Discharge capacitor... alternating polarity.... Slave Clock Driver - Page 1
Forum section: electronics / projects

author: NivagSwerdna
author_rank: Super Contributor
timestamp: January 08, 2019, 11:43:30 am
content: Due to poor performance of my DCF77 receiver (due to local interference) I am thinking of re-doing my slave clock driver...Basically my slave clock has a solenoid that needs to be driven every second with a pulse (<100ms) of 24V and each second the polarity changes.My current naive circuit uses a boost module to get from 9V to 24V and switches using a L298N H-bridge module. This is, I think, totally overkill.I was wondering if rather than an H-bridge I could achieve similar with two capacitors in series, charged to 48V and then to have a common at the junction and then to collect the other wire of my clock to the top and bottom of the pair (with transistors, of which max one was open at any time)? (Not sure how the snubber diode would fit it).Am I overthinking this?* I think what I am asking is... can I make a poor man's H-bridge but using only 1-N and 1-P type transistor?

author: Ian.M
author_rank: Super Contributor
timestamp: January 08, 2019, 12:50:53 pm
content: A continuation ofhttps://www.eevblog.com/forum/beginners/boost-converter-to-charge-a-c-to-activate-a-solenoid/You'll have the problem of the top end of the pair bouncing around which may make providing it with reliable base drive *interesting*.Optocoupling its base drive should work OK. OTOH L298 chips are pretty cheap and you only need one big cap that way, so for a one-off I see no advantage over the mess of discretes you are considering.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: January 08, 2019, 01:30:45 pm
content: You have a better memory than me! But indeed... this time I thought I should make it manufacture-able... I'm older and wiser now.Indeed.. the plan would be for deep sleep between the minutes and a boost just prior to the pulse.Since this is no-longer a one off I thought I would try and reduce BOM cost... not convinced a H-bridge is cheap. (e.g. L398N is more than 300 pennies at 10-off)

author: Ian.M
author_rank: Super Contributor
timestamp: January 08, 2019, 02:00:32 pm
content: You need to consider the extra assembly costs as well as the base BOM cost. However you *may* be able to save a bit if you can find a cheaper H-bridge with enough current capability.Another factor if you are doing this as a kit or bare PCB is parts availability. A cheaper H-bridge may be viable for a one-off production run, but unless its something really common, you risk it becoming unobtanium or going on allocation if you don't buy all you need up front.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: January 08, 2019, 04:58:09 pm
content: ZXMHC6A07T8 looks like a possibility

author: David Hess
author_rank: Super Contributor
timestamp: January 08, 2019, 05:31:34 pm
content: What you suggest with a half bridge driving the solenoid into a pair of capacitors will work. Or a single capacitor in series with the solenoid will also work which is sometimes seen when driving a grounded speaker. With the capacitors sized correctly, this effectively becomes a half-bridge driving the load to ground from a bipolar power supply.