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EPE Motor controller March 2015

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mikeb
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Username: mikeb

Post Number: 1161
Registered: 02-2008

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Posted on Wednesday, 01 April, 2015 - 04:17 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I have bashed together the above. All tested at 15V from a DC power supply and PIC and MOSFET driver have nice pwm.

When I power from mains, I can only get 8.6V DC on the 15V rail. The only difference is that I am using a 7805 for the 5V regulator.

I know reactive-capacitive voltage droppers do not provide large amounts of power. The 7805 needs about 5mA. Am I missing something here?

mb
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mikeb
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Post Number: 1162
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Posted on Wednesday, 01 April, 2015 - 04:29 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I measure a 3.5V drop across one of the 470 ohms, which is about 7.5mA. That seems low for a PIC, OPAMP and (admittedly off) MOSFET driver.

When I take out the MOSFET driver the rail goes up to 11V.

I'm beginning to think the micro power voltage regulator was specified for a reason ... hmmmm ...
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pebe
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Post Number: 174
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Posted on Wednesday, 01 April, 2015 - 05:28 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Where is the circuit?
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mikeb
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Post Number: 1163
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Posted on Wednesday, 01 April, 2015 - 05:34 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

In the mag
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gordon
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Post Number: 1059
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Posted on Wednesday, 01 April, 2015 - 06:41 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

There seems to be 2 220nF capacitor droppers in the bridge rectifier circuit supplying the low voltage supply. I make the impedance about 14.5K per capacitor, so 29K in total. With 240 volts AC, you get about 8mA. Looks like you might be right about the reason for the low standby current 5 volt regulator. You also lose a bit of current from the 2 470R resistors in series with the capacitors, and the 15 volt zener. There was a very good artical years ago in EE, which explained mains dropper supplies very well, but I can't remember when!
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basementboy
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Post Number: 237
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Posted on Wednesday, 01 April, 2015 - 08:41 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I'm never keen on these so called wattless power supplies. When the caps go short circuit, they reliably blow up everything downstream.
I used to reckon on getting around 10mA total out of a supply like that, so the 7805 is stealing too much.
Just take a small transformer out the junk box and plumb it in.
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bruce
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Post Number: 1088
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Posted on Wednesday, 01 April, 2015 - 09:03 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I thought those wattless supplies provided 7mA for every 100n, so 400n should supply 30mA. However, this type of supply is tricky if you aren't careful. That 30mA flows whatever you do, so the zener needs to be able to cope ( in this case with 0.5W )
Also, I don't know the current draw of the 7805, but 30mA seems a bit stingy for everything that follows.
I reckon basementboy is right: use a wall-wart and ditch that silly psu. You're attached to the mains anyway and it will make the circuit live and troubleshooting risky. With a trivial circuit they can be OK, but a complex project needs lots of fiddling with.
Bruce
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mikeb
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Post Number: 1164
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Posted on Wednesday, 01 April, 2015 - 09:20 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

As usual - lots of helpful suggestions and ideas - I have a transformer salvaged from an old transistor radio that might fit the bill.

cheers

mb
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james
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Post Number: 507
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Posted on Wednesday, 01 April, 2015 - 11:04 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

If the transformer idea doesn't work out you could try changing the two 220nF X2 capacitors for 470nF X2 capacitors.
That should double the available current.

Cheers

James
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colin55
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Username: colin55

Post Number: 7
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Posted on Thursday, 02 April, 2015 - 12:04 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Use 78L05. It takes 1mA
I have covered cap-fed power supplies in full detail on my website: Talking Electronics.com

(Message edited by Colin55 on 02 April, 2015)
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colin55
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Post Number: 8
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Posted on Thursday, 02 April, 2015 - 12:10 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

The mistake John Clarke made is this:
The input capacitance is 110n and this will provide about 7.5mA. There are lots of mistakes in the magazine and this is just one more that no-one has picked up.
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mikeb
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Post Number: 1166
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Posted on Thursday, 02 April, 2015 - 12:58 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Colin55 - So I guess you are saying the 2off 220nF are in series giving 110nF.

Yes - I can see that ..... and that would tie up with my measurements. I have a couple of old 470nF X2 caps around. I will do a lash up and see if it works.

I have found EPE circuits work well, with few mistakes, but perhaps not in this case?
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colin55
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Post Number: 9
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Posted on Thursday, 02 April, 2015 - 03:13 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

You cannot use 7805 AT ALL in the circuit.

Use 78L05.
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james
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Post Number: 508
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Posted on Thursday, 02 April, 2015 - 09:46 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi Colin.

Impressive website, I expect an awful lot of work went into that.

In the capacitave power supply in question, I think that the effective series capacitance when the mosfet is switched on is 220nF. This is because two diodes in BR1 short out an alternate capacitor on each alternate half cycle of the mains. ie. Only one capacitor charges and discharges on each half mains cycle.
I wouldn't expect John Clarke to make a simple mistake like you suggest - too much of a class act!

Mike, just a friendly warning - something that you are probably well aware of anyway. Even when using an isolating transformer to produce the 15V DC supply, the "15V" will still be floating at 230V potential.

Cheers

James
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mikeb
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Post Number: 1167
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Posted on Thursday, 02 April, 2015 - 11:58 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

James

Yep thanks - the 'earthy' side will still be at 240V. I will bung this into Spice and check.

Colin

An amazing website - well done - you are doing your part to keep the home hobby alive.
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mikeb
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Post Number: 1168
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Posted on Friday, 03 April, 2015 - 04:06 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I have taken the 7805 and MOS driver out of circuit and the rail has risen to 14.63V.

So a reactive voltage dropper is very sensitive to power demands as you have advised. I don't think the zener is conducting, so I expect the rail to droop when the other devices are attached.

The recommended 5V regulator is only £1.80 so I will not stray any further from Johns design and I have learnt something to boot.

Thanks all .....
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mikeb
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Post Number: 1169
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Posted on Friday, 03 April, 2015 - 04:09 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

BTW - The MOS driver turns off at 10V so it cant be lower then that.
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mikeb
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Post Number: 1176
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Posted on Wednesday, 08 April, 2015 - 03:16 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Thanks for help - everything now working.

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