any relay do this?

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KPierson

Platinum - Posts: 3,527

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Joined: April 14, 2005

Location: Ohio, United States

Posted: April 30, 2009 at 9:40 PM / IP Logged

and don't forget the diode across the coil of the relay.

Kevin Pierson

stupidpig

Member - Posts: 31

Joined: February 17, 2009

Location: California, United States

Posted: April 30, 2009 at 9:43 PM / IP Logged

1/2 watt for the 20k too? Also, which type of capacitor I should use? Electrolytic 12V? The only one 12V 100uF I found at Digikey is NLW100-12-ND, but it is not stocked.

stupidpig

Member - Posts: 31

Joined: February 17, 2009

Location: California, United States

Posted: April 30, 2009 at 9:49 PM / IP Logged

Oh BTW, is there a limit on how big the two resistor I can use to get longer delay? I guess I can't just keep increasing those to get really long delay, like 60s, right?

i am an idiot

Platinum - Posts: 13,716

Joined: September 21, 2006

Location: Louisiana, United States

Posted: April 30, 2009 at 9:56 PM / IP Logged

60 seconds will probably require a larger cap too. If you have problems with popping on your audio system when the relay disengages, then you may want to put the diode across the relay. The 2n6488 will not be damaged by the relay. The delay keeps it from collapsing fast and causing the spike.

Use at least a 16 volt cap I would feel much better if you used a 25 or 35 volt. If you are having to order all of this, order a 220 and a 330 mic also. You keep talking of more time. It sounds as though you may need the 220. You may be able to just use a 220 or 330 mic cap and leave the resistors as they are. A 1/4 watt should be fine for the 20K.

The transistor does not run hot at all. There is no need to sink it. However there is going to be 12 volts on the metal tab.

stupidpig

Member - Posts: 31

Joined: February 17, 2009

Location: California, United States

Posted: May 01, 2009 at 1:31 AM / IP Logged

Sorry, I'm not very good in electronics, does "220 mic" means "220pF mica"? Can you take a look of the following list from Digikey and see whether they are correct? Thanks.

Index	Quantity	Part Number	Description	Unit Price USD	Extended Price USD
1	5	P10294-ND	CAP 100UF 35V ELECT FC RADIAL	0.38000	$1.90
2	2	338-1046-ND	CAP 220PF 500V MICA RADIAL	1.51000	$3.02
3	2	338-1045-ND	CAP 330PF 500V MICA RADIAL	2.06000	$4.12
4	2	2N6488GOS-ND	TRANS PWR NPN 15A 80V TO220AB	1.10000	$2.20
5	2	PB681-ND	RELAY AUTO SPDT 30A 12VDC QC RES	2.93000	$5.86
6	10	47KH-ND	RES 47K OHM 1/2W 5% CARBON FILM	0.05800	$0.58
7	10	2.2KH-ND	RES 2.2K OHM 1/2W 5% CARBON FILM	0.05800	$0.58

i am an idiot

Platinum - Posts: 13,716

Joined: September 21, 2006

Location: Louisiana, United States

Posted: May 01, 2009 at 2:26 AM / IP Logged

Mic = MicroFarad. I do not know where the Mu or Mue symbol is on the keyboard. Notice above, the 100UF is a 100MicroFarad. The 220 and 330 are PF P is for PicoFarad.

Also just so you know, an Axial lead capacitor means that there is a leg coming out of each end of the cap. A Radial lead cap has both legs coming out of the bottom of the capacitor. There is no right or wrong one, just depends on how you plan to build it. Radial is also known as PC (Printed Circuit).

KPierson

Platinum - Posts: 3,527

Joined: April 14, 2005

Location: Ohio, United States

Posted: May 01, 2009 at 5:19 AM / IP Logged

i am an idiot wrote:

The transistor does not run hot at all. There is no need to sink it. However there is going to be 12 volts on the metal tab.

Is there a specific reason you went with such a big transistor? I'm going to build this tonight but I will have to sub a transistor in place of the 2n6488. According to the datasheet the 2n6488 has an hfe of ~150 at 200mA so I'll try to find something with similar characteristics.

Kevin Pierson

stupidpig

Member - Posts: 31

Joined: February 17, 2009

Location: California, United States

Posted: May 01, 2009 at 5:23 AM / IP Logged

Thanks. Learn something new :)

For the cap, the Axial version cost a lot more than the Radial one. Since I was planning to have the circuit (except the relay) on a PCB, I think Radial is ok. You said the transistor will not generate a lot of heat, but how about the resistor? As this is going to put under the hood, I'm planing to have the circuit sealed in a smal plastic box, butI'm not sure it will cause any heat problem or not.

i am an idiot

Platinum - Posts: 13,716

Joined: September 21, 2006

Location: Louisiana, United States

Posted: May 01, 2009 at 11:30 AM / IP Logged

The only reason for the 6488 was that we have lots of them left over from the old punch 150 and punch 75 amps. And also, overengineering is always a good thing.. Figured the reduced size of the capacitors would allow the 6488 to fit somewhere.

There is no heat issue at all.

dualsport

Silver - Posts: 983

Joined: September 27, 2005

Location: United States

Posted: May 04, 2009 at 10:53 PM / IP Logged

One possible consideration in switching the relay so slowly may be the high resistance of the relay contacts when the drive current is right at the threshold as it switches on and off.

If the relay has to carry a lot of current, it could be a problem with the contacts heating up. Ideally relays are switched on and off quickly for minimum contact resistance.

Some hysteresis in the drive circuitry or even just using the relay to drive a second relay might be a way to avoid this, if it proves to be any kind of issue. Using the first relay to drive a second one would basically allow the power relay to snap on and off fully. The first relay can be just a small relay since it only drives the coil of the power relay.

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