay coil will have a fairly wide operating range, down to at least 80% nominal, so you don't need to worry about the VCE(sat) drop across the switcher transistor. Just make sure the transistor is saturated by appropriate choice of base resistor.

Take a 2N3904 with min gain of 100; force gain to 50 to ensure saturation, then 200mA load current = 4mA base current, base resistor (on a 12V high) = (12 - 0.7)/.004 = 2825, so 2k7 or less is fine for any load up to the maximum rating.
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I think you'll find that the DC current gain is quite a bit lower than the AC current gain figure you are assuming. If I recall the rule of thumb correctly, DC current gain is about 1/10 of AC current gain (beta).

I_F
 
imix500

imix500

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Joined 2004
Hello all,
gmphadte, I have a ton of 1n914 so those will be across all the coils.

So, the mercury relays arrived today. The good news is they all work. The bad news is they pull 600mA at 6v. I will be using a 5v switcher jacked up to 5.5v, but this is RIGHT on the edge of capability for the good ole 2n2222. So I guess I need to move up to a small power transistor. I was thinking of a TIP, as they can handle more than enough current and they are cheap. I will of course take suggestions of a better one.
So for a TIP29B/C for example, how do I take the info off the data sheet and figure the amount of drive current the base will need at 6v/600mA? (This is one of the many holes in my electronics knowlege) I'm worried the Flip-Flop might not be able to drive the larger transistor directly. Thanks!
 
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gmphadte

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Joined 2003
The darlington TIP120 will be fine and u will not need any other driver. The TIP has HFE of 1000. It means if u feed the gate with 1mA, the Ic will be 1000mA.
U should select the base current a little higher than required. Take your liberty.
HFE=Ic divided by Ib.
Each output in CMOS ics can drive upto 20 mA.

1N400x is prefered and they should be cheap.

Now that your load current is so high, (24x0.6=14.4A)u should have separate supply or separate filter cap and regulator for the ICs and go unregulated(preferably) for the relays.

U should have gone for higher voltage relays. But I understand u were not aware of this.
In order to reduce holding current of the relays, u can connect a parallel RC in series with the coil.

Gajanan Phadte
 
I_Forgot

I_Forgot

Member
Joined 2005
Digi-key has MOSFETs that will switch 30A with a logic level gate voltage. I have used them to switch 10A when driven by a PIC uC. They should work for you, too. Look up RFP30N06LE. They cost about $1 each. On resistance is VERY low so there is very little voltage drop across them.

I_F
 
imix500

imix500

Member
Joined 2004
Thanks guys.
gmphadte, I will use some 400X diodes across the coils.
Higher coil voltage would have been good but for $3 ea I couldn't pass them up.
Wow, didn't realize base current math was that easy.
So what are the advantages of using a mosfet over a darlington like the tip for switching? Is it lower ON resistance? Thanks.
 
C

cpemma

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Joined 2003
cpemma.co.uk
imix500 said:
So what are the advantages of using a mosfet over a darlington like the tip for switching? Is it lower ON resistance? Thanks.
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Generally the much lower voltage drop, though being a resistance it goes up with load current. They really score at around 3-5A where a TIP31 or TIP41 would need too much base current for an opamp or logic driver and a TIP120 would drop too much voltage. Snag is you really need the logic-level type for highish current on 5V supplies.
 
I_Forgot

I_Forgot

Member
Joined 2005
imix500 said:
Ok, so it looks like Mosfets it is- no room for heatsinks. We usually order from Mouser- sometimes Allied, so I'll check out what they've got. Would one of the new QFET's be ok or is there better? Thanks.
Click to expand...

I guess it depends on what you mean by better. You're driving a relay. It ain't hi-fi. If the MOSFET has a low threshold voltage, low on-resistance, and no problem switching the current your relays need at the voltage at which they operate, it will work just fine. If you were designing a high freq switch-mode power supply you'd have to worry about switching times and other things, but your relays are going to be about 1000 times slower than even the slowest MOSFET, so switching times won't matter.

I see digi-key and mouser are both out of stock on the RFP30N06LE. I suspect they are waiting for RoHS compliant versions of the part. Try searching for "logic input MOSFET". "low threshold MOSFET", etc.

I_F
 
imix500

imix500

Member
Joined 2004
Hey guys, so I'm finally building this project in stages and I've come to the reset on power on timer using a cd4541be. I have my rc values at .1uf and 20k to get a turn on time of .5888 sec. But one component value I can't figure out is the resistor that goes from Ctc to rs. The data sheet (which is pretty darn obscure I must say) suggests equal or greater than 10k, but what shoud it be? 10k? 100k? Thanks!
 
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