6 way FSW Relay PS Problem

[David Root]

Started up my #183 w/HRM & 6-way footswitch. I have a 6V3-0/350mA PT winding supplying 7.2VAC to a RC201 bridge rectifier. I'm only getting 6.6VDC out of it where I should be getting about 9.0VDC. The 6.6 volts is going into the 5V fixed regulator, a 1086. 0.0VDC coming out.

This is all on Jason's combined Relay/Bias PS board. The bias side works fine. The relay positive off the board is feeding six relays, four in series and two in series from the same point, to the cathode side of each relay diode. I have six 1K resistors on this line feeding the footswitch jack and back from there to the other side of the relay diodes. The LEDs in the FSW each have a 150 ohm resistor in parallel.

I have a common ground line from the six FSW switches going back to the FSW chassis jack, (Cat5e) and from that to the same ground point as the relay PS ground.

I thought 350mA would be plenty for six relays and I can't figure out what I did that is screwing things up.

[ToneMerc]

Are sure it's not a LM1086-adjustable, rather than a LM1086-5? Disconnect everything from the positive out and see if now there's an output. Not trying to be funny, but you do know the output is the center pin right? It's not pined out the same as the LM780x series. Also, the heat sink tab has to be isolated from the chassis, if not it will short to ground.

TM

[Structo]

Did you make sure to isolate the tab from chassis ground?

[David Root]

Duh! And I looked at the datasheet too and didn't see it! Jason's PS board is NOT set up for the 1086. I'll have to use a different 220 pack regulator.

Thanx guys!

[ToneMerc]

Duh! And I looked at the datasheet too and didn't see it! Jason's PS board is NOT set up for the 1086. I'll have to use a different 220 pack regulator.

Thanx guys!

David, the board has no bearing whatsoever on the type of regulator and could care less about what type of regulator is used. You just have to wire the 1086 reg accordingly and isolate the the metal tab from the chassis using a mica spacer and rubber screw bushing. On a LM780x, the tab is ground and of course you found out that on a 1086 the tab is hot.

Using Jasons boards, wire the 1086 as follows:

Pin 1: ground
Pin 2-center: from voltage regulator output
Pin 3: to voltage regulator input

Isolate the mounting tab and you are golden.

TM

[David Root]

TM, I don't see how that can work without hacking the PCB traces and adding jumpers on the backside of the board.

[bepone]

Started up my #183 w/HRM & 6-way footswitch. I have a 6V3-0/350mA PT winding supplying 7.2VAC to a RC201 bridge rectifier. I'm only getting 6.6VDC out of it where I should be getting about 9.0VDC. The 6.6 volts is going into the 5V fixed regulator, a 1086. 0.0VDC coming out.

hello david , just to note

you cant get 9VDC with 6.3VAC winding, it is not possible because of some losses in bridge rectifier (and internal PT secondary resistance).

true math is (if we have 6.3V winding)
6.3V AC at secondary PT
1.4V from AC we loose at the bridge rectifier (two diodes in forward pass)

so we have 6.3-1.4=4.9VAC for rectifiing

4.9*1.4 (filter cap after bridge rec.)=6.86 VDCmax (peak value)
4.9*1.22 (mean value) =5.98 VDC (possible, measured with multimeter)
----

so, with 6.3VAC transformer you can get cca 6VDC max (i did not calculate secondary resistance from PT, but voltage will be less than 6V because of that, depends of current which draws relay circuit)

regards,

[bepone]

7805 and 1086 are TO220 reg. with different pinouts,
7805
[img:362:344]http://www.king27.freeserve.co.uk/graphics/7805.gif[/img]
1086
[img::]http://www.chinaicmart.com/uploadfile/i ... 123939.jpg[/img]

for which one is mentioned circuit board constructed? if different, little knife and some PCB "rewiring" will do the job

[ToneMerc]

TM, I don't see how that can work without hacking the PCB traces and adding jumpers on the backside of the board.

David, you are correct! I was under the impression that you were using his clone board with the external regulator. Yep, drop in LM7805 or 7806.

TM

[ToneMerc]

hello david , just to note

you cant get 9VDC with 6.3VAC winding, it is not possible because of some losses in bridge rectifier (and internal PT secondary resistance).

true math is (if we have 6.3V winding)
6.3V AC at secondary PT
1.4V from AC we loose at the bridge rectifier (two diodes in forward pass)

Bepone, I have to disagree with your math(5.98V) being I have 5 amplifiers with 6.3V relay supplies that prove otherwise. All of them have at least 8.6-8.9VDC at the input to a LM7806 regulator.

TM

[martin manning]

You need to multiply the RMS AC x 1.414 first, then subtract the diode drop, I think, and the diode drop may only be 0.6V or so. Then, 6.3*1.414 - 1.2=7.7V. Also, the RMS voltage might be as much as 10% or more higher than the nominal 6.3VAC when the transformer secondary is lightly loaded. That'd get you to 8.6V.

[bepone]

You need to multiply the RMS AC x 1.414 first, then subtract the diode drop, I think,V.

you have right,
first multiply with 1.41 and then substract.. yes,

i was writing my post in 6 AM, very sleepy condition, sorry folks for confusion

[David Root]

Thanks all, I will pick up a couple of 7805 at my local electronics store.

Looking on Mouser I see the ST4940 has dropout below 0.5A draw of <0.5V, this might be useful when most of the relays are on. But they're $2.00 each.