Simply relay circuit - help please

[nemo1966]

Hi guys, I did some electronics years (and years) ago but now find myself with a little problem.

Last year I bought a satellite box, it is brilliant but in the summer has a tendency to over-heat. The manufacturer built in a header for a fan that is supposed to be 5v, however it turned out the header only outputs 3v. The 3v isn't enough to drive the fan, so I have fitted a fan using 5v from the usb connector and this works fine apart from the fact that the fan is on permanently as opposed to the fan header that can be temperature controlled.

So my idea is to make a 3v to 5v relay using the fan headers 3v to switch on and off the usb's 5v supply.

Can anyone help with a diagram and parts? I am in the UK

thanks
nemo

 

[KrisBlueNZ]

You can probably just use a transistor for this. I'm making a few assumptions here.

1. The negative terminal of the 3V fan connector is connected to the negative supply from the USB port.

2. The 3V fan connector is either ON or OFF; it doesn't have a variable voltage to vary the fan speed.

Take a wire from the positive connection of the fan connector through a resistor (start with, say, 220 ohms), to the base of an NPN transistor. Connect the emitter of the transistor to the negative of the USB voltage. Connect the positive of the USB voltage to the fan, and the other side of the fan to the collector of the transistor.

When the 3V appears on the fan connector, the transistor will be turned ON by current through the resistor, and will conduct current from its collector to its emitter, completing the circuit and turning the fan ON.

A suitable transistor would be the ZTX650, which has a fairly high gain, a low saturation voltage (low collector-emitter voltage drop when ON) and a 2A maximum collector current rating.

Check that the current rating of the fan is less than one amp.

If the transistor gets warm while the fan is running, it may not be saturating fully; in this case, reduce the base resistor (try 100 ohms instead of 220).

If you discover that the fan does not run at full speed, it's possible that the fan connector provides a 5V PWM (pulse width modulation) voltage, and that the duty cycle is about 60%. This would explain why the connector claims to be for a 5V fan but you only measure 3V on it. The unit may start the fan at 60% duty cycle and increase the duty cycle if it continues to get hotter. You can check for this possibility using an oscilloscope on the fan connector.

 

[nemo1966]

At present the fan is simply connected to the USB positive and negative, fan is rated at 0.2amp

Thanks for the reply, just getting my head around it.

Nemo

 

[CDRIVE]

If you don't have a scope you can build this simple circuit to test if the voltage is actually Pulse Width Modulated 5V. If the voltage is actually only 3V the LED won't light. Or if your DVM has a Frequency function you can use that.instead.

Chris

 

[nemo1966]

If you don't have a scope you can build this simple circuit to test if the voltage is actually Pulse Width Modulated 5V. If the voltage is actually only 3V the LED won't light. Or if your DVM has a Frequency function you can use that.instead.

Chris

ah I see now thanks... I take it the resistors are 120 and 68 ohms?

thanks

 

[KrisBlueNZ]

Good idea Chris

 

[CDRIVE]

Good idea Chris

Ha, I must have experienced a brief moment of cogent thought. I've been suffering with a nasty case of Snowbird Flu.

Chris

 

[nemo1966]

Thanks very much for the help guys - much appreciated. Just waiting for transistor.

thanks again
nemo

 

[CDRIVE]

If the transistor gets warm while the fan is running, it may not be saturating fully; in this case, reduce the base resistor (try 100 ohms instead of 220).

If you discover that the fan does not run at full speed, it's possible that the fan connector provides a 5V PWM (pulse width modulation) voltage, and that the duty cycle is about 60%. This would explain why the connector claims to be for a 5V fan but you only measure 3V on it. The unit may start the fan at 60% duty cycle and increase the duty cycle if it continues to get hotter. You can check for this possibility using an oscilloscope on the fan connector.

If you don't have a scope you can build this simple circuit to test if the voltage is actually Pulse Width Modulated 5V. If the voltage is actually only 3V the LED won't light. Or if your DVM has a Frequency function you can use that.instead.

Chris

Thanks very much for the help guys - much appreciated. Just waiting for transistor.

thanks again
nemo

Are you sure you understood what my little test circuit was for?

 

[nemo1966]

I had assumed that as you stated if the voltage of the fan terminal was 3v then the LED wouldn't light up. Then if it was 5v PWM then the led would light after a while when the voltage went to 5v?????

 

[CDRIVE]

I had assumed that as you stated if the voltage of the fan terminal was 3v then the LED wouldn't light up. Then if it was 5v PWM then the led would light after a while when the voltage went to 5v?????

My point is If the port deliverers 5V (PWM) then the LED would always light. Then you can connected the 5V fan directly to the port. No need for the transistor.

Chris

 

[nemo1966]

Ah I see, the voltage is "supposed" to be 5v but has already been verified at 3v hence having to use the USB to drive the fan. The downside is that connecting the fan to the usb leaves the fan in an on position all the time. e,g, you can't use the control functions to turn it on and of when the HDD gets hot etc.

thanks

 

[KrisBlueNZ]

nemo, I don't think that's what anyone here is trying to suggest.

You said there is a fan connector, which is supposed to be 5V, but when you measured it, you measured only 3V.

It is possible that this connector IS supposed to be connected to a 5V fan. The voltage could have been 3V for two possible reasons.

First, the board may supply a variable DC voltage, to vary the fan speed. Initially it may start the fan at 60% voltage, i.e. 3V, and only increase the voltage all the way up to 5V if the temperature continues to increase, i.e. more air is needed.

Second, the board may do the same thing but using PWM, which is pulse width modulation. A 5V supply is cycled ON and OFF at a certain frequency, typically something like 100 Hz, and the duty cycle, that is the percentage of the time that the voltage is ON, is chosen to give a certain proportion of the fan's maximum speed. In this case, with a duty cycle of 60%, a multimeter will read the average voltage, which will be 3V. Again, the board would increase the duty cycle if it doesn't start to cool down.

These are just possible explanations for why your connector might be intended to drive a 5V fan but you only measured 3V on it.

Chris's circuit is an easy way to test whether there is a 5V PWM signal on that connector. If there is just 3V DC on the connector, the LED won't light up, but if there's a signal that alternates between 5V and 0V, the LED will light up at the times when the voltage is 5V.

If you have an oscilloscope you could also use that to check for a PWM voltage on the connector.

If you have a PWM signal on your connector, then using that signal to control a transistor that switches a 5V supply from your USB port will give you only 3V (average) across the fan, i.e. there's no point adding the transistor and using the USB supply.

Look up pulse width modulation on Wikipedia for more information.

 

[nemo1966]

That's what I understood.

However a 5v PWM still won't drive the fan as the fan needed is a 60x60x10mm @ 5v. These are like rocking horse dung to obtain and most people are forced to run a 12v fan at 5v (decreased performance but OK because they are very quiet). The 3v isn't however enough to start the fan, again hence wanting to use the fan header as a control rather than the actual power supply.

Just knocking up the test circuit anyway to check if it's 5v PWM or an actual 3v.

thanks
Nemo

 

[KrisBlueNZ]

OK, I thought you misunderstood.

Right, 3V wouldn't be enough to start a 12V fan. I'd be concerned about it starting at 5V as well, once it has accumulated some gunk. I guess there isn't a 12V rail available?

 

[CDRIVE]

Is this the first time we're hearing that the fan is a 12V model?

Chris

 

[KrisBlueNZ]

I think so Chris. I had assumed it would be a 5V fan since he wants to run it from a 5V supply.

 

[CDRIVE]

I think so Chris. I had assumed it would be a 5V fan since he wants to run it from a 5V supply.

I think that famous quote from Struther Martin said it best.

On the up side this failure to communicate doesn't come close to this unrelated topic.
https://www.electronicspoint.com/flashing-5watt-led-t256005p10.html#post1525837

Chris

 

[nemo1966]

90% of the worlds desktop PC's have 12v fans running at much lower voltages, usually using variable resistance controllers. Even the 5v supply we are talking about could be a PWN running at a startup of 3v.

 

[nemo1966]

OK I'm thinking of going in a slightly different direction. I am thinking of using a 12v external power supply for the fan(s). However I would still like to use the internal fan header to simply turn on and off the supply to the 12v fans.

Now will it matter if the header is 3v or 5v PWM?

thanks
Nemo