Power to turn on an automobile relay

**MatrixPC** · 01-28-2005, 05:26 PM

I am running into a problem and I can't find a solution.
I am building a Sproggy PSU and the main relay for the Sproggy that Mastero uses only can provided 16A max. Base on the 4.5 design, this PSU draw more than 16A. I need to replace this relay.
I have search and can't find one that is like 25A or 30A that suitable for a replacement.
I try to use the standard DPDT auto relay (one with 85, 86, 30, 87, 87a). The mobo can't turn on this relay.
If you guys are not familiar with the Sproggy design, the basic is that a transitor 2N3904 get it power from 5V STB via a 10K resistor combine with the PSOn signal. The output is to turn on the relay (- leg).

Is it because the mobo PSOne signal when set to ground, doesn't provide enought return current to turn on the relay?

I used a old ATX mobo (P3) and AC PSU (out put rated 12V@4.5) to test the relay by using the PSOne signal as Ground and 12V output from the drive power connector. When the relay click, the 12V drop almost 1.5V.

What is the reason and how can I fixed the problem?

**grahamrb** · 01-28-2005, 05:43 PM

This is a bit of a work around but why not use the 16Amp relay to switch on the automotive relay. You would then be able to use the car battery to turn on the automotive relay.

**KyferEz** · 01-28-2005, 06:47 PM

Matrix, the reason you cannot drive the automotive relay is most of them pull around 160mA (I think that is exactly what the radio shack one does and is likely much more current than the relay for the sproggy needs) and a 3904 does not have enough gain to drive one with a 10k resistor on the base.

Quick and dirty calculations:
With 10k on the input, the base current is 5v/10k=0.5mA. Now, the standard gain on the 3904 when you will need 100mA collector current is 30 (hFE). So to calculate maximum current you can dirve: 0.5mA*30=15mA

To get 160mA, you need 160mA/30=5.3mA, which gives you R=5v/5.3mA=937ohms. So, from all this, you see that you need at most a 1k ohm resistor on the base, _and_ that the circuit that is driving the transistor must be able to source 5.3mA.

Now, you can either replace the 10k with a 1k, or swap it out for a small darlington if the circuit driving the transistor cannot source 5.3mA. A good darlington that should work for this is the MPS6725 (MPS6725OS-ND from www.digikey.com).

KyferEz

**zootjeff** · 01-28-2005, 11:17 PM

I thought a 3904 had a hfe of more like 100-200. Where do you see 30?

http://rocky.digikey.com/WebLib/Fair...ata/2N3904.pdf

**DallasCamaro** · 01-28-2005, 11:34 PM

I would have to say the absolute easiest way around all of this would still be to use the output from the relay you have now to power the coil of a 30A Automotive relay, which will then power your load. Sure, it's an extra relay, but it's the simplest way of solving the problem. I'm not familiar with the device you are using, but at least this method would completely avoid any modifications to your device. <good to take into account for warrany or resale issues>

**KyferEz** · 01-29-2005, 01:13 AM

Jeff:
Yes, the 3904 has an hFE of 100, minimum, with a collector current of 10mA. Using the datasheet you provided, the hFE is 30 with collector currents of around 100mA.

Everyone else:
When dealing with transistors, you should always use the worst case scenario to ensure that the transistor will work. The reason for that is the hFE cannot be fully controlled during manufacturing, so some transistors will have a higher hFE, others with a much lower hFE. That is why the minimum and maximums vary so wildly for a part, and why, to be safe, you always calculate with the minimums for the current you will be driving. That said, you _often_ can get away with using the typical values.

KyferEz