# help me analyze my project circuit

#### WINPINPH1

Joined Sep 27, 2010
24
It was posted before and have so many questions.

I don't need to use enable/disable from 4017 all i need is set and reset.

The main purpose of 10 output is to switch an ac circuit by triggering relay.

Let us concentrate on one output since it's all thesame.

what's bothering me is how to trigger the relay i'll be using power relay at 220vac.

from output of the decoder which is 5v since high state then how come i need to use 1k and 2N3904?

From the circuit what is CE current to drive the relay?

I hope my questions are clear.

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#### campeck

Joined Sep 5, 2009
194
You are using the transistor so that your 5v can switch the 12v relay. You cant turn a 12v relay on with 5v. The 1k resistor is there to make sure it is in saturation as a switch.

I have my own question. That transistor is rated at 200mA collector current. Does the relay have enough resistance to limit the current through it? Or it's own internal resistor? Wouldn't that transistor fry otherwise?

#### SgtWookie

Joined Jul 17, 2007
22,230
Are you using 9v to power the 4017? You know that 4000 series CMOS can be powered by 12v, right? Why not just use 12v to power everything, including your logic inputs?

How much current do the relay coils require, or what is their resistance?

4017 counters do not have a "set" input, only a "reset" input. When the IC is reset, output 0 will go high, and all other outputs will go low.

#### campeck

Joined Sep 5, 2009
194
How much current do the relay coils require, or what is their resistance?
Both.
These aren't ratings that are on the relay itself are they? If they need a certain current then are you supposed to supply a resistor to limit the current? Or does the relay do it itself. Because if that relay only has like 5ohm of resistance then he's pushing 1A through the transistor rated for .2A.

#### SgtWookie

Joined Jul 17, 2007
22,230
A 2N3904 is rated for 200mA max, but it's best to not push it much over 100mA.

In order to saturate a transistor, Ib=Ic/10, or the base current needs to be 1/10 of the desired collector current.

Rbase = (Vin - Vbe) / (Ic / 10)

4000 series CMOS has pretty limited current source/sink capability. You would really be better off using N-channel power MOSFETs to sink the current from the relay coil.

If our OP can supply a manufacturer and part number for the relays, we can find out what their current requirement is.

They could likely use solid-state relays (SSR's) to eliminate the power usage in electromechanical relays.

#### campeck

Joined Sep 5, 2009
194
regardless of their current requirement though...

When the transistor is on and the relay is just a long wire wrapped around a bunch of times. wouldn't the 12v just push whatever current it could through the resistance of the relay? If this resistance is low wouldn't you expect more than 100mA to flow? Have you ever had to use a resistor on a relay?

#### SgtWookie

Joined Jul 17, 2007
22,230
Wires used in relays are very small gauge, and there are a lot of turns wrapped around the core. For example, AWG46 wire is .00157" in diameter (finer than a human hair), and measures roughly 4.2 Ohms per foot.

Once a relay has been energized, the current flow can actually be reduced somewhat, as it doesn't need as much current to keep it energized as it did to energize it in the first place.

Using a resistor in series with a relay coil could result in the relay not energizing at all.

#### WINPINPH1

Joined Sep 27, 2010
24
as of now i don't have specifications for relay please give me suggestions what to use and how to calculate the right current to energized the relay.

relay is power relay to switch 220vac load is one nokia charger only.

#### WINPINPH1

Joined Sep 27, 2010
24
ok from the suggestions i modified my clock voltage to 12v regulated.

whats next?

if my output is high therefore that is 12v then 1k resistor in series

Ib = V/R = 12/1K = 12ma am i correct?

but from the specifications of 2N3904 the Veb should not exceed 6v. am i correct?

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#### SgtWookie

Joined Jul 17, 2007
22,230
Unless you can give us some specifications, you're wasting our time.

I don't have time to waste.

If yo don't want to provide all of th e specifics, then I invite you to go someplace else.

#### WINPINPH1

Joined Sep 27, 2010
24
please be patient to me. I'm just learning electronics.

can you tell me what are the things you need to know to help me?

schematic was already posted. Basically all i need to know is about the schematic diagram. About the specifications,

I dont have specifications.

I'm asking help maybe you could give one.

Am i in the right section of the forum for newbie? or newbie is not welcome in this forum?

#### wayneh

Joined Sep 9, 2010
17,498
please be patient to me. I'm just learning electronics.

can you tell me what are the things you need to know to help me?
The concerns raised so far are about the current that will flow when the positive trigger is received at the base of the transistor; whether that current (collector-emitter) will fry the transistor and/or the relay. A few specs about the relay (does it have no label at all?) might alleviate this concern.

If you don't care about toasting either of those two components, you could just proceed with caution. But you posted here for a reason, presumably to avoid a puff of smoke.

I double the recommendation to use an N-type power MOSFET to accomplish the switching. Find one rated at 1A or more and you should be fine, assuming the relay draws much less than that.

Have you tried switching the relay with low (ie. safe) voltages? You could try a AA battery first and move up until you get reliable switching, then design around providing that voltage across the relay.

#### WINPINPH1

Joined Sep 27, 2010
24
If i'll use a power relay from that website above. R45 power relay.

please tell me the computation from output of 4017 to the current / voltage going through the transistor and the amt.

current that will trigger the relay.

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#### wayneh

Joined Sep 9, 2010
17,498
I have no idea what that means. But look on p. 48 of the catalog and it likely gives the current draw at 12v. If that is more than about 1/2 of what the transistor is rated for, then you're cutting it close. If it exceeds the rating, it'll probably fry. It looks to me like it will draw under 50mA, but you need to verify.

#### WINPINPH1

Joined Sep 27, 2010
24
thank you.

can you please show me the computation how am i going to prevent such situation of destroying my transistor.

beginning from the output of the 4017. Its supplied voltage / current to the transistor and transistor output to trigger the relay.

where i can find the catalog?

#### SgtWookie

Joined Jul 17, 2007
22,230
It would help us to help you if you would edit your profile by clicking on the "User CP" link at the top of the page, and put your country and state or province or island nation in the place provided.

Without knowing where you are, we have no idea what vendors you might be able to order from. I only suspect that you are not from the USA.

#### wayneh

Joined Sep 9, 2010
17,498
where i can find the catalog?
Same link you posted above - the relay mfg's site.

#### n1ist

Joined Mar 8, 2009
189
You will also need suitable pulldowns and debouncing on your input switches. and of course power, ground, and a decoupling cap on the 4017.
/mike

#### marshallf3

Joined Jul 26, 2010
2,358
ok from the suggestions i modified my clock voltage to 12v regulated.

whats next?

if my output is high therefore that is 12v then 1k resistor in series

Ib = V/R = 12/1K = 12ma am i correct?

but from the specifications of 2N3904 the Veb should not exceed 6v. am i correct?
So long as you don't go above (or way below) the base current specs for the transistor the BE junction will drop ~0.6 - 0.7V and the resistor will take care of the rest. 12 mA into the base x the gain of the transistor is roughly what it's capable of switching as far as relay current. In a 12V circuit it isn't unusual to use between 4.7K - 10K to drive a 2N3904 to ensure it's in saturation.